IB Biology topic 6.5


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6.5.1 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.
The nervous system consist of the central nervous system (CNS) - the spinal chord and the brain - and peripheral nerves - connect all parts of the body to the CNS.
The nervous system is composed of cells called neurones; these are typically elongated cells and they can carry messages by rapid electrical impulses.
6.5.2 Draw and label a diagram of the structure of a motor neuron.
6.5.3 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.
Neurones carry electrical impulses long distances in the body using elongated structures called nerve fibres (axons).

Sensory neurones: carry nerve impulses from receptors (sensory cells) to the CNS.
Motor neurones: carry impulses from the CNS to effectors (muscle and gland cells).
Relay neurones: carry impulses within the CNS, from one neurone to another.
6.5.4 Define resting potential and action potential (depolarization and repolarization).
Resting potential: the electrical potential across the plasma membrane of a cell that is not conducting an impulse.
Action potential: the reversal and restoration of the electrical potential across the plasma membrane of a cell, as an electrical impulse passes along it (depolarisation and repolarisation)
6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron.
(1) At the resting potential ion channels for sodium ions and potassium ions are both closed (sodium outside, potassium inside) = positive charge outside and negative charge inside the cell.
(2) When the cells is disturbed by a stimulus the sodium channels open and sodium ions diffuse in.
(3) The interior of the axon becomes increasingly positively charged compared to the outside and the membrane is depolarised (negative outside, positive inside)
(4) The depolarisation moves along the axon = action potential.
(5) Behind the action potential sodium channels start closing and potassium channels open to let potassium ions out of the cells - the interior of the axon becomes less positive again - repolarising
(6) Sodium/potassium pumps start working to re-establish the resting potential.
6.5.6 Explain the principles of synaptic transmission.
Synapse: junction between two neurones. The plasma membranes of the neurones are separated by a narrow fluid-filled gap called the synaptic cleft.
Neurotransmitters: chemicals that transfer messages across the synapse.

(1) Nerve impulse reaches the end of the pre-synaptic neurone.
(2) Calcium diffuses into the pre-synaptic neurone through calcium tunnels.
(3) Vesicles of neurotransmitter is released into the synaptic cleft.
(4) Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic neurone.
(5) Sodium ions enter the post-synaptic neurone - cause depolarisation.
(6) The depolarisation passes down the post-synaptic neurone and initiates an action potential.
(7) Calcium is pumped out. Neurotransmitter is broken down rapidly in the synaptic cleft and is reabsorbed into the vesicles.
6.5.7 State that the endocrine system consists of glands that release hormones that are transported in the blood.
The endocrine system consists of glands that release hormones that are transported in the blood.
6.5.8 State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.
Homeostasis is maintaining the internal environment of the body between limits: parameters controlled include:
- body temperature
- blood pH
- carbon dioxide concentration
- blood glucose concentration
- water balance
6.5.9 Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.
Homeostasis involves monitoring levels of variables, if there are small fluctuations above and below the set point no response is cause, however, if the level rises significantly above the set point, it is reduced by negative feedback and oppositely if the level falls significantly below the set point, it is increased by negative feedback.
Negative feedback has a stabalising effect as a change in levels always causes the opposite change. A rise in levels feeds back to decrease production and reduce the level, whereas a decrease in levels feeds back to increase production and raise the level.
6.5.10 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.
The hypothalamus in the brain monitors temperature of the blood, as heat is transferred in the blood, and compares it with a set point (close to 37 degrees celsius). If the blood temperature is lower or higher than the set point the hypothalamus signals parts of the body by neurones to bring the temperature back to the set point (negative feedback).

Responses to overheating:
- Skin arterioles become wider - more blood flows through the skin - this transfers the heat from the core of the body to the skin losing the heat to the environment.
- Skeletal muscles remain relaxed and resting so they do not generate heat.
- Sweat glands secrete sweat to make the surface of the skin damp - water evaporates which has a cooling effect.

Responses to chilling:
- Skin arterioles become narrower - less blood is brought to the skin - the temperature of the skin falls and less heat is lost to the environment.
- Skeletal muscles do many rapid contractions to generate heat (shivering).
- Sweat glands do not secrete sweat - the skin remains dry.
6.5.11 Explain the control of blood glucose concentration, including the roles of glucagon, insulin and α and β cells in the pancreatic islets.
Cells in the pancreas monitor the concentration of blood glucose. It send hormone messages to target organs when the level is low or high. The responses by target organs affect the rate at which glucose is loaded or unloaded to/from the blood. Mechanisms involved are examples of negative feedback.

Responses to high blood glucose levels:
- Beta cells in the pancreatic islets produce insulin.
- Insulin stimulates the liver and muscle cells to absorb glucose from the blood and convert it into glycogen.
- Other cells are stimulated to absorb glucose for the use in cell respiration instead of fat.
These processes reduces the blood glucose level.

Responses to low blood glucose levels:
- alpha cells in the pancreatic islets produce glucagon.
- Glucagon stimulates liver cells to break glycogen down into glucose and release it into the blood.
This raises the blood glucose level.
6.5.12 Distinguish between type I and type II diabetes.
Diabetes is a condition in which the control of blood glucose does not work effectively.
There are two types:
Type I:
- onset usually during childhood
- beta cells produce insufficient insulin
- insulin injections controls glucose levels
- diet is not sufficient to control the condition

Type II:
- onset usually after childhood (old people usually)
- target cells become insensitive to insulin
- insulin injection usually not needed
- Low carb diet can control the condition