Communication and Homeostasis

Stimulus - any change in the environment that causes a response

Response - a change in behaviour or physiology as a result of a change in the environment

Can either use neuronal system or hormonal system for communication.

Neuronal - interconnected network of neurones that signal to each other across synapse junctions. the neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly.

Hormonal - uses blood to transport its signals. Cells in an endocrine organ release the signal directly into the blood. It is carried all over the body but is only recognised by specific target cells. The hormonal system enables longer-term responses to be coordinated.

Homeostasis - maintainance of the internal environment in a constant state despite external changes

Negative Feedback - process that brings about a reversal of change in the internal environment to return to a steady state or optimum position

Positive Feedback - process that increases the original change, causing the system to destabilise and cause harm

In order to maintain a constant internal environment a number of processes must occur:

• any change to the internal environment must be detected
• the change must be signalled to other cells
• there must be a response that reverses the change

Stimulus - Receptor - Communication Pathway - Effector - Response

There are a number of structures required for this pathway to work:

• Sensory receptors, such as temperature receptors or glucose conc. receptors. These receptors are internal and monitor conditions inside the body. If they detect a change they will be stimulated to send a message.
• A communication system such as the nervous system or the hormonal system. This acts by signalling between cells. It is used to transmit a message from the receptor cells to the effector cells. The message may or may not pass through a coordination centre such as the brain.
• Effector cells, such as hepatocytes or muscle cells. These cells will bring about a response that reverses the change detected by the receptor cells.

Positive feedback is less common than negative feedback.

When positive feedback occurs the response is to increase the original change. This destabilises the system and is usually harmful. Eg when the body gets too cold, the enzymes become less active. If they are less active the exergonic reactions that release heat are slower and release less heat. This allows the body to cool further and slows down the enzyme-controlled reactions even more, so that the body temperature spirals downwards.

There are some occasions when positive feedback can be beneficial, such as at the end of a pregnancy to bring about dilation of the cervix - change is signalled to the anterior pituitary gland, stimulating it to secrete oxytocin. 

Ectotherm - an organism that relies on external sources of heat to regulate its body temperature

Advantages to being an ectotherm:

• use less of their food in respiration
• need to find less food and may be able to survive for long periods without eating
• a greater proportion of the energy obtained from food can be used for growth

Disadvantages:

• less active in cooler temperatures, and may need to warm up in the morning before they can be active
• may not be capable of activity during the winter as they never warm up sufficiently

Adaptations that help regulate temperature:

• Thermal gaping is used by some larger ectotherms. The open mouth allows heat loss by evaporation from the moist mucus surfaces. Some tortoises have bee observed tp use a similar principle by spreading saliva over the neck and front legs.
• Reorientation of the body with respect to solar radiation can vary the surface area exposed to heating. 
• Colour changes of the skin may alter the ability of the body to absorb radiated heat energy. A dark-bodied individual will absorb heat more rapidly than a light-bodied one.
• Body raising is used by ectotherms to minimise heat gains by conduction from hot surfaces such as rocks and sand.
• Burrowing is widely used as a behavioural device which enables ectotherms to avoid the greater temperature fluctuations at the surface of their habitat. Amphibious and semi-aquatic reptiles may return to water rather than burrow.

Endotherm - organism that can use internal sources of heat to maintain its body temperature

Advantages of being an endotherm:

• fairly constant body temperature whatever the temperature is externally
• activity possible when external temperatures are cool
• ability to inhabit colder parts of the planet

Disadvantages:

• significant part of energy intake used to maintain body temperature in the cold
• more food required 
• less of the energy from food is used for growth, or more food is needed in order to grow

Behavioural mechanisms to maintain body temperature when too hot:

• move into shade or hide in burrow
• orientate body to decrease surface area exposed to sun
• remain inactive and spread out limbs to increase surface area

Physiological mechanisms to maintain body temperature when too hot:

• Sweat is secreted onto skin and changed into vapour, taking latent heat of vaporisation from the body to do this.
• Panting increass evaporation of water from lungs, tongue and other moist surfraces, using latent heat
• Pilo-erector muscles relaxed: hair shafts 'flatten' and allow heat to be lost by convection and radiation
• Vasodilation allows more blood into capillaries near the skin surface, so more heat lost by radiation
• Rate of metabolism is reduced in liver cells; less heat is generated from exergonic reactions such as respiration
• No spontaneous contractions from skeletal muscles

Physiological mechanisms to maintain body temperature when too cold:

• Less sweat is secreted; less evaporation of water, so less loss of latent heat
• The endotherm doesn't pant, so less water evaporates
• Pilo-erector muscles contract, so hair shafts are raised to trap a layer on insulating air, reducing the loss of heat from the skin
• Vasocontriction reduces the flow of bool through capillaries near the surface of skin, so less heat is radiated
• Rate of metabolism is increased, therefore respiration generates more heat, which is transferred to the blood
• Spontaneous contractions generate heat as muscle cells respire more

• Sensory neurones carry the action potentioal from a sensory receptor to the Central Nervous System (CNS)
• Motor neurones carry an action potential from the CNS to an effector such as a muscle or gland
• Relay neurones connect sensory and motor neurones

Structure:

• Many are very long so that they can transmit the action potential over a long distance
• The cell surface membrane has many gated ion channels that control the entry or exit of sodium, potassium or calcium ions
• They have sodium/potassium ion pumps that use ATP to actively transport sodium ions out of the cell and potassium ions in
• They maintain a potential difference across their cell surface membrane
• They are surrounded by a myelin sheath that insulates the neurone from the electrical activity in nearby cells - gaps in between called nodes of Ranvier
• They have a cell body that contains the nucleus, many mitochondria and ribosomes

Motor neurones have their cell body in the CNS and have a long axon that carries the action potential out to the effector.

Sensory neurones have a long dendron carrying the action potential from a sensory receptor to the cell body, which is positioned just outside the CNS. They then have a short axon carrying the action potential into the CNS.

Both sensory and motor neurones have numerous dendrites connected to other neurones.

Sensory receptors are specialised cells that can detect changes in our surroundings.

• The membrane starts in its resting state - polarised with the inside of the cell being -60 mV compared to the outside.
• Sodium ion channels open and some sodium ions diffuse into the cell.
• The membrane depolarises - it becomes less negative with respect to the outside and reaches the threshold value of -50 mV.
• Voltage-gated sodium ion channels open and many sodium ions flood in. As more sodium ions enter, the cell becomes positively charged inside compared with outside.
• The potential difference across the plasma membrane reaches +40 mV. The inside of the cell is positive compared with the outside.
• The sodium ion channels close and the potassium channels open.
• Potassium ions diffuse out of the cell brining the potential difference back to negative inside compared with outside - repolarisation.
• The potenital difference overshoots slightly, making the cell hyperpolarised.
• The original potential difference is restored so that the cell returns to its resting state.

For a short time after each action potential it is impossible to stimulate the cell membrane to reach another action potential - refractory period.

Resting Potential

Potential difference across the neurone cell membrane while the neurone is at rest (-60 mV).

• When a neurone is at rest, it is actively transporting ions across its cell surface membrane. Sodium/potassium ion pumps use ATP to pump 3 sodium ions out of the cell for every 2 potassium ions pumped into the cell. The plasma membrane is more permable to potassium ions than to sodium ions and many diffuse out again. The cell membrane is polarised.

Action Potential

Depolarisation of the cell membrane so that the inside is more positive than the outside, with a potential difference across the membrane of +40 mV.]

• A small depolarisation will have no effect on the voltage-gated channels, however if the depolarisation is large enough to reach threshold potenital, it will open some nearby voltage-gated channels. This causes a large influx of sodium ions and the depolarisation reaches +40 mV, which is an action potential - all or nothing.

Sodium and potassium ions cannot diffuse through the myelin sheath. Therefore the ionic movements that create an action potential cannot occur over much of the lenght of the neurone. The gaps  in the myelin sheath are gaps between the Schwann cells that make up the myelin sheath called nodes of Ranvier. The ionic exchanges that cause an action potential only occur at the nodes of Ranvier. In myelinated neurones the local currents are elongated and sodium ions diffuse along the neurone from one node of Ranvier to the next, meaning that the action potential appears to jump from one node to the next. This is saltatory conduction.

• An action potential arrives at the synaptic knob
• The voltage-gated calcium ion channels open
• Calcium ions diffuse into the synaptic knob
• The calcium ions cause the synaptic vesicles to move to and fuse with the presynaptic membrane
• Acetylcholine is released through exocytosis
• Acetylcholine molecules diffuse across the synaptic cleft
• Acetylcholine molecules bind with receptors sites on the sodium ion channels in the postsynaptic membrane 
• Sodium ion channels open
• Sodium ions diffuse across the postsynaptic membrane into the postsynaptic neurone
• A generator potential is created
• If sufficient generator potentials combine then the potential across the postsynaptic membrane reaches the threshold potential
• A new action potential is created in the post synaptic neurone

Neurotransmitter - chemical that diffuses across the cleft of the synapse to transmit a signal to the postsynaptic neurone

Cholinergic Synapses - use acetylcholine as their transmitter substance

Synaptic Knob - swellign at the end of the presynaptic membrane

Acetylcholinesterase - enzyme that breaks down acetylcholine into ethanoic acid and choline to stop transmission of signals

• Several presynaptic neurones might converge to one postsynaptic neurone.
• One presynaptic neurone might diverge to several postsynaptic neurones.
• Synapses ensure that signals are transmitted in the correct direction.
• Synapses can filter out unwanted low-level signals.
• Low-level signals can be amplified by a process called summation. If a low-level stimulus is persistent it will generate several successive action potentials in the presynaptic neurone.
• Acclimatisation - after repeated stimulation a synapse may run out of vesicles containing the transmitter substance. The synapse is said to be fatigued. This means the nervous system no longer responds to the stimulus.
• The creation of specific pathways within the nervous system is thought to be the basis of concious thought and memory. 

Frequency of Transmission

• When a stimulus is at higher intensity the sensory receptor will produce more generator potentials. This will cause more frequent action potentials in the sensory neurone. When these arrive at a synapse they will cause more vesicles to be released. In turn, this creates a higher frequency of action potentials in the postsynaptic neurone.

Myelination

• Myelinated neurones can transmit an action potential much more quickly than non-myelinated neurones can
• The increased speed of transmission means that the signal reaches the end of the neurone much more quickly
• This enables a more rapid response to stimulus
• Non-myelinated neurones tend to be shorter and carry signals only over a short distance - used in coordinating body functions such as breathing and the action of the digestive system

Secretes horomones directly into blood

• The cells receiving a hormone signal must possess a specific complementary receptor on their plasma membrane
• Each hormone is different from all the others
• This means that a hormone can travel around in the blood without affecting cells that do not possess the correct specific receptor
• The cells that possess the correct specific receptor are called target cells 

Two types of hormone:

• protein and peptide hormones, and derivatives of amino acids (adrenaline, insulin and glucagon)
• steroid hormones, such as sex hormones 

These two types of hormone work in different ways. Proteins are not soluble in the phospholipid membrane and do not enter the cell. Steriods can pass through the membrane and actually enter the cell to have a direct effect on the DNA in the nucleus

Adrenaline is an amino acid derivative. It is unable to enter the target cell. Therefore, it must cause an effect inside the cell without entering the cell itself.

The adrenaline receptor on the outside of the cell surface membrane has a shape complementary to the shape of the adrenaline molecule. This receptor is associated with an enzyme on the inner surface of the cell surface membrane - adenyl cyclase 

• Adrenaline in the blood binds to its specific receptor on the cell surface membrane.
• The adrenaline molecule is called the first messenger. 
• When it binds to the receptor it activates the enzyme adenyl cyclase.
• The adenyl cyclase converts ATP to cyclic AMP. 
• cAMP is a second messenger. 
• The cAMP can then cause an effect inside the cell by activating enzyme action.

Adrenal glands are found lying anterior to the kidneys - one on each side of the body. Each gland can be divided into a medulla region and a cortex region.

Adrenal Medulla

• Manufacture and release adrenaline in response to stress such as shock or pain - most cells have adrenaline receptors
• The effect of adrenaline is to prepare the body for activity

Adrenal Cortex

• Uses cholesterol to produce certain steroid hormones
• Mineralocorticoids help to control the concentrations of sodium and potassium in the blood
• Glucocorticoids (eg cortisol) helo to control the metabolism of carbohydrates and proteins in the liver

Pancreas

The pancreas has both exocrine and endocrine functions

• The majority of cells in the pancreas manufacture and release digestive enzymes
• The cells are found in small groups surrounding tiny tubules into which they secrete digestive enzymes
• The tubules join to make up the pancreatic duct which carries the fluid containing the enzymes into the first part of the small intestine 

Islets of Langerhans

• Alpha cells manufacture and secrete the hormone glucagon
• Beta cells manufacture and secrete the hormone insulin
• The islets are well supplied with blood capillaries and these hormones are secreted directly into the blood

If blood glucose rises too high

• Detected by beta cells in the islets of Langerhans
• Beta cells secrete insulin into the blood
• Insulin detected by receptors on liver and muscle cells
• Liver and muscle cells remove glucose from blood and convert glucose to glycogen (glycogenesis)
• Glucose concentration falls

If blood glucose drops too low

• Detected by alpha cells in the islets of Langerhans
• Alpha cells secrete glucagon into the blood
• Glucagon detected by receptors on liver cells
• Liver cells convert glycogen into glucose (glycogenolysis) and release glucose into the blood

Glucose can also be produced by converting amino acids and fats (gluconeogenesis)

• Cell membranes of the beta cells contain both calcium ion channels and potassium ion channels.
• The potassium ion channels are normally open and the calcium ion channels are normally closed. Potassium ions diffuse out of the cell making the inside of the cell more negative; at rest the potential difference across the cell membrane is about -70 mV. 
• When glucose concentrations outside the cell are high, glucose molecules diffuse into the cell.
• The glucose is quickly used in metabolism to produce ATP.
• The extra ATP causes the potassium channels to close.
• The potassium can no longer diffuse out and this alters the potential difference across the cell membrane - it becomes more negative inside.
• This change in potential difference opens the calcium ion channels.
• Calcium ions enter the cell and cause the secretion of insulin by making the vesicles containing insulin move to the cell surface membrane and fuse with it, releasing insulin by exocytosis

Diabetes mellitus - a disease in which blood glucose concentrations cannot be controlled effectively

Hyperglycaemia - the state in which the blood glucose concentration is too high

Hypoglycaemia - the state in which the blood glucose concentration is too low

• Also known as insulin-dependent diabetes.
• Usually starts in childhood. 
• It is thought o be the result of an autoimmune response in which the body's own immune system attacks the beta cells and destroys them. 
• It may also result from a viral attack. 
• The body is no longer able to manufacture sufficient insulin and cannot store excess glucose as glycogen.

Treatment

Type I diabetes is treated using insulin injections. The blood glucose concentration must be monitored and the correct dose of insulin must be administered to ensure that the glucose concentration remains fairly stable.

• Also known as non-insulin-dependent diabetes.
• A person with type II diabetes can still produce insulin, however as people age, their responsiveness to insulin declines. 
• This is probably becayse the specific receptors on the surface of the liver and muscle cells decline and the cells lose their ability to respond to the insulin in the blood. 
• The levels of insulin secreted by beta cells may also decline. 
• Some factors may bring an earlier onset of type II diabetes, such as obesity, family history, being asian/afro-carribean.

Treatment
Type II diabetes is usually treated by careful monitoring and control of the diet. Care is taken to match carbohydrate intake and use. This may eventually be supplemented by insulin injections or use of other drugs which slow down the absorption of glucose from the digestive system.

• The heart muscle is myogenic
• The heart contains its own pacemaker - this is called the sinoatrial node. The SAN is a region of tissue that can intiate an action potential, which travels as a wave of excitation over the atria walls, through the AVN (atrioventricular node) and down the Purkyne fibres to the ventricles, causing them to contract.
• The heart is supplied by nerves from the medulla oblongata. These nerves connect to the SAN. These do not initiate a contraction, but they can affect the frequency of the contractions. Action potentials sent down the accelerator nerve increase the heart rate. Action potentials sent down the vagus nerve reduce the heart rate.
• The heart muscle responds to the presence of adrenaline in the blood.

Factors that affect the heart rate

• movement of limbs is detected by stretch receptors in muscles - increase heart rate
• exercise produces more carbon dioxide, which can react with water and decrease pH, which is detected by chemoreceptorsin the carotid arteries - increase heart rate
• presence of adrenaline increases heart rate
• blood pressure rises too high - reduces heart rate