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 Inner fiber = intrafusal fiber Wrapped around the intrafusal fiber is a sensory nerve that picks up the sensation of stretch. This represents only one muscle fiber - a muscle has many fibers

Outer fiber = extrafusal fiber Inner fiber = intrafusal fiber 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. Alpha motor neuron Gamma Motor Neuron

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 Ca 2+ release, leads to fiber contraction Relaxation, Ca 2+ levels lowered by organelle reuptake

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 pre- motor 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 STRIATUMVENTRAL ANTERIOR NUCLEUS OF THALAMUS SUBTHALAMIC NUCLEUS GLOBUS PALLIDUS SUBSTANTIA NIGRA BRAINSTEM SPINAL CORD DIRECT ACTIVATION PATHWAYS FINAL COMMON PATHWAY INDIRECT ACTIVATION PATHWAYS KEY: DOPAMINE GABA GLUTAMATE SCHEMA OF DIRECT AND INDIRECT ACTIVATION PATHWAYS

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices The Corticothalamic Loop 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. Thalamus Basal Ganglia Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices From Stimulus to Action Here are the basal ganglia nuclei laid out for clarity. Let’s suppose 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. Basal Ganglia Thalamus Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices A Decision is Born 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. Basal Ganglia Thalamus Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices The Duality of the Striatum The striatum consists mainly of medium spiny neurons, that are 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. Basal Ganglia Thalamus Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices The Direct Pathway 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. Basal Ganglia Thalamus Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices The Indirect Pathway 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. Basal Ganglia Thalamus Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices Inhibiting the Inhibitor The subthalamic nucleus (STN) is normally under tonic inhibition of the external segment of the globus pallidus (GPe). 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. Basal Ganglia Thalamus Globus Pallidus

VA/VL complex of Thalamus Motor Cortex Spinal Cord Internal Globus Pallidus External Globus Pallidus Striatum Subthalamic Nucleus pars reticularispars compacta Substancia nigra Motivation and association cortices A Black Brake The STN also excites the substantia nigra pars reticulata, causing it to also inhibit the motor thalamus. 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. Basal Ganglia Thalamus Globus Pallidus

Cerebellum Functions Maintenance of EquilibriumMaintenance of Equilibrium - balance, posture, eye movement - balance, posture, eye movement Coordination of half-automatic movement of walking and posture maintenanceCoordination of half-automatic movement of walking and posture maintenance - posture, gait - posture, gait Adjustment of Muscle ToneAdjustment of Muscle Tone Motor Leaning – Motor SkillsMotor Leaning – Motor Skills Cognitive FunctionCognitive 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.