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The nervous system
The nervous system is divided into two main parts: The central nervous system and the peripheral nervous
The central nervous system (CNS) comprises the brain and spinal chord. Its role is to process the
information from the peripheral nervous system and initiate a response.
The peripheral nervous system is made up of pairs of nerves that originate from the brain or spinal cord.
These consist of sensory neurones which carry impulses from receptors to the CNS, and motor neurones
which carry impulses away from the CNS to effectors.
A stimulus is a detectable change in the internal or external environment of an organism that produces a
response in that organism.
Receptors range from specialised sensory cells, such as those in the skin, to more complex sense organs
such as the ear or the eye. Receptors detect the information from inside the body and from the
surroundings. Sensory receptors detect one form of energy and convert it into electrical energy. They act
as transducers. The electrical impulses travel along nerves and are called nerve impulses.
Effectors may be muscles or glands and bring about responses.
The simplest type of nervous response to a stimulus is a reflex arc. An example of a reflex arc is the
withdrawal reflex, when you immediately withdraw your hand, for example if you place it on a hot object.
A reflex action is a rapid, involuntary response resulting from nervous impulses initiated by a stimulus. The
action is involuntary in that the brain is not involved. Reflex actions are generally protective in function.
The sequence of a reflex arc is as follows (example: withdrawal from a hot surface):
Stimulus the hot surface
Receptor temperature and pain receptors
in the skin
Effector muscle in the arm
Response arm muscle contracts and hand
is removed from surface
Neurones (nerve cells) are specialised cells adapted to rapidly carrying nerve impulses from one part of the
body to another. There are three types: sensory, motor, relay (or intermediate).
Sensory neurones bring impulses from the sense organs or receptors into the CNS.
Relay neurones receive impulses from sensory neurones or other intermediate neurones and relay them to
motor neurones or other intermediate neurones.
Motor neurones carry impulses from the CNS to the effector organs.
Each nerve cell consists of:
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A cell body containing a nucleus and granular cytoplasm containing many ribosomes. These ribosomes
are grouped together forming Nissl granules which are concerned with the formation of
Dendrites many thin extensions which carry impulses towards the cell body. These receive
impulses from other nerve cells. Some neurones also have an axon which is a long membrane covered
cytoplasmic extension which transmits impulses from the cell body. At its end, an axon divides into
branches which form synapses with other neurones.…read more
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Most of the channels that allow the potassium ions to diffuse in are open while most of the sodium
channels are closed
As a result, the axon membrane is more permeable to potassium ions that sodium ions, so these
diffuse out faster than sodium ions diffuse in
The result is that the outside of the membrane is positive compared to the inside,
The action potential
When a stimulus is received by a receptor or nerve ending, a reversal of the charges on the axon membrane…read more
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The action potential causes a small electric current across the membrane and as a portion of the membrane
is depolarised, depolarisation of the next portion is initiated. There is a series of local currents propagated
along the axon.
The sodium/potassium pump is active all the time and behind the transmission, the pump restores the resting
potential. Once the resting potential is restored, another impulse can be transmitted.
The `all or nothing principle'
The size of the impulse is independent of the size of the stimulus.…read more