NROC64H3 Lecture Notes - Lower Motor Neuron, Alpha Motor Neuron, Gamma Motor Neuron

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16 Mar 2012

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Chapter 13 Spinal Control of Movement
The motor system consists of all our muscles and neurons that control them; complex patterns of behaviour can be generated
without the participation of the brain
Spinal cords command and control of co-ordinated muscle contraction
Brain’s command and control of motor programs in spinal cord
CNS axon branch muscle fiber skeletal muscle somatic motor system (voluntary control)
Cardiac muscles
e.g. heart muscle
Skeletal muscles
e.g. move bones around joints, move eyes within head, respiration etc.
Digestive tract, arteries, structures innervated by nerve fibers in the autonomic nervous system
*important for peristalsis, control of blood pressure and blood flow
Major muscle that causes flexion BRACHIALIS, whose tendons insert into the humerus at one end and into the ulna at the other.
Flexion Flexors
Extension Extensors
Triceps brachii
Biceps brachii
Flexors and extensors pull on the joint in the opposite direction therefore
they are ANTAGONISTS to one another
Axons of motor neurons bundle together form ventral roots joins with dorsal root form spinal nerve exits cord
through notches between vertebrae
Skeletal muscles are not spread out evenly throughout the body and neither are the lower motor neurons within the SC
The greater number of motor neurons, the larger the size of the ventral horn for accommodation (e.g. cervical and lumbar
where distal and proximal muscles are located)
Flexing of an elbow joint
CONTRACTION of synergistic flexor muscles + RELAXATION
of the antagonistic extensor muscles
AXIAL trunk (posture)
PROXIMAL shoulder, elbow, pelvis, knee
DISTAL hands, feet, fingers (manipulation of
small objects)
Upper motor neurons of the brain supply input to the spinal
Lower motor neurons are the only ones that directly
command muscle contractions; they are somatic motor
neurons in the ventral horn of SC
Alpha Motor Neurons
Lower motor neurons are classified into two types (1) ALHPA MOTOR and (2) GAMMA MOTOR
Alpha motor neurons directly trigger the generation of force by muscles
Motor unit one alpha motor neuron + all muscle fibers it innervate
Motor neuron pool many alpha motor neurons + single muscle that it innervates
Graded Control of Muscle Contraction by Alpha Motor Neurons
Alpha motor neurons release ACh at the neuromuscular junction the specialized synapse
between a nerve and skeletal muscle presynaptic AP EPSP in muscle fiber “endplate
potential” postsynaptic AP twitch = rapid sequence of contraction & relaxation
CNS controls muscle contraction via the following:
1) Variable firing rate of motor neurons summation of twitches cause sustained
contraction as the number and frequency of incoming AP increase
2) Recruiting additional synergistic motor units
e.g. antigravity muscles of legs LARGE motor units = many muscle fibers per alpha
motor neuron large alpha motor neurons
muscles controlling rotation of eyes SMALL motor units = few muscle fibers per
alpha motor neuron; muscles with large # of small motor units are FINELY controlled
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