Neuromuscular Communication PDF

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