motor pathways

parts of CNS and PNS specialized for control of limb, trunk, and eye movements

motor system

from simple reflexes (knee jerk) to voluntary movements

motor system

only muscles

contract

3 major types of movements

-reflexes

-postural adjustments

-voluntary movements

motor systems involve

1st motor cortex

premotor cortex

red nucleus

reticular formation

vestibular nuclei

superior colliculus

reflexes

spinal cord circuits

spinal and brainstem

postural adjustments

spinal cord, brainstem, and cortex

voluntary movements

postural adjustments and voluntary movements depend more on

cerebellar and BG function than reflexes

types of reflexes

monosynaptic and polysynaptic

reflex involving 2 neurons

monosynaptic reflex

quadricep DTR

jaw jerk

monosynaptic reflex

more complex response

polysynaptic reflex

crossed extension, withdrawal reflex

polysynaptic reflex

dynamic controlo

postural adjustments

feedback control is

reactive

error correction

feedback control

issues arise when response lags stimulus; sometimes too late

feedback control

response anticipates stimulus

feedforward control

more timely, but depends on internal models

feedforward control

practice, learning

feedforward control

organized around purposeful acts

voluntary movements

modifiable input-output relationships

voluntary movements

limitless, whole brain is involved

voluntary movements

lower motor neurons location

ventral horn to NMJ

a-LMN supply

skeletal muscle fibers (extrafusal) and regulate skeletal muscle contraction to produce movement

y-LMN supply

intrafusal fibers in the muscle spindles, regulating the sensitivity of the muscle spindles for modulation of the alpha LMN excitability

LMN influenced by descending projections from

brainstem and cerebral cortex

no direct synaptic link to muscles

UMN

supra-spinal system

UMN

If LMN cell bodies and/or axons are destroyed, then the affected muscles can undergo:

-decrease or loss of reflexes

-paresis or paralysis

-atrophy

-decrease or loss of muscle tone

-fibrillations/fasciculations

b/c LMN is the only route from spinal cord to muscle, LMN lesions

interrupt the efferent limb of reflexes

loss of decreased ability to generate muscle force is common consequence of

MN lesions

complete lesion of a peripheral nerve produces paralysis because

LMNs are the only pathway from the CNS to skeletal muscle

loss of muscle bulk

atrophy

results from lack of muscle use

disuse atrophy

caused by damage to nervous system

neurogenic atrophy

LMN lesions cause two types of abnormal muscle

hypotonia/flaccidity

UMN signs

loss of function vs gain of function

loss of function signs (UMN)

-paresis and paralysis

-impaired selective motor control

-absent or decreased muscle tone due to immediate post injury and spinal shock

-flaccidity and hypotonia

gain of function sign (UMN)

-spasticity: myoplasticity, hyperreflexia, excess reticuclospinal drive (abnormal synergies)

-rigidity

-abnormal reflexes

-compensatory and pathologic co-contraction

LMN syndrome

-weakness/paralysis/paresis

-hyporeflexia

-hypotonia (flaccidity)

-atrophy of muscles

-fibrillations and fasciculations

UMN syndrome

-weakness/paralysis/paresis

-HYPERreflexia

-HYPERtonia (spasticity)

-minimal (disuse) atrophy

-extensor plantar response (Babinski)

planning and execution of complex voluntary activity

cerebral cortex

initiation of movement and modulation of motor cortex

basal ganglia

timing, speed, direction and precision of motor activity

cerebellum

integrates cortical, visual, auditory, vestibular, and somatosensory systems to plan, modify, and coordinate movement

cerebellum

both influence motor activity INDIRECTLY and SUBCONSCIOUSLY through the thalamus

cerebellum and BG

influencer voluntary motor activity via DIRECT PROJECTIONS to either brainstem cranial nerve motor nuclei, spinal cord motor neurons or to the reticular formation

primary and secondary motor cortices

precentral gyrus of the frontal lobe

primary motor cortex

-brodmann's area 4

controls execution of well-defined, distinct voluntary motor activity of the contralateral side of the body

PMC

each cortical neuronal group control the motor activity of a

single muscle, proportional to its complexity

motor homonculus

PMC

programming of complex motor activity by groups of muscles performing a task

secondary motor cortex

secondary motor cortex, 3 areas in frontal lobe

-supplementary motor area

-premotor cortex

-frontal eye field

secondary motor cortex, one area in parietal lobe

posterior parietal motor area

programming phase of patterns and sequences of elaborate movements

supplementary motor area

coordination of movements occurring on the two sides of the body

supplementary motor area

muscle contractions of the trunk (axial) and proximal extremity (limb girdle) muscles, controlling arm and thigh, brodmanns area 6

supplementary motor area

lateral frontal lobe

premotor cortex

motor control of the axial and proximal (limb girdle) musculature of the extremities, planning/early initiation phase

premotor cortex

turning/gliding the body and upper limbs toward a certain direction

premotor cortex

rostral to the premotor area

frontal eye field

coordination of eye movements, particularly mediating voluntary visual tracking of a moving object

frontal eye field

tactile discrimination and stereognosis (3-dimensionality)

posterior parietal motor area

movements that require visual guidance

posterior parietal motor area

terminate in the spinal cord or cranial nerve nuclei

cortical descending motor paths

main output neurons

pyramidal cells

located in layers 5 of motor cortex

pyramidal cells

release glutamate to interneurons or less commonly LMNs directly

pyramidal cells

large pyramidal cels projecting to the corpus striatum, brainstem, and spinal cord

inner pyramidal layer

2/3 of the fibers originate from the pyramidal cell layer of the frontal cortex

-1/3 from the motor cortex

-1/2 from SMA+premotor (brodmann's 6)

corticospinal tract

1/3 originate from the posterior parietal motor area and the somatosensory cortex

corticospinal tract

pyramidal cells are referred to as the

UMNs

UMNs travel through the

corona radiata and the posterior limb of the internal capsule to the brainstem medulla where almost all cross to form the lateral corticospinal tract

UMNs synapse onto interneurons, alpha motor neurons (to skeletal muscle), and

gamma motor neurons (to muscle spindle stretch receptors- contractile portion) in the cord

descending pathways modulating

pain sensory mechanisms

synapse with second order neurons in the somatic relay nuclei of the ascending sensory pathways and in the second order sensory neurons in the dorsal horn

descending somatosensory neurons

influence motor activity by modulating the transmission of sensory information to higher brain centers

descending somatosensory neurons

their fibers travel with but are not considered part of the UMN pathway

descending somatosensory pathway

start in pons, through medulla, ends in spinal cord

descending somatosensory neurons

corticospinal tract-

motor cortex (homonculus -> spinal cord)

most UMNs terminate in the lateral intermediate zone gray matter of the cord on interneurons

lateral corticospinal tract

some UMNs terminate

directly on LMNs in the anterior horn gray matter of the cord (many to hand muscles) lateral corticospinal tract

UMN synapses are

excitatory (lat. corticospinal tract)

interneurons may be

excitatory or inhibitory (lat. corticospinal tract)

lateral corticospinal tract crosses at

medulla

most important for controlling unwanted movement of skeletal muscle

lateral corticospinal tract

~2-3% of fibers descend ipsilaterally

lateral corticospinal tract

synapse with spinal cord interneurons mediating movement of trunk (axial) and proximal extremity (limb girdle) musculature

uncrossed lateral corticospinal tract

maintenance of upright posture and general orientation of the limbs

uncrossed lateral corticospinal tract

10-15% of corticospinal tract axons that do not decussate and descend ipsilaterally create the _______..

anterior corticospinal tract

descend ipsilaterally in the anterior funiculus

anterior corticospinal tract

most fibers decussate in the anterior white commissure just before their termination on interneurons or directly on LMNs mainly in the anterior horn gray matter of the cervical and upper thoracic spinal cord

anterior corticospinal tract

axial (neck, shoulder, trunk) and upper limb girdle

anterior corticospinal tract

90% of fibers of corticospinal tract cross as

lateral corticospinal tract

8% of fibers of corticospinal tract stay

ipsilateral as anterior corticospinal tract

2% of fibers in corticospinal tract remain ipsilateral as

uncrossed lateral corticospinal tract

55% of corticospinal fibers terminate at

cervical cord levels (for fine hand control/movement)

20% of corticospinal fibers terminate at

thoracic levels