Motor units have an “all-or-Motor units have an “all-or-none” responsenone” response

All of the fibers innervated by the All of the fibers innervated by the motor neuron contractmotor neuron contractOr none of them doOr none of them do

1974 Hennenman’s Size 1974 Hennenman’s Size PrinciplePrinciple

Smaller motor units are recruited firstSmaller motor units are recruited first

The frequency of motor unit recruitment The frequency of motor unit recruitment (use/firing) is directly related to the size (use/firing) is directly related to the size and ease of triggering an action potential and ease of triggering an action potential in the soma (neuron cell body)in the soma (neuron cell body)

smaller cell bodies (slow motor units) will smaller cell bodies (slow motor units) will be recruited first, and overall, most be recruited first, and overall, most frequentlyfrequently

Control of MovementControl of Movement

CNS: brain and spinal cordCNS: brain and spinal cord

PNS: all the nerves extending from the PNS: all the nerves extending from the brain and spinal cordbrain and spinal cord

afferent nerves: direction of AP is toward afferent nerves: direction of AP is toward the spinal cord, usually involves sensory the spinal cord, usually involves sensory informationinformation

efferent nerves: direction of AP is away efferent nerves: direction of AP is away from the spinal cord, usually involves from the spinal cord, usually involves motor informationmotor information

reflexes are “all-or-none” contractions, reflexes are “all-or-none” contractions, monosynapticmonosynaptic

Resting membrane potential: Resting membrane potential: ~70 mV~70 mV

EPSP: excitatory post-synaptic EPSP: excitatory post-synaptic potentialpotential

release of ACH increases post-synaptic release of ACH increases post-synaptic permeability to Napermeability to Na++, potential rises but , potential rises but may not reach depolarizing threshold may not reach depolarizing threshold levellevel

IPSP: inhibitory post-synaptic IPSP: inhibitory post-synaptic potentialpotential

another neurotransmitter is released which another neurotransmitter is released which decreases the Nadecreases the Na++ permeability, reducing permeability, reducing the chance for potential reaching the the chance for potential reaching the thresholdthreshold

Activity in neuron within CNSActivity in neuron within CNS

Depends on the sum total of EPSPs and Depends on the sum total of EPSPs and IPSPs IPSPs

Occurring in a narrow range of time and Occurring in a narrow range of time and spacespace

Muscle SpindlesMuscle Spindles

Connective tissue capsules in the shape of Connective tissue capsules in the shape of footballs that are filled with lymph and footballs that are filled with lymph and specialized fibers, implanted between specialized fibers, implanted between muscle fibersmuscle fibers

Afferent and efferent innervationsAfferent and efferent innervations

Provide information about absolute length Provide information about absolute length of muscle and rate of change in length of of muscle and rate of change in length of the fiber the fiber

When muscle is contracted, activation of When muscle is contracted, activation of the the motor neurons occurs, these efferent motor neurons occurs, these efferent -neurons cause contraction of the muscle -neurons cause contraction of the muscle fibers within the muscle spindlefibers within the muscle spindle

this “takes up the slack” within the fibers of this “takes up the slack” within the fibers of the capsule, allowing it to respond to the capsule, allowing it to respond to further stretchfurther stretch

Golgi Tendon Organs (GTOs): Golgi Tendon Organs (GTOs): respond to tensionrespond to tension

exert inhibitory effects on the agonist and exert inhibitory effects on the agonist and facilitate the effects on the antagonist facilitate the effects on the antagonist musclesmuscles

is possible to disinhibit GTOs (minimizing is possible to disinhibit GTOs (minimizing the effectiveness of the GTOs) with the effectiveness of the GTOs) with trainingtraining

allows athlete to push the limits of the allows athlete to push the limits of the tissuetissue

wrist wrestling: fractures and ruptures wrist wrestling: fractures and ruptures muscles and tendons occurmuscles and tendons occur

MyoplasticityMyoplasticity

Capacity of skeletal muscle for adaptive Capacity of skeletal muscle for adaptive changechange

Due to training, nutrition, endocrine milieu Due to training, nutrition, endocrine milieu

HypertrophyHypertrophy

Increase in fiber sizeIncrease in fiber size

HyperplasiaHyperplasia

Increase in fiber numberIncrease in fiber number

Gender differencesGender differences

men typically have greater mass cross-men typically have greater mass cross-sectional area than womensectional area than women

men are able to generate more absolute men are able to generate more absolute strength than womenstrength than women

not true when strength is expressed not true when strength is expressed Str/BW or Str/FFW, gender differences Str/BW or Str/FFW, gender differences disappeardisappear

when examined for the ability to generate when examined for the ability to generate force and power (normalized for cross-force and power (normalized for cross-sectional area) there are no gender sectional area) there are no gender differencesdifferences