UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 20/05/2023 [SEARCH]

Nervous system: 4. The structure and function of the central nervous system (CNS)

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INDEX of biology notes on the nervous system

Keywords: structure function components central nervous system CNS brain spinal cord motor neurotransmitter molecules relay neurones synapses nerve cells neurons axon nerve fibres dendrites axon sensory neurons receptor cells effector neurone cells


(4) The structure and function of the central nervous system (CNS)

All vertebrates, those animals with backbones, have a CNS consisting of the brain and spinal cord.

Information in the form of an electrical signal, from receptors, passes along cells in nerves (neurones) to the brain through the central nervous system (spinal cord ==> brain)  and ...

... the brain then coordinates the response,

... reflex actions are automatic and rapid,

... and often involve sensory, relay and motor neurones.

The 'components' of the nervous system

As mentioned already, the CNS of vertebrates consists of the brain and spinal cord only.

The CNS is then connected to the body by sensory neurones and motor neurones.

Brain reminder!

The spinal cord is a long column of neurones (nerve cells) that runs from the base of the brain down through the spine - physically protected by the bones of the vertebral column.

It is the spinal cord that relays information between the brain and the rest of the body.

The electrical nerve impulses ('information') are relayed via sensory neurones, relay neurones and motor neurones AND pretty fast too in a fraction of a second!

The structure and function of different parts of the nervous system are described below.

Nerve cells, also called neurons/neurones, are, elongated cells that carry electrical signals or impulses all around the body.

The diagram on the right shows the basic structure of a nerve cell or neurone.

Neurones can be very long! The cell body, containing the nucleus, is about 0.1 mm across, but the axon can be a meter long (1000 mm) - this single long nerve cell acts faster in relaying electrical signals than a series of individual smaller cells connected together.

The axon, which carries the electrical signal, is covered in a protective electrically insulating myelin sheath (not shown here, but see other cell diagrams below).

The cell body connects to lots of other neurones.

Some general points about nerve cell (neurone) structure:

(i) All neurones, like most other cells, have a cell body containing the nucleus in a membrane surrounded cytoplasm and other subcellular structures.

The cell body of all neurones is found in the central nervous system (CNS)

The cell body has fine tip extensions called dendrites/dendrons that connect to other neurones and carry the electrical impulses of the nerve signals.

As well as the dendrites for nerve cell communication, neurones have an extended shape so they can carry electrical nerve impulses from one part of the body to another.

(ii) Dendrites (dendrons) are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project.

(iii) An axon (nerve fibre), is a long, slender projection of a nerve cell (neuron), in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body.

(iv) Neurones are relatively long cells which helps the fast electrical impulse transfer between one neurone and another - one long nerve cell transfer is faster than through lots of smaller-shorter cells.

(v) The myelin sheath is a fatty electrically insulating tissue layer around the axon connections between neurones. The myelin sheath also helps speed up the electrical impulse transfer and the axon in the neurone cells carries the electrical signal - if there was no myelin insulation, the signal will be lost.

Axon endings (axon terminals) are button-like endings of axons through which axons make synaptic contacts with other nerve cells or with effector cells.

Receptors - groups of cells that respond to a particular stimulus - e.g. they detects stimuli such as heat, light, pain, sound, taste, smell, pressure (see previous section for more details).

Receptors often form part of a larger complex organs e.g. the taste buds on your tongue or the retina cells of the eye which respond to light.

Receptors start what is known as the 'reflex arc' described in the next section.

Sensory neurones - the nerve cells that transmit the electrical impulse signal from the receptors in the sense organs to the spinal cord and brain of the central nervous system.

Diagram of the structure and function of a sensory neurone

A long dendron carries nerve signals from receptor cells to the cell body which is at the centre of the neurone.

A shorter axon then transfers the electrical impulse from the cell body to the axon terminals that connect to the CNS.

The CNS processes the information and coordinates how the body should respond to the information received from nerve impulses.

Relay neurones (connector neurones) - the nerve cells that transmit the electrical signals through the CNS (brain + spinal cord) from sensory neurones to the motor neurones.

Relay neurones have lots of dendrites spreading out that carry the nerve signals from the sensory neurones to the cell body and an axon carries the nerve impulses to the motor neurones.

A synapse (diagram below) is a connection (junction) between two neurones e.g. the thin gap of the junction between a sensory neurone and a relay neurone, it enables the electrical impulse signals from receptors  to reach the spinal cord and brain (i.e. the central nervous system) and on to the effectors.

Between the end of one neurone, and the start of another, chemicals are released in the gap that rapidly diffuse across the gap in the synapse, triggering the transfer the electrical signal.

You can think of the released chemical as 'messenger molecule', and technically it is called a neurotransmitter because it triggers the electrical signal from one nerve cell (neurone) to another.

The transfer of the nerve impulses is quite fast, but the diffusion of the neurotransmitter molecules across the synapse gap does take a short time, so things are slowed down a bit.

Diagram of the structure and function of a synapse

Neurotransmitter - chemicals produced that transmit the electrical signal across a synapse gap between one neurone cell and another (see text and diagram under synapse).

Motor neurones (effector neurones) - the nerve cells that transmit the electrical signals from the central nervous system of the brain and spinal cord from one neurone to another to the effector cells of the muscles or glands (see diagram above).

The signals trigger the appropriate response by the muscles (e.g. contract) or gland (e.g. hormone secretion)

Motor neurone structure and function

A motor neurone has many short dendrites that carry nerve impulses from the CNS to the cell body, then one long axon carries the signal from the cell body to the effector cells.

The branches (dendrites) connect with other nerve cells.

Receptor cells have already been discussed.

Effector cells in the muscles or glands that respond in a variety of ways to the electrical signal from the brain or spinal cord (CNS) - the cells respond to the nervous impulses and cause things to happen.

Nervous impulses cause muscles to respond and contract e.g. from receptors detecting heat or pain.

Nerve impulses cause glands to secrete hormones - chemical messengers to effect a response.

Reflex arc

Effectors complete what is known as the 'reflex arc' described in the next section which fully describes the sequence:

stimulus  ===>  receptor  ===>  CNS coordinator  ===>  effector  ==> response

More on the reflex arc in Part (5)


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