Motor units have an “all-or- none” response All of the fibers innervated by the motor neuron contract Or none of them do
1974 Hennenman’s Size Principle Smaller motor units are recruited first The frequency of motor unit recruitment (use/firing) is directly related to the size and ease of triggering an action potential in the soma (neuron cell body) smaller cell bodies (slow motor units) will be recruited first, and overall, most frequently
Control of Movement CNS: brain and spinal cord PNS: all the nerves extending from the brain and spinal cord afferent nerves: direction of AP is toward the spinal cord, usually involves sensory information
efferent nerves: direction of AP is away from the spinal cord, usually involves motor information reflexes are “all-or-none” contractions, monosynaptic
Resting membrane potential: ~70 mV
EPSP: excitatory post-synaptic potential release of ACH increases post-synaptic permeability to Na +, potential rises but may not reach depolarizing threshold level
IPSP: inhibitory post-synaptic potential another neurotransmitter is released which decreases the Na + permeability, reducing the chance for potential reaching the threshold
Activity in neuron within CNS Depends on the sum total of EPSPs and IPSPs Occurring in a narrow range of time and space
Muscle Spindles Connective tissue capsules in the shape of footballs that are filled with lymph and specialized fibers, implanted between muscle fibers Afferent and efferent innervations Provide information about absolute length of muscle and rate of change in length of the fiber
When muscle is contracted, activation of the motor neurons occurs, these efferent -neurons cause contraction of the muscle fibers within the muscle spindle this “takes up the slack” within the fibers of the capsule, allowing it to respond to further stretch
Golgi Tendon Organs (GTOs): respond to tension exert inhibitory effects on the agonist and facilitate the effects on the antagonist muscles is possible to disinhibit GTOs (minimizing the effectiveness of the GTOs) with training
allows athlete to push the limits of the tissue wrist wrestling: fractures and ruptures muscles and tendons occur
Myoplasticity Capacity of skeletal muscle for adaptive change Due to training, nutrition, endocrine milieu
Hypertrophy Increase in fiber size
Hyperplasia Increase in fiber number
Gender differences men typically have greater mass cross- sectional area than women men are able to generate more absolute strength than women not true when strength is expressed Str/BW or Str/FFW, gender differences disappear
when examined for the ability to generate force and power (normalized for cross- sectional area) there are no gender differences