Communication and Homeostasis

• Light sensitive cells in the retina detect light intensity and range of wavelengths (colour).
• Olfactory cells in the nasal cavity detect the presence of volatile chemicals.
• Tastebuds detect the presence of soluble chemicals.
• Pressure receptors in the skin detect pressure on the skin.
• Sound receptors in the cochlea detect vibrations in the air.
• Muscle spindles detect the length of muscle fibres.
• These are all transductors and convert the stimulus to a nerve impulse.

• The motor neurone has:
  A cell body at the end with a large nucleus and lots of rough ER and golgi bodies
  Many short dendrites that carry impulses to the cell body
  A long axon which carries an impulse from the cell body to the effector
• The sensory neurone has:
  Long processes on either side of the cell body
  A dendron carrying nerve impulses from a receptor to the cell body
  An axon carrying an impulse from the cell body to the central nervous system.

When not conducting an impulse, the potential difference across the membrane is -60mV.
Sodium-Potassium pumps actively transport 3Na+ ions out for every 2 K+ ions in.
The axon contains organic anions, which the membrane is impermeable to.
Slight loss of K+ ions through the permeable membrane.
Membrane impermeable to Na+ ions.

1. The membrane is at resting state; -60mV inside compared to outside. Polarised.
2. Na+ ion channels open and some Na+ ions diffuse out.
3. The membrane depolarises- it become less negative with respect to the outside and reaches the threshold potential of -50mV.
4. Voltage-gated sodium ion channels open and many Na+ ions enter. As more Na+ ions enter, the more positively changed the cell becomes, compared to outside.
5. The potential difference across the membrane reaches +40mV. The inside is now positive compared to the outside.
6. The Na+ ion channels shut and the K+ ion channels open.
7. K+ ions diffuse out of the cell, bringing the potential difference back to negative compared with the outside-repolarisation.
8. The potential difference overshoots slightly, making the cell hyperpolarised.
9. The original potential difference is restored, so the cell returns to its resting state.

The myelin sheath is an insulating layer of fatty material which Na and K ions cannot pass through. Between the Schwann cells are gaps- called the Nodes of Ranvier, which contain Voltage-gated Sodium and Potassium
ion channels, allowing ionic exchange to occur. The action potential ‘jumps’ from one node to the next-Saltatory conduction.

1. The membrane is at its resting state- polarised with the inside being -60mV compared to the outside.
2. Sodium ion channels open and some sodium ions diffuse in.
3. The membrane depolarises- it becomes less negative with respect to the outside and the threshold value of -50mV is reached.
4. Voltage gated sodium ion channels open, and sodium ions flood in. As more sodium ions enter, the membrane becomes positively changed on the inside compared to the outside.
5. The potential difference across the membrane reaches +40mV. The inside is positive compared to the outside.
6. The sodium ion channels close and the potassium ion channels open.
7. Potassium ions diffuse out of the cell, bring the potential difference back to negative inside compare to outside. This is called repolarisation.
8. The potential difference overshoots slightly making the cell hyperpolarised.
9. The original potential difference is restored so the cell returns to its resting state.

• A stimulus at the higher intensity will cause the sensory neurons to produce more generator potentials.
• More frequent action potentials in the sensory neurone, More vesicles released at the synapse,
• A higher frequency of action potentials in the postsynaptic neurone
• A higher frequency of signals to the brain
• A more intense stimulus

• Myelinated neurones:
  100-120ms-1
  Up to 1m transmission distance
  Fast response time
  Used in movement
  1/3 of all neurones
  One neurones is surrounded by one Schwann cell, wrapped round many times
• Non-myelinated neurones:
  2-20ms-1
  mm or cm transmission distance
  Slow response time
  Used in breathing and digestion
  2/3 of all neurones
  Many neurones are surrounded by one Schwann cell

The synaptic knob contains:
• Many mitochondria
• A large amount of smooth ER
• Vesicles containing acetylcholine
• There are also voltage gated sodium ion channels in the membrane
The postsynaptic membrane contains:
• Specialised sodium ion channels that will only open when acetylcholine binds to them

• A neurotransmitter is a chemical that diffuses across the cleft of the synapse to transmit a signal to the postsynaptic neurone. They cause the generation of a new action potential in the postsynaptic neurone. In cholinergic synapses the neurotransmitter is acetylcholine. It is stored in vesicles in the synaptic knob, and when the action potential arrives, the voltage gated sodium ion channels open, so calcium ions diffuse out.
• This causes the vesicles to fuse with the synaptic membrane, so acetylcholine is released by exocytosis. It diffuses across the cleft and binds to receptor sites on the sodium ion channels on the postsynaptic membrane. Sodium ions diffuse across the synaptic membrane into the postsynaptic neurone, creating a generator potential. If the generator potential is sufficient, the potential across the membrane reaches thethreshold potential, and a new action potential is created.

• Several presynaptic neurones may converge together to allow signals from different parts of the nervous system to create the same response.
• One presynaptic neurone may diverge to several post synaptic neurones to allow one signal to betransmitted to several parts of the nervous system- one may elicit a response, and one may inform the brain.
• They ensure that signals are transferred in only one direction- only the presynaptic knob contains acetylcholine in vesicles.
• They can filter out unwanted low-level signal, possibly created by a low level stimulus. Several vesicles of acetylcholine must be released for an action potential to be created in the post synaptic neurone.
• Low level signals can be amplified by summation (when several small potential charges combine to produce one larger charge in the potential membrane). If a low-level stimulus is persistent, it can generate several successive action potentials in the presynaptic neurone. The release of many vesicles of acetylcholine in a short space of time will enable the postsynaptic generator potentials to combine together to produce an action potential.
• Acclimatisation- after repeated stimulation, a synapse may run out of vesicles containing the transmitter substance. The synapse is said to be fatigued. This helps avoid overstimulation of an effector, which could damage it.
• The creation of specific pathways in the nervous system is thought to be the basis of conscious thought and memory.

• Endocrine gland: a gland that secrets hormones directly into the blood. Endocrine glands have no ducts.
• Exocrine gland: a gland that secrets molecules directly into a duct that carries the molecules to where they are used.
• Hormone: a molecule released into the blood which acts as a chemical messenger
  Target tissue: a group of cells that have receptors embedded in the plasma membrane that are complementary in shape to specific hormone molecules. Only these cells will respond to the specific hormone.

The first messenger is the hormone that transmits a message around the body, e.g. adrenaline. The second messenger, e.g. cAMP transmits a signal inside the cell.

The adrenal glands have two distinct regions- the cortex region and the medulla region. The adrenal medulla releases adrenaline, which:
• Relaxes smooth muscle in the bronchioles
• Increases the stroke volume of the heart
• Increases heart rate
• Causes general vasoconstriction- raising blood pressure
• Stimulates conversion of glycogen to glucose
• Dilates the pupils
• Increases mental awareness
• Inhibits the action of the gut
• Causes body hair to erect
The adrenal cortex releases cholesterol. Cholesterol is used to make steroid hormones in the body;
• Mineralalocorticoids help control the concentrations of Na and K in the blood
• Glucocorticoids help control the metabolism of carbohydrates and proteins in the liver

The cells surrounding exocrine gland of the pancreas secretes digestive enzymes into the pancreatic duct,which then goes onto the small intestine. This is the majority of the pancreas.
The exocrine cells- the Islets of Langerhans- consist of α and β cells. The α cells manufacture and secrete glucagon, whereas the β cells manufacture and secrete insulin. They are involved in the regulation of blood glucose levels.

• Detected by a cells
• The fall inhibits insulin production
• They secrete glucagon into the blodd
• Bind to receptors on hepatocytes where these reaction occur:
• Glycogenolysis-conversion of glycogen to glucose
• More fatty acids are used in respiration
• Gluconeogensis-conversion of amino acids and fats to glucose

• Detected by b cells
• the rise inhibits glucagon production
• Secrete insulin into the blodd
• Bind to receptors on hepatocytes, in the liver
• This activates adenyl cyclase in the cell
• Converts ATP to cAMP
• The cAMP activates a series of enzyme catalysed reactions within the cell;

• More glucose channels are placed in the cell surface membrane
• More glucose enters the cell
• Glycogenesis-glucose in the cell is converted to glycogen
• More glucose is converted to fats
• More glucose is used in respiration

1. The cell membranes of the β cells contain Ca2+ and K+ ion channels.
2. The K ion channels are normally open, and the Ca ion channels are normally shut. K ions diffuse out of the cell, making the inside more negative.
3. When glucose concentrations outside of the cells are high, glucose molecules diffuse into the cell.
4. The glucose is quickly metabolised to ATP.
5. The extra ATP causes the K ion channels to close.
6. The K ions can no longer diffuse out, so the cells become less negative inside.
7. This change in potential difference opens the Ca ion channels.
8. Ca2+ 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.

Type 1 Diabetes:

• Cause-Auto-immune response in which body's beta cells are attacked and so insulin is not produced
• Treatment- Injections and blood glucose concentrations are closely monitored

Type 2 Diabetes:

• Cause- Body can produce insulin receptors lose ability to detect and respond to insulin
• Treatment- Monitoring and controlling diet and may be supplemented with injections

GM bacteria:
• Exact copy of human insulin.
o Faster acting.
o More effective.
• Less chance of developing tolerance.
• Less chance of rejection.
• Cheaper.
• More adaptable to demand.
• Less likely to have moral objections.

Stem cells
• Could be used to produce new β cells.
• Scientists have found stem cells in the pancreas of adult mice.
• Undifferentiated.

Action potentials sent down the Accelerator Nerve to the heart; from the Cardiovascular centre of the medulla oblongata cause the heart to speed up. This may be because of:
• Movement of limbs detected by stretch receptors in muscles
o Extra oxygen may be needed
• Drop in pH detected by chemoreceptors in the carotid arteries, the aorta and the brain (when we exercise we produce CO2, this may react w/ H2O in the blood and reduce the pH).
o CO2+ H2O → H2CO3
o H2CO3→H++HCO3- Action potentials sent down the Vagus Nerve decreases the heart rate. This may be because of:
• Blood pressure rising When the concentration of CO2 in the blood falls, it reduces the activity of the Accelerator Nerve, slowing the heart rate.

The presence of Adrenaline increases the heart rate to prepare the body for activity.