Summary

This document details the neuromuscular communication system, explaining concepts such as the peripheral and central nervous systems, motor units, and principles of muscle activation. It thoroughly discusses excitation, motor unit function characteristics, and provides definitions of concepts and terminology.

Full Transcript

Neuromotor System Organization - PNS: gives info back to CNS Neuromotor System Function - CNS: brain (including retinas), spinal cord, integrative/control centers - Afferent division (sensory): somatic and visceral neurons which conducts impulses from receptors to CNS - PNS: cranial nerves III-XII a...

Neuromotor System Organization - PNS: gives info back to CNS Neuromotor System Function - CNS: brain (including retinas), spinal cord, integrative/control centers - Afferent division (sensory): somatic and visceral neurons which conducts impulses from receptors to CNS - PNS: cranial nerves III-XII and spinal nerves - Efferent division (motor): motor neurons which conduct impulses from CNS to effectors - Somatic nervous system: voluntary; conducts impulses from CNS to skeletal muscles - Autonomic nervous system: involuntary; conducts impulses from CNS to cardiac muscle, smooth muscle, and glands - Sympathetic and parasympathetic Peripheral Nervous System Structure - Contains 31 pairs of spinal nerves and 12 pairs of cranial nerves - Cranial nerves contain fibers that transmit sensory and/or motor info. - Spinal nerves include: - 8 pairs of cervical nerves - 12 pairs of thoracic nerves - 5 pairs of lumbar nerves - 5 pairs of sacral nerves - 1 pair of coccygeal nerves Peripheral Nervous System Function - Afferent neurons: relay sensory info. from receptors in periphery to CNS - Efferent neurons: transmit info. away from brain to peripheral tissues - 1. Somatic nerves (motor neuron): innervate skeletal muscle and produce excitatory response to activate muscle 2. Autonomic nerves (involuntary): produce either excitatory or inhibitory effect on smooth or involuntary muscles Autonomic Nervous System Structure - Sympathetic fibers: (fight or flight aspect) supply heart, smooth muscle, sweat glands, and viscera - Parasympathetic fibers: (rest and digest) supply thorax, abdomen, and pelvic regions (leave brainstem and sacral segments of spinal cord) - Regions of medulla, pons, and diencephalon exert control over autonomic nervous system Motor Unit Anatomy - Motor unit: makes up functional unit of movement; consists of motor neuron and specific fibers it innervates - Each muscle fiber generally receives input from ONLY 1 motor neuron - A motor neuron may innervate many muscle fibers - Number of muscle fibers per motor neuron related to muscle’s particular movement function - Motor neuron pool: ALL the alpha-motor neurons that innervate one muscle Microanatomy of Neuromuscular Junction (NMJ) Excitation - 1. Impulse arrives at NMJ as existing occurs only at NMJ - 2. ACh releases from vesicles in terminal axons into synaptic cleft - 3. ACh then combines w/ a transmitter-receptor complex in postsynaptic membrane - 4. Changes in electrical properties elicits endplate potential that spreads from motor endplate to extra-junctional sarcolemma of muscle - a) AP travels over muscle fiber length - b) Enter deep via T-tubule system - c) Spreads to inner structures of muscle fiber to prime the contractile machinery for excitation Motor Unit Function Characteristics - MUs contain: - a) Only 1 specific muscle fiber type (type I or type II) - b) OR subdivision of the type II fiber w/ the same metabolic profile - Motor units classify based on 3 physiologic and mechanical properties of muscle fibers they innervate: - 1. Twitch characteristics - 2. Tension characteristics - 3. Fatigability Motor Unit Speed, Force, and Fatigue - Motor unit designation → influences muscle fiber type in motor unit BUT not other way around - W/ prolonged aerobic training: fast-twitch muscle fibers b/c fatigue resistant as type I fibers - Motor neurons have stimulating effect on muscle fibers they innervate - MNs modulate fiber properties and adaptive response to stimuli All-or None Principle of Tension Generation - Stimulus strong enough to trigger a motor neuron action potential → activates ALL muscle fibers in motor unit to contract synchronously - A MU does NOT exert a force gradation → either the impulse elicits an action or it does not - All of nothing type of capacity - After the motor neuron fires and impulse reaches the NMJ → all fibers of motor unit fire simultaneously Gradation of Force Production - Force of muscle action varies from slight to max. via 2 mechanisms - 1. INCREASE # of motor units recruited - Muscle generates considerable force when activated by all its motor units - May decrease when muscle size increases (hypertrophy) - 2. INCREASE frequency of motor unit discharge - Repetitive stimuli that reach muscle b/4 it relaxes → increases total tension Affected by nerve’s conduction velocity - Increase velocity in power vs. endurance-trained indvs. Frequency decrease at rest Pos. relationship w/ amt. of force produced Increase firing rate depends on muscle size Size Principle of Motor Unit Recruitment - Increase muscle forces required → motor neurons w/ larger axons recruited - Size principle: - Orderly recruitment of specific motor units to produce smooth muscle action - Allows CNS to fine tune skeletal muscle activity to meet motor task demands - Differential control of motor unit firing patterns represents major factor distinguishing skilled from unskilled performance Size Principle and Selective MU Recruitment - Increase muscle force required → motor neurons w/ larger axons recruited - Size principle: - Orderly recruitment of specific motor units to produce smooth muscle action - Allows CNS to fine tune skeletal muscle activity to meet motor task demands - Selective recruitment: - Preferential recruitment of type II motor units - Can occur during change in direction of exerted forces and explosive muscle actions Fatigue Resistance - Fatigue: decrease in force capacity w/ repeated stimulation or during a given time period - 4 components impact voluntary muscle action: - 1. Central nervous system (CNS) - 2. Peripheral nervous system (PNS) - 3. Neuromuscular junction (NMJ) - 4. Muscle fiber - Fatigue occurs from disruption in chain of events b/w CNS and muscle fiber - Any delays or interference w/ that → decrease in performance

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