Chapter 13 Flashcards
Common signs of lower motor neuron lesions
Decreased or absent reflexes
Flaccid paralysis
Atrophy
Low motor neurons convey
Signals to extrafusal and intrafusal skeletal muscle fibers
Cell bodies of spinal LMN are located in
Ventral horn
Motor pools
A cluster of Cell bodies whose axons project to a single muscle
Medially located motor pools
Innervate axial and proximal muscles
Laterally located motor pools
Innervate distal muscles
Anteriorly located motor pools
Innervate extensors
Posterior motor pools
Innervate flexors
Myotome
A group of muscles innervated by a single spinal nerve
Two types of motor neurons
Alpha and gamma
Alpha motor neurons
Have large cell bodies and large myelinated axons
Project to extrafusal skeletal muscle
Gamma motor neurons
Have medium sized myelinated axons
Project to intrafusal fibers in the muscle spindle
Alpha-gamma coactivation 
When the CNS wants a muscle to contract, signals are sent to alpha motor neurons, and also instructs gamma motor neurons to contract intrafusal fibers when the extrafusal muscle fibers actively contract
Motor unit
An alpha motor neuron and the muscle fibers it innervates
Motor units are classified as
Slow twitch, or fast twitch
Slow twitch muscle fibers
Innervated by smaller in diameter, slower conducting alpha motor neurons
Fast twitch muscle fibers
Innervated by larger diameter, faster conducting alpha motor neurons
Reciprocal inhibition
The inhibition of antagonist muscles during agonist contraction
Avoids cocontraction, one gets stimulated/excited, one gets inhibited
Reflexes involve
A receptor, an afferent limb, a synapse, an efferent limb, and an effector
Spinal reflexes require
Sensory receptors, primary afferents, synapses between primary afferents and LMNs, and muscles
Phasic stretch reflex / Deep Tendon Reflex / Myotatic Reflex
Muscle contraction in response to quick stretch
Tapping the knee reflex
Withdrawal reflex / cutaneous reflex
Person steps on tack → lower limb flexs automatically lifting the foot before person is consciously aware of the pain
Circuitry in spinal cord
Muscle spindle output
Not linearly related to changes in muscle length or rate of change in length
Modified by sensitivity adjustments and by recent movements and contractions the muscle has undergone
Contracture
The adaptive shortening of a muscle-tendon unit
Prolonged immobility causes contracture, muscle loses elasticity and thickens
Loss of sarcomeres
Spontaneous involuntary muscle contraction include
Muscle cramps
Fasciculations
Myoclonus
Tremors
Fibrillations
Abnormal movements
Fasciculations
Quick twitches of all muscle fibers in a single motor unit
Eyelid twitch
Myoclonus
Brief involuntary contraction of a muscle or group of muscles
Hiccups or muscle jerks
Fibrillations
Random, spontaneous, brief contractions of single muscle fibers snot visible on the surface of the skin and are always pathologic
Detectable by EMG (electromyography)
Muscle atrophy
The loss of muscle bulk
Disuse atrophy
Results from lack of muscle use
Neurogenic atrophy
Caused by damage to the nervous system
Hypotonia
Abnormally low muscular resistance to passive stretch
Occurs with decreased LMN input
Flaccidity
Total lack of muscle tone
Occurs with complete LMN lesions
Poliovirus
Infection that affects only LMNs
Selectively invades lower motor neuron cell bodies and destroys some of them denervating muscle fibers
Postpolio syndrome
Occurs years after illness
Due to overextended surviving neurons not being able to support the abnormal number of axonal branches, causing some distal branches to die
Symptoms include increased muscle weakness, joint and muscle pain, fatigue and breathing problems
