Motor Systems

Motor System Hierarchy

Control Systems (Basal nuclei and cerebellum) -->initiator (cortex)--> Executers (UMNs)-->LMNs-->Muscle

LMN cell bodies are located

in the anterior horn of the spinal cord and certain cranial nerve nuclei

LMNs consists of 2 types of neurons

-alpha and gamma motor neurons

alpha motor neurons

innervate the extrafusal muscle fibers and generate muscle force (are able to cause the contraction to perform actual work)

gamma motor neurons

innervate muscle fibers (intrafusal fibers) within the muscle spindle (gamma motor neurons are responsible for the muscle tone)

to generate more force, one can

increase the firing rate of the motor unit or recruit more motor units to become activated

the primary determinant of muscle tone is the level of activity in

the stretch reflex arc

Stretch Reflex
-->type of reflex
-->consists of

a monosynaptic reflex
-consists of:
-->sensory apparatus (muscle spindle)
-->afferent neuron
-->efferent neuron
-->muscle

the muscle spindle

-a spindle/fusiform shaped organ deep inside the extrafusal muscle fibers

muscle spindle consists of

specialized muscle fibers that has both sensory and motor innervation

Each muscle spindle consists of

specialized muscle fibers that has both sensory and motor innervation

Each muscle spindle is attached at each end to

extrafusal muscle fibers

There are sensory (sensitive to stretch) in the ______ of each spindle and contractile fibers at _______ of the spindle

There are sensory receptors (sensitive to stretch) in the middle of each spindle and contractile fibers at each end of the spindle.

How the muscle spindle works in the stretch reflex

-a tendon is tapped-->extrafusal muscle fibers stretch-->stretches muscle spindle
-sensory fibers in spindle activated-->send APs via afferent axons into posterior aspect of the spinal cord to synapse on LMNs--both alpha motor neruons that will innervate extrafusal muscle fibers for contraction and gamma that will innervate intrafusal muscle fibers to maintain muscle tone

damage to muscle spindle will affect

muscle tone

damage to gamma motor neurons will affect

muscle tone

UMNs

project and synapse on LMNs and to interneurons in the spinal cord
-can influence muscle tone
-help maintain posture while sitting and moving

LMN signs

-Flaccid paralysis (paresis)
-hyporeflexia or areflexia
-decreased muscle tone
-atrophy
-fasciculations

The 5 groups of UMNs

-Reticular formation (brainstem)- Medial and Lateral Reticulospinal tracts
-Vestibular nuclei - Medial and Lateral Vestibulospinal tracts
-Superior colliculus - Tectospinal tracts
-Red nucleus - Rubrospinal tracts
-Motor cortex- corticospinal, corticobulbar tracts

pyramidal system vs extrapyramidal system

-The pyramidal system = corticospinal tracts
-The extrapyramidal system = all other pathways and basal nuclei and cerebellar paths

the lateral pathways will influence

distal limbs

the medial pathways will influence

axial muscles

lateral pathways

-descend and terminate laterally
-involved in movements of the DISTAL LIMBS
-includes the lateral cortciospinal and rubrospinal tracts

lateral pathways include what tracts

-lateral corticospinal
-rubrospinal

Lateral corticospinal tract

-UMN axons in the spinal cord
-lateral pathway

Rubrospinal tract

-from red nucleus (cell bodies) of midbrain and axons decussate and project to cervical levels of spinal cord only
-innervates the LMNs of the cervical spinal cord which activates the flexors of the upper extremity.

Rubrospinal tract: pathway--from and to where?

-from red nucleus (cell bodies) of midbrain--> axons decussate and project to cervical levels of spinal cord only

Rubrospinal tract innervates LMNs of ______spinal cord which activates what?

innervates LMNs of cervical spinal cord which activates FLEXORS of the UPPER extremity

Vestibulospinal tracts

-consists of medial and lateral vestibulospinal tracts (both medial pathways)
-excites the extensors antigravity musculature)

Vestibulospinal tracts excite

the extensors antigravity musculature)

Tectospinal tract
-pathway
-function

-cell bodies in superior colliculus--> axons project to upper spinal cord
-turns the head and the visual gaze to an area of interest

function and origin of tectospinal tract

-turns the head and the visual gaze to an area of interest
-origin-superior colliculus

Reticulospinal tracts

-consists of medial and lateral reticulospinal pathways
-these tracts enhance tone in the antigravity muscles (axial and proximal limb)
-Medial reticulospinal tracts have cell bodies in the pons
-Lateral reticulospinal tracts have cell bodies in the medulla

SMILe (superior-pons=medial; inferior-medulla=lateral)

Medial reticulospinal tracts have cell bodies

in the pons

Lateral reticulospinal tracts have cell bodies

in the medulla

function of the Reticulospinal tracts

enchances tone in the antigravity muscles (axial and proximal limb)

Cortices involved in motor activity include

-Primary motor cortex
-Lateral premotor area
-Supplementary motor cortex

function of motor cortex

Initiates, coordinates, and plans voluntary motor movements

Primary motor cortex function

involved in voluntary motor movement/ fine control of indvidual muscles

lesion of primary motor cortex

spastic paresis/paralysis (UMN signs)

Lateral premotor cortex
-function
-receives inputs from

-involved in sensorimotor intergration (ex. catchign a ball)
-receives inputs from the frontal eye fields

Supplementary motor cortex
-function

involved in initiating and coordinating internally generated movements ("I want to reach down and pick up my pen from the floor")

Lesion of the lateral premotor and supplementary motor cortices will result in

inability to initiate movement or perform a series of movements or make movements guided by sensory
inputs

Describe the corticobulbar pathway

-originate in the motor cortex (lateral area devoted to the face)--> project contralaterally to brainstem nuclei that contain LMNs that will innervate the face
-"bulb"= brainstem

lesion in either the Corticospinal or Corticobulbar tracts will lead to UMNs on what side

contralateral

UMN signs

-spastic paresis
-hypertonia
-hyperreflexia
-Positive Babinski's sign
-clonus

Spasticity

innvolves an increase in muscle tone and and increase in the stretch reflex, therefore the limbs become stiff and this can be seen when trying to passively move a patient's muscle

Rigidity is often used in place of the word spasticity

spastic paresis

when trying to passively flex a patient's limb that has spastic paresis, after a brief period of applied force, the muscle resistance suddenly collapses

This phenomenon is probably due to the strong drive of the Golgi tendon organs which fire more as the muscle is stretched and leads to inhibition of the motor neurons that innervate the muscle that is being stretched.

Decorticate rigidity

arms are flexed

Decorticate rigidity cause

Bilateral damage above the midbrain.

arms are flexed because the rubrospinal tracts are still functioning

Decerebrate rigidity

arms are extended

Decerebrate rigidity cause

bilateral damage that include the midbrain

arms are extended because the rubrospinal tracts are lesioned

spinal shock

-immediately after a spinal cord injury
-the UMN signs do not appear immediately: will experience flaccid paralysis and lack of reflexes that can last days to months

apraxia

total or partial loss of the ability to perform coordinated movements or manipulate objects in the absence of motor or sensory impairment

fasciculations

muscular twitching of contiguous groups of muscle fibers

clonus

rapidly alternating muscular contraction and relaxation

Babinski sign

when the sole of the foot is stroked, the toes--particularly the big toe goes upward)

UMN vs LMN lesion signs/symptoms

UMN vs LMN lesion signs/symptoms UMNs:
-spastic paresis (weakened voluntary movement)
-hyperreflexia
-NO muscle atrophy (except by misuse)
-Babinski sign

LMNs:
-flaccid paralysis
-hyporeflexia
-muscle atrophy
-fasiculuations