Somatic Motor Systems
Motor Units are the basic element of motor
control in the somatic division
• A motor unit consists of a single spinal or
cranial motor neuron plus the skeletal muscle
fibers that it innervates.
• Cell bodies of spinal somatic motor neurons are
in lamina 10 of the spinal gray; cranial motor
neurons are in nuclei of cranial nerves.
• When a motor neuron is turned on, all of the
muscle fibers it innervates are turned on.
Basic facts about mammalian
skeletal muscle
• Cells are large in diameter (up to 200 mu), and long (up to meters in
larger animals)
• Skeletal muscle is one of the two main forms of striated muscle: the
contractile machinery is organized into the form of sarcomeres.
• Each cell receives one and only one synapse from one and only one
motorneuron.
• Contractile activity is usually triggered by bursts of action potentials
in motor neurons, which causes a smooth, sustained contraction
called a tetanus.
• There is no spontaneous contractile activity in the absence of input
from motor neurons
• Skeletal muscle is adapted by its structure for delivering maximal
force at lengths close to its rest length.
Structure of
Muscle
Ultrastructure of contractile machinery in
striated muscle
Skeletal muscles fall into three functional
classes
• Type I slow twitch (oxidative) - this fiber
type predominates in postural muscles.
These fibers are characterized by smaller
diameter, numerous mitochondria, little
glycogen, numerous capillaries, and
abundant myoglobin which gives them a
red color. They can sustain contraction for
long periods before fatiguing.
Fast twitch fibers are used at higher effort levels.
• Type IIa fast twitch (oxidative) These fibers are
used where moderate force must be sustained. They
have a mixed metabolic strategy and intermediate
fatigue properties.
• Type IIb fast twitch (glycolytic) These fibers
have the largest diameters and are characterized by few
mitochondria, no myoglobin, few capillaries and large
glycogen stores. They develop high forces but fatigue
quickly.
Somatic units consist of single muscle fiber types
• Smaller cell bodies are more easily brought to threshold;
the smallest cell bodies belong to motorneurons that
serve type I units, the next size category serves Type IIa
units, and the largest sizes serve Type IIb units
• As muscular effort is increased, additional units are
recruited in the order
• I
IIa
IIb
• Probably in learned, high-speed activities, individual type
II motor units in any individual muscle can be activated
by higher centers through a motor program, without
having to activate the type I fibers.
What kinds of synaptic inputs does a spinal
motor neuron receive?
• Sensory inputs from the ipsilateral body surface
• Sensory inputs from muscle and joint receptors
that activate stretch, withdrawal and tendon
reflexes
• Inputs from neurons in same or adjacent
segments of the contralateral cord – for crossed
reflex pathways
• Descending pathways from higher motor centers
that modulate spinal reflexes, regulate posture
and apply motor programs for voluntary
movements.
Spinal motorneurons integrate a large
amount of information
• The English neurophysiologist Sherrington
described spinal motorneurons as “the
final common path” of motor control to
skeletal muscle. You could think of them
as funnels with a wide mouth for collecting
information (dendrites) but a narrow neck
for delivering it to muscle fibers (axon).
Motor Control at the Spinal Level: The
Stretch (Myotatic) Reflex
What does it do? Activates motor units in a muscle
in response to stretch of the muscle.
Functions:
• Maintains muscle length at a setpoint –
particularly important for posture maintenance
• Modulates force as needed to move muscle
length to meet a new setpoint – important for
load compensation in voluntary movements
Anatomical Components of the reflex arc
• Sensors: muscle spindles (myotatic organs) containing
tiny modified muscle fibers (intrafusal fibers) and two
types of stretch receptor endings:
Annulospiral endings (rapidly adapting) –
associated with nuclear bag fibers
Flower spray endings (slowly adapting) –
associated with nuclear chain fibers
• Afferents: Ia (largest size) axons connected to rapidlyadapting stretch receptors (annulospiral endings; II
axons connected to slowly adapting receptors (flower
spray endings)
• Efferents: Alpha motor neurons to extrafusal fibers
Nuclear
chain fiber
Details of the muscle
spindle
Nuclear bag fiber
Spindle capsule
Flower spray ending
Annulospiral ending
Type II afferent
Type Ia afferent
Static gamma efferent
Dynamic gamma efferent
Gamma motorneurons are for resetting
spindle length and are not within the
stretch reflex arc
Extrafusal fiber (for size comparison)
Extrafusal fibers make up by far the greatest part of the muscle mass and
provide all of the force. They are innervated by alpha motorneurons.
Spindle afferents are stimulated by muscle
stretch
• Stretching a muscle also stretches
individual muscle spindles. The center of
the spindle is the most mechanically
compliant part, so that is where the most
stretch occurs. This stretch activates the
two kinds of afferents
The Stretch Reflex Arc
The stretch reflex arc is monosynaptic; this is the only
known monosynaptic reflex. All integration is carried out by
the alpha motor neurons.
the reciprocal inhibition of the opponent muscle is
polysynaptic – there is at least one interneuron in the
pathway
These are alpha motor neurons
Stretch reflexes are the power steering of
voluntary movements
• Fact 1: the intrafusal fibers cannot shorten the
spindle unless the muscle also shortens
• Fact 2: spindle shortening and muscle
shortening must match each other.
• If muscle shortens and spindle doesn’t, the
spindle becomes slack and unresponsive.
• If spindle is activated and shortens but muscle is
also activated and doesn’t shorten, the center
part of the spindle gets stretched and the
afferents are stimulated.
Steps in a reflex-assisted voluntary movement:
• 1. Higher motor centers send descending signals that
activate particular muscle groups – this is believed to
involve coactivation of alphas to initiate muscle activity
and gammas to reset length setpoint of spindles.
• 2. Question: Is muscle at new setpoint length yet?
• YES: activity in Ia and II afferents from muscle spindles
is at baseline level – no additional motor unit recruitment
• NO: activity in spindle afferents is above baseline –
spindle inputs and descending central inputs summate
on alphas, recruiting additional motor units.
The Tendon Reflex Arc
The Golgi tendon organ reflex is believed to mediate overload
protection by inhibiting the motor units of the stressed muscle
and activating those of the opponent muscle.
The axons of Golgi
tendon organs are
Type II afferents
The Withdrawal Reflex Arc
The flexion or withdrawal
reflex mediates
protective withdrawal of
an injured appendage.
The crossed extension
component of the reflex
increases activity in
extensors of the
contralateral limb when
the flexion reflex is
activated.
What can a spinal animal do?
(Cord is transected at a level just below the medulla)
• 1. generate muscle tone if alpha motor neurons are modulated at
correct level – but this generally doesn’t happen if cord is transected
below the medulla.
• 2. generate alternate stepping movements if the animal is supported
– due to mutual inhibitory connections between extensors and
flexors and between opposite limbs – in fact such animals can run
on a treadmill, changing gaits as the treadmill is speeded up, in a
way indistinguishable from intact animals.
http://www.youtube.com/watch?v=wPiLLplofYw
• 3. perform spinal reflexes (withdrawal, genitourinary reflexes) – but
in human patients reflex responsiveness depends very much on the
management of the patient after the injury.
• No voluntary movements are possible for myotomes below the level
of section.
Higher Somatic Motor Control
From the Vestibular Nucleus of
the Brainstem Upward
Higher Motor Control is distributed between
4 interacting brain areas
• Brainstem – antigravity reflexes, including
responses to vestibular inputs
• Motor Cortex – distal muscles of appendages for
highly controlled manipulative movements
• Cerebellum – coordination of rapid movements
• Basal Ganglia – execution of motor programs
involving multiple large muscle groups
• The interneurons in these areas are sometimes called
upper motor neurons, in contrast to spinal motor
neurons, which are called lower motorneurons.
Vestibular System
• Components:
Semicircular canals: responsive to
rotational acceleration of head relative to
the inertial frame of reference
Saccule and utricle: detect head position
relative to pull of gravity and linear
acceleration of head
Semicircular canals detect head rotation relative to
the inertial frame of reference
endolymph lags behind
as head rotates,
displacing barrier and
stimulating hair cells
endolymph
Gelatinous
barrier
Hair
cells
Branch of vestibular
nerve
The two sets of 3 canals are oriented to detect
head rotation in 3 planes
The saccule and utricle (one set on each side of
the head) detect head position relative to gravity,
linear acceleration, and low-frequency vibration
Acceleration in any
direction creates a
direction and
magnitude specific
pattern of stimulation
of the hair cells
Gelatinous
filling
Head
acceleration
Branch of
vestibular nerve
Calcium carbonate
weight
Filling
displaced
in opposite
direction
Vestibular System and brainstem postural reflexes
• Brainstem modulates outflow to extensors to
maintain postural setpoints, particularly in
response to changes in head position.
• Deviations relative to pull of gravity are detected
by muscle/joint receptors and the vestibular
system, activating righting responses.
• Vestibulo-ocular and vestibulo-collic reflexes
allow you to fix your gaze on a target
irrespective of head and body movements
Midbrain transection creates a decerebrate animal
• One classic sign of damage to pathways between
medulla and more rostral centers is a tonic “decerebrate
rigidity”. This arises from the fact that the vestibular
nuclei of the brainstem mostly provide excitatory input to
spinal motor neurons that serve extensors, whereas the
higher centers provide some mixture of excitatory and
inhibitory input. When the inhibitory component is
removed, excitatory inputs predominate, so appendages
are rigidly extended.
Like the primary somatosensory cortex, the primary motor
cortex has a somatotopic organization
The motor cortical
homunculus
Cortical columns in motor cortex
Within the motor cortex, individual columns
consist of cells that are involved in
controlling motion around a single joint.
Within a column, there are cells that
become active for specific angles of
movement of that joint. As a result of this,
the motor cortex is particularly important for
finely controlled manipulations in primates.
It is less important in animals that do not
use their appendages for manipulation.
The cerebellum is involved in predicting and
controlling the future position of rapidly moving
body parts
• Decomposition of complex movements is a
symptom of cerebellar damage
• Cerebellar neurons only interact with other brain
motor areas and do not connect directly with
spinal motor neurons.
The basal ‘ganglia’ include 3
interior nuclei
• Globus pallidus
• Caudate nucleus
• Putamen
Basal ganglion diseases cause uncontrolled movement,
suggesting that motor programs are organized and stored
primarily by the basal ganglia
• Athetosis – writhing
movements
• Ballisms – flinging
movements
• Chorea – ‘dance-like’
movements
• Parkinson’s disease –
rigidity and difficulty in
initiating movements
• In many cases these
seem to involve
inappropriate playing out
of motor programs
• Parkinson’s disease
involves death of
dopaminergic neurons in
the substantia nigra –
dopamine is an important
modulator of neuronal
activity in the putamen.
Two main kinds of pathways lead from brain
motor areas down to the spinal cord
• 1. Pyramidal tract = direct corticospinal
tract
Axons coming from pyramidal cells in the
motor cortex pass through the thalamus,
decussate in the medullary pyramids and
descend in lateral corticospinal tract in
cord – these axons mainly serve distal
muscles that are involved in reaching and
manipulation.
Pyramidal
and
ventromedial
tracts
The ventromedial tract
carries a small number
of fibers that run direct
from the motor cortex to
the spinal cord – these
mostly innervate axial
muscles.
Extrapyramidal Pathways
2. The extrapyramidal tracts contain axons emanating from the basal
ganglia and brainstem nuclei – some decussate in the brainstem –
some don’t decussate at all (!). In contrast to the direct corticospinal
tract, these are polysynaptic pathways. These pathways are mainly
involved in control of axial and girdle muscles. They are mainly
involved in movements that can occur without conscious control
a. Tectospinal – control head movements, reflexive responses to visual
and auditory threats, including escape responses.
b. Vestibulospinal – modulate stretch reflexes, match posture to head
position
c. Rubrospinal- an additional pathway from motor cortex to spinal
motorneurons - innervates mainly flexors – so works together with
vestibulospinal outputs to regulate posture
d. Reticulospinal – from reticular formation – adjust posture to reflect
alertness
Some extrapyramidal pathways
What can a decerebrate animal do?
• Brain is transected above midbrain but below
thalamus
• Righting reflexes – driven by vestibular system –
but not those driven by visual system
• Static responses – modifications of posture in
response to the needs of a particular situation –
eg tonic neck responses
What can a decorticate (high midbrain
transection) animal do?
• Nonprimates (rats, cats): walk, eat, drink, copulate, raise
pups (but not make a nest), learn to some extent (eg,
conditioned reflexes), show “sham” emotion, respond to
touch, pain and sound
– decorticate cats attack moving objects – even though
they can’t see.
• Primates – decortication causes very serious global
disability – one of the ways we know about this is from
anencephalic children, who in almost all cases enter a
chronic vegetative state.
Diseases of central motor pathways
• “Upper motor neuron” lesions (i.e. in cerebral white matter that
interrupt descending pathways): paralysis on contralateral side,
increased muscle tone and stretch reflex, + Babinski sign and other
infantile reflexes reappear.
• Basal nuclei diseases: Parkinson’s D., Huntington’s chorea –
impoverished movement repertoire (bradykinesia)
http://www.youtube.com/watch?v=0E7x1mPa3iM&feature=related
http://www.youtube.com/watch?v=OveGZdZ_sVs&feature=related
• Corticospinal tract lesions (i.e. in spinal cord) paralysis on ipsilateral
side, decreased muscle tone and reflex strength; muscle wasting.
• “Lower motor neuron disease” (i.e. lesions to single spinal
segments): paralysis of specific muscle groups, decreased muscle
tone; muscle wasting.