motor unit Flashcards
major role of motor control and processing the brain
to make goal directed movements of body and limbs
Motor systems: translate neural signals into contractile force in muscles to produce movement
functions of skeletal muscles
to move bones joints and skin
execute voluntary movements, maintain posture, produce heat and energy
skeletal muscles are effectors of movements:
Muscles are specialized to generate tension by contraction: shortening contractions, isometric contractions (equal length), lengthening contraction
sk muscles are made up of fascicles (parallel), fascicles made of muscle fibers
synergistic and antagonistic muscles
muscles that work across the joint together (produce similar action)- synergystic
Muscles that work in opposite direction (work against each other)- antagonistic
muscles generate tension by pulling
each joint require at least 2 muscles pulling in opposite directions:
Extensor and a flexor
Flexor moves limb toward body, extensor move limb away from body
motor neurons
efferents, are the final common pathway for movement, muscles are activated by motor units
motor unit
one motor unit= one motor neuron + all the muscle fibers it innervates
the motor unit is the smallest functional element of the motor system, smallest unit of muscle under neural control. Provides the smallest increment of tension that can be generated
sizeof motor unit varies with precision of neural conrol over muscle movement
motor neurons communicate with skeletal muscles via the NMJ, release of Ach, and activation of nicotinic Ach receptors, contraction of contractile proteins in muscle
motor units in normal
normal muscle: 1 MN innervates many muscle fibers in different fascicles within one muscle (innervated fibers are not just adjacent to each other)
provides efficient way to activate a muscle. during low force movement a few motor units are recruited across the muscle. as the force needed to generate a larger movement increases, additional motor units are recruited
smooth, graded increase in force so that limb movement
different motor units are activated asynchonously- smooth fluid movements
pathological muscle
sometimes a motor unit innervates many muscle fibers that are all in one fascicle–> making the muscle unit much larger. can be jerkey and not finely graded
motor neurons location
located in spinal cord (ventral horn) and brain stem
Sp cd has motor neurons that control neck trunk and limbs
Brainstem has motor neurons that control muscles in face and head
Motor neurons in the sp cd are organized into a vertical column (extending 1-4 segments) called a motor nucleus
motor neurons that innervate the same muscle are found in the same motor nucleus (motor neuron pool). 1 muscle can be innervated by many motor neurons in one nucleus
somatotopic organization
in ventral horn of sp cd: motor neurons are arranged according to individual muscles. Medial motor nuclei: motor neurons that innervate the axial muscles of the neck and back- connected across many segments by propriospinal neurons (long axons, branch extensively)
Lateral motor nuclei: innervate limb muscles: most medial innervate proximal limbs most lateral innervate distal limbs. Connected across few segmentts by propriospinal neurons (short axons, branch less)
Flexors are near the center of spinal cord, extensors are more peripheral
lower vs upper motor neurons
lower: 1’ motor neurons in sp cd ventral horn and brainstem nuclei, that directly innervate and control muscles
upper: neurons that originate in brainstem and motor cortex that descend and synapse on lower mootor neurons
difference between motor unit types
differ in speed at which they are recruited to fire, strength of contraction, fatigability
slow twitch motor units
innervate red, slow twitch muscle fibers (type 1)
smaller fibers, small MN, slow contraction, smaller tension output. rely on oxidative catabolism, depend on glucose and oxygen from blood stream to regenerate ATP. Large numbers of mitochondria, myglobin fatigue resistant
produce small tension for long duration without fatigue
specialized for endurance esp posture standing and walking
fast fatigable motor units
innervate white fast twitch fibers
larger fibers, large mn fast large contraction
rely on anaerobic catabolsim, depend on glycogen stores that are converted into lactic acid
fatigue because glycogen is rapidly depleted
produce brief burst of force, takes hours to recover fully
specialized for strength and speed, esp running, jumping and vigorous exersice
fast fatigue-resistant
innervate intermediate fibers (properties between red and white muscle fibers)
combines fast twitch dynamics and contraction with enough aerobic capacity to resist fatigue for several minutes. specialized for exercise with endurance
most muscles contain a mix of all 3 types but the proportions vary
