Download Motor System & Behavior

Motor System
Why is the Motor System Important?
• All observable behavior is directly related to
activity in the motor system.
• Without the motor system, we could
experience sensation, think, reason, problem
solve, read, write, and do mental math, but
we would not be able to communicate our
thoughts and abilities to anyone.
1. Skeletal Muscles (vs. smooth muscles or
cardiac muscle)
- striated (striped) appearance because
they are comprised of muscle fibers
- move through a pull action (contraction)
- work in pairs with a reciprocal muscle
(bicep contracts and triceps relaxes)
- stimulated by a Motor Neuron
2. Anatomy of the Muscle
striated muscles are made of muscle
fibers that have two parts, outer and
inner:
Outer fiber = extrafusal fiber
This represents
only one
Inner fiber = intrafusal fiber
muscle
fiber - a muscle
has many fibers
Wrapped around the intrafusal fiber is a sensory nerve
that picks up the sensation of stretch.
Outer fiber = extrafusal fiber
Inner fiber = intrafusal fiber
Gamma Motor
Neuron
Alpha motor
neuron
Each muscle fiber has a gamma motor neuron that synapses on the
intrafusal fiber. The alpha motor neuron synapses on the extrafusal
fibers. One alpha motor neuron can stimulate numerous fibers. This
is called the motor unit. The neural link between the alpha motor
neuron and the muscle fiber is called the neuromuscular junction.
Dorsal horn for
sensory input
Ventral horn for motor
output
• The ratio between the alpha motor neuron
and the number of muscles fibers it
innervates is associated with the degree of
dexterity needed in the movement
high ratio (1:150) = contraction of large muscles
low ratio (1: 10) = contraction of small muscles needed
for fine movements
Motor Homunculus is related to the number of
alpha motor neurons needed to innervate
muscles of various regions of our body.
3. Comparing the Anatomy of the CNS with the
Anatomy of the Neuromuscular Junction
Motor Unit
•
•
•
•
•
•
•
•
Alpha Motor Neuron
=
Muscle Fiber
=
Endplate
=
=
NT is Acetylcholine
Nicotinic Receptors
=
Calcium enters
=
Endplate Potential (EPP) =
Muscle Contraction or =
Muscle Action Potential &
movement
CNS Synapse
Presynaptic Neuron
Postsynaptic Neuron
Dendrite
Many different NTs
Many different receptors
Sodium enters
EPSP
Action Potential & release of
NT
How is limb position maintained?
• Involuntary movement (i.e. posture):
continual contraction and relaxation of the
muscles in our feet and calves.
• Voluntary movement:
Stretch of the intrafusal fiber causes contraction of
the extrafusal fiber via alpha motor neuron.
Keeping the movement at this position requires a
direct signal from the brain.
Remember: muscles work in pairs; so if
one contracts, the other relaxes
This is referred to as reciprocal innervation.
What if both muscles contracted at the
same time?
The neural mechanism of reciprocal
innervation is a bit tricky…
4. Alpha Motor Neuron is the Final Common Path
for all movement. Movement can be generated
from:
- sensory signals in the muscle spindle like the
stretch reflex
- sensory signals from skin as in the pain
withdrawal response
- involuntary signals from the brainstem for
posture, keeping us upright without conscious
attention
- signals from the brain for voluntary movement
But, regardless of where the signal
originates, all movement is the
result of activity in the alpha motor
neuron – making this the Final
Common Path
What would happen if the alpha motor
neuron stopped working?
Excitation-Contraction Coupling
Muscle contraction
•Alpha motor neurons release Ach
•ACh produces large EPSP in muscle fibers (via
nicotinic Ach receptors
•EPSP evokes action potential
•Action potential (excitation) triggers Ca2+
release, leads to fiber contraction
•Relaxation, Ca2+ levels lowered by organelle
reuptake
Excitation-Contraction Coupling
Excitation-Contraction Coupling
Voluntary Movement: Instructions
from Cerebral Cortex
• Dorsolateral Prefrontal Cortex: directs movement of
our limbs (as in reaching) and movements of our
fingers.
• Actual signal for movement must go through premotor cortex, then motor cortex.
• From motor cortex, signal travels down spinal cord
eventually reaching the alpha motor neuron.
• BUT, the instructions for this movement ultimately
comes from our Parietal lobe, which receives sensory
input.
Another view of the
cerebrospinal track
Of course, this is really too simple…
• Other brain areas involved in movement:
1. ventromedial frontal cortex – involved in
body control, posture and whole body
movements
2. Cerebellum
3. Basal Ganglia
4. Brainstem
• In the end, all movement funnels through
the alpha motor neuron (final common
path)
PREMOTOR AND
SUPPLEMENTARY
MOTOR CORTEX
PRIMARY
MOTOR
CORTEX
VENTRAL
ANTERIOR
NUCLEUS OF
THALAMUS
SUBTHALAMIC
NUCLEUS
GLOBUS
PALLIDUS
STRIATUM
SUBSTANTIA NIGRA
BRAINSTEM
DIRECT ACTIVATION
PATHWAYS
INDIRECT
ACTIVATION
PATHWAYS
SPINAL CORD
KEY:
DOPAMINE
GABA
FINAL COMMON PATHWAY
GLUTAMATE
SCHEMA OF DIRECT AND INDIRECT ACTIVATION
The Corticothalamic Loop
Thalamus
As the cortex determines that a voluntary movement is
needed, the basal ganglia become engaged in selecting
and presenting the motor cortex with the right motor
programs needed to perform the movement. The basal
ganglia integrates all the necessary data streams for the
various cortex areas, processes them, and the result is
served back to the frontal motor cortex as a buffet of
carefully chosen motor programs, ready to be performed
in a synchronized symphony of muscle contractions.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Globus
Pallidus
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
From Stimulus to Action
Thalamus
Globus
Here are the basal ganglia nuclei laid out for clarity. Let’s suppose Pallidus
that the body is idle, so that no voluntary movement occurs. Now
assume a ball has been spotted, and the motivation to grab the ball
is born within the motivation areas of the cortex. The motor has
currently no idea of how to actually get the ball, and cannot
execute any movement yet because the motor thalamus, that acts
as a motion “gatekeeper,” is inhibited. Without this inhibition, wild
and random movement would occur. So, before a motion is
started, the thalamus is prohibited to allow any movements
because one of the efferent parts of the basal ganglia, the internal
segment of the globous pallidus, is inhibiting it.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
A Decision is Born
Thalamus
The cortical and subcortical motivation and association
cortices decide that a certain action is to be taken, e.g. to get
the ball, but cannot execute the “reach” and “grasp” motor
programs on its own. Of course, there are different reaching
and grasping programs for different types of objects at
different positions, and the programs need not only be
chosen and started—they must also be halted at the right
time. Thus, the motor cortex needs to have the correct motor
programs chosen and unlocked by the basal ganglia.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Globus
Pallidus
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
The Duality of the Striatum
Thalamus
Globus
The striatum consists mainly of medium spiny neurons, that are Pallidus
usually silent because they require strong input signals to fire an
action potential. The inputs are not only from the cortex, but also
from the dopaminergic neurons of the substantia nigra pars
compacta. The striatum’s dopamine receptors are both of
excitatory D1 and inhibitory D2 types, which selects for the
balance between the motion starting the direct and indirect
pathways. Keep in mind that the caudate and putamen are parts of
the striatum, and that both are reached by inhibitory and excitatory
nigral neurons. But for now, let’s just focus on the motion starting
the direct pathway.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
The Direct Pathway
Thalamus
When the striatum receives the input from the cortex,
together with dopamine from the 5Nc to the D1 receptors.
Its GABAergic neurons will inhibit the GPi. The GPi has in
itself a tonically inhibitory effect on the motor thalamus,
and this is the “gate” for preventing unwanted movements.
With the GPi inbihited, the motor thalamus is now
disinhibited, and it can now present the frontal motor cortex
with the appropriate motor programs for the desired
movement, their temporal sequence and the strength of the
muscle contactions.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Globus
Pallidus
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
The Indirect Pathway
Thalamus
Let’s say the brain changes its mind about the ball, and
decides it’s best not to grasp it afterall. But the movement to
reach out and grasp the ball has already begun. This is
where the indirect pathway kicks in. It serves as a way to
nullify the disinhibitory actions of the direct pathway. In
short, it acts as a brake, restoring the inhibition of the motor
thalamus. The key structure in accomplishing this brake, is
the subthalamic nucleus.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Globus
Pallidus
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
Inhibiting the Inhibitor
Thalamus
The subthalamic nucleus (STN) is normally under tonic
inhibition of the external segment of the globus pallidus
(GPe).
Globus
Pallidus
When this inhibition is lifted by the striatum, the STN,
excited the inhibitory GPi, which means that the GPi will
“brake” the motor thalamus to its original state.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
A Black Brake
Thalamus
The STN also excites the substantia nigra pars reticulata,
causing it to also inhibit the motor thalamus.
Globus
Pallidus
This way, by influencing both the substantia nigra the GPi,
the STN performs as an effective 2-way brake that stops the
thalamus from permissing the cortex to execute motor
programs.
VA/VL complex
of Thalamus
Motor Cortex
Spinal Cord
Basal Ganglia
Internal
Globus Pallidus
Subthalamic
Nucleus
External
Globus Pallidus
Striatum
Substancia nigra
pars reticularis
pars compacta
Motivation and
association
cortices
Cerebellum Functions
• Maintenance of Equilibrium
- balance, posture, eye movement
• Coordination of half-automatic movement of
walking and posture maintenance
- posture, gait
• Adjustment of Muscle Tone
• Motor Leaning – Motor Skills
• Cognitive Function
Interaction
of CNS
areas
involved in
movement
Output via alpha
motor neuron for
movement
Disorders of the Motor System
• Amyotrophic lateral sclerosis – motor neurons of the
brainstem & spinal cord are destroyed.
• Huntington’s Disease – progressive destruction of the basal
ganglia (GABA).
• Muscular Dystrophy – biochemical abnormality affecting the
utilization of Ca++ causing wasting away of muscles.
• Myasthenia gravis – autoimmune disorder that destroys Ach
receptors (starts with head as in drooping eyelids then
progresses to swallowing & respiration).
• Parkinson’s disease – degeneration of neurons in the striatum
due to loss of cells in the substantia nigra that
synthesis/release dopamine.