3.8 The nervous system

The nervous system:

• Detects changes or stimuli inside the body and in the environment.

• Processes and stores information.

• Initiates responses.

What is a stimulus?

A detectable change in the internal or external environment of an organism that produces a response in that organism.

What are the two main parts of the nervous system?

• The central nervous system (CNS)

• The peripheral nervous system (PNS)

What comprises the CNS?

the brain and spinal cord

What is the function of the CNS?

Processes information provided by a stimulus and coordinates a response.

What comprises the PNS?

• The somatic nervous system

• The autonomic nervous system

What is the somatic nervous system?

Pairs of nerves that originate in the brain or spinal cord, and their branches. These nerves contain the fibres of sensory neurones, which carry impulses from receptors to the CNS, and motor neurones, which carry impulses away from the CNS to effectors.

What is the autonomic nervous system?

Provides unconscious control of the functions of internal organs e.g. heartbeat, digestion.

What is a nerve net?

The simplest type of nervous system. It is a diffuse network of cells that group into ganglia, but do not form a brain.

What are the two types of nerve net?

• Ganglion cells provide connections in several connections.

• Sensory cells detect stimuli e.g. light, sound, touch.

Comparing a Hydra nerve net to the mammalian nervous system.

	Hydra	Human
Nervous system type	Nerve net	CNS
Number of cells in NS	2	Many
Regeneration of neurones	Rapid	Slow if at all
Myelin sheath	Absent	Present
Conduction speed	Slow	Fast
Direction of impulse	All directions	One direction

What are neurones?

Specialised cells adapted to rapidly carry nervous impulses from one part of the body to another.

What are the three types of neurone?

• Sensory

• Motor

• Relay

What is the function of the sensory neurone?

Carries nerve impulses from the receptor cells in the sense organ to the CNS.

What is the function of the motor neurone?

Transports the nerve impulse from the CNS to the effectors (muscles and glands)

What is the function of the relay neurone?

Recieves impulses from sensory neurones or other relay neurones and transmits them to motor neurones or other relay neurones.

Define resting potential

The potential difference across the membrane of a cell when no nervous impulse is being conducted.

What is an excitable cell?

A neurone is an excitable cell, meaning it can change its resting potential.

What is the potential difference across an axon cell membrane during a resting potential?

-70mV

Why is the cell membrane described as polarised?

There is a potential difference across the membrane.

Why is the resting potential negative?

The membrane is more negative inside because of the negative ions of large proteins, of organic acids such as pyruvate, and of organic phosphates e.g. ATP^4- in the cytoplasm, and from the uneven distribution of inorganic ions.

The inside of the cell has

• a higher concentration of potassium ions than the outside

• a lower concentration of sodium ions than the outside

Why is the axon membrane 100x more permeable to potassium ions than sodium ions?

• Some of the channels that allow the 2K⁺ ions to diffuse out are open.

• Most of the channels that allow the 3Na⁺ ions to move in are closed.

Therefore, K⁺ ions diffuse out faster than Na⁺ ions diffuse back in.

How is the concentration and uneven distribution of ions maintained?

Sodium-potassium exchange pumps pump K⁺ ions back into the cell and Na⁺ ions back in.

What are sodium-potassium exchange pumps?

Trans-membrane proteins with ATPase activity that transport K⁺ and Na⁺ ions across the membrane against a concentration gradient, by active transport.

What is a nervous impulse?

The transmission of a change in potential along a nerve fibre associated with the movement of sodium ions.

Define action potential

The rapid rise and fall of the electrical potential across a nerve cell membrane as a nervous impulses passes.

What is a voltage gated channel?

One which opens and closes in response to a particular voltage across the membrane.

How does depolarisation of the axon occur?

• The energy of the stimulus causes some of the voltage-gated sodium channels in the axon membrane to open.

• The sudden increase in permeability of the membrane to Na⁺ ions allows them to rapidly diffuse into the axon, down their concentration gradient.

• The negative charge of -70mV inside the axon rapidly becomes a positive charge of +40mV.

• Once the potential inside the cell is +40mV, the sodium ion channels close, preventing further influx of sodium ions.

How does repolarisation of the axon occur?

• The potassium channels open and K⁺ ions diffuse down their concentration gradient. The cell becomes less positive inside as more diffuse out and the membrane is repolarised.

How does hyperpolarisation occur?

More K⁺ ions diffuse out than Na⁺ ions diffused in, so the potential difference across the membrane becomes even more negative than the resting potential.

How is the resting potential restored?

The sodium-potassium pump pumps K⁺ ions back in and Na⁺ ions back out, restoring the ion balance of the resting potential.

Define depolarisation

A temporary reversal of potential across the membrane of a neurone such that the inside becomes less negative than the outside as an action potential is transmitted.

How does the action potential travel along an axon?

By a wave of depolarisation

Define the absolute refractory period

• Period during which no new action potential can be initiated.

• As at the site of the initial action potential, the sodium channels are inactivated and cannot open again until the resting-potential has been re-established, so a new action potential cannot be initiated there.

What is the ‘all or nothing’ law?

If the intensity of a stimulus is below a certain threshold value, no action potential is initiated. But, if the intensity of the stimulus exceeds the threshold value, an action potential is initiated. A nervous impulse is either initiated or not and it is always the same size (+40mV).

What are the three major factors affecting the speed of conduction of the nerve impulse?

1. Temperature

2. The diameter of the axon

3. Myelination

How does temperature affect the speed of conduction of a nerve impulse?

• Ions move faster at higher temperatures than at lower temperatures as they have more kinetic energy.

How does the diameter of the axon affect the speed of transmission of a nerve impulse?

• The greater the diameter of the axon, the greater its volume in relation to the area of the membrane. More sodium ions can flow through the axon, so impulses travel faster.

How does myelination affect the speed of conduction of the nerve impulse?

Speeds up the rate of transmission by insulating the axon. The action potential can jump from node to node (of Ranvier) along a myelinated axon. This is saltatory conduction. The greater the distance between the nodes, the greater the rate of transmission.

Describe synaptic transmission

1. When an action potential reaches the synaptic end bulb of the presynaptic neurone, voltage-dependent alcium ion channels open and calcium ions diffuse into the end bulb, down their concentration gradient.

2. The influx of calcium ions causes the synaptic vesicles to move towards and fuse with the presynaptic membrane. This releases the neurotransmitter acetylcholine into the synaptic cleft by exocytosis.

3. The neurotransmitter rapidly diffuses across the synaptic cleft and binds to a receptor in the postsynaptic membrane, opening a channel and allowing sodium ions to diffuse in, down their concentration gradient.

4. The post synaptic neurone is depolarised. The neurotransmitter substance is removed from the receptors by enzyme action , which breaks it down into inactive products. These products diffuse back into the presynaptic knob and are regenerated and repackaged into synaptic vesicles. This requires ATP, hence why the synaptic knob has many mitochondria.