6.5: Nerves, Hormones and Homeostasis

Question 1

State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses.

Answer 1

The nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses.

Question 2

State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.

Answer 2

Nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.

Question 3

Define resting potential and action potential (depolarisation and repolarisation):

Answer 3

Resting potential: The charge difference across the membrane when a neuron is not firing (-70mV), as maintained by the sodium-potassium pump.

Action potential: The charge difference across the membrane when a neuron is firing (30mV).

Depolarisation: The change from a negative resting potential to a positive action potential (caused by opening of sodium channels).

Repolarisation: The change from a positive action potential back to a negative resting potential (caused by opening of potassium channels).

Question 4

Explain how a nerve impulse passes along a non-myelinated neuron:

Answer 4

1. Generation of a Resting potential:
   - The sodium-potassium pump (Na+/K+ pump) maintains the electrochemical gradient of the resting potential (-70mV).
   - It is a transmembrane protein that uses active transport to exchange Na+ and K+ ions across the membrane.
   - It expels 3 Na+ ions for every 2 K+ ions admitted.
   - This makes the inside of the membrane relatively negative when compared to the outside (-70mV = resting potential)
2. Transmission of an Action potential:
   - Sodium and potassium channels in nerve cells are voltage-gated, meaning they can open and close depending on the voltage across the membrane.
   - In response to a signal at a sensory receptor or dendrite, sodium channels open and sodium enters the neuron passively.
   - The influx of sodium (Na+ in) causes the membrane potential to become positive (depolarisation).
   - If a sufficient change in membrane potential is achieved (threshold potential), adjacent voltage-gated sodium channels open, generating a wave of depolarisation (action potential) that spreads down the axon.
   - The change in membrane potential also activates voltage-gated potassium channels, causing potassium to exit the neuron passively.
   - The efflux of potassium (K+ out) causes the membrane potential to become negative again (repolarisation).
   - Before the neuron can fire again, the original distribution of ions (Na+ out, K+ in) must be re-established by the Na+/K+ pump.
   - The inability to propagate another action potential during this time (refractory period) ensures nerve impulses only travel in one direction.

Question 5

Explain the principles of synaptic transmission:

Answer 5

1. Action potential reaches the end of a presynaptic neuron.
2. Voltage gated calcium channels open.
3. Calcium ions flow into the presynaptic neuron.
4. Vesicles with neurotransmitters inside the presynaptic neuron fuse with the plasma membrane.
5. Neurotransmitters diffuse in the synaptic cleft and bind to receptors on the postsynaptic neuron.
6. The receptors are channels which open and let sodium ions into the postsynaptic neuron.
7. The sodium ions cause the postsynaptic membrane to depolarize.
8. This causes an action potential which passes down the postsynaptic neuron.
9. Neurotransmitters in the synaptic cleft are degraded and the calcium ions are pumped back into the synaptic cleft.

Question 6

State that the endocrine system consists of glands that release hormones that are transported in the blood.

Answer 6

The endocrine system consists of glands that release hormones that are transported in the blood.

Question 7

State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.

Answer 7

Homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.

Question 8

Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms:

Answer 8

Homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance. Blood and tissue fluid (derived from blood) make up the internal environment. Negative feed back is used to keep the internal environment between limits. It uses the nervous and endocrine system to do so. If blood glucose levels rise above the set point, this will feed back to decrease production and reduce the level back around the set point. Negative feed back is only triggered when there are significant increases or decreases from the set point.

Question 9

Explain the control of body temperature, including the transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering:

Answer 9

• The hypothalamus is responsible for monitoring the temperature of the blood which is normally close to 37 degrees. If there are significant fluctuations from this set point, the hypothalamus sends signals (messages carried by neurons) to different parts of the body to restore the temperature back to the set point. This is done through negative feedback.

If blood temp. increases above set point:

• Skin arterioles increase in diameter so that more blood flows to the skin. By doing so it transfers heat from the core of the body to the skin and this heat is then lost to the external environment, cooling down the body in the process.
• Skeletal muscle stays relaxed so that more heat is not generated.
• Sweat glands secrete large amounts of sweat which makes the surface of the skin moist. When water evaporates from the moist skin it cools down the body.

Question 10

Explain the control of blood glucose concentration, including the roles of glucagon, insulin and ‘a’ and ‘ß’ cells in the pancreatic islets:

Answer 10

Blood glucose concentration does not have a specific set point like blood temperature. Blood glucose levels drop and rise through the day and so the body usually tries to keep blood glucose levels around 4 to 8 millimoles per dm3 of blood. Once again, negative feedback is used to do so. There are responses by target organs which affect the rate at which glucose is taken up from the blood or loaded into the blood.

Response to blood glucose levels above the set point:
- β cells in the pancreatic islets produce insulin. Insulin stimulates muscle cells and the liver cells to take up glucose from the blood and convert it into glycogen. These are then stored in the form of granules in the cytoplasm of cells. Also, other types of cells are stimulated to take up glucose and use it for cell respiration instead of fat. All of these processes lower the levels of glucose in the blood.

Response to blood glucose levels below the set point:
-α cells in the pancreatic islets produce glucagon. Glucagon stimulates the liver cells to convert glycogen back into glucose and release this glucose into the blood. This raises the glucose levels in the blood.

Question 11

Distinguish between type I and type II diabetes:

Answer 11

Type I diabetes:

• The onset is usually early, sometime during childhood.
• β cells do not produce enough insulin.
• Diet by itself cannot be used to control the condition. Insulin injections are needed to control glucose levels.

Type II diabetes:

• The onset is usually late, sometime after childhood.
• Target cells become insensitive to insulin.
• Insulin injections are not usually needed. Low carbohydrate diet can control the condition.