Motor Control
Motor Control
• In order to learn about how the nervous
system can coordinate complex movement
patterns we must look at all the players
involved.
• Key brain structures.
• Spinal cord structures.
Spinal Cord Pathways
Motor Neuron Topography of
the Spinal Cord
• Flexors vs. extensors:
motor neurons that
innervate extensors
muscles are located
anterior to the motor
neurons that innervate
the flexor muscles.
Proximal vs. distal:
motor neurons that
innervate proximal
muscles are located
medial to motor neurons
that innervate distal
muscles.
Three Ascending Pathways
• The nonspecific (Anterior Lateral) and specific
(Dorsal Column Ascending Pathway) send impulses to
the sensory cortex
– These pathways are responsible for discriminative
touch, pain and conscious proprioception
• The spinocerebellar tracts send impulses to the
cerebellum.
– This pathway is important in providing procrioceptive
feed back from muscles, joints necessary from proper
coordination
– This will not contribute to sensory perception since it
doesn’t ascend to the cortex
Spinothalamic Pathway
• Detects pain,
pressure,
temperature, crude
touch, tickle and itch
• Cross occurs in
spinal cord usually
1-2 levels above
where afferent input
entered.
• Synapses in the
thalamus and
continue to cerebral
cortex
Dorsal Column Ascending Pathway
• Detect deep touch,
pressure, vibration,
and proprioception.
• Carry signals from arm
and leg
• Cross in the medulla
• Synapses in the
thalamus carries signal
to cerebral cortex
Spinocerebellar/ Dorsal Column tracts
• Dorsal Column is the
primary pathway that
allows us to perceive
sensory input.
• Spinocerebellar tracts:
allow our cerebellum to
interpret sensory
information from muscles
and joints to help
coordinate descending
cortical spinal pathways
(Motor) Systems
• Descending tracts deliver efferent impulses from the brain to the
spinal cord, and are divided into two groups
– Direct pathways (pyramidal): tracts which originate in the
cerebral cortex.
• Initiate movement from premotor and prefrontal areas that
are receiving sensory information ( Multimodal) from many
areas of the brain.
• controls contra lateral side of body.
– Indirect pathways, (extra pyramidal) originate in other area of the
brain. i.e. midbrain.
• This system is involved in executing of subconscious motor
programs i.e. arm swing.
• Helps maintain appropriate level skeletal muscular tone.
• Cerebellum contain ½ of the brains neurons(50 billion) and 75% of
the surface area of the cerebral cortex. It allows us to correct errors in
movement on an ongoing basis
The Direct (Pyramidal) System Lateral
Corticospinal Tract
• Controls and initiates voluntary
movement. Originate with the
pyramidal neurons in the
precentral gyri
• Impulses project down through
internal capsule to the cerebral
peduncles to the medulla.
• 90% of fibers decussate to the
opposite side and continue to
descend down through the
lateral corticospinal tracts. It will
enter the gray matter and
synapse in the anterior horn
• Stimulation of anterior horn
neurons activates skeletal
muscles
Corticospinal Tract
• Voluntary coordinated
movements for striated
muscle has a 2 neuron
pathway
– upper motor neuron
runs from the cerebral
cortex to the alpha
motor neuron.
– lower motor neuron
runs from the alpha
motor neuron in spinal
cord to the target
muscle.
• Upper motor neurons
cross in the medulla which
means the left side of your
brain controls the right
side of your body.
Indirect (Extrapyramidal)
System
• These motor pathways are complex and
multisynaptic, and regulate:
– Axial muscles that maintain balance and posture
– Muscles controlling coarse movements of the
proximal portions of limbs
– Head, neck, and eye movement
– Has no projection to the spinal cord.
–
Extra Pyramidal Pathways
•
•
•
Tectospinal tract :colliculi of
midbrain
– reflex turning of head in
response to sights and sounds
Reticulospinal tract (reticular
formation)
– No conscience control during
surprise. (Upper truck
extensor tone)
– controls limb movements
important to maintain posture
and balance
Vestibulospinal tract (brainstem
nuclei)
– postural muscle activity in
response to inner ear signals
Basal Ganglia
Kinesthetic Sense
• Provides constant feedback to the brain
about what activity has just occurred.
– Muscles Spindles,
– Joints and Ligament receptors
– Golgi Tendon organs.
• These are critical in motor planning
The Stretch (Myotatic) Reflex
• When a muscle is stretched, it contracts and
maintains increased tonus (stretch reflex)
– helps maintain equilibrium and posture
• head starts to tip forward as you fall asleep
• muscles contract to raise the head
– stabilize joints by balancing tension in extensors and
flexors smoothing muscle actions
• Very sudden muscle stretch causes tendon reflex
– knee-jerk (patellar) reflex is monosynaptic reflex
– testing somatic reflexes helps diagnose many diseases
• Reciprocal inhibition prevents muscles from
working against each other
Nature of Somatic Reflexes
• Quick, involuntary, stereotyped reactions of glands
or muscle to sensory stimulation
– automatic responses to sensory input that occur without
our intent or often even our awareness
• Functions by means of a somatic reflex arc
–
–
–
–
–
stimulation of somatic receptors
afferent fibers carry signal to dorsal horn of spinal cord
one or more interneurons integrate the information
efferent fibers carry impulses to skeletal muscles
skeletal muscles respond
The Patellar Tendon Reflex Arc
Monitoring of Muscle Length and Tension
• Skeletal muscle has 2 sensory receptors that monitor muscle
length and tension
• Extrafusal fibers are those that contract to produce
tension and movement ( Actin /Myosin)
• Muscle spindles are called intrafusal muscle fibers that are in
the center of extrafusal fibers. They are wrapped around by an
afferent neuron which sends information about the length of a
muscle and the speed at which the length changes during
contraction or stretching.
• Golgi tendon organs are afferent neurons that are wrapped
around the collagen fibers of a tendon near the attachment to
muscle which sends information about the tension that a
muscle produces during contraction.
Sensory Receptors
of Muscle
• Muscle spindles change
shape depending if the
muscle is stretched or
shortened
– the afferent neurons
wrapped around the
spindle will increase or
decrease AP frequency
respectively
• Afferent neurons synapse
with interneurons and
motor neurons in the
spinal cord
Activity of Muscle Spindles
• Regardless of the reason for a change in length,
the stretched spindle in (a) generates a burst of
action potentials as the muscle is lengthened in
(b), the shortened spindle produces fewer action
potentials from the spindle
Alpha/Gamma Co-Activation
• Since muscle spindle activity would decrease during muscle contraction
(Spindle on slack) the spindle would not be providing proprioceptive
input.
• To avoid this problem both the smaller gamma motor neurons( Ў ) and
the larger alpha motor neuron are both innervated simultaneously.
• Gamma motor neurons originate in the brain stem. Regulate the resting
muscle tone. Dynamic nuclear bag fibers detect initiation of stretch while
nuclear chain fibers monitor sustained stretch.
• ( Ў ) maintain tension within the spindle apparatus while the muscle is
contracting to provide continuous afferent input.
Golgi Tendon Organ
• Compared to
when a muscle is
contracting,
passive stretch of
the relaxed
muscle produces
less stretch of the
tendon and fewer
action potentials
from the Golgi
tendon organ
Golgi Tendon Reflex
• Contraction of the extensor
muscle of the thigh tenses
the Golgi tendon organ and
activates it to fire action
potentials. Responses
include:
– inhibition of the motor
neurons that innervate
this muscle (A)
Autogenic Inhibition
– excitation in the
opposing flexor’s motor
neurons (B)
Withdrawal (Flexion)
and Crossed Extensor
Reflexes
• Pain sensory afferents
detect pain in foot and
send APs to the spinal
cord
• Interneurons in the cord
activate flexors and inhibit
extensors on the “pained”
side of the body
– lift leg to move away
from painful stimulus
• Interneurons in the cord
activate extensors and
inhibit flexors on the
opposite side of the body
– body weight is
supported by one leg