Neurones and Neural Pathways

Chapter 27

The nervous system consists of a complex network of nerve cells called NEURONES. The diagram shows the three types of neurone. SENSORY, ASSOCIATION and MOTOR

Structure of a Neurone

Neurones all share the same basic structure. They consist of a cell body, axon and dendrite

Information from receptors is transmitted along several dendrites which gather into one fibre to carry the information towards the cell body

DENDRITES

CELL BODY

The Cell body of a neurone contains the nucleus and most of the cytoplasm. It is the control centre of the cell’s metabolism and contains ribosomes. These are required to make proteins including enzymes for synthesis of neurotransmitters.

AXONS

An axon is a single nerve fibre which carries nerve impulses away from a cell body and, in the case of sensory and association neurones, on to the next neurone in the sequence.

AXONS

The axons of motor neurones are extremely long. They can be more than a metre long if reaching body parts like the toes.

Each axon from a motor neurone carries a message from the cell body to an effector

The direction in which a nerve impulse travels is always: dendritescell bodyaxon

DENDRITE

CELL BODY

AXON

Myelin Sheath

This is a jacket of fatty material around a nerve fibre. A nerve fibre lacking myelin is described as unmyelinated.

The presence of the myelin sheath greatly increases the speed at which impulses can be transmitted along the axon of a neurone

Post-Natal Development of Myelination

• Myelination, the development of myelin around axon fibres of individual neurones, takes time and is not complete at birth but continues during post–natal development.

Infants

Multiple Sclerosis

Chemical Transmission at a Synapse

• A synapse is a tiny region of functional contact between an axon ending of one neurone and the membrane of the dendrite (or sometimes the cell body) of the next neurone

• The nerve cell before the synapse is called the presynaptic neurone; the one after is called the postsynaptic neurone

• It is at the synapse that information is passed on by means of a chemical called a neurotransmitter

A synapse

Presynaptic neurone

Postsynaptic neurone

A region of contact between a motor neurone and an effector is called a neuro-effector junction

Neurotransmitters

• There are many neurotransmitters passed on at the synapse. Two examples of the many neurotransmitters are:

• ACETYLCHOLINE

• NORADRENALINE

The two neurones at a synapse are separated by a narrow space called the synaptic cleft

A synaptic knob is full of vesicles of one type of neurotransmitter

When a nerve impulse passes through the presynaptic neurone and reaches the synaptic knob, several vesicles fuse with the knob surface membrane and release neurotransmitter from the vesicles into the synaptic cleft

The neurotransmitter molecules briefly combine with receptor molecules at sites on the postsynaptic dendrite and a nerve impulse is able to be passed on

Since vesicles containing neurotransmitter occur on one side only of a synapse, this ensures that nerve impulses are transmitted in one direction only

Threshold

Fate of neurotransmitter after transmission of impulse

• As soon as an impulse has been transmitted the neurotransmitter is rapidly removed.

• Acetylcholine is broken down into non-active products by an enzyme present on the postsynaptic membrane, as in the following equation:

acetylcholinesterase• Acetylcholine Non-active products The non-active products are then reabsorbed by the presynaptic

neurone and resynthesised into active neurotransmitter stored in vesicles ready for reuse. Mitochondria present in the presynaptic knob provide the energy.

Noradrenaline is rebsorbed by the presynaptic membrane and stored in vesicles ready for reuse.

Frequency of impulses

• The nerve impulses transmitted are equal in size, however the number of impulses transmitted per second can vary depending on the intensity of the original stimulus

Loud Music Causes More Impulses to be Sent

It is important to remove neurotransmitter quickly between impulses. If it doesn’t happen then only a limited number of impulses can be passed. We would therefore be unable to distinguish between loud and soft sounds or similarly between mild and severe pain

Excitatory and inhibitory signalsIn the CNS, one postsynaptic neurone normally forms synapses with many presynaptic axons from several different neurones

At some of these synapses, the receptor sites in the postsynaptic membrane respond to the arrival of neurotransmitter( e.g. acetylcholine) by having an excitatory effect which increases the chances of reaching threshold and transmitting a nerve impulse

Excitatory and Inhibitory SignalsAt other synapses the receptor sites respond to the neurotransmitter (e.g. acetylcholine) by having an inhibitory effect which reduces the chance of reaching threshold and transmitting a nerve impulse

When the sum of the excitatory effects from the postsynaptic membrane is greater than the inhibitory effects and threshold is reached, a nerve impulse is transmitted.

When the inhibitory effects are in excess, no signal is fired

Heart rate decreases as receptor sites receive acetylcholine and cause inhibition

Peristalsis rate increases as acetylcholine combines with receptor sites that cause excitation

Complex Neural Pathways

• Neurones are found to be connected to one another in many different ways in the CNS. Two examples of neural pathways are:

• DIVERGING NEURAL PATHWAY

• CONVERGING NEURAL PATHWAY

Diverging Neural PathwayTo diverge means to branch out from a common point. In a diverging neural pathway, the route along which an impulse is travelling divides. This allows information to be transmitted to several destinations

e.g.Temperature Control

Examples of a diverging neural pathways would be the hypothalamus having a neural pathway that diverges into branches which lead to sweat glands, skin arterioles and skeletal muscles.

Diverging Neural Pathway

Fine Motor Control

Another example of a diverging neural pathway would be the cerebrum transmitting impulses to different muscles in the hand. This allows the fine motor control of the fingers and thumbs and allows them to work in unison

Converging Neural Pathway

To converge means to come together and meet at a common point. In a converging neural pathway, impulses from several sources are channelled towards one point

Rods and ConesRods and cones are visual receptors present in the retina of the eye. They contain pigments which break down in the presence of light. In each case, this breakdown forms a chemical which triggers off nerve impulses along a pathway of neurones

The pigment in cones is not very sensitive to light and needs bright light to break it down and trigger nerve impulses

The pigment in rods is so sensitive to light that dim light triggers off its breakdown and sends impulses. It is inactive in bright light

Convergence of Signals from Rods

• As the intensity of light entering the eye decreases, cones cease to respond and rods take over. Unlike cones several rods form synapses with the next neurone in the pathwaySeveral rods form synapses with the next neurone in the pathway

The nerve impulse transmitted by one rod in dim light is weak. It would mean not enough neurotransmitter would be released to carry on the impulse. Several rods are needed to allow enough neurotransmitter to be released. A nerve impulse is then passed through the optic nerve to the brain

Plasticity of response of the Nervous System

Investigating the brain’s capacity to suppress the blinking reflex

Blinking the eye is an example of a reflex action. A reflex action is a rapid, automatic response designed to protect the body from danger

In this experiment, ten attempts are made to make the volunteer blink using their right eye. A ten second interval is allowed between each attempt to allow the volunteer the opportunity to summon maximum willpower. Some can suppress the blinking but others cannot resist blinking no matter how hard they try

Investigating the ability of the brain to suppress sensory impulses

If a person is given a task to do that requires a lot of concentration and is subjected to auditory and visual distractions, some people are good at suppressing the sensory impulses from the distractions and perform well each time. Other people find it hard to block out the sensory impulses

Plasticity of Response

• Sometimes the brain can be persuaded to temporarily suppress a reflex action or block out certain sensory impulses. This demonstrates Plasticity of Response of the nervous system.

Plasticity of Response

• Plasticity is thought to occur as you have two conflicting messages- one saying to blink and the other not to blink meeting in a convergent pathway.

• If the overall effect at the synapse is excitatory then the nerve impulse is fired and blinking occurs.

• If the overall effect is inhibitory then no impulse is fired and blinking fails to occur.

• This explains why some people can resist blinking whilst others can’t help themselves