The Motor System

How do we move

Movement is orchestrated by the coordinated action of the peripheral, spinal brain stem, cerebellar, and cerebral regions, shaped by a specific context, and directed by the intentions of the performer

Why do we move

Accomplish a task or achieve a goal
Locomotion is one of the most important attributes of living things because it liberates an organism from some of the constraints imposed on it by the environment
More than half of the nervous system is directly involved in motor performance
Disruption of motor control mechanisms is the most common manifestation of disease in the nervous system

What are the two types of lower motor neurons

Alpha motor neuron
Gamma motor neuron

Alpha motor neuron: cell body

Ventral horn of spinal cord

Alpha motor neuron: exits

Spinal cord via ventral root

Alpha motor neuron: innervates

(Extrafusal) skeletal muscle

Alpha motor neuron: neurotransmitter

Acetycholine

Alpha motor neuron: axons

Large myelinated Aα

Gamma motor neurons: cell bodies

Ventral horn

Gamma motor neurons: exits

Ventral root

Gamma motor neurons: innervates

Intrafusal fibers of muscle spindle

Gamma motor neurons: axons

Medium myelinated Ay

What is a motor neuron

A single Alpha motor neuron and the muscle fibers it innervates

What type of motor unit are slow twitch fibers

Type I

Slow twitch

Postural and slowly contracting muscle

What type of motor unit are fast twitch fibers

Type IIA or IIB

Fast twitch

Phasic, power producing muscles

Fast and slow twitch motor units

Dependent on its nerve innervation
Slow twitch muscle fibers actually fire first during most movements because their smaller cell bodies in anterior horn depolarize faster

Peripheral sensory input to motor neurons

Sensory information from GTO’s and muscle spindles give information regarding tension, muscle length, and rate of change in length
Information can be processed at multiple levels to adjust activity of motor output

Alpha-Gamma Coactivation

During most movements, the alpha and gamma motorneuron systems function simultaneously

What does coactivation maintains

The stretch on the central region of the muscle spindle

Myotomes

Groups of muscle innervated by a single spinal nerve

Motor neuron pools

Axons from a motor pool project to one muscle

Types of motor neuron pools

Medial pool
Lateral pool
Anterior pool
Posterior pool

Medial pool

Axial and proximal muscle

Lateral pool

Distal musculature

Anterior pool

Extensors

Posterior pool

Flexors

Spinal region coordination

Muscle synergies
Stepping Pattern Generators (CPG’s)
Reflexes
Reciprocal inhibition

Reflexes

Thought to be the sole basis of movement by early neuroscientists
Most movement is voluntary with some contributions of reflexive movement

Reflex examination

Can give some idea of the integrity of the motor system at the peripheral and spinal cord level

Stretch reflexes

Stretch reflex, Myotactic reflex, Deep tendon reflex (DTR)
Monosynaptic
Autogenic facilitation
Phasic vs. Tonic

Role of GTO

Information regarding tendon tension
Helps to modulate muscle contraction/tension of tendon
Works with other proprioceptors
May facilitate or inhibit based on task

Cutaneous reflexes

Flexor withdrawal reflex: multisynaptic
Crossed extension
Abdominal reflex
Plantar reflex (Babinski)

H-Reflex

Can be used to quantify excitability (or lack of) of alpha motor neurons
Electrical equivalent of stretch reflex

Posture and gross movement

Tend to synapse on motor neurons located medially in the ventral horn which supply proximal postural muscles
Tend to be extensor biased
4 tracts brain stem, one cortex

What are the major pathways of the medial system

Medial corticospinal tract
Lateral vestibulospinal tract
Medial vestibulospinal tract
Tectospinal tract
Pontine (medial) reticulospinal tract

Medial (anterior/ventral) corticospinal tract: function

Axial and proximal joint control, neck, shoulder, and trunk

Medial (anterior/ventral) corticospinal tract: begins

Areas 4 and 6 of cortex

Medial (anterior/ventral) corticospinal tract: descends

Corona radiata, posterior limb internal capsule, crus cerbri, ventral pons, pyramids, medial/anterior corticospinal tract

Medial (anterior/ventral) corticospinal tract: terminates

Contralateral motor neurons via anterior white commissure, also has some ipsilateral/bilateral connections
Ends mid-thoracic

Medial vestibulospinal tract: function

Regulates activity of upper back and neck muscles in response to vestibular input

Medial vestibulospinal tract: begins

Medial vestibular nuclei

Medial vestibulospinal tract: descends

In medial longitudinal fasiculus (MLF)/medial vestibulospinal tract into upper thoracic cord

Medial vestibulospinal tract: terminates

Ipsilateral/bilateral motor neurons to cervical and upper back muscles

Lateral vestibulospinal tract: function

Facilitates antigravity muscles of ipsilateral extremities and inhibits non-antigravity muscles

Lateral vestibulospinal tract: begins

Lateral vestibular nuclei

Lateral vestibulospinal tract: descends

In lateral vestibulospinal tract

Lateral vestibulospinal tract: terminates

Ipsilateral upper and lower extremity anti-gravity muscles

Tectospinal tract: function

Mediates reflexive movement of head and neck to visual stimuli

Tectospinal tract: begins

Superior colliculus

Tectospinal tract: crosses

Dorsal tegmental decussation in mid-brain

Tectospinal tract: descends

Medial longitudinal fasiculus (MLF) then tectospinal tract

Tectospinal tract: terminates

Contralateral motor neurons of cervical muscles

Pontine (medial) reticulospinal tract: function

Activates ipsilateral lower motor neurons of postural muscles and limb extensors
Functions normally with minimal input from cortex

Pontine (medial) reticulospinal tract: begins

Pontine reticular formation

Pontine (medial) reticulospinal tract: descends

In pontine (medial) reticulospinal tract

Pontine (medial) reticulospinal tract: terminates

Ipsilateral motor neurons of postural muscles and limb extensors

Lateral activation pathways

Descend in lateral spinal cord and synapse with motor neurons in the lateral portion of the ventral horn

Fine movement, fractionation of movement

Lateral corticospinal
Medullary (lateral) reticulospinal tract
Rubrospinal

Lateral corticospinal tract: function

Fractionation of movement
The ability to activate individual muscles independently of other muscles

Lateral corticospinal tract: begins

Areas 4 and 6 of cortex

Lateral corticospinal tract: descends

Corona radiata, posterior limb internal capsule, crus cerbri, ventral pons, pyramids, lateral corticospinal tract

Lateral corticospinal tract: crosses

85% at pyramidal decussation in low medulla

Lateral corticospinal tract: terminates

On motor neurons or interneurons in ventral horn to supply distal muscles
55% UE’s, 20% trunk, 25% LE’s

Medullary (lateral) reticulospinal tract: function

Regulation of tone (more inhibitory than faciliatory) requires input from cortex to function appropriately
Facilitates flexors and inhibits extensors
But during walking this effect is reversed

Medullary (lateral) reticulospinal tract: begins

Medually reticular formation (nucleus gigantocellularis)

Medullary (lateral) reticulospinal tract: descends

Ipsilaterally/bilaterally in lateral reticulospinal tract

Medullary (lateral) reticulospinal tract: terminates

Motor neurons of postural muscles and limb extensors

Rubrospinal tract: function

Closely associated with lateral corticospinal tract, flexors of UE
More developed in monkeys

Rubrospinal tract: begins

Red nucleus in midbrain

Rubrospinal tract: crosses

Immediately in ventral tegmental decussation of midbrain

Rubrospinal tract: descends

Tegmentum pons and medulla
Joins lateral corticospinal tract in spinal cord

Rubrospinal tract: terminates

On motor neurons or interneurons in ventral horn to supply various muscle groups with emphasis on UE flexors
Needs cortical input to function properly

Corticobulbar tracts

Motor supply from cortex to cranial nerves innervating musculature of head and neck
All of these pathways are crossed
Muscles of upper face receive bilateral innervation

Nonspecific activating pathways

Ceruleospinal and Raphespinal tracts
General effects on motor performance and motivation

Primary motor area (4)

Movement execution and fractionation
More than 50% hand and speech

Supplementary motor area (6)

Active prior to bimanual tasks and sequential movements

Premotor area (6)

Generates patterns of movement involving multiple muscle groups and joints