Nervous System & Nerve Impulses

Neurones are single cells, and even though there are different types they all have the same basic structures: cell bodies contain a nucleus, dendrites which conduct impulses towards the cell body and the axon which transmits impulses from the cell body.

There is an insulating layer called the myelin sheath around the axon which is made up of Schwann cells wrapped around the axon - this affects how quickly nerve impulses travel through the axon.

Not all neurones have myelinated axons.

This is a motor neurone.

The cell body is always situated within the CNS, the axon conducts impulses from the CNS to the effectors (muscles or glands).

They are long neurones.

Sensory neurones carry impulses from sensory cells (receptors) to the CNS.

Mostly found in the CNS, they have a large number of connections with other nerve cells.

Reflex arcs are responsible for reflexes - rapid, involuntary responses to stimuli.

Receptors firstly detect a stimulus (e.g. hot saucepan) and generate a nerve impulse.

A sensory neurone conducts the nerve impulse to the CNS along the sensory pathway.

The sensory neurones enter the spinal cord and from a synapse with a relay neurone. Relay neurones from a synapse with a motor neurone that leaves the spinal cord.

The motor neurone carries the impulse to an effect (such as a muscle) which produces a response (movement).

The iris controls the size of the pupil; it contains a pair of antagonistic muscles called radial and circular muscles (controlled by the autonomic nervous system).

When the pupil constricts the radial muscles relax while the circular muscles contract (bright light).

When the pupil dilates the radial muscles contract while the circular muscles relax (low light).

When high light strikes the photoreceptors present in the retina (back of the eye) nerve impulses pass along the optic nerve to sites within the CNS.

This sends impulses to constrict the pupil, reducing the amount of light entering the eye.

The resting potential of an axon is -70 mV. It is caused by the uneven distribution of ions across the cell surface membrane - it is achieved by the action of sodium-potassium pumps in the cell surface membrane.

Na+ is pumped out of the cell while K+ is pumped into the cell - act against concentration gradients and uses energy from ATP.

Once concentration gradient is set up, K+ diffuses out of the cell down the gradient making the outside of the cell membrane positive and the inside negative. More K+ that diffuses out the larger the potential difference across the membrane.

• Depolarisation occurs once an electrical current changes the potential difference - this causes the shape of the Na+ gate to change. Na+ enters the axon and more depolarisation occurs
• causing more gates to open. The potential difference across the membrane is now +40mV.

• Repolarisation occurs when the Na+ channels close. K+ channels open due to depolarisation of the membrane. K+ ions move out of the axon causing a negative charge inside the membrane.

• Hyperpolarisation the K+ ions continue to move out the cell making the potential difference more negative than the resting potential - this is called hyperpolarisation of the membrane. The resting potential is re-established by the closing of K+ channels causing the ions to move back into the membrane.

A new action potential cannot be generated in the same section of the membrane for about 5 milliseconds, this is known as the refractory period, which lasts until sodium and potassium channels have returned to their normal state in which they are closed and resting potential is restored.

This ensures that nerve impulses only go in one direction.

The speed of impulse conduction is very fast, allowing fast responses to stimuli. They are affected by 3 main things:

• Temperature - the higher the temperature, the faster the speed. Warmer blooded animals have faster responses than colder blooded ones.

• Axon diameter - the wider the diameter, the faster the impulse travels. Marine invertebrates who live at low temperatures have developed thick axons to speed up their responses.

• Myelin Sheath - only vertebrates have a myelin sheath which acts as an electrical insulator along most of the axon, it prevents any flow of ions across the membrane. Gaps known as nodes of Ranvier occur in the myelin sheath at regular intervals and they are places where depolarisation can occur. The action potential can jump from one node to the next - this process is known as saltatory conduction.

A synapse is where two neurones meet. The gap between two neurones is a synaptic cleft.

Calcium ion channels open and diffuse into the membrane, causing the synaptic vesicles which contain neurotransmitters to fuse with the presynaptic membrane and release their contents into the synaptic cleft (by exocytosis).

The neurotransmitter diffuses across and reaches the specific protein receptor sites on the post synaptic membrane. The neurotransmitters bind to the receptors changing the shape of the proten and making the membrane permable to Na+ ions. this causes depolarisation producing an action potential which travels along the postsynaptic neurone.

Acetylcholinesterase is an enzyme at the post synaptic membrane which breaks down the neurotransmitter so it can no longer bind to receptors. This prevents constant depolarisation.

Summation - refers to the idea that each impulse adds to the effect of others. There are two main types:

• Spatial Summation - this is where the impulses from different synapses occur from different neurones.

• Temporal Summation - this is where several impulses arrive at a synapse after travelling along a single neurone one after another. The combined release of neurotransmitter generates an action potential in the post synaptic membrane.

Nervous Control

- Electrical transmission is fast acting

- Associated with short term changes e.g. muscle contraction.

- Responses is very local, such as a specific muscle cell or gland.

Hormonal Control

- Chemical transmission through the blood, so is slow acting.

- Associated with long term changes such as growth or sexual development.

- Blood carries the hormones to all cells but only specific cells can respond.

- Response may be widespread such as in growth and development.