Motor Systems

Upper Motor Neuron

motor neuron that originates in the brain, neuronal cells bodies are in the primary motor cortex and the brainstem, travel down the white matter of the ventral spinal cord and synapse via an interneuron to a second motor neuron

Lower Motor Neuron (LMN)

a motor neuron that targets muscles directly, seuronal cell bodies are in the cranial nerve nuclei of the brainstem and the gray matter of the spinal cord, axons leave the central nervous system to become the motor division of the peripheral nervous system

Cranial Nerve III

aka oculomotor nerve, motor function to the extraocular muscles, levator palprebrae superioris, iris sphincter and ciliary body

Cranial Nerve IV

aka trochlear nerve, motor function to the extraocular muscles (dorsal oblique)

Cranial Nerve V

aka trigeminal, only branch 3 (mandibular branch) has motor function for mastication (chewing), nuclei in the pons

Cranial Nerve VII

aka facial nerve, motor function for facial expression (orbicularis oculi) and parasympathetic autonomic innervation of glands, nuclei in rostral medulla

Cranial Nerve XI

aka accessory, motor function to extrinsic shoulder muscles, nucleus in the caudal medulla

Cranial Nerve XII

aka hypoglossal, motor function for the tongue, nuclei in the caudal medulla

Dorsal Rectus

innervated by CN III, elevates globe, nuclei in midbrain

Ventral Rectus

innervated by CN III, depresses globe, nuclei in midbrain

Medial Rectus

innervated by CN III, adduction of the globe, nucleus in midbrain

Lateral Rectus

innervated by CN VI, abduction of the globe, nucleus in the medulla

Dorsal Oblique

innervated by CN IV, top of globe toward midline, nucleus in midbrain

Ventral Oblique

innervated by CN III, top of globe away from midline, nucelus in midbrain

Retractor Bulbi

innervated by CN VI, retracts orbit, nucleus in medulla

Cranial Nerve VI

aka abducens, motor function to extraocular muscles (lateral rectus and retractor bulbi)

Nystagmus

an involuntary/unconscious movement of the eye, can be normal or abnormal

Strabismus

an abnormal position of the eye, can be neuromuscular is static or fixed in all orientations, or vestibular if inducible or positional when orientation is changed

Orbicularis Oculi

muscle innervated by CN VII that is responsible for closing the eye

Cranial Nerves IX and X

aka glossopharyngeal and vagus, motor output to skeletal muscles of larynx, pharynx, and esophagus, parasympathetic to salivary glands and thoracic/abdominal viscera, nuclei in caudal medulla

Muscle Spindles

modified myofibers that are encapsulated and detect muscle stretch, detect the velocity of muscle length change and absolute length of the muscle (proprioceptors)

Type Ia Fibers

type of fiber in a muscle spindle that detects the velocity of muscle length changes

Type II Fibers

type of fiber within a muscle spindle that detects the absolute length of the muscle at a given time

Golgi Tendon Organs

high-threshold stretch receptors in tendons where the collagen fascicles join the myofibers, serve as proprioceptors and prevent overcontraction

Myotatic Reflexes

local muscle stretch reflexes essential for maintaining posture and support against gravity, providing muscle tone and preventing over-stretching, can positively innervate the agonist muscle while negatively innervating the antagonist

C7-T1/2

cervical intumescence location where the radial nerve exits

L4-6

location in the lumbosacral intumescence where the femoral nerve exits

L6-S1

location in the lumbosacral intumescence where the sciatic nerve exits

Muscle mass

the total weight or size of muscle tissue, the number of myofibers is typically fixed but the cells themselves can grow

Muscle Tone

the muscle’s tension or resistance to passive stretch, determined by continuous low-level state of contraction and intrinsic physical properties of muscle

Muscle Hypertrophy

increased muscle mass from an increase in individual muscle fiber size

Muscle Atrophy

decreased or reduced muscle mass from a decrease in individual muscle fiber size

Disuse Atrophy

gradual loss of skeletal muscle mass from lack of physical activity or mechanical load

Neurogenic/Denervation Atrophy

severe loss of skeletal muscle mass and contractility from lack of lower motor neuron innervation

Pyramidal System

motor pathways important for skilled movement originate in the cortex and descend spinal cord in an uninterrupted monosynaptic pathway, varies across species but not super important in quadrapeds

Extrapyramidal System

motor pathways for rhythmic and subconscious movement are multineural and multisynaptic, rely on motor nuclei in brainstem and basal nuclei

Reciprocal Inhibition

process by which the contraction of one muscle reflexively triggers the relaxation of its opposing muscle

Crossed Extensor Reflex

occurs during the withdrawal reflex, flexor muscles activate and ipsilateral extensor muscles relax via reciprocal inhibition, can also activate the contralateral extensor muscles to maintain support against gravity if weight bearing

Paresis

decreased or reduced voluntary movement

Paralysis

absence or loss of voluntary movement

Monoparesis/plegia

only one limb is affected

Hemiparesis/plegia

ipsilateral thoracic and pelvic limbs affected

Paraparesis/plegia

both pelvic limbs affected

Tetraparesis/plegia

all four limbs affected

Signs of UMN Dysfunction

voluntary movement: decreased to absent caudal to lesion

spinal reflexes: normal to increased

muscle atrophy: disuse atrophy, gradual to mild

muscle tone: increased, spastic paresis/paralysis

Signs of LMN Dysfunction

voluntary movement: decreased to absent caudal to the lesion

spinal reflexes: decreased to absent

muscle atrophy: neurogenic atrophy, rapid and severe

muscle tone: decreased, flaccid paresis/paralysis