NROC64H3 Chapter Notes - Chapter 13: T-Tubule, Mitochondrion, Gastrocnemius Muscle

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17 Mar 2012
Chapter 13 Spinal Control of Movement
- Motor system has muscles and neurons that control
them, “to move things is all the mankind can do… for
such the sole executants is muscle, whether in
whispering a syllable or in telling a forest”
- SC contains motor programs for generation of
coordinated movements and that these programs are
accessed, executed, and modified by descending
commands from the brain
- Motor control has 2 parts: 1) SC’S command and
control of coordinated muscle contraction, 2) brain’s
command and control of motor programs in SC
- Peripheral somatic motor system: joints, skeletal
muscles, spinal motor neurons and how they
communicate w/ each other
The Somatic Motor System
- Muscles divided into 2 categories:
1. Smooth muscle: Digestive tract, arteries and related
Innervated by nerve fibers from ANS, role in peristalsis
(movement of material through intestines) and control of
blood pressure and blood flow
2. Striated muscle: 2 types
A. Cardiac muscle: Heart muscle, contracts rhythmically even
in absence of innervation (from ANS functions to
accelerate/slow down heart rate)
B. Skeletal muscle: Bulk of muscle mass of body and functions
to move bones around joints, move eyes w/I head, control
respiration, control facial expression, produce speech
Enclosed by connective tissue sheath that forms tendons.
Each muscle has 100s of muscle fibres (cells of skeletal
muscle), innervated y 33 paired somites
These muscles, and parts of NS that control them = somatic
motor system
i.e. Elbow joint
Joint is where humerus/upper arm bone is bound by
ligaments to radius of ulna/lower arm bone function like
hinge on pocket knife
Close knife flexion (muscle= brachialis, other biceps brachii
and coracobrachialis) Muscles = flexors, all 3 muscles =
Open knife-extension, muscles=triceps brachii and anconeus
= extensors
Muscles that are responsible for movements of trunk = axial
muscles= maintaining posture
Muscles moving shoulder, elbow, pelvis and knee =
proximal/gridle muscles = locomotion
Muscles moving hands, feet, digit = distal muscles = manipulation of objects
Think axial-close, proximal, next close, distal-FAR
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The Lower Motor Neuron
- Lower motor neuron: Innervated by ventral horn of SC
- Upper motor neuron: Supplies input to SC
- Ventral Horn:
Cervical and lumbar are swollen
Medial lateral: axial distal
Dorsal ventral: flexors extensors
The Segmental Organization of Lower Motor Neurons
- Axons lower motor neurons bundle together ventral roots joins w/
dorsal root spinal nerve
- Mix spinal nerves contain sensory and motor fibers, motor provides
fibers from 1 spinal nerve belong to spinal segment (cervical 1-8.
Thoracic 1-12, lumbar 1-5. Sacral 1-5)
- Motor neurons innervate distal and proximal musculature, found in
cervical and lumber-sacral segments
- Motor neurons innervating axial musculature are found at all levels
- LMN found in ventral horn
Alpha Motor Neurons
- Alpha motor neurons: Trigger generation of force by muscles make up elementary component of motor
control = motor unit
- Muscle contraction = individual and combined actions of motor units
- Collection of alpha motor neurons innervating single muscle is motor neuron pool
Graded Control of Muscle Contraction by Alpha Motor Neurons
- Important to exert right amount of force during movement
1. CNS controls muscle contraction by varying firing rate of motor neurons
Alpha motor neuron releases acetylcholine/Ach at neuromuscular junction (specialized synapse b/w nerve and
skeletal muscle)
Ach released cases excitatory postsynaptic potential (EPSP)/endplate potential large enough to trigger 1 PS
2. CNS grades muscle contraction is by recruiting additional synergistic motor units
Extra tension depends on how many muscle fibers are in that unit
Inputs to Alpha Motor Neurons
Alpha motor neurons excite skeletal muscles we need to understand what regulates motor neurons
Controlled by synaptic inputs in ventral horn
3 sources of input to an alpha motor neuron
1. Dorsal root ganglion cells w/ axons the innervate a specialized sensory apparatus embedded w/I the muscle
muscle spindle MUSCLE LENGTH
2. Upper motor neurons in motor cortex and brain
stem initiation and control of voluntary movement
3. Interneurons in SX excitatory/inhibitory SPINAL
Types of Motor Units
- Each motor unit contains muscle fibers
- Fast motor Units: Rapidly fatiguing white fibers,
larger diameter, faster-conducting axons, high-
frequency bursts of AP = White muscle fibers:
Little mito, anaerobic metabolism, contract and
fatigue rapidly
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