Biology 12
The Nervous System
Neurons (aka Nerve Cells)
A neuron is made up of three main parts: dentrites, axon and cell body
o Dentrite: conducts impulse toward a cell body
o Axon: conducts impulse away from a cell body
o Cell body: contains the nucleus, which __________________ the cell.
There are 3 kinds of neurons
Type
Diagram
1. Sensory
Neuron
(aka afferent
neuron)
2. Interneuron
(aka association
neurons)
3. Motor Neuron
(aka efferent
neuron)
Sensory neurons
o transmits information about changes in the internal and external environment to the
CNS.
o have long ______________________ that extend from receptors to the CNS.
o cell body is located outside of the CNS
Interneuron
o confined to the grey matter of the CNS
o provide a link between a sensory neuron and a motor neuron
o relatively short
Motor neuron
o transmits nerve impulases from the CNS to an effector organ (e.g. muscle or gland)
thereby initiating a physiological response (e.g. muscle contraction or hormone
sectretion)
o have long ____________________ that extends from the cell body of the motor neuron
(located within the CNS to an effector
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Biology 12
The Nervous System
Nerves
comprised of many nerve fibres (axon plus supporting tissues) enclosed in a connective-tissue
sheath
connect CNS with organs and tissues of body
three types
i. motor nerve: carries only ____________ nerve fibres
ii. sensory nerve: carries only ______________ nerve fibres
iii. mixed nerve: carries __________________ nerve fibres
o dendrites of sensory neurons and axons of motor neurons can be found in the
same mixed nerve as each other
o there can be hundreds of long fibres in the mixed fibres
o these fibres are covered with a fatty tissue called myelin sheath.
 myelin sheath
 composed of Schwann cells that wrap around each fibre
 insulates long fibres of neruons from each other within the
nerve and surrounding
 allows for faster travel of impulses
 points between the Schwann cells is called the nodes of Ranier
o note: invertebrates generally lack myelin sheath and
therefore are not capable of fast nerve responses
spinal nerves are connected to the CNS either on the dorsal or ventral side of the spinal cord
o sensory nerves are connected through the dorsal (towards back) root
 cell bodies of sensory nerves are found in the dorsal root ganglia
o motor nerves are connected through the ventral (towards front) root
 cell bodies of motor nerves are located in the spinal cord
 cell bodies of motor neurons coupled with those of interneurons, gives the
inner regions of the spinal cord a darker colour, called grey matter
 in contrast the outer regions are white because of the
___________________ of the axons
Comparison of Types of Neurons
Bases of
Sensory Neuron
Interneuron
Motor Neuron
Comparison
Structure
Function
Location
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Biology 12
The Nervous System
Reflex Arch: an automatic and innate response to a particular stimulus
basic functional unit of the nervous system
generally involves all three types of neurons: _____________________, ____________________
and _______________________
extremely rapid response
does not involve the brain to carry out
o instead, sensory input is passed through the spinal cord and back out to the motor
output to an effector organ
o effector organ could be a muscle (causing movement) or a gland (resulting in
secretion)
where neurons connect with other neurons or effector organs is called a synapse
e.g. the stretch reflex (one being the knee-jerk reflex)
o sensor senses a stretch in the muscle of the leg
o impulse is sent by sensory neurons to the spinal cord in the ____________ root
o sensory neuron synapse with motor neurons and interneurons
o motor neurons leave the spinal cord through the ______________ root
o initiate contraction of quadriceps and relaxation of the hamstrings
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