ES - Neuromuscular Anatomy and Phys - Neuromuscular Anatomy Flashcards
Motor Unit
A motor neuron and the muscle fibers it innervates.
All muscle fibers of a motor unit contract together when they’re stimulated by the motor neuron.
Typically several hundred muscle fibers per motor unit.
Motor Unit structure
Nucleus Dendrites Axon Nodes of Ranvier Myelin Sheath NMJ on muscle
Motor Unit and Muscle Activation
when a motor neuron fires an impulse or AP, all fibers it innervates are simultaneously activated and develop force.
Motor units and precision movements
ex. eye muscles
The extent of control of a muscle depends on the number of muscle fibers within each motor unit.
Muscle req. greater precision can have motor units w/ as few as 1 muscle fiber per unit.
Changes in number of active motor units in small muscles can produce the extremely fine gradations in force that are necessary for precise movements of the eyeball.
Motor units and less precise movements
ex. quad muscles
may have several hundred fibers served by one motor neuron.
Motor unit abnormalities
weakness or loss of contraction strength of the muscle.
can result in muscle:
Atrophy
Fasciculations (involuntary twitches)
Hypotonia (decrease in tone of skeletal tone)
Hyporeflexia (decreased muscle stretch reflexes; rapid stretching)
Muscle Fiber Types
Type I (slow twitch) Type IIa (fast twitch) Type IIx (fast twitch)
Fast Twitch Motor Unit
develops force and also relaxes rapidly and thus has a short twitch time.
Slow Twitch Motor Unit
develops force and relaxes slowly and has a long twitch time.
Type I characteristics
motor neuron size recruitment threshold nerve conduction velocity contraction speed relaxation speed fatigue resistance endurance force production power output aerobic enzyme content anaerobic enzyme content SR complexity capillary density myoglobin content mitochondrial size, density fiber diameter Color
Small motor neuron size Low recruitment threshold Slow nerve conduction velocity Slow contraction speed Slow relaxation speed High fatigue resistance High endurance Low force production Low power output High aerobic enzyme content Low anaerobic enzyme content Low SR complexity High capillary density High myoglobin content High mitochondrial size, density Small fiber diameter Color red
Type IIa characteristics
motor neuron size recruitment threshold nerve conduction velocity contraction speed relaxation speed fatigue resistance endurance force production power output aerobic enzyme content anaerobic enzyme content SR complexity capillary density myoglobin content mitochondrial size, density fiber diameter Color
Large motor neuron size Intermediate/High recruitment threshold Fast nerve conduction velocity Fast contraction speed Fast relaxation speed Intermediate/Low fatigue resistance Intermediate/Low endurance Intermediate force production Intermediate/High power output Intermediate/Low aerobic enzyme content High anaerobic enzyme content Intermediate/High SR complexity Intermediate capillary density Low myoglobin content Intermediate mitochondrial size, density Intermediate fiber diameter Color White/Red
Type IIx characteristics
motor neuron size recruitment threshold nerve conduction velocity contraction speed relaxation speed fatigue resistance endurance force production power output aerobic enzyme content anaerobic enzyme content SR complexity capillary density myoglobin content mitochondrial size, density fiber diameter Color
Large motor neuron size High recruitment threshold Fast nerve conduction velocity Fast contraction speed Fast relaxation speed Low fatigue resistance Low endurance High force production High power output Low aerobic enzyme content High anaerobic enzyme content High SR complexity Low capillary density Low myoglobin content Low mitochondrial size, density Large fiber diameter Color White
Types of specialized sensory receptors (proprioceptors) (2)
Muscles Spindles
Golgi Tendons Organs (GTO)
Muscle Spindles
Consist of several modified muscle fibers (infrafusal) enclosed in a sheath of connective tissue and run parallel to normal fibers (extrafusal).
Provide info concerning muscle length and the rate of change in length.
Spindles indicate the degree to which the muscle must be activated in order to overcome a given resistance
Muscle Spindles activity during stretch (4)
1) muscle lengthens and spindles stretch
2) this deformation activates sensory neuron of spindle, sending an impulse to spinal cord
3) impulse to spinal cord synapse (connect) w/ motor neurons
4) this activates motor neuron that innervates the same muscle (muscles contracts)
