The structure of the human brain
- The cerebrum, which is the largest part and organises most of our higher thought processes, such as conscious thought and memory.
- The cerebellum, which coordinates movement and balance.
- The hypothalamus and pituitary complex, which organises homeostatic responses and controls various physiological processes.
- The medulla oblongata, which coordinates many of the autonomic responses.
Parasympathetic nervous system
- Consists of a few nerves leading out of the CNS, which divide up and lead to different effectors.
- Ganglia in the effector tissue
- Long pre-ganglionic neurones (length dependent of position of the effector).
- Short post-ganglionic neurones
- Uses acetylcholine as the neurotransmitter
- Decreases activity- conserves energy
- Most active in sleep or relaxation
- Effects include: decreases heart rate, constricts pupils, reduces ventilation rate, increases digestive activity, sexual arousal.
The cerebrum has 2 cerebral hemispheres, which are connected via major tracts of neurones called the corpus callosum. The outermost layer of the cerebrum consists of a thin layer of nerve cell bodies called the cerebral cortex. The cerebrum is much more highly developed in humans than any other organism. It controls the higher brain functions such as conscious thought, conscious actions, emotional responses, intelligence, reasoning, judgement and decision making and factual memory.
The cerebral cortex is subdivided into areas responsible for specific activities and body regions:
- Sensory areas receive APs indirectly from the sensory receptors. The sizes of the regions allocated to receive input from different receptors are related to the sensitivity of the area that inputs are received from.
- Association areas compare sensory inputs with previous experience, interpret what the input means, and judge an appropriate response.
- Motor areas send APs to various effectors. The sizes of the regions allocated to deal with different effectors are related to the complexity of the movements needed in the parts of the body. Motor areas on the left side of the brain control effectors on the right side of the body and vice versa.
It contains over half of all of the neurones in the brain. It is involved with balance and fine coordination of movement. To do this it must receive information from many sensory receptors and process the information accurately. The sensory receptors that supply information to the cerebellum include the retina, the balance organs in the inner ear, and spindle fibres in the muscles, which give information about muscle length and the joints.
The conscious decision to contract voluntary muscles is initiated in the cerebral cortex. However, the cerebral cortex does not provide the complex signals required to coordinate complex movements. The cerebellum coordinates the fine control of muscular movements such as maintaining body position and balance, judging the position of objects and limbs when moving, tensioning muscles and coordinating contraction and relaxation of antagonistic skeletal muscles.
This control often requires learning. Once learnt, actions may become second nature and involve much unconscious control. It is 'programmed' into the cerebellum and neurones from the cerebellum conduct APs to the motor areas so that motor output to the effectors can be finely controlled.
The cerebrum and cerebellum are connected by the pons.
The hypothalamus and pituitary complex 1
The hypothalamus controls homeostatic mechanims in the body. It contains its own sensory receptors and acts by negative feedback to maintain a constant internal environment.
- Temperature regulation: the hypothalamus detects changes in core body temperature. However, it also receives sensory input from temperature receptors in the skin. It will initiate responses to temperature change that regulate body temperature within a narrow range. These responses may be mediated by the nervous system or by the hormonal system (via the pituitary gland).
- Osmoregulation: the hypothalamus contains osmoreceptors that monitor the water potential in the blood. When the water potenial changes, the osmoregulatory centre initiates responses that bring about a reversal of this change. The responses are mediated by the hormonal system via the pituitary gland.
Sympathetic nervous system
- Consists of many nerves leading out of the CNS, each leading to a separate effector.
- Ganglia just outside CNS
- Short pre-ganglionic neurones
- Long post-ganglionic neurones (length dependent of position of the effector)
- Uses noradrenaline as the neurotransmitter
- Increases activity- prepares body for activity
- Most active at times of stress
- Effects include: Increases heart rate, dilates pupils, increases ventilation rate, reduces digestive activity, ******.
The autonomic nervous system
The autonomic nervous system consists of motor neurones that conduct action potentials from the CNS to effectors that are not under voluntary control. This includes glands, the cardiac muscle and smooth muscle in the walls of the blood vessels, the airways and the wall of the digestive system. The control of many of these effectors does not require rapid responses, and the neurones are mostly non-myelinated. There are at least 2 neurones involved in the connection between the CNS and the effector. These neurones are connected at small swellings called ganglia.
Autonomic means 'self-governing', and the autonomic nervous system operates to a large extent independently of conscious control. It is responsible for controlling the majority of the homeostatic mechanisms and so plays a vital role in regulating the internal environment of the body.
It can be further divided into the sympathetic, which prepares the body for action, and parasympathetic nervous system, which conserves energy. The 2 systems differ in both structure and action. They work antagonistically, as the action of one system opposes the action of the other. At rest, APs pass along the neurones at a low frequency. This is controlled by subconscious parts of the brain.
Peripheral nervous system
The role of the PNS is to ensure rapid communication between the sensory receptors, CNS and the effectors.
The PNS is composed of sensory and motor neurones. These are usually bundled together in a connective tissue sheath to form nerves.
Sensory nervous system
The sensory fibres entering the CNS are dendrons of the sensory neurones.
These neurones conduct action potentials from the sensory receptors into the CNS.
These neurones have their cell body in the distal root leading into the spinal cord and a short axon connecting to other neurones in the CNS.
Motor nervous system
The motor nervous system conducts action potentials from the CNS to the effectors.
It is further subdivided according to the functions of the motor nerves:
•Somatic nervous system
•Autonomic nervous system
The somatic nervous system
The somatic nervous system consists of motor neurones that conduct APs from the CNS to the effectors that are under voluntary control, such as the skeletal muscles.
These neurones are mostly myelinated, so that responses can be rapid.
There is always one single motor neurone connecting the CNS to the effector.
The hypothalamus and pituitary complex 2
The pituitary gland acts in conjunction with the hypothalamus. The pituitary gland consists of two lobes:
- The posterior lobe is linked to the hypothalamus by specialised neurosecretory cells. Hormones such as ADH, which are manufactured in the hypothalamus, pass down the neuroscretory cells and are released into the blood from the pituitary gland.
- The anterior lobe produces its own hormones, which are released into the blood in response to releasing factors produced by the hypothalamus. These releasing factors are hormones that need to be transported only a short distance from the hypothalamus to the pituitary. Hormones from the anterior pituitary control a number of physiological processes in the body, including reponse to stress, growth, reproduction and lactation
The medulla oblongata
The medulla oblongata controls the non-skeletal muscles (the cardiac muscles and involuntary smooth muscles) by sending action potentials out through the autonomic nervous system. The medulla oblongata contains centres for regulating several vital functions, including:
- The cardiac centre, which regulates heart rate
- The vasomotor centre, which regulates circulation and blood pressure
- The respiratory centre, which controls the rate and depth of breathing.
These centres receive sensory information and coordinate vital functions by negative feedback.
The 'fight or flight' response
Detecting a threat to survival stimulates the 'fight or flight' response.
In mammals, this leads to a range of physiological changes that prepare the animal for activity.
The activity may be running away, or it may be a direct challenge to the perceived threat.
Simulus and response
The brain must assimilate changes to the internal and external environment and coordinate a response that ensures survival.
Responses may be short term (e.g. the homeostatic meachanisms of temperature control), or longer term responses (e.g. the behaviours associated with reproduction).
The brain coordinates responses throughout output to the receptors. This output may include:
- action potentials in the somatic nervous system
- action potentials in the sympathetic and parasympathetic parts of the autonomic nervous system
- release of hormones via the hypothalamus and pituitary gland
Pupils dilate - Allows more light to enter the eyes, making the retina more sensitive
Heart rate and blood pressure increase - Increases rate of blood flow to deliver more oxygen and glucose to the muscles and to remove cabon dioxide and other toxins.
Aterioloes to the digestive system and skin are constricted, whilst those to the muscles and liver are dilated - Diverts blood flow away from the skin and digestive system and towards the muscles.
Blood glucose levels increase - Supplies energy for muscular contraction
Metabolic rate increases - Converts glucose to useable forms of energy such as ATP
Erector pili muscles in the skin contract - Makes hairs stand up (sign of aggression)
Ventilation rate and depth increase - Increases gaseous exchnage so that more oxygen enters the blood and supplies aerobic respiration.
Endorphins are released in the brain - Wounds inflicted on the mammal do not prevent activity.
The action of the hypothalamus
Coordination of the fight or flight response
Receptors that can detect an external threat include the eyes, ears and nose. Internal receptors may detect a threat, such as pain or a sudden increase or decrease in blood pressure. The cerebrum uses such sensory input to coordinate a suitable response:
1. Inputs feed into the sensory centres in the cerebrum.
2. The cerebrum passes signals to the association centres.
3. If a threat is recognised, the cerebrum stimulates the hypothalamus.
4. The hypothalamus increases activity in the sympathetic nervous system and stimulates the release of hormones from the anterior pituitary gland.
Knee jerk 2
As happens with other reflex actions, the higher parts of the brain are informed that the reflex is occurring. However, because there is no relay neurone, the brain cannot inhibit the reflex.
Inhibition relies on rapid myelinated neurones carrying the inhibitory action potentials to the synapse before the motor neurone is stimulated. In the absence of a relay neurone, the motor neurone is stimulated directly by the sensory neurone and there is insufficient delay to enable inhibition. This is why doctor test your reflexes by tapping the tendon below the knee cap. It causes an immediate response that cannot be inhibited.
While we are walking or running, the knee must bend and will stimulate the muscle spindles. However, the complex pattern of nervous impulses coming from the cerebellum is able to inhibit the reflex contractions.
As action potentials are sent to the hamstring muscles, stimulating them to contract, inhibitory action potentials are sent to the synapse in the reflex arc to prevent the reflex contraction of the opposing muscle.
Knee jerk reflex 1
It is a spinal reflex - the nervous pathway passes through the spinal cord rather than through the brain.
The knee jerk reflex is involved in coordinated movement and balance. The quadriceps contract to straighten the leg. This muscle is attached to the lower leg via the patella tendon that connects the patella to the lower leg bones at the front of the knee. When the muscles at the front of the thigh are stretched, specialised stretch receptors called muscle spindles detect the increase in length of the muscle. If this stretching is unexplained, a reflex action causes contraction of the same muscle.
This is part of the mechanism that enables us to balance on two legs. Consider a situation where you are stating still. Under these conditions, the muscle in front of the thigh will stretch if the knee is bending or the body is starting to lean backwards. Contraction of the muscle straightens the knee or brings the body back above the legs. Such as response must be very rapid, so that the body can remain balanced.
The knee jerk reflex is unusual in that the nervous pathway consists of only the sensory neurone and the motor neurone. This makes the response quicker as there is one less synapse involved.
What is a reflex action?
Reflex actions are responses to change in the environment that do not involve any processing in the brain to coordinate the movement. The nervous pathway is as short as possible so that the reflex is rapid. Most reflex pathways consist of just 3 neurones:
Sensory neurone ---> Relay neurone ---> Motor neruone
The brain may be informed that the reflex has happened, but is not involved in coordinating the response.
Reflex actions always have a survival value. A reflex may be used to get out of danger, to avoid damage to part of the body, or it may be used to maintain balance.
Reflex actions include the blinking reflex and the knee jerk reflex.
The blinking reflex causes temporary closure of the eyelids to protect the eyes from damage.
The nervous pathway for the blinking reflex passes through a part of the brain - the reflex is a cranial reflex. However, the pathways is direct that does not involve any thought processes in the higher parts of the brain. Since the receptor and the effector are in the same place, this is called a reflex arc.
Blinking may be stimulated by sudden changes in the environment such as:
- A forgein object touching the eye (the corneal reflex)
- Sudden bright light (the optical reflex)
- Loud sounds
- Sudden movements close to the eye
This reflex is mediated by a sensory neurone from the cornea, which enters the pons. A synapse connects the sensory neurone to a relay neurone, which passes the action potential to the motor neurone. The motor neurone passes back out of the brain to the facial muscles, causing the eyelid to blink. This is a very short and direct pathway, so the corneal reflex is very rapid - it takes about 0.1 seconds. The corneal reflex usually causes both eyes to blink, even is one cornea is affected.
The sensory neurone involved also passes the action potential to myelinated neurones in the pons. These myelinated neurones carry the action potential to the sensory region in the cerebral cortex, to inform the higher parts of the brain that the stimulus has occurred. This allows the reflex to be overridden by concious control. The higher parts of the brain can send inhibitory signals to the motor centre in the pons. The myelinated neurones carrying impulses to and from the cerebral cortex transmit action potentials much more rapidly than the non-myelinated relay neurone in the pons. Therefore, the inhibitory action potentials can prevent the formation of an action potential in the motor neurone.
This protects the light-sensitive cells of the retina from damage.
The stimulus is detected by the retina and the reflex is mediated by the optical centre in the cerebral cortex.
The optical reflex is a little slower than the corneal reflex.
Plants respond to external stimuli
It seems obvious that animals respond to the biotic (living) and abiotic (non-living) components of their environment.
Plants also respond to the external stimuli.
Central nervous system
The CNS consists of the brain and spinal cord.
The human brain contains about 86 billion neurones. Much of the brain is composed of relay neurones, which have multiple connections enabling complex neural pathways.
Most of these cells are non-myelinated cells and the tissue looks grey in colour (grey matter).
The spinal cord also has many non-myelinated relay neurones making the central grey matter.
However, the spinal cord also contains large numbers of myelinated neurones making up an outer region of white matter. These myelinated neurones carry action potentials up and down the spinal cord for rapid communication over longer distances.
The spinal cord is protected by the vertebral column.
Between each of the vertebrae peripheral nerves enter and leave the spinal cord carrying action potentials to and from the rest of the body.
Stem elongation 2
Then the researchers chose a pea plant with a mutation that blocks gibberelling production between ent-kaurene and GA12-aldehyde in the synthesis pathway for gibberellins.
Those plants produce no gibberellin and grow to only about 1cm.
The researches grafted a shoot onto a homozygous le plant (which cannot convert GA20 to GA1) and it grew tall.
Such a shoot, with no GA20 of its own, does have the enzyme to convert G20 to GA1, and it can use the unused GA20 from the normal plant.
Because the shoot grew tall, this confirmed that GA1 causes stem elongation.
Further studies have shown that gibberellins cause growth in the internodes by stimulating cell elongation (by loosening cell walls) and cell division (by stimulatingproduction of a protein that controls the cell cycle).
Stem elongation 1
In Japan, a fungus causes a disease which makes rice grow very tall. The fungal compound involved are gibberelins and include gibberellic acid (GA3).
Scientists tested gibberellic acid on many different plants. When they applied it to dwarf varieties of plants such as maize and peas, or to rosette plants (like cabbages), they grew taller. This suggests that gibberellic acid is responsible for plant stem growth. However, just because GA3 can cause stem elongation, it doesn't mean that it does so in nature. An experiment like this needs to work within concentrations of gibberellins naturally found in plants, and in parts of the plant that gibberelin molecules normally reach.
Researchers found a way to meet these criteria. They compared GA1 levels (another member of the giberellin family) of tall pea plants (homozygous for the dominant Le allele) and dwarf pea plants (homozygous for the recessive le allele), which were otherwise genetically identical. They found that plants with higher GA1 levels were taller.
However, to show that GA1 directly causes stem growth, researchers need to know how GA1 is formed. They worked out that the Le gene was responsible for producing the enzyme that converted GA20 to GA1.
Gibberellins promote seed germination.
When the seed absorbs water, the embryo releases gibberellin, which travels to the aleurone layer in the endosperm region of the seed.
The gibberellin enables the production of amylase, which can break down starch into glucose.
This provides a substrate for respiration for the embryo, and so it grows.
The glucose is also used for protein synthesis.
Where in a plant does growth occur?
In plant cells the cell wall limits the cell's ability to divide and expand. Growth only happens in particular places in the plant where there are groups of immature cells that are still capable of dividing, called meristems.
- Apical meristems are at tips or apices of roots and shoots, and are responsible for the roots and shoots getting longer.
- Lateral bud meristems are found in the buds. These can give rise to side shoots.
- Lateral meristems forming a cylinder near the outside of roots and shoots and are responsible for the roots and shoots getting wider.
- In some plants, intercalary meristems are located between the nodes, where the leaves and buds branch off the stem. Growth between the nodes is responsible for the shoot getting longer.
Investigating phototropic and geotropic responses
The experiment has an experimental plant and a control plant (with 10 replicates). The control plant is illuminated from all sides, while the experimental plant has illumination from just one side. In each plant, the shoots and roots are marked every 2mm at the start.
Look at the results after several days. The shoot has bent towards the light, because the shady side of the shoot has elongated more than the illuminated side. The mean and standard deviation of the lengths between the marks has increased on the shady side.
We can also investigate geotropic responses. A control plant is constantly spun (very slowly) by a machine called a klinostat to ensure the effect of gravity is applied equally to all sides of the plant. For the experimental plant, the klinostat is not switched on, so gravity is only applied to one side.
In the experimental plant, the root bends downwards, because the upper side of the root has elongated more that the lower side. The shoot bends upwards, because the lower side of the shoot has elongated more than the upper side. In the control, both root and shoot grow horizontally.
Take care not to handle mains plugs, switches, lamps or the klinostat with wet hands.
Gibberellins are plant hormones which are responsible for control of stem elongation and seed germination.
Hormones involved with regulating plant growth
- Abscisic acid inhibts bud growth. High auxin in the shoot may keep abscisic acid levels high in the bud. When the tip (the source of auxin) is removed, abscisic acid levels drop and the bud starts to grow. Abscisic acid also inhibits seed germination and growth and causes stomatal closure when the plant is stressed by low water availability.
- Cytokinins promote bud growth- directly applying cytokinin to buds can override the apical dominance effect. High levels of auxin make the shoot apex a sink for cytokinics produced in the roots- most of the cytokinin goes to the shoot apex. When the apex is removed, cytokinin spreads evely around the plant. Cytokinins also promote cell division, delay leaf senesence and promote cell expansion.
Types of stimuli
Responding to the environment may help plants to survive long enough to reproduce. For example, in higher temperatures, plants may deposit thicker layers of wax on their leaves; in very windy conditions, they may have vascular tissue which is more heavily lignified. Plants show specific responses to the threat of herbivores, employing the following chemical defences:
- Tannins- toxic to microorganisms and larger herbivores. In leaves, they are found in the upper epidermis, and make the leaf taste bad. In roots, they prevent infiltration by pathogenic microoorganisms.
- Alkaloids- derived from amino acids. In plants, scientists think they are a feeding deterrent to animals, tasting bitter. They are located in growing tips and flowerese, and peripheral cell layers of stems and roots.
- Pheromones- chemicals which are released by one individual and which can affect the behaviour or physiology of another.
Types of response
Tropisms are directional growth response of plants. They include:
- phototropism- shoots growth towards light (they are positively phototropic), which enables them to photosynthensise.
- geotropism- roots grow towards the pull of gravity. This anchors them in the soil and helps them to take up water, which is needed for support (to keep cells turgid), as a raw material for photosynthesis and to cool the plant. There will also be minerals, such as nitrate in the water, needed for the synthesis of amino acids.
- chemotropism- on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
- thigmotropism- shoots of climbing plants, such as ivy, wind around other plants or soil structures to gain support.
If a plant responds towards a stimulus, it is a positive tropic response. If a plant responds away from a stimulus, it is a negative tropic response.
Non-directional responses to external stimuli are called nastic responses. E.g. Mimosa pudica responds to touch with a sudden folding of the leaves, this is an example of thigmonasty.
Control of responses - the role of plant hormones
Hormones coordinate plant responses to environmental stimuli. Like animal hormones, plant hormones are chemical messengers that can be transported away from their site of manufacture to act in other parts (target cells or tissues) of the plant. They are not produced in endocrine glands, but by cells in a variety of tissues in the plant.
When hormones reach their target cells, they bind to receptors on the plasms membrane. Specific hormones have specific shapes, which can only bind to specific receptors with complementary shapes on the membranes of particular cells. This specific binding makes sure that the hormones only act upon the correct tissues. Some hormones can have different effects on different tissues; some can amplify each other's effects, and some can even cancel out each other's effects. Hormones can influence cell division, cell elongation or cell differentiation.
Hormones can move around the plant by:
- active transport
- mass flow in the phloeum sap or in xylem vessles
Auxins are plant hormones which are responsible for regulating plant growth. If you break off the shoot tip (apex), the plant starts to grow side brances from lateral buds that were previously dominant.
Researches suggested that auxins from the apical bud prevent lateral buds from growing. When the tip is removed, auxin levels in the shoot drop and the buds grow. To test hypothesis applied a paste containing auxins to the cut end of the shoot, and the lateral buds did not grow. However, scientists' manipulation of the plants could have had an unexpected effect; upon exposure to oxygen, cells on the cut end of the stem could have produced a hormone that promoted lateral bud growth. Because of this, scientists applied a ring of auxin transport inhibitor below the shoot. The lateral buds grew.
Based on this result, scientists suggested that a normal auxin level in lateral buds inhibits growth, whereas low auxin levels promote growth. However, the 2 variables- auxin levels and growth inhibition- may have no effect on each other, but could be both affected by a third variable. Years later, a different scientist remarked that auxin levels in lateral buds of the kidney bean actually increased when the shoot tip was cut off. Now scientists think 2 other hormones are involved.
Divisions of the nervous system
Investigating the effect of plant hormones on phot
Darwin's experiments confirmed that the shoot tip was responsible for phototropic responses. Boysen-Jensen's work confirmed that water and/or solutes need to be able to move backwards from the shoot tip for phototropism to happen. When a permeable gelatine block was inserted behind the shoot tip, the shoot still showed positive phototropism. When an impermeable mica block was inserted, there was no phototropic responses.
Use of gibberellins in sugar production
Spraying sugar cane with gibberellins stimulates growth between the nodes, making the cells elongate.
This is useful because sugar cane stores sugar in the cells of the internodes, making more sugar available from each plant.
Use of gibberellins in plant breeding
A plant breeder's job is to produce plants with desired characteristics, by breeding together other plants, usually takes many generations.
However, in conifer plants this can take a particularly long time, because conifers spend a long as juveniles before becoming reproductively active.
Gibberellins can speed up the process by inducing seed formation on young trees.
Seed companies that want to harvest seeds from biennial plants (only flower in their 2nd year of life) can add gibberellins to induce seed production.
Stopping plants from making gibberellins is also useful.
Spraying with gibberellin synthesis inhibitors can keep flowers short and stocky, and ensures that the internodes of crop plants stay short, helping to prevent lodging.
Lodging happens in wet summers- stems bend over because of the weight of water collected on the ripened seed heads, making the crop difficult to harvest
Because ethene is a gas, and cannot be sprayed directly, scientists have developed 2-chloroethylphosphonic acid, which can be sprayed in solution, is easily absorbed, and slowly releases ethene inside the plant. Commercial uses of ethene include:
- speeding up fruit ripening in apples, tomatoes and citrus fruits
- promoting fruit drop in cotton, cherry and walnut
- promoting female sex expression in cucumbers, reducing the chance of self pollination (makes cucumbers taste bitter) and increasing yield
- promoting lateral growth in some plants, yielding compact flowering stems.
Restricting ethene's effects can also be useful. Storing fruit at a low temperature, with little oxygen and high carbon dioixde levels, prevents ethene synthesis and thus prevents fruit ripening.
This means fruits can be stored for longer - essential when shipping unripe bananas from the Caribbean. Other inhibitors of ethene synthesis, such as silver salts, can increase the shelf life of cut flowers.
The mammalian nervous system responding to the env
A successful organism must be able to respond to changes in the environment. These changes could be in the external or in the internal environment. The communication system must enable:
•detection of changes in the environment
•cell signalling to occur between all parts of the body
•coordination of a range of effectors to carry out responses to the sensory input
Many environmental changes require rapid and well-coordinated responses to ensure survival.
This may involve a wide array of responses such as coordinated muscle action, control of balance and posture, temperature regulation and coordination with the endocrine system. This is the role of the nervous system.
Use of gibberellins in brewing
To make beer you need malt, which is usually produced in a malt house at a brewery.
When barely seeds germinate, the aleurone layer of the seeds produces amylase enzymes that break down stored starch into maltose.
Usually, the genes for amylase production are switched on by naturally occuring gibberellins.
Adding gibberellins speeds up the process.
Malt is then produced by drying and grinding up the seeds.
Use of gibberellins in fruit production
- Gibberellins delay senescence in citrus fruit, extending the time fruits can be left unpicked, and making them available for longer in the shops.
- Gibberellins acting with cytokinins can make apples elongate to improve their shape.
- Without gibberellins, bunches of grapes are very compact; this restricts growth of individual growth. With giberellins, the grape stalk elongates, they are less compact, and the grapes get bigger.
The mechanism of auxin's effect
Auxin increases the stretchiness of the cell wall by promoting the active transport of H+ by an ATPase enzyme on the plasma membrane, into the cell wall. The resulting low pH provides optimum conditions for wall-loosening enzymes (expansins) to work. These enzymes break bonds within the cellulose (at the same time, the increased H+ ions also disrupt hydrogen bonds within cellulose), so the walls become less rigid and can expand as the cell takes in water.
How the light causes redistribution of auxin is still uncertain. 2 enzymes have been identified- phototropin 1 and 2- whose activity is promoted by blue light. Blue light is the main component of white light that causes phototropic response. Hence, there is lots of phototropin 1 activity on the light side, but progressively less activity towards the dark side. This gradient is thought to cause the redistribution of auxins through their effect on PIN proteins. These transmembrane proteins can be found dorsally, ventrally or laterally on the plasma membrane of cells, and they control the efflux of auxin from each cell, essentially sending auxin in different directions in the shoot, depending upon their location on the plasma membrane.
The activity of PIN proteins is controlled by a different molecule, PINOID. One theory suggests that phototropins affect the activity of PINOID, which then affects PIN activity. However, recent evidence from Arabidopsis suggests this may only work for pulse-induced phototropism (short bursts of light), which another independent mechanism able to operate in continuous light.
Auxin in geotropic responses
Auxin is also involved in the geotropic responses of roots.
In a root lying flat, Went discovered that auxin accumulates on the lower side, where it inbits cell elongation.
The upper side continues to grow and the root bends downwards.
This effect of auxin in roots is in contrast to that in the shoot, where auxin promotes cell elongation on the lower side, making the shoot lying flat bend upwards.
This happens because root and shoot cells in the elongation zone exhibit different responses to the same concentrations of auxin.
The commercial use of auxin
Artificial auxins can be used to prevent leaf and fruit 'drop' and to promote flowering for commercial flower production. This is useful if there are too many small fruit that will be difficult to sell - the plant then produces fewer, larger fruit. Other uses include:
- Dipping the end of a cutting in rooting powder (auxins and talcum powder) before planting encourages root growth.
- Treating unpollinated flowers with auxin can promote growth of seedless fruit (parthenocarpy). Applying auxin promotes ovule growth, which triggers automatic production of auxin by tissues in the developing fruit, helping to complete the developmental process.
- Auxins are used as herbicides to kill weeds. Because they are man made, plants find them more difficult to break down, and they can act within the plant for longer. They promote shoot growth so much that the stem cannot support itself, buckles and dies.
The chemical messenger: auxin
To confirm the role of auxin as a chemical messenger, agar blocks impregnated with different concentrations of auxin give the same result. In fact, using a series of blocks of different concentrations of auxin (indole-3-acetic acid) created by serial dilution gives a shoot curvature in proportion to the amount of auxin.
Auxins are produced at the apex of the shoot. The auxin travels to the cells in the zone of elongation, causing them to elongate, and making the shoot grow. When light is equal on all sides, the auxin simply promotes shoot growth evenly.
Light shining on one side of the shoot appears to cause auxins to be transported to the shaded side, causing the cells there to elongate more quickly, making the shoot bend towards the light.
The extent to which cells elongate is proportional to the concentration of auxins.
The commercial use of cytokinins
Because cytokinins can delay leaf senescence, they are sometimes used to prevent yellowing of lettuce leaves after they have been picked.
Cytokinins are used in tissue culture to help mass-produce plants.
Cytokinins promote bud and shoot growth from small pieces of tissue taken from a parent plant.
This produces a short shoot with a lot of side branches, which can be split into lots of small plants.
Each of these is then grown separately.
The role of the sympathetic nervous system
The autonomic nervous system controls many physiological mechanisms on card 48.
Increasing stimulation of the sympathetic nervous system will increase the activity of the effectors, as described on card 32.
However, nervous communication is used for rapid response rather than prolonged response. A fight or flight from danger may need to be a prolonged response.
This is achieved by the endocrine system.
The sympathetic nervous system stimulates the adrenal medulla.
Adrenaline released from the adrenal medulla has a wide range of effects on cells (see hormonal communication topic).
The mechanism of adrenaline action
Adrenaline is a first messenger. It is an amino acid derivative and is, therefore, unable to enter the target cells; it must cause an effect inside the cell, without entering the cell itself.
1. Adrenaline binds to the adrenaline receptor on the plasma membrane. This receptor is associated with a G protein on the inner surface of the plasma membrane, which is stimulated to activate the enzyme adenyl cyclase.
2. Adenyl cyclase converts ATP to cyclic AMP (cAMP), which is the second messenger inside the cell.
3. cAMP causes an effect inside the cell by activating enzyme action. The precise effect depends upon the cell that the adrenaline has bound to.
The release of hormones from the anterior pituitar
The hypothalamus secretes releasing hormones (releasing factors) into the blood. These pass down a portal vessle to the pituitary gland and stimulate the release of tropic hormones from the anterior part of the pituitary gland. These stimulate activity on a variety of endocrine glands.
- Corticotropin-releasing hormone (CRH) from the hypothalamus causes the release of adrenocorticotropin hormone (ACTH). ACTH passes around the blood stream, and stimulates the adrenal cortex to release a number of different corticosteroid hormones. These include glucocorticoids such as cortisol, which regulates the metabolism of carbohydrates. As a result, more glucose is released from glycogen stores. New glucose may also be produced from fat and protein stores.
- Thyrotropin-releasing hormone (TRH) causes the release of thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to release more thyroid hormone (thyroxine). Thyroxine acts on nearly every cell in the body, increasing the metabolic rate and making the cells more sensitive to adrenaline.
The human heart
The heart pumps blood around the circulatory system. This cicrulation has many important roles such as the transport of oxygen and nutrients (glucose, fatty acids and amino acids) to the tissues, removal of waste products (CO2), transport of urea from liver to kidneys and to distribute heat around the body.
The requirements of the cells and tissues vary according to their level of activity. When you are being physically active, your muscle cells need more oxgyen and glucose so that they can respire more, releasing the energy for contraction. Your heart muscle cells also need more oxygen and fatty acids. All the muscles will also need to remove more carbon dioxide and heat.
It is essential that the circulatory system can adapt to meet the needs of the tissues. Part of this adaption is controlling the activity of the heart. The heart action can be modified by:
- Raising or lowering the heart rate. This is the number of beats per minute.
- Altering the force of the contractions of the ventricular walls.
- Altering the stroke volume (volume of blood pumped per beat).
The rate at which the heart beats is affected by a number of factors.
Cardiac muscle in the heart is myogenic. This means that it can initiate its own beat at regular intervals. However, the atrial muscle has a higher myogenic rate than the ventricular muscle.
The 2 pairs of chambers must contract in a coordinated fashion or the heart action will be ineffective. Therefore, a coordination mechanism is essential.
The heart contains its own pacemaker, the sinoatrial node. The SAN initiates waves of excitation that usually override the myogenic action of the cardiac muscle. The SAN is a region of tissue that can initiate an action potential, which travels as a wave of excitation over the atrial walls, through the AVN and down the Purkyne fibres to the walls of the ventricles, causing them to contract.
The heart muscle also responds directly to adrenaline in the blood, which increases the heart rate.
Control of heart rate by the cardiovascular centre
At rest, the heart rate is controlled by the SAN. This has a set frequency, varying from person to person, at which it initiates waves of excitation. The frequency of excitation is typically 60-80 per minute. However, the frequency of these excitation waves is altered by the output from the cardiovascular centre in the medulla oblongata.
Nerves from the cardiovascular centre in the medulla oblongata of the brain supply the SAN. These nerves are part of the autonomic nervous system. The nerves do not initiate a contraction, but can affect the frequency of the contractions:
- Action potentials sent down a sympathetic nerve (the accelerans nerve) cause the release of the neurotransmitter noradrenaline at the SAN. This increases the heart rate.
- Action potentials sent down the vagus nerve release the neurotransmitter acetylcholine, which increases the heart rate.
A range of environmental factors affect heart rate. Input from sensory receptors is fed to the cardiovascular centre in the medulla oblongata. Some inputs increase heart rate, others decrease it. The interaction of these inputs is coordinated by the cardiovascular centre to ensure that the output to the SAN is appropriate to the overall conditions.
Control of heart rate by the cardiovascular centre
Sensory input to cardiovascular centre includes:
- Stretch receptors detect movement of the limbs (proprioreceptors). These send impulses to the cadiovascular centre (CC), informing it that extra oxygen may soon be needed. This leads to an increase in heart rate (HR).
- Chemoreceptors in the carotid arteries, aorta and the brain monitor the pH of the blood. When we exercise, the muscles produce more carbon dioxide. Some of this reacts wuth the water in the blood plasma to produce a weak acid (carbonic acid). This reduces the pH of the blood, which will affect the transport of oxygen. The change in pH is detected by the chemoreceptors, which send action potentials to the CC. This tends to increase the HR.
- The chemoreceptors also detect when the level of carbon dioxide falls when we stop exercising. This reduces the activity of the accelerator pathway, decreasing the heart rate.
- Stretch receptors (baroreceptors) in the walls of the carotid sinus monitor blood pressure. The caroid sinus is a small swelling in the carotid artery. An increase in blood pressure, prehaps during vigorous exercise, is detected by baroreceptors. If pressure rises too high, they send action potentials to the CC, leading to a reduction in heart rate.
Artificial control of heart rate
If the mechanism controlling the heart rate fails, then an artificial pacemaker must be fitted.
A pacemaker delivers an electrical impulse to the heart muscle.
A pacemaker is implanted under the skin and fat on the chest (or sometimes within the chest cavity itself).
It may be connected to the SAN or directly to the ventricle muscle.
Three types of muscle
Muscles are composed of cells arranged to form fibres.
These fibres can contract to become shorter, which produces a force.
Contraction is achieved by interaction between two protein filaments (actin and myosin) in the muscle cells.
Muscle cannot elongate without an antagonist.
Therefore, muscles are usually arranged in opposing pairs, so that one contracts as the other elongates.
In some cases, the antagonist may be elastic recoil or hydrostatic pressure in a chamber.
There are 3 types of muscle: involuntary (smooth), cardiac, and voluntary (skeletal or striated muscle).
Involuntary (smooth muscle)
It consists of individual cells, tapered at both ends (spindle shaped).
At rest, each cell is about 500 um long and 5um wide.
Each cell contains a nucleus and bundles of actin and myosin.
This type of muscle contracts slowly and regularly.
It does not tire quickly.
It is controlled by the autonomic nervous system.
It is found in the walls of tubular structures, such as the digestive system and blood vessles.
The muscle is usually arranged in longitudinal and circular layers that oppose each other.
Cardiac muscle forms the muscular part of the heart. The individual cells form long fibres, which branch to form cross-bridges between the fibres.
These cross-bridges help to ensure that electrical stimulation spreads evenly over the walls of the chambers. When the muscle contracts, this arrangement also ensures that the contraction is a squeezing action rather than one-dimensional.
The cells are joined by intercalated discs. These are specialised cell surface membranes fused to produce gap junctions that allow free diffusion of ions between the cells. Action potentials pass easily and quickly along and between the cardiac muscle fibres.
Cardiac muscle contracts and relaxes continuously throughout life. It can contract powerfully and does not fatigue easily. Some muscle fibres in the heart (Purkyne fibres) are modified to carry electrical impulses. These coordinate the contraction of the chamber walls. Heart muscle is myogenic - it can initiate its own contraction. However, the rate of contraction is normally controlled by the SAN.
Cardiac muscle appears striated (striped) when viewed under the microscope.
Voluntary (skeletal or striated) muscle
Skeletal muscle occurs at the joints in the skeleton. Contraction causes the movement of the skeleton by bending or straightening the joint. The muscles are arranged in pairs called antagonistic pairs. When one contracts, the other elongates.
The muscle cells form fibres of about 100um in diameter. Each fibre is multinucleate and is surrounded by a membrane called the sarcolemma.
Muscle cell cytoplasm is known as sarcoplasm, and is specialised to contain many mitochondria and extensive sarcoplasmic reticulum (specialised endoplasmic reticulum).
The contents of the fibres are arranged into a number of myofibrils, which are the contractile elements. These myofibrils are divided into a chain of subunits called sarcomeres. Sarcomeres contain the protein filaments actin and myosin.
Actin and myosin are arranged in a particular banded pattern, which gives the muscle a striped or striated appearance.
Dark bands are known as the A bands and lighter bands are the I bands.
The neuromuscular junction
Skeletal muscle is under voluntary control.
Its contractions are stimulated by the somatic nervous system.
The junction between the nervous system and the muscle is called a neuromuscular junction.
It has many similarities to a synapse.
Stimulation of contraction
1. Action potentials arriving at the end of the axon open calcium ion channels in the membrane. Calcium ions flood into the end of the axon.
2. Vesicles of acetylcholine move towards and fuse with the end membrane.
3. Acetylcholine molecules diffuse across the gap and fuse with receptors in the sarcolemma.
4. This opens sodium ion channels, which allow sodium ions to enter the muscle fibre, causing depolarisation of the sarcolemma.
5. A wave of depolarisation spreads along the sarcolemma and down transverse tubules into the muscle fibre.
The motor unit
Some motor neurones stimulate single muscle fibres.
However, many motor neurones divide and connect to several muscle fibres.
All these muscle fibres contract together, providing a stronger contraction.
This is called a motor unit.
Structure of the myofibril
Myofibrils are the contractile units of skeletal muscle and contain two types of protein filament:
- Thin filaments, which are aligned to make up the light band; these are held together by the Z line.
- Think filaments, which make up the dark band.
The thick and thin filaments overlap, but in the middle of the dark band there is no overlap. This is called the H zone.
The distance between two Z lines is called a sarcomere.
This is the functional unit of muscle.
At rest, a sarcomere is about 2.5um long.
The thick and thin filaments are surrounded by sarcoplasmic reticulum.
The thin filaments are actin.
Each filament consists of 2 chains of actin subunits twisted around each other.
Wound around the actin is a molecule of tropomyosin to which are attached globular molecules of troponin.
Each troponin complex consists of three polypeptides: one binds to actin, one to troposmyosin and the third binds to calcium when it is available.
Tropomyosin and troponin are part of the mechanism to control muscular contraction.
At rest, these molecules cover binding sites to which the thick filaments can bind.
Each thick filament consists of a bundle of myosin molecules.
Each myosin molecule has 2 protruding heads, which stick out at each end of the molecule.
These heads are mobile and can bind to the actin when the binding sites are exposed.
The sliding filament hypothesis
During contraction, the light band and the H zone get shorter.
Therefore, the Z lines move closer together and the sarcomere gets shorter.
This observation led to the sliding filament hypothesis.
During contraction, the thick and thin filaments slide past one another.
The mechanism of contraction
The sliding action is caused by the movement of the myosin heads.
When the muscle is stimulated, the tropomyosin is moved aside, exposing the binding sites on the actin.
The myosin heads attach to the actin and move, causing the actin to slide past the myosin.
Control of contraction
1. When the muscle is stimulated, the action potential passes along the sarcolemma and down the transverse tubules (t-tubules) into the muscle fibre.
2. The AP is carried to the sarcoplasmic reticulum, which stores calcium ions, and causes the release of calcium ions into the sarcoplasm.
3. The calcium ions bind to the troponin, which alters the shape pulling the tropomyosin aside. This exposes the binding sites on the actin.
4. Myosin heads to bind to the acid, forming cross-bridges between the filaments.
5. The myosin heads move, pulling the actin filament past the myosin filament.
6. The myosin heads detach from the actin and can bind again further up the actin filament.
Millions of cross-bridges can be formed between the actin and the myosin filaments. Once contraction has occurred, the calcium ions are rapidly pumped back into the sarcoplasmic reticulum allowing the muscle to relax.
The role of ATP
ATP supplied the energy for one contraction. Part of the myosin head acts as ATPase and can hydrolyse the ATP to ADP and inorganic phosphate (Pi), releasing energy:
1.The myosin head attaches to the actin filament, forming a cross-bridge.
2. The myosin head moves (tilts backwards), causing the thin filament to slide past the myosin filament. This is the power stroke. During the power stroke, ADP and Pi are released from the myosin head.
3. After the power stroke, a new ATP molecule attaches to the myosin head, breaking the cross bridge.
4. The myosin head then returns to its original postition (swings forwards again) as the ATP is hydrolysed, releasing the energy to make this movement occur. The myosin head can now make a new cross-bridge further along the actin filament.
Maintaining the supply of ATP
As there are millions of myosin heads involved in muscle contraction, there is a huge requirement for ATP. The ATP is available in muscle tissue is only enough to support at most 1-2 seconds' worth of contraction. ATP must be regenerated very quickly in order to allow continued contraction. 3 mechanisms are involved in maintaining the supply of ATP:
- Aerobic respiration in mitochondria. Muscle tissue contains a large number of mitochondria in which aerobic respiration can occur. The Bohr effect helps to release more oxygen from the haemoglobin in the blood. However, during intense activity, the rate at which ATP can be produced will be limited by the delivery of O2 to the muscles.
- Anaerobic respiration in the sarcoplasm of the muscle tissue. Anaerobic respiration can release a little more ATP from the respiratory substances. However, it leads to the production of lactate, which is toxic. Anaerobic respiration can only last a few seconds before lactica acid build-up starts to cause fatigue.
- Creatine phosphate in the sarcoplasm acts as a reserve store of phosphate groups. The phosphate can be transferred from the creatine phosphate to ADP molecules, creating phosphotransferase is involved. The supply of creatine phosphate is sufficient to support muscular contraction for a further 2-4 seconds.