Chapter 8 – Control of Movement
Skeletal Muscle
The muscles that move our skeletons
Most are attached to bones at each end
Fastened to bones via tendons
Movement types:
o Flexion: contraction of a flexor muscle
The drawing in of a limb
o Extension: contraction of extensor muscles
Antigravity muscles
o Muscles contract, limbs flex
Anatomy
o Skeletal muscles are composed of 2 types of muscle
fibers
Extrafusal muscle fibers: served by axons of the
alpha motor neurons
Provide the muscle’s motive force
Found outside muscle spindles
Intrafusal muscle fibers: sensory organs that are
served by 2 axons (1 sensory, 1 motor)
Called muscle spindles (within the spindles)
Central region (capsule) contains sensory
endings sensitive to stretch
Gamma motor neuron causes intrafusal
muscle fiber to contract (very little force) to
modify the sensitivity to stretch
o 1 myelinated axon of alpha motor neuron serves many
Extrafusal muscle fibres (from 1 per 10 to 1 per 100s)
o motor unit: 1 alpha motor neuron + its axon +
associated extrafusal muscle fibres
1 muscle fiber has:
a bundle of myofibrils (with overlapping
strands of actin and myosin) (overlap zone
is called a striation)
Muscular Contraction
o Neuromuscular junction: the synapse between the
terminal button of an efferent neuron and the
membrane of a muscle fiber
Terminal buttons synapse on motor endplates
Endplate potential: Acetylcholine is released from
terminal buttons and depolarizes the postsynaptic
membrane Very large potential, always causes muscle
fiber to fire, induces a twitch/contraction
Opens the calcium channels so calcium ions
enter the cytoplasm, triggering the
contraction
Ca2+ are a cofactor that permits
myofibrils to take energy from the
ATP in the cytoplasm
Cross bridges row along actin
filaments
* Muscles not covered in course
Control of Movement by the Brain
Primary Motor Cortex
o On the precentral gyrus, rostral to the central sulcus
o Shows somatotopic organization
Disproportionate amount of cortical area devoted
to fingers and speech muscles
Organized in terms of movements
o Frontal association cortex: principal cortical input to
primary motor cortex (rostral to PMC)
o Supplementary motor area (SMA) and premotor cortex
important for control of movement
Both regions receive sensory info from the
parietal and temporal lobes, both send efferent
axons to PMC
SMA is on medial surface of the brain, rostral to
PMC Premotor cortex is on the lateral surface, rostral
to the PMC
The Descending Pathways
o Lateral group: independent limb movements, incl.
hands and fingers
1. Corticospinal tract
Has axons of cortical neurons from PMC that
terminate in the gray matter of the spinal
cord
Also has neurons from parietal and temporal
lobes
Path: cortex –> subcortical white matter –>
ventral midbrain (cerebral peduncles in
medulla) –> form the pyramidal tracts –>
most fibers decussate (cross over) to
contralateral spinal cord (form the lateral
corticospinal tract)
The rest of the fibers descend thru the
ipsilateral spinal cord (form the
ventral corticospinal tract) * actually
part of the ventromedial group
Most axons in lateral Cor-Spi tract control
distal parts of limbs (arms, hands, fingers,
lower legs, feet, toes)
Form synapses with motor neurons in
the gray matter of the spinal cord (in
lateral part of ventral horn)
Axons in ventral Cor-Spi tract control
muscles of upper legs and trunk
send terminal buttons to both sides of
gray matter in spinal cord
2. Corticobulbar tract
Projects to the medulla th th
Terminates in the motor nuclei of 5 , 7 ,
9 , 10 , 11 thand 12 thcranial nerves
Control movements of the face, neck,
tongue and some eye muscles
3. Rubrospinal tract
Originates in the red nucleus of the
midbrain Red nucleus receives most important
inputs from motor cortex via
corticorubral tract from cerebellum
Controls independent movements of the
forearms and hands (not fingers)
o Independent limb movements: right and left limbs
make different movements, or 1 limb moves while the
other stays still
o Ventromedial group:
1. Vestibulospinal tract
Cell bodies are in vestibular nuclei
Controls posture
2. Tectospinal tract
Cell bodes in superior colliculus
Coordinate head and trunk w/ eye
movements
3. Reticulospinal tract
Cell bodies in nuclei in brain stem &
midbrain reticular formation
Control automatic functions (muscle tonus,
respiration, coughing, sneezing) and
walking
4. Ventral corticospinal tract
o Control automatic movements: gross movem
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