The nervous system detects and
interprets changes in conditions inside
and outside the body and responds
to them. The central nervous system
analyses information and initiates
responses; the peripheral nervous
system gathers information and carries
the response signals. Some responses
are involuntary; others are dictated by
conscious thought. All nervous system
activity consists of signals passed
through pathways of interconnected
neurons (nerve cells).
M e d i a n n e r v e
U l n a r n e r v e -
F e m o r a l n e r v e
P o s t e r i o r t ib ia l n e r v e
Peripheral nervous system
This system comprises allthe nerves connecting
the brain and spinal cord to the rest ofthe body.
Ofthese, 31 pairs (the spinal nerves) connectto
the spinal cord and 12 pairs (cranial nerves)
connect to the brain. The main nerves ofthe
limbs are labelled.
Structure ofthe spinal cord
The grey matter contains nerve cell bodies;
white matter contains their conducting fibres.
On joining the cord, spinal nerves split into
two. The dorsal root carries sensory fibres;
the ventral root carries motor fibres.
Central nervous system
This system consists ofthe brain and spinal cord,
protected bythe skull and spine. The CNS
receives input from sense organs and receptors
and sends signals to muscles and glands, via the
peripheral nervous system.
Some possible events in response
to a finger touching a hot object
are shown. A receptor sends a
message, via a sensory fibre, to the
spinal cord. This triggers a signal
that travels, via a motor fibre, back
to a muscle, which contracts to
move the finger away from the
heat. This action is called a reflex
arc. Other signals pass to the brain.
S e n s o r y r e c e p t o r s
S e n s o r y f i b r e
Sensoryand motor fibres
Although the sensory and motor fibres
carrysignals in opposite directions,
along part oftheir lengths they may
occupy exactlythe same nerve.
B r a in
W h e n i m p u l s e s r e a c h t h e c e r e b r a l c o r t e x ,
p a i n i s f e l t . O t h e r a c t i v i t y ( c o n c e r n e d , f o r
e x a m p l e , w it h m e m o r i z i n g t h e p a i n f u l
s t im u lu s ) m a y a l s o b e in it ia t e d .
C e r e b r a l c o r t e x
B r a in s t e m
E y e m o v e m e n t
S i g n a l s a r r iv in g a t t h e b r a i n s t e m
m a y c a u s e m o r e a c t i o n , s u c h
a s t h e e y e s m o v i n g t o w a r d s
t h e s t im u lu s .
N e r v e f i b r e s t o b r a in
G r e y m a t t e r
Spinal cord
The signal entering
the spinal cord passes
to other nerve cells by
separate connections.
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