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 synaptic gap
(synaptic cleft) 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.
