Co-ordination and response / sensitivity
Motor Neurone
Living organisms are able to respond to
both internal and external stimuli.
Co-ordination is the process whereby a
living organism gives the correct
response at the correct time to a
particular stimulus. This allows the
organism to adapt, to change, and
increase their chance of survival.
For example, when animals/humans get
hungry, they go to find food – so that
their chance of survival may increase.
myelin sheath
axon
A stimulus is a change in the
environment. This can be internal (e.g.
hunger) or external (e.g. light, sound).
Many stimuli are forms of energy and
are detected by organs. These organs
have receptors or receptor cells. A
receptor detects a stimulus/change in the
environment. It sets up an electrical
current in the form of a nerve impulse.
The electrical current will travel to a coordinating centre (e.g. brain/spinal
cord), which would relay the impulse to
an effector.
nerve fibre
direction of impulse
motor nerve endings in
nerve muscle
Effectors are either muscles or glands.
stimulus
(change in environment)
Synapse (gap)
nucleus
receptor
direction
of impulse
branches of one fibre
near cell body of next
nerve cell
nerve fibre
co-ordinating centre
effector
response
cell body
Co-ordination is achieved by:
1.
2.
the nervous system
the endocrine system
The Nervous system
Receptors change particular types of energy into a nerve impulse.
Receptors are transducers (i.e. they change energy from one form to another). For
example, the eyes, ears, nose, tongue and skin – all our sense – are receptors. In order to
make it an efficient communication system, the messages must travel rapidly and directly.
The general structure of the nervous system consists of:
1.
Central nervous system (C.N.S.)
-
brain and spinal cord
2.
Peripheral nervous system (P.N.S.)
-
paired nerves arising form spinal cord
3.
Autonomic nervous system (A.N.S.)
-
the part of the nervous system that
maintains
a
constant
internal
environment (e.g. heartbeat)
The basic unit of the nervous system: the neurone – nerve cell
1.
Sensory neurone
transmits a nerve impulse from the receptor cell
to the central nervous system
2.
Motor neurone
transmits nerve impulse from central nervous
system to effector
3.
Relay/link/intermediate neurone
carries nerve impulses from a sensory neurone
to a motor neurone
All of these transmit electrical impulses. They are found in different parts of the nervous
system. Their structure varies.
Sensory neurone
Motor neurone
Cell body
Side of axon
At end of axon
Cell body
In white matter of spinal
cord (outer part)
In grey part of spinal cord
(central)
axon
axon
Cell body
cell body
cell body
Nerve cells
are up to 1 metre long and are very thin (about 0.0005 mm in
diameter). This allows for less diffusion and so a more rapid
transmission of the impulse. The nerve impulses travel at
approximately 100 metres per second!
Axon
relays nerve impulses over long distances. They are surrounded by a
myelin sheath.
Myelin sheath
insulates the neurone so that there is little/no lateral transmission.
Node of Ranvier
speeds up the nerve impulses as they jump from one neurone to
another. This is where electrical and chemical changes take place.
Cell body
in a motor neurone, this is where the impulse stars from
Dendrites
these are thin, cytoplasmic extensions by which nerve impulses are
carried from one axon to effectors or the cell bodies of other
neurones.
Motor end plate
this is where the impulses end (when going to muscle)
Synapse
At the end of each neurone there is a gap (there is no physical
connection between one neurone and the next) between it and the
next nerve cell or effector. This gap is called the “synapse”. The
dendrites on one neurone do not directly touch the dendrites on the
next neurone.
Transmission across synapse
ACH (acetylcholine) is the transmitter substance/chemical
this is released from what is called the “synaptic knob”
it is released into the synaptic gap
it diffuses across the synapse
in one direction
it then combines with receptor sites on the “post synaptic membrane”
this makes the membrane permeable to Na+ ions (causes depolarisation)
the process is terminated by an enzyme: acetylcholine esterase
N.B.: There is no physical connection between one neurone and the next. Nerve impulses
are transmitted by chemical diffusion to cause de-polarisation in the next neurone (i.e. to
make the next neurone send the same impulse).
What is the nerve impulse?
The nerve impulse is the electro-chemical change in the axon. When Na+ goes in, K+
immediately leaves. This causes a disturbance further along. To restore this to normal, there
is a pump.
+
+
Na
K
axon
Nerve
A nerve is a “collection (or bundle) of nerve fibres bound by connecting tissue”. Nerves
may be mixed (i.e. have sensory and motor neurones)
An outline of the “reflex arc”:
dorsal root ganglion
receptor detecting
stimulus
cell body
synapse
sensory neurone
white matter
cell body
relay neurone
grey matter
motor neurone
effect created in
muscle or gland
Notice that all synapses are in the grey matter.
Reflex action
Reflex actions do not require the brain for its initiation. It is voluntary (i.e. unconsciously
done). However, it can be overridden by the brain and conscious thought.
Definition:
“
Rapid, automatic response by an organ / system of organs in response to a stimulus
which does not require the brain for its initiation.
You must know:
the reflex arc – including what each part is
the definition of a reflex action
how nerve impulses are transmitted across the synapse
”
Reflex centres
There are two reflex centres:
1.
Brain
The medulla oblongata. This controls the facial expressions such as blinking,
dilation of the pupil, yawning (yes, it is a reflex – so tell that to a teacher which
blames you for yawning...it is automatic!) and sneezing.
The medulla also controls reflexes to do with homeostasis (i.e. reflexes which are
required to keep everything in the same state) e.g. breathing, heart rate, peristalsis
and blood pressure.
2.
Spinal cord
This redirects the sensory impulses from the sensory neurone directly to the motor
neurones via reflectors. This is for bodily reflexes such as knee jerks and what
happens when you place your hand on a hot plate.
There are three types of response to a stimulus:
Type of response
What it is
Control
Involuntary
cannot be consciously controlled
medulla / spinal cord
Voluntary
result of a conscious decision
brain – e.g. action of kicking a ball
Conditioned response
association of two unrelated stimuli
story of dog...see below!
Example of controlled response
Stretch receptors in the muscle inform the brain of how relaxed/contracted the muscle is.
A high degree of control is needed when two or more muscles are working in antagonistic
harmony.
Nerve fibres in the stretch receptors of the muscle send nerve impulses / information to the
spinal cord to indicate the degree to which the muscle is being stretched. This is then
relayed to the brain which sends an impulse telling the muscle to contract/relax.
Example of conditioned response
There once was a dog. The person who owned the dog always rang a bell when he/she
wanted to give the dog food. At first, the dog did not produce saliva when it heard the food,
but it salivated (i.e. produce saliva) when it saw the food. After a while, it began to salivate
when it heard the bell and saw the food. Then, it salivated when the bell was rung without
any food being there. This is a conditioned response. The dog thought that there would be
food when the bell rang and so it started to prepare itself by producing saliva.