Motor Control Systems Flashcards
What are the 2 major systems of motor control?
Somatic and Visceral
Somatic motor system
-Controls skeletal muscle
-Voluntary
Visceral motor system
-Aka autonomic nervous system
-Controls smooth & cardiac muscle and glands
-Mostly involuntary
Upper motor neurons
From control regions rostral to ventral horn of spinal cord
➢ basically, everything other than the lower motor neurons
Primary motor area
All three motor cortical areas contribute to the corticospinal tract (CST)
▪ CST fibers decussate at pyramidal decussation
▪ influence lower motor neurons
Extrapyramidal systems
Forebrain & brainstem areas that are not part of the CST
➢ vestibulospinal
➢ reticulospinal
➢ tectospinal
➢ rubrospinal
Lower motor neurons
Both α- and γ- motor neurons
▪ in ventral horn of the spinal cord
➢ note differences in UMN & LMN lesion symptoms
Upper motor neuron modulators
Regions that have few, if any, direct inputs on lower motor neurons, but have a significant influence on motor activity
-Basal ganglia
-Cerebellum
-Association cortex
-Thalamus
Corticospinal tract (CST)
Premotor/supplementary/primary motor areas–>from cortex via post. limb of internal capsule–>decussate (pyramidal) at caudal medulla –>SC descend in the lateral funiculus =LCST
Lateral CST
Decussate in pyramids (~85%)
Descend in CL spinal cord in lateral funiculus
Act on more lateral lower motor neurons in ventral horn of spinal cord
Influence peripheral/limb muscles
Medial (anterior) CST
Fibers stay ipsilateral at caudal medulla (~15%)
Travel in ventral funiculus of ipsilateral spinal cord
Decussate low in spinal cord
Influence more medial ventral horn neurons; axial/trunk muscles
Both CST pathways
Project to both α- and γ- motor neurons
➢ alpha = extrafusal; gamma = intrafusal (muscle spindles)
▪ alpha/gamma coactivation
Project directly and indirectly (by way of interneurons)
Premotor cortex and supplementary motor cortex
➢ premotor area is just rostral to primary motor cortex in frontal lobe
➢ supplementary is similar, but on medial surface of hemisphere
▪ both regions are involved with the initiation of voluntary movement
➢ neural activity seen before movement
▪ activity even if movement is just imagined or contemplated
▪ both regions receive inputs from prefrontal cortex (decision making) and somatosensory association areas (spatial/kinesthetic information about body and objects)
Primary motor area
Precentral gyrus of frontal lobe
▪ ‘final output’ from cerebral cortex
Somatotopic: motor homunculus
Ventral horn
α-motor neurons in ventral horn of spinal cord
▪ innervate skeletal muscle (extrafusal fibers)
➢ at least three types of muscle fibers based on metabolic pathways and physiological properties
Organization of ventral horn
▪ medial = axial (proximal) muscles; lateral = peripheral (distal) muscles (limbs)
▪ dorsal = flexors; ventral = extensors
Motor Unit
➢ one α-motor neuron and all the muscle fibers it innervates
▪ function of muscle helps determine the size of the motor unit
Each neuron innervates a single type of muscle (I, IIa, IIb)
Small motor units
▪ relatively few muscle fibers innervated
* as low as 1:3 (!)
▪ allows very small, precise contractions
* e.g., ocular muscles
Large motor units
▪ many muscle fibers innervated by a single neuron
* 1:1,000+
▪ allows larger, stronger (although less precise) contractions
* e.g., gastrocnemius
Basics of Spinal reflexes
➢ simplest demonstration of interplay between PNS & CNS
▪ sensation→integration→response
Stretch reflexes
▪ e.g., Q.f. extension after hitting the patellar ligament with a hammer
➢ sensor = muscle spindles
➢ Info ascends via spinocerebellar tract
➢ collaterals of neurons signaling the stretch sensation reach LMN in the ventral horn
▪ reciprocal innervation
common feature of almost every somatic reflex
* one muscle (or group of muscles) is activated while the antagonist muscle(s) are inhibited
* allows limb movement
Flexor (withdrawal) reflexes
➢ sensor = nociceptors
➢ Info ascends via spinothalamic tract
➢ some collaterals ascend to innervate ventral horn at appropriate level (via interneurons)
▪ but respond to (potentially) damaging stimuli
Crossed extensor reflexes & Central pattern generators
➢ more obvious in lower limbs
➢ as one limb is flexed, other limb is extended
▪ signals must decussate to other side of cord (thus the name)
➢ this basic circuitry of alternating flexion and extension of limbs forms the basis of locomotion
▪ movement pattern is generated in the central nervous system
▪ may allow decreased specificity from the upper motor neurons
* i.e., may decrease computational demands on the motor cortex
Vestibulospinal tract
(extrapyramidal pathway)
From vestibular system in inner ear
Two pathways; lateral and medial
Lateral vestibulospinal tract
Helps control anti-gravity muscles in whole body
Compensation for tilt and movement of body (head movement)
▪ sensors also responsible for feelings of dizziness
Medial vestibulospinal tract
Bilateral projection to cervical spinal cord
Coordinates head and eye movements
▪ e.g., stabilizes head & eyes while moving/walking
Reticulospinal tract
(extrapyramidal pathway)
Major alternative to pyramidal pathway
from reticular formation
postural control & postural controls during locomotion
Medullary (lateral) reticulospinal tract
inhibits trunk extensors
Pontine (medial) reticulospinal tract
activates trunk extensors (anti-gravity)
Tectospinal tract
(extrapyramidal pathway)
From superior and inferior colliculus
Mediate muscles controlling reflexive head turning to ocular and auditory stimuli, respectively
Rubrospinal tract
(extrapyramidal pathway)
From red nucleus
Minor in humans (major in lower mammals)
- is primary in babies’ crawling (before myelination of CST)
▪ note that the red nucleus is critical in human cerebellar→cerebral motor pathways
Corticobulbar Pathway
Upper motor neurons for Cranial nerves V, VII, & XII (trigeminal, facial & hypoglossal)
▪ controls most facial muscles
▪ facial expression & mastication
-usually bilateral in upper face and to muscles of mastication
-usually unilateral to lower face
Corticobulbar Pathway
1st Order: pre- supple-, and primary motor areas travels with CST fibers decussate at appropriate brainstem level depending on what they are controlling
2nd Order: Trigeminal (V), Facial (VII), Nucleus ambiguus (X), Spinal accessory nucleus (XI), and hypoglossal nucleus (XII)
Damage to UMN
weakness, spasticity, increased tendon reflexes, increased Babinski sign, little atrophy, and tend to have greater influence on:
▪ affect extensors & ABductors of arm
▪ flexors of leg
Examples of Lesion to the UMN
Cerebral cortex (stroke)
▪ focal motor deficit to contralateral face, hand, limb, etc
Internal capsule (lacunar stroke)
▪ contralateral hemiparesis
▪ often widespread
➢ fig 13-8 of Greenberg
Lesion rostral to red nucleus
▪ contribute to decorticate rigidity
➢ flexion of elbows, wrists & fingers
➢ extension & internal rotation of legs
Lesion caudal to red nucleus
▪ contribute to decerebrate rigidity
➢ extension of arms & legs (esp. elbow)
➢ internal rotation of both arms & legs
Brainstem
▪ often bilateral
▪ often associated sensory and cranial nerve disturbances
Spinal cord (white matter tract)
▪ increases tone (spasticity)
▪ often sensory deficits
➢ includes other pathways
Damage to LMN
weakness, flaccid paralysis (hypotonia), atrophy (ipsilateral, along myotome), usually involves adjacent white matter
▪ so upper motor neuron damage symptoms seen below level of injury
Visceral Motor (Autonomic) nervous system (ANS)
▪ motor control of smooth muscle, cardiac muscle, & glands
▪ homeostatic control
3 systems of the visceral motor [ANS]
1 Sympathetic division
2 Parasympathetic division
3 Enteric nervous system
Enteric NS
Neurons and plexuses that control the gastrointestinal tract
▪ e.g., myenteric & submucosal plexus
Influenced by sympathetic & parasympathetic divisions
▪ but separate & independent
➢ helps explain GI tract function even in patients with high spinal cord injuries
Large, but not well understood
▪ as many or more neurons than the spinal cord!
Similarities between sympathetic & parasympathetic divisions
Both influence internal organs
usually dual opposing innervation
▪ at each tissue, one system inhibits and the other activates
➢ depends on tissue
▪ there are several major exceptions (only symp, only parasym, cooperative)
Both PNS and SNS have two neuron chains that synapse in a PNS ganglia
Preganglionic neurons
▪ originate in CNS
▪ myelinated
▪ cholinergic
Postganglionic neurons
▪ originate in autonomic ganglia
▪ unmyelinated
▪ have nicotinic receptors on soma & dendrites
▪ act on target tissues
Sympathetic nervous system
▪ ‘fight or flight’
▪ energy expenditure
SNS Preganglionic neurons
originate from lateral horn [intermediolateral nucleus (IML)]
▪ only from T2-L1
➢ “thoracolumbar system”
➢ control of head must re-ascend outside of spinal cord
▪ have shorter distance to travel to synapse in ganglia
SNS Ganglia
Sympathetic chain ganglia
▪ majority of synapses
▪ close to spinal cord on either side
Prevertebral ganglia
➢ celiac, inferior & superior mesenteric ganglia
▪ site of synapses other than chain ganglia
▪ preganglionic axon travelling to these prevertebral ganglia = splanchnic nerves
Postganglionic neurons: longer distance to terminal tissue
SNS Adrenergic
▪ use norepinephrine (NE) and/or epinephrine (Epi) as neurotransmitter
➢ released onto target tissues
➢ tissues respond due to adrenergic receptors
▪ α- & β- receptors
* many subtypes, various sensitivity and response to NE/Epi
Parasympathetic nervous system
▪ ‘rest and relax’/ ‘rest and digest’
▪ energy conservation
PNS Preganglionic neurons
Originate from brainstem and sacral spinal cord
➢ ‘craniosacral’
▪ CN III (oculomotor)
➢ iris
▪ CN VII (facial)
➢ lacrimal & submandibular salivary glands
▪ CN IX (glossopharyngeal)
➢ parotid salivary gland
▪ CN X (vagus); 90%+ of all parasympathetic axons
➢ innervates most thoracic and abdominal organs
▪Sacral: lateral horn of T12-L1
PNS Ganglia
▪ terminal ganglia
➢ located next to within target tissues
PNS Postganglion neurons
➢ shorter
▪ cholinergic
➢ release Ach onto target tissues
▪ tissues respond due to muscarinic receptors
