Somatic Nervous System: Motor Functions

Motor Neurons and the Somatic Nervous System

• The somatic nervous system controls motor functions through motor neurons.

Motor Functions of the Nervous System

• Motor functions involve the organism responding to actual situations and controlling effector activity.
• Effectors are innervated by the efferent portion of the nervous system.
• Skeletal muscles are effectors related to the somatic nervous system.
• Inner organs, glands, and vessels are effectors of the autonomic nervous system.

Motor Skills

• Motor skills are basic functions, involving maintaining position and movement in space to get food/work.
• Skeletal muscles are essential in motor skills
• The somatic nervous system controls motor skills.
• Coordination of different muscles results in reflex responses, posture/locomotion, and voluntary movement.

Motor Activity Types

• Motor activity is categorized by will, being either voluntary or involuntary (posture-regulating).
• Motor activity, categorized by aim, is either postural or intentional.
• Motor activity, categorized by origin, is reflexive or due to a central motoric program.

Motor System Output

• The motor system has three classes of movement: reflex responses, rhythmic/repetitive motions, and voluntary movement.
• Reflex responses are stereotyped, involuntary movements initiated by a stimulus.
• Rhythmic, repetitive motions are sequences of relatively stereotyped and almost automatic voluntary movements like walking.
• Voluntary movement is purposeful, goal-directed, complex, and learned.

Neural Systems Controlling Body Movement

• The spinal level involves local movement patterns like programmed withdrawal reflexes and rhythmical motions.
• The midbrain level maintains axial tone for standing and continuously modifies tone in different muscles using information from the vestibular apparatus, thus maintaining equilibrium.
• The motor cortex sends activating motor signals to the spinal cord and is involved in selecting movement goals and strategies.

Control of Voluntary Movement

• Central motor neurons are localized in motor brain centers, cross the midline, and terminate on MNs in the brainstem and spinal cord.
• Peripheral motor neurons found in motor centers in the brainstem and spinal cord, exit the CNS via cranial and spinal nerves.
• Peripheral motor neurons initiate movement by commanding skeletal muscles.

Peripheral Motor Neurons (MNs)

• Peripheral MNs transmit CNS activation to skeletal muscle, but can only be contractile.
• Relaxation of muscles is caused by the inhibition of MNs.
• Peripheral MNs control of skeletal muscle contraction, as the final common pathway, and produce trophic effects.
• They originate in the ventral horn of the spinal cord and brain stem.
• A motor neuron pool is a collection of MNs that innervate a single muscle, like the triceps.
• Lesions lead to the inability of the muscle to contract, unlike with smooth and cardiac muscle.

Types and Functions of Motor Neurons

• Alpha-MNs innervate extrafusal muscle fibers and are the most numerous.
• Alpha-MNs are arranged in well-defined columns.
• Alpha-MNs facilitate muscle contraction.
• Gamma-MNs innervate intrafusal muscle fibers, which are part of muscle spindles.
• Gamma-MNs keep muscle spindles taut.

Ventromedial and Dorsolateral Motor Neuron Pools

• The dorsolateral MN pool controls voluntary movement of the extremities, especially the hands (manipulation).
• The central input forms the dorsolateral system.
• The ventromedial MN pool controls posture and balance of the trunk.
• The central input forms the ventromedial system.

Motor Neuron Arrangement in the Spinal Cord

• Proximal-distal rule: Motoneurons for proximal muscles are medial, while those for distal muscles are lateral.
• Extensor-flexor rule: Motoneurons for flexor muscles are dorsal, while those for extensor muscles are ventral.

Propriospinal Neurons and Spinal Interneurons

• Neurons intrinsic to the spinal cord make connections with MNs, afferent neurons and other neuronal groups in the spinal cord.
• Spinal interneurons are localized in the spinal intermediate zone (laminae V-VIII) and are used with different names.
• Spinal interneurons regulate voluntary and reflex motor acts.
• Spinal interneurons transmit information from the periphery and higher centers to MNs.
• Spinal interneurons modulate afferent impulses, allowing or preventing transmission from the periphery and local spinal circuits.
• Spinal interneurons are vital to sensorimotor integration and motor coordination.

Propriospinal Neurons

• Axons traverse several spinal cord segments.
• Propriospinal neurons are subdivided into long (posture), intermediate, and short types.
• They are localized in cervical and lumbar regions.
• They integrate descending motor commands and afferent feedback from the periphery.
• They deliver the results to the peripheral MNs.

Types of Spinal Interneurons

• Types of spinal interneurons include Ia inhibitory interneurons, Ib inhibitory interneurons, and Renshaw cells.

Ia Inhibitory Interneurons Inputs

• They get input from Ia afferents of the muscle spindle.
• They receive collateral fibers of central MN axons, which make direct excitatory connections to peripheral MNs.

Ia Inhibitory Interneurons Involvement

• They are involved in the stretch reflex(reciprocal inhibition) and coordination of muscle contraction during voluntary movements.
• Ia Inhibitory Interneurons organization simplifies voluntary movements, as higher centers do not have to send separate commands to opposing muscles.

Characteristics of Ib Inhibitory Interneurons

• They have input from Golgi tendon organs, joint and cutaneous receptors, and descending pathways.
• They are involved in the autogenic inhibition reflex, which coordinates extensor and flexor muscles during a reflex response.

Renshaw Cells

• Renshaw cells are inhibitory interneurons.
• They get excitatory collateral input from α-MNs ("informed" of how vigorously that neuron is firing).
• They output to Ia inhibitory interneurons, other Renshaw cells, and the cell body of the initial a-MN and/or an α-ΜΝ of the same MN pool.
• They control the level of excitation of MNs.
• They modulate recurrent inhibition produced by other Renshaw cells.
• They modulate Ia reciprocal inhibition between antagonist motor pools.
• They mediate recurrent inhibition of spinal α-MNs.

Peripheral Motor Neurons of the Brainstem

• Peripheral MNs control the muscles of the head and face, organized in two lines.
• Medial: close to the midline, motor efferent nuclei innervating oculomotor muscles and muscles of tongue.
• Lateral: special visceral motor column innervating mimic, chewing, pharyngeal and laryngeal muscles.

Features of Motor Pathways

• Motor pathways are characterized by the location of cells of origin, trajectory of axons, terminal distribution fields in the spinal gray matter, and muscle groups toward which their main action is directed.

Location of Cells of Origin in Motor Pathways

• The origin of the anterior and lateral corticospinal tracts and corticobulbar tract is the cortex.
• The origin of the rubrospinal tract is the midbrain.
• The origin of the vestibulospinal tract is the pons.
• The origin of the reticulospinal tract is the rostral medulla.

Trajectories of Axons: Two Main Motor Systems

• The ventromedial motor system controls posture and balance.
• Ventromedial tracts include the anterior corticospinal, vestibulospinal, reticulospinal, and tectospinal tracts.
• The dorsolateral motor system controls skilled (fractionated) movement of contralateral limbs.
• Dorsolateral tracts include the corticobulbar, lateral corticospinal, and rubrospinal tracts.

Medial Brain Stem Pathways

• The origin is brainstem nuclei.
• The course is reticulospinal, tectospinal and vestibulospinal tracts.
• The terminations: interneurons and long propriospinal neurons project to proximal and axial MNs.
• The function: basic postural control system and affect axial muscles in ventromedial portion of spinal cord.

Anterior Corticospinal Tract

• Origin: pyramidal cells of the primary motor cortex.
• Course: only in the upper part of the spinal cord, gradually diminishing in size.
• Terminations: interneurons and long propriospinal neurons; and project to proximal and axial MNs.
• Function: voluntary motor impulses to the axial muscles.

Lateral Corticospinal Tract

• The origin is pyramidal cells of the primary motor cortex, fibers decussate in the medulla.
• The terminations are MNs in the spinal cord.
• The function controls MNs and interneurons in the spinal cord.
• It is the only descending pathway that has direct innervation on alpha MNs allowing the cortex to control the fine movements of fingers and hands.

Corticobulbar Tract

• Origin: pyramidal cells of the primary motor cortex.
• Terminations: brainstem motor nuclei of cranial nerves on the opposite side of the brain.
• Function: control of voluntary movement of the head's and neck's muscles.

Rubrospinal Tract

• The origin is the red nucleus with brainstem nuclei.
• The course: is the rubrospinal tract.
• The terminations are interneurons in the dorsolateral part of the spinal cord.
• It influences distal muscles and is more concerned with goal-directed limbic movements (reaching and manipulating/grasping).

Dorsolateral vs. Ventromedial Motor Systems

• The dorsolateral system functions to create skilled (fractionated) movements of contralateral limbs.
• The ventromedial system is responsible for posture and balance.
• The dorsolateral system tracts cross the midline, while the ventromedial system tracts can be both crossed and uncrossed.
• the corticospinal, rubrospinal and corticobulbar are in the dorsolateral system
• the restibulospinal , reticulospinal , tectospinal and ventral corticospinal tracts are in the ventromedial system.
• The dorsolateral system projects in the lateral column and dorsolateral MN pool.
• The ventromedial system projects in the ventral column and ventromedial MN pool.
• The dorsolateral system effects the distal limb segments, while the ventromedial system affects the trunk and proximal limb segments.

Central Motor Neuron Termination

• Majority: 90% terminate on interneurons (polysynaptic connection).
• Minority: 10% make monosynaptic connection.

Influence of Higher Centers on Spinal Reflexes

• Direct input to α-MNs.
• Excitation of segmental inhibitory interneurons.
• Actions on propriospinal neurons that travel to other segmental levels.
• Input to γ-MNs.
• Synapses on afferent terminals.

Final Common Pathway

• Alpha-MNs receive and integrate inputs from various parts of the brain as well as from sensory receptors.
• Serve as the final common pathway to the muscle.

Peripheral Motor Neuron Syndrome

• Occurs with damage from cutting your finger, as the body adapts to repair damage outside the CNS.
• Consequences: paralysis (loss of movement) or paresis (weakness), areflexia, hypotonia, and atrophy.

Injury of Peripheral Motor Neuron Axons

• The axon degenerates downstream from the site of injury.
• Injury leads to an immune response to quickly clean up damaged tissue.
• The injured neurons produce growth-promoting genes.
• The axon starts to grow again.
• Schwann cells promote axon growth along original path to target cells: by secreting neurotrophins and by providing a surface for them to grow along.

Regeneration of Axons: PNS vs CNS

• PNS axon regeneration grows slowly at a rate of 3-4 mm per day.
• The new axon may reestablish the former connection, but full function is not guaranteed.
• CNS axon regeneration, distal portion of the axon degenerates, the process of degeneration takes longer, and myelin-producing oligodendrocytes fail to proliferate following an injury.
• Regeneration is unlikely in the CNS.

Neural Stem Cells

• Neural stem cell offer a promising solution for regeneration.
• The CNS do contain stem cells.
• Neural Stem cells may eliminate the need for neural tissue.
• Ethical concerns of stem cell research and the source must be addressed.

Motor Function of the Spinal Cord

• Discussed through simple reflex arc circuits and more complex influences of higher regulatory centers.

Basic Reflex Circuitry

• Reflex arcs involve a receptor, afferent neuron, center, efferent neuron, and effector.
• The functional unit of the nervous system involves a response of an organism to a receptor stimulus mediated by a reflex arc.

Proprioceptive Reflexes

• A proprioceptive reflex arc originates and ends in the same muscle.
• It is a mono- or bi- synaptic reflex.
• Receptors: muscle spindle, Golgi tendon organ.
• Includes: myotatic reflexes, inverse myotatic reflexes.

The Myotatic (Stretch) Reflex

• Muscle contraction happens in response to passive stretching.
• The reflex is monosynaptic and involves a 2-neuron loop.
• Stretching the muscle causes the muscle spindle to stretch