Communication and Hormones

Keeping cells active:

Living organisms need to maintain certain limited conditions inside their cells.

Cellular activities rely on enzymes that require specific condtitions to work efficiently.

Stimulus and response - External environments:

Changes in external environment may place stress on organism.

Changes must be monitored and organism's behaviour or physiology must change accordingly.

Environmental change is stimulus. Way in which organism changes is response.

Many cells in multicellular organism protected by epithelial tissues and organs e.g. skin.

Animals - internal cells and tissues bathed in tissue fluid -> environment of the cells.

Metabolic activities use up and produce substances - waste diffuses into tissue fluid.

Build up of carbon dioxide could affect pH of environment and therefore enzyme activity.

Accumulation of excess waste/toxins acts as stimulus, causing removal of wastes so cell can survive.

Cells respond by by reducing their activities so less waste is produced.

Stimulus and response - Maintaining the internal environment of cells:

Composition of tissue fluid maintained by blood, which transports substances to and from cells.

To prevent accumulation of wastes or toxins in blood, they must be excreted from body.

Important that concentrations of substances monitored closely - ensures too much of useful substances not excreted but enough waste removed.

Ensures cells supplied with substances they need.

Coordination:

Multicellular organism more efficient as cells can be differentiated - cells specialised to perform particular function.

Cells that monitor blood may be away from cells that release substance into blood.

Good communication system required:

• Covers whole body
• Enables communication between cells
• Allows specific communication
• Enables rapid communication
• Enables short and long-term responses.

Cell signalling:

Process by which cells communicate with each other - one cell releases chemical detected by another cell.

Two major systems of communication are neuronal and hormonal systems.

Neuronal system enables rapid responses and hormonal system enables longer-term responses.

Homeostasis:

The maintenance of the internal environment in a constant state depsite external changes.

Conditions needed to be kept constant:

• Temperature
• Glucose concentration
• Water potential of blood
• Blood pressure
• Carbon dioxide concentration

Negative Feedback:

To maintain a constant environment:

• Change in internal environment must be detected
• Change must be signalled to other cells
• Response to reverse change

Negative feedback is reversal of a change in internal environment to return to a steady state or optimum position.

Complex arrangement of structures that are all coordinated by cell signalling for negative feedback to work effectively. Pathway:

STIMULUS -> RECEPTOR -> CELL SIGNALLING -> EFFECTOR -> RESPONSE

• Sensory receptors are internal and monitor internal conditions. Will be stimulated to send message if change detected.
• Communication system like hormonal or nervous systems acts as signalling between cells. Used to transmit message from receptor to effector cells.
• Effector cells e.g. muscle or liver cells bring about response to reverse change detected.

Positive feedback:

Increases any change detected by receptors. Tends to be harmful, does not lead to homeostasis.

Examples -

When body gets too cold, enzyme activity decreases, exergonic reactions are slower and release less heat, cools body further, enzyme activity decreased even more etc.

Positive feedback beneficial during pregnancy with dilation of cervix.

Dilation detected and signalled to anterior pituitary gland, oxytocin secreted, increases contractions, cervix stretches more, more oxytocin secreted etc.

The meaning of constant:

Negative feedback maintains reasonably constant conditions - never remain perfectly constant

As long as variation not too great, conditions will be acceptable

The need to maintain body temperature:

Change in temperature can have effect on structure of proteins and enzymes. Affects enzymes' ability to function inside cells.

Level of activity achieved by organism altered if enzymes not functioning properly.

Core temperature important as vital organs withing core part of body.

Endotherms

Maintain the temperature of their body within fairly strict limits. Largely independent of the external temperature.

Ectotherms:

Rely on external sources of heat to regulate body temperature.

Not able to increase respiration to generate heat internally.

Advantages:

• Use less food in respiration
• Need less food and can survive long periods without eating
• Greater proportion of energy from food used for growth

Disadvantages:

• Less active in cooler temperatures, need to warm up before becoming active. Greater risk of predation.
• Not always capable of activity during winter, must have sufficient energy stores to survive winter.

Temperature regulation in ectotherms:

Once active, muscle contractions generate some heat from increase respiration.

Will change its behaviour or physiology to increase or decrease absorption of heat from its environment depending on temperature.

Exposing body to sun -> Enables more heat to be absorbed

Orientating body to sun -> Exposes larger surface area for more heat absorption

Orientating body away from sun -> Exposes lower surface area so less heat is absorbed

Hide in burrow -> Reduces heat absorption by keeping out of sun

Alter body shape -> Exposes more or less surface area to sun

Increase breathing movements -> Evaporates more water

Endotherms:

Can use internal sources of heat, like heat generated from metabolism in liver, to maintain body temperature.

Can use behavioural and physiological mechanisms as well to maintain body temperature.

Advantages:

• Fairly constant body temperature despite external temperature
• Activity possible when external temperature is cool
• Ability to inhabit colder parts of the planet.

Disadvantages:

• Significant part of energy intake used to maintain tmperature in cold.
• More food required
• Less energy from food used for growth. More food needed to grow.

Temperature regulation in endotherms:

Sweat glands in skin -> Water in sweat evaporates, heat from bloody to supply latent hear of vaporistion

Lungs, mouth and nose -> Panting increase evaporation of water from lungs, tongue etc.

Hairs on skin -> Hairs raised to trap insulating layer of air, reduces loss of heat from skin. Hairs like flat, little insulation, more heat lost by radiation and convection.

Arterioles leading to capillaries in skin ->Vasodilation allows more blood near skin's sufrace, more heat radiated from skin. Vasoconstriction reduces blood flow near surface so less heat radiated.

Liver cells -> Rate of metablism reduced, less heat generated from exergonic reactions. Rate of metabolism increased, respiration generates more heat which transferred to blood.

Skeletal muscles ->Spontaneous contractions generate heat as muscle cells respire more.

Behavioural mechanisms:

• Moving into shade/burrow or into sunlight
• Orientating body to decrease or increase surface area exposed to sun
• Remaining inactive and spreading out limbs to increase surface area or moving about to generate heat in muscles.

Control of temperature regulation:

Monitor temperature of blood in hypothalmus in brain.

If core temperature drops or rises above optimum, hypothalmus sends signals to reverse change (negative feedback).

The role of peripheral temperature receptors:

Thermoregulatory centre in hypothalmus monitors blood temperature and detects change.

Peripheral temperature receptors monitor temperature in extremities, send info to hypothalmus.

Sensory receptors:

Specialised cells that detect changes in surroundings - energy tranducers.

Stimulus is the changes in energy levels in the environment. Convert energy into electrical energy (nerve impulse)

Generating nerve impulses - Changing nerve impulses:

Neurones have specialised channel proteins specific to sodium or potassium ions.

When gate on channel open, permeability of membrane to specific ion increases.

Nerve cell membranes have carrier proteins - actively transport sodium ions out and potassium ions into cell.

Membrane is polarised. Inside of cell negatively charged compared to outside.

Impulse created by alteration of permeability of membrane.

Sodium ion channels open, membrane permeability increased, movement of ions down concentration gradient creates change in potential difference, cell less negative - depolarisation.

Generating nerve impulses - Generator potentials:

Small change in potential caused by one or two sodium ion channels opening is called generator potential.

Larger the stimulus, more gated channels will open.

If enough sodium ions enter cell, potential difference changes significantyly and initiates action potential.

Sensory and motor neurones:

Impulse transmitted to other parts of body along neurones as an action potential.

Types of neurone:

• Sensory neurones - from sensory receptor to CNS
• Motor neurones - from CNS to effector cells
• Relay neurones - connect sensory and motor neurones.

Neurones have similar basic structur to help carry out function

• Very long - can transmit action potentials over long distances
• Plasma membrane has many gated ion channels - control entry/exit of ions
• Have sodium/potassium pumps - actively transport sodium ions in and potassium ions out
• Maintain potential difference across their cell surface membrane
• Surrounded my myelin sheath - insulate neurone
• Have a cell body - contains nucleus, mitochondira, ribosomes
• Numerous dendrites - connect to other neurones
• Motor have cell body in CNS and long azon that carries impulse to effector
• Sensory have long dendron carrying impulse from receptor to cell body outside CNS, then short axon carryin impulse into CNS.

A resting neurone:

Sodium/potassium pumps actively transport 3 sodium ions out and 2 potassium ions into cell

Membrane more permeable to potassium ions - some diffuse out again

Interior of cell maintained at negative potential compared to outside - polarised

Resting potential is about -60mV

An action potential:

At rest, sodium gated channels closed.

If some open, sodium ions diffuse down concentration gradient into cell, causing depolarisation of membrane.

Gated channels in receptor cell opened by energy changes in environment. Gates further along opened by changes in potential difference (voltage-gated channels)

All or nothing:

Generator potentials in sensory receptor are depolarisations of membrane

Small depolarisation has no effect on voltage-gated channels.

If depolarisation large enough to reach threshold potential, will open nearby voltage gated channels

Causes influx of sodium ions, depolarisation reaches +40mV (action potential).

Once value reached, action potential transmitted because many gated channels open.

Action potential is self-perpetuating - continues along to end of neurone

Ionic movements:

After action potential, ion concentrations must be restored by sodium/potassium pumps

Refactory period allows cell to recover and ensures impulses only transmitted in one direction

Impossible to stimulate cell membrane to reach another action potential during this period.

Events of an action potential:

1. Membrane in resting state - polarised - inside of cell -60mV compared to outside.

2. Sodium ion channels open and somme sodium ions diffuse into cell.

3. Membrane depolarises - becomes less negative - reaches threshold potential of -50mV.

4. Voltage-gated sodium ion channels open, sodium ions flood in. Cell becomec more positively charged inside compared to outside.

5. Potential difference across membrane reaches +40mV

6. Sodium ion channels close and potassium channels open.

7. Potassium ions diffuse out of cell briging potential difference back down - repolarisation.

8. Potential difference overshoots slightly - hyperpolarised cell

9. Original potential difference is restored so cell returns to resting state.

Local currents:

Opening of sodium ion channels at one point upsets resting potential created by ion pumps.

Creates local currents in cytoplasm. Cause sodium ion channels further along membrane to open

• When action potential occurs, sodium channels at one point open
• Allows sodium ions to diffuse across membrane down gradient
• Movement of sodium ions upsets balance of ionic concentrations created by pumps
• Concentration of sodium ions inside neurone rises at point where channels are open
• Causes sodium ions to diffuse sideways away from this region of increased concentration
• Movement of charged particles is local current.

Voltage-gated sodium ion channels:

Gates on channels further along the neurone are operated by changes in voltage

Movement of sodium ions aong neurone alters potential difference - when reduced, gates open

Allows sodium ions to enter neurone further along, action potential has moved along neurone

The myelin sheath:

Insulating layer of fatty material.

Sodium and potassium ions cannot diffuse through it

Nodes of Ranvier are gaps between Schwann cells that make up myelin sheath

Ionic exchanges that cause action potential only occur at nodes of Ranvier.

Local currents are elongated in myelinated neurones and sodium ions diffuse along neurone from one node to the next - saltatory conduction.

Advantages of saltatory conduction:

Action potentials only occur at nodes, speeds up transmission of the action potential

Myelinated neurones conduct actiona potentials more quickly than non-myelinated neurones.

The structure of a cholinergic synapse:

ynapse is where one neurone can communicate with or signal to another neurone.

Action potential cannot bridge gap between neurones

Presynpatic action poential causes release of transmitter substance that diffuses across gap, generates new action potential in postsynaptic neurone.

Cholinergic synapses use acetylcholine as transmitter substance.

The synaptic knob:

Swelling at end of presynaptic neurone

Contains many mitochondira - active processes involved

Has large amount of SER

Has vesicles of acetylcholine

Has voltage-gated calcium ion channels in the membrane

The postsynaptic membrane:

Contains specialised sodium ion channels that respond to transmitter substance

Channels have specific receptor site specific to acetylcholine - complementary

When acetylcholine binds, sodium ion channels open.

Role of acetylcholinesterase:

Enzyme found in synaptic cleft - hydrolyses acetylcholine to ethanoic acid and choline.

Stops transmission signals so synapse doesn't continue to produce action potentials in postsynaptic neurone.

Ethanoic acid and choline recycled. Re-enter synaptic knob by diffusion

Recombined to acetylcholine using ATP. Recycled acetylcholine stored in vesicles for future use.

Transmission axross the synapse:

• Action potential arrives at synaptic knob
  
• Voltage-gated calcium ion channels open
• Calcium ions diffuse into synaptic knob
• Calcium ions cause synaptic vesicles to move to and fuse with presynaptic membrane.
  
• Acetylcholine released by exocytosis
• Acetylcholine molecules bind to receptor sites on sodium ion channels in postsynaptic membrane.
• Sodium ion channels open.
• Sodium ions diffuse across membrane into postsynaptic neurone.
• Generator potential created
• If sufficient generator potentials combine, potential across postsynaptic membrane reaches threshold potential
• New action potential created in postsynaptic neurone.

Action potentials and cell signalling:

Action potential is all or nothing response.

Once it starts, neurone will conduct it along its whole length, No variation in size or intensity

Processes are same in all neurones and cholinergic synapses

The frequency of transmission:

When stimulus is at higher intensity, sensory receptor will produce more generator potentials which causes more frequent action potentials in sensory neurone.

More vesicles released at synapse, creates higher frequency of action potentials in postsynaptic neurone.

Brain determines intensity of stiumlus from frequncy of signals arriving.

Higher frequency means more intense stimulus.

The Roles of synapses:

Main role to connect two neurones together so that signal can be transmitted.

• Several presynaptic neurones might converge to one postsynaptic neurone
• One presynaptic neurone might diverge to several parts of the nervous system
• Ensure signals are transmitted in correct direction
• Can filter out unwanted low-level signals
• Low-level signals can be amplified by summation
• Acclimatisation - nervous system no longer responds to certain stimulus.

Pathways created by synapses that enable nervous system to convey wide range of messages.

Myelinated and non-myelinated neurones:

Scwann cells wrapped around neurone so sheath consists of several layers of membrane and thin cytoplasmfrom Schwann cell.

Neurones found in CNS are non-myelinated. Still associated with Schwann cells but several neurones may be enshrouded in one loosely wrapped Schwann cell.

Means action potential moves along neurone in wave rather than jumping from node to node as in myelinated neurones.

Advantages of myelinated neurones:

Can transmit action potentials much more quickly than non-myelinated neurones.

Myelinated neurones carry signals from sensory receptors to CNS and then to effectors - carry signals over long distance, allow rapid response.

Signalling by using hormones

Endocrine system is communication system in body.Uses blood to transport signals

Signals released by endocrine system are hormones - directly into blood

Released directly into blood from endocrine glands - ductless glands

Endocrine or exocrine:

Endocrine secrete hormones directly into blood

Exocrine glands don't release hormones. Have small duct that carries secretion to another place

Targeting the signal:

Cells receiving hormone signal have specific complementary receptor that hormone binds to on their membrane - target cells/tissue

Hormone can travel round without affecting all cells because of this.

The nature of hormones:

Two types - protein + peptide hormones and steroid hormones.

Work in different ways. Proteins not soluble in phospholipid membrane, do not enter cell. Steroids can pass through membrane and have direct effect on nucleus of cell.

The action of adrenaline:

An amino acid derivative, unable to enter target cell. Receptor on outside of cell

Receptor associated with enzyme on inner surface - adenyl cyclase

Adrenaline (first messenger) in blood binds to receptor, activating adenyl cyclase.

Enzyme converts ATP to cAMP (second messenger) inside cell. cAMP then causes effect inside cell by activating enzyme action.

The functions of the adrenal glands:

Lie anterior to kidneys. Have medulla region and cortex region

Adrenal medulla - centre of gland. Cells release adrenaline in response to stress

Adrenaline prepares body for activity e.g. increase heart rate, dilate pupils

Adrenal cortex uses cholesterol to produce steroid hormones. Mineralocorticoids and glucocorticoids.

The pancreas:

Small organ below stomach. Has exocrine and endocrine functions.

Secretion of enzymes:

Majority of pancreatic cells release digestive enzymes - exocrine function of pancreas

Pancreatic duct carries fluid containing enzymes to small intestine.

Fluid contains: amylase, trypsinogen and lipase

Secretion of hormones:

Islets of Lanegerhans contain a cells - maufacture and secrete glucagon, and B cells - manufacture and secrete insulin.

Islets well supplied with blood capillaries, hormones secreted directly into blood - endocrine function

The control of blood glucose:

Cells in islets of Langerhans monitor blood glucose concentration. B and a cells detect changes and respond by releasing a hormone.

If blood glucose rises too high: Detected by B cells which secrete insulin in response. Target cells are hepatocytes.

Insulin binds to receptors, activates adenyl cyclase which produces cAMP which activates enzyme-controlled reactions in cell. Effects of insulin on cell:

• More glucose channels put in cell surface membrane so more glucose enters
  
• Glucose in cell converted to glycogen for storage (glycogenesis)
• More glucose converted to fats or used in respiration

If blood glucose drops too low: Detected by a cell, secrete glucagon in response. Target cells are hepatocytes. Effects of glucagon:

• Conversion of glycogen to glucose (glycogenolysis)
• Use of more fatty acids in respiration
• Production of glucose by conversion from amino acids and fats (gluconeogenesis)

The importance of regulating insulin levels:

If blood glucose concentration is too high then important that insulin released from B cells to reduce it.

If blood glucose concentration is too low then important insulin secretion stops.

The control of insulin secretion:

• Cell membranes of B cells contain calcium and potassium ion channels
  
• Potassium channels normally open, calcium normally closed. Potassium ions diffuse out making it more negative
• When glucose concentration outside cell are high, glucose molecules diffuse into cell
• Glucose used in metabolism to produce ATP
• Extra ATP causes potassium channels to close
• Potassium no longer diffuse out, alters potential difference across membrane - less negative inside
• Change in potential difference opens calcium ion channels
• Calcium ions enter cell and cause secretion of insulin by making vesicles containing insulin move to cell surface membrane and released by exocytosis.

Diabetes Mellitus:

Negative feedback enables body to keep blood concentration fairly well controlled within certain limits

Diabetes mellitus disease where body no longer able to control blood glucose concentration - can lead to hyperglycaemia (high concentrations) or hypoglycaemia (low concentrations) after eating or exercise.

Type 1 diabetes:

Usually starts in childhood. Insulin-dependent diabetes.

Result of autoimmune response where body's immune system attacks B cells and destroys them

Body no longer able to manufacture sufficient insulin and cannot store excess glucose as glycogen

Type 2 diabetes:

Non-insulin dependent diabetes. Can still produce insulin but as people age, responsiveness to insulin declines.

Specific receptors on surface of liver and muscle cells decline and cells lose ability to respond to insulin.

Levels of insulin secreted decline. Obesity, family history, diet high in sugars bring on earlier onset of this type.

Treatment of diabetes:

Type 2 usually treated by careful monitoring and control of diet. Care taken to match carbohydrate intake and use.

Type 1 diabetes usually treated by insulin injections.

- Blood glucose concentration must be monitored and correct dose of insulin must be administered to ensure glucose concentration remains stable.

The source of insulin:

Insulin can be produced by bacteria that have been genetically engineered to produce human insulin. Advantages:

• Exact copy of human insulin so faster acting and more effective
• Less chance of developing tolerance to insulin
• Less chance of rejection due to an immune resoponse
• Lower risk of infection
• Cheaper to manufacture than to extract insulin from animals
• Manufacturing process more adaptable to demand
• Some people less likely to have moral objections

The human heart: Requirements of cells vary according to their level of activity. When physically active, muscles need more oxygen and glucose and heart muscles need more oxygen and fatty acids. Also need to remove carbon dioxide.

Therefore important that heart can adapt to meet requirements of the body. How the heart adapts to supply more oxygen and glucose:

• Increase in heart rate
• Heart can increase strength of its contractions
• Can increase volume of blood pumped per beat (stroke volume)

Control of heart rate: Rate at which heat beats affected by number of things:

• Heart muscle is myogenic
• Heart contains pacemaker - sinoatrial node. Can initiate action potential which travels as wave of excitation over atria walls.
• Heart supplied by nerves from medulla oblongata of brain. Connect to SAN. Do not initiate contraction but can affect frequency of contractions. Action potentials sent down accelerator nerve increase heart rate. Action potentials sent down vagus nerve reduce heart rate.
• Heart muscle responds to presence of hormone adrenaline in blood.

Interaction between control mechanisms:

Various factors affecting heart rate must interact in coordinated way to ensure heart beats at most appropriate rate

Under resting conditions, SAN controls heart rate. Has set frequency at which it initiates wave of excitation

Frequency of these waves of excitation can be controlled by cardiovascular centre in medulla oblongata

Factors affecting heart rate:

• Movement of limbs detected by stretch receptors in muscles. Send impulses to cardiovascular centre when in need of extra oxygen, increases heart rate
• During exercise, muscles produce more carbon dioxide. Some reacts with watrer in plasma and reduces pH. Change in pH detected by chemoreceptors in carotid arteris, aorta and brain. Send impulses, increses heart rate
• When stop exercising, carbon dioxide concentration falls. Reduces activity of accelerator pathway, heart rate decreases
• Adrenaline secreted in response to stress/shock. Its presence in blood increases heart rate to prepare body.
• Blood pressure monitored by stretch receptors in carotid sinus walls (small swelling in carotid artery). If too high, stretch recpetors send signals to cardiovascular centre, responds by reducing heart rate.