20. Somatic Motor System

Precommand Level

• Cerebellum
• Basal nuclei

Projection Level

• Primary motor cortex
• Brain stem nuclei

Segmental Level

• Spinal cord

Objectives

• Describe the somatic motor system.
• Describe how the somatic motor system is controlled.

Functions of Somatic Motor Systems

• Movement: Change body position to perform desired act.
• Posture: Maintain body position.
• Coordination: Control the pattern and sequence of muscle contraction in complex movements.

Important Structures of the SMS

• Skeletal muscle fibers (effectors): Innervated by alpha motor neurons in the spinal cord and cranial motor neurons.
• Alpha motor neurons (lower motor neurons): Deliver impulse in PNS.
  • Located in spinal nerves emerging from the spinal cord, innervate muscles of movement and posture.
  • Located in cranial nerves emerging from the brain stem, controlling eye movement, chewing, facial expression, larynx, soft palate, muscles supporting the head, and tongue.

Structures That Influence Motor Neurons

• Reflexes: Simple, involuntary movements.
• Brain stem nuclei: Muscle tone.
• Brain stem: Control Central Pattern Generators (CPGs) - complex movements involving many muscles (e.g., chewing, swallowing, walking).
• Primary Motor Cortex: Voluntary movement.
• Premotor Cortex: Planning and coordinating voluntary movement.
• Cerebellum: Feedback to motor cortex; coordination, balance, posture/tone.
• Basal Nuclei: Feedback to motor cortex; starting and stopping movement, suppression of unwanted movements, preventing overshoot, arm swinging when walking.

Organization of the SMS

• Skeletal muscle (effector) receives information from pyramidal (direct, corticospinal) and extrapyramidal (indirect) tracts.
• Motor neurons innervating peripheral tissue have cell bodies in the ventral horn of the spinal cord grey matter.
• Motor neurons exit the spinal cord via the ventral root.
• The spinal cord is organised to allow reflex arcs.

Spinal Cord Motor Neurons

• The motor neurons in the spinal cord are called the LOWER MOTOR NEURONS
• Each muscle is served by at least one motor nerve.
• Each motor nerve contains axons of up to hundreds of motor neurons.
• Each axon branches into a number of endings, each forming a neuromuscular junction with a muscle fibre.

The Motor Unit

• One motor neuron may innervate a few or several hundred muscle fibres.
• The motor neuron + the muscle fibres it innervates = motor unit.
• Recruitment of more motor units increases contractile force.
• Muscles that have fine muscle control have small motor units (eye muscles), muscles that are less precise (hip) have large motor units.

Innervation of Skeletal Muscle

• Skeletal muscles are stimulated by somatic (lower) motor neurons.
• Axons travel from the central nervous system to skeletal muscle.
• Each axon divides into many branches as it enters muscle.
• Axon branches end on the muscle fibre, forming a neuromuscular junction or motor end plate.

Events at the Neuromuscular Junction (NMJ)

• Axon terminal (end of axon) and muscle fibre are separated by the synaptic cleft.
• Stored within axon terminals are membrane-bound synaptic vesicles.
• Synaptic vesicles contain the neurotransmitter acetylcholine (ACh).
• Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors (nicotinic).
• NMJ consists of axon terminals, synaptic cleft, and junctional folds.

Events at the Neuromuscular Junction (2)

• Nerve impulse arrives at axon terminal, causing ACh to be released into synaptic cleft.
• ACh diffuses across cleft and binds with receptors on sarcolemma.
• ACh binding leads to electrical events that ultimately generate an action potential through muscle fibre.
• ACh is quickly broken down by the enzyme acetylcholinesterase, which stops contractions.

Generation of an Action Potential Across the Sarcolemma

• Resting sarcolemma is polarised, meaning a voltage exists across the membrane.
• Inside of the cell is negative compared to outside.
• Action potential is caused by changes in electrical charges.
• Occurs in three steps:
  1. End plate potential
  2. Depolarisation
  3. Repolarisation

1. End Plate Potential

1. ACh released from motor neuron binds to ACh receptors (nicotinic) on sarcolemma
2. Causes chemically gated ion channels (ligands) on sarcolemma to open
3. $Na+$ diffuses into muscle fibre
4. Because $Na+$ diffuses in, the interior of sarcolemma becomes less negative (more positive)
5. Results in a local depolarisation called the end plate potential

2. Depolarisation: Generation and Propagation of an Action Potential (AP)

1. If the end plate potential causes enough change in membrane voltage to reach a critical level called the threshold, voltage-gated $Na+$ channels in the membrane will open.
2. A large influx of $Na+$ through channels into the cell triggers AP that is unstoppable and will lead to muscle fibre contraction.
3. AP spreads across the sarcolemma from one voltage-gated $Na+$ channel to the next one in adjacent areas, causing that area to depolarise.

3. Repolarisation: Restoration of Resting Conditions

• $Na+$ voltage-gated channels close, and voltage-gated $K+$ channels open.
• $K+$ efflux out of the cell rapidly brings the cell back to its initial resting membrane voltage.
• Refractory period: muscle fibre cannot be stimulated for a specific amount of time until repolarisation is complete.
• Ionic conditions of resting state are restored by the $Na+ - K+$ pump where $Na+$ that came into the cell is pumped back out, and $K+$ that flowed outside the cell is pumped back into the cell.

Hierarchy of Motor Control

• Cerebellum and basal nuclei are the ultimate planners and coordinators of complex motor activities, not the cerebral cortex.
• Complex motor behaviour depends on complex patterns of control at three levels:
  1. Segmental level
  2. Projection level
  3. Precommand level

1. Segmental Level

• The lowest level of the motor hierarchy, consists of reflexes and spinal cord segmental circuits that control automatic movements.
• Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles.
• For repeated motor activities.
• Network of oscillating inhibitory & excitatory neurons in & around ventral horn, which set crude rhythms & alternating patterns of movement.
• Controls locomotion and specific, repeated motor activity.

2. Projection Level

• The spinal cord is under direct control of the projection level of motor control.
• Consists of:
  • Upper motor neurons from the motor cortex to spinal cord interneurons (the direct (pyramidal neurons) system) to produce voluntary skeletal muscle movements, travels down the corticospinal tract.
  • Brain stem motor areas that oversee the indirect (extrapyramidal) system (reticular, vestibular, red nuclei etc.) - to modify and control the activity at the segmental level, posture, balance, skilled movements, control basic rhythm of CPGs.
• Sends a copy of information it sent to lower levels, to the higher command levels.

3. Precommand Level

• Highest level, neurons in the cerebellum and basal nuclei are involved in the Planning & Coordination of Complex Movements, they control the outputs from the motor cortex
• Cerebellum
  • Receives ascending input from receptors (proprioceptors, equilibrium, vision)
  • Regulates motor activity, acts on motor pathways on the motor cortex via the Thalamus (Lacks direct connections with the spinal cord)
  • Coordinates movements with posture, involved in complex movements
  • Monitor and maintain muscle tone
  • Perform unconscious planning and discharge in advance of intentional willed movement
• Basal nuclei
  • Receives input from all cortical areas and outputs to premotor and prefrontal cortex via the thalamus
  • Precisely start or stop movements
  • Blocks unwanted movements
  • Inhibit various motor centres under resting conditions

Levels of Motor Control and Their Interactions

• Precommand Level (highest)
  • Cerebellum and basal nuclei
  • Programs and instructions (modified by feedback)
• Projection Level (middle)
  • Motor cortex (pyramidal pathways) and brain stem nuclei (vestibular, red, reticular formation, etc.)
  • Conveys instructions to spinal cord motor neurons and sends a copy of that information to higher levels
• Segmental Level (lowest)
  • Spinal cord
  • Contains central pattern generators (CPGs)
  • Sensory input Motor output Reflex activity

Example of How the Motor Hierarchy Works

• The motor cortex wants to move the fingers in a sequenced pattern; it informs the precommand centres.
• Basal nuclei and cerebellum (precommand areas) unconsciously plan and discharge in advance of the willed movements (timing and patterns (coordination) of the movements while maintaining posture).
• Precommand centres control the motor cortex and provide its readiness to initiate the act.
• The motor cortex then chooses to act or not act, but the groundwork has already been laid.