THE NERVOUS
SYSTEM
The Nervous System
Central Nervous
Peripheral Nervous
System
System
Brain
Spinal
cord
Cranial
Nerves
Spinal
Nerves
Ganglia
Neuron = Nerve cell
• Sensory Neuron
(afferent)
• Carry electrical
impulses from
sense organs to the
C.N.S
• Cell body is outside
C.N.S in ganglia
• Motor Neuron
(efferent)
• Carry electrical
impulses from the
C.N.S to effector
(muscle/Gland)
• Cell body inside
C.N.S
Interneurons
• Occur in the C.N.S only
• May connect sensory to motor neurons
Direction of impulse
Diagrams
• Draw diagram of sensory neuron and
interneuron into notes
Functions of parts
• Dendrites – receive information and carry electrical
impulses towards the cell body
• Axons – carry impulses away from a cell body
• Cell body – contains a nucleus and it forms
neurotransmitter chemicals
• Myelin sheath – is a fat rich material that insulates
electrical impulses
• Schwann cells – form myelin sheath
• Neurotransmitter swellings (vesicles) – produce
neurotransmitter chemicals.
• Dendron – brings impulse to the cell
body
• Nodes of Ranvier – are gaps in the
myelin sheath. Electrical impulse ‘jumps’
from one gap to the next
 Speeds up transmission of impulse
Stimulus
• A change in the neurons environment that,
if strong enough, will generate an impulse
e.g. red light at traffic lights
Movement of an Impulse
• Impulses travels along an neuron in the
form of an electrical impulse
• In order for the impulse to travel it is
necessary for ions to move in and out of
dendrites and axons
• The movement of ions requires energy in
the form of ATP
Threshold level
• The minimum intensity of stimulus
needed to generate an impulse
• It does not matter if the stimulus is
above the required strength the
same message is sent
• Message sent in all neurons is the
same
All or nothing Law – an impulse is only
generated if the stimulus is at or above the
threshold.
Functions of parts
• Cerebrum – major sensory and
motor control, language, memory,
intelligence and consciousness.
• Cerebellum – muscle co-ordination,
movement and balance.
Functions of parts
• Medulla oblongata – regulates
breathing and heart rate.
• Hypothalmus – overall body
homeostasis
osmoregulation, temperature control
regulates hormone secretion by the
pituitary gland.
• Pituitary gland – master gland
- secretes 8 hormones
which control other
glands in the body
The Spinal Cord
• Bones of the backbone enclose the spinal
cord. It carries impulses to and from the
brain.
• It is also involved in many reflex actions
T.S. of spinal cord
Functions of parts
• The Meninges – are membranes that
surround (the brain) and Spinal cord.
• Spinal nerves – 31 pairs of spinal
nerves carry impulses to and from the
spinal cord.
• Dorsal root – carries impulses into the
spinal cord along sensory neurons.
• Ganglion – (pl. Ganglia) located on the
dorsal root is a swelling that contains
cell bodies of sensory neurons
Functions of parts
• White matter – contains axons
• Grey matter – contains cell bodies
and dendrites.
• Central canal – contains
cerebrospinal fluid.
• Ventral root – contains motor
neurons that carry impulses out of the
spinal cord
Reflex Action
A reflex action is an automatic,
unconscious or involuntary response
to a stimulus.
The brain is NOT involved in reflex
actions
Examples – knee jerk reaction, blinking for
protection, raising your hands when falling
Significance/Benefit allows faster
responses than normal
REFLEX ACTION --- The Reflex Arc
The path of a reflex action
•
The path taken by a reflex action is
called a reflex arc.
Example : Dropping a hot object
 Receptors in finger detect hot object
 Impulse travels along sensory neuron
to spinal cord
 Impulse travels into spine via dorsal
root
 In the spinal cord the impulse splits
The path of a reflex action
(a) It crosses a synapse onto an interneuron
(b) It crosses another synapse and passes on
up to the brain.
 From the interneuron the impulse crosses
another synapse and travels out of the
ventral root along a motor neuron
 The motor neuron connects to a muscle to
a