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Peripheral Nervous System Peripheral Nervous System Organization of Nervous System: Involuntary reflexes (spinal cord); Motor Units: voluntary actions (higher brain centers) Motor Unit: Nervous system A single motor neuron and all the muscle fibers innervated by it (motor unit = all-or-none) Integration Motor unit size dictates control: Central nervous system Peripheral nervous system (CNS) (PNS) Fine Control / Rapid Reaction: Motor Sensory 1-10 fibers / MU (e.g., ocular muscles) output input Gross Control / Slow Reaction: Brain Spinal cord Motor division Sensory division 1000’s fibers / MU (e.g., quadriceps) (efferent) (afferent) Recruitment: Addition of motor units to produce smooth, steady Autonomic nervous system Somatic nervous system (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle) muscle tension (multiple fiber summation) Motoneuron Pool: Set of motor neurons innervating Small large motor units activated… muscle fibers within the same muscle Sympathetic division Parasympathetic division • Varying thresholds Motor units overlap; provides coordination Marieb & Hoehn – Figure 9.13 Guyton & Hall – Figure 54.2 Peripheral Nervous System Peripheral Nervous System Types of Motor Neurons: Muscle Spindle – Anatomy: 1) Alpha () motor neurons: • Give rise to large Type A alpha (A) motor nerve fibers (~ 14 µm diameter) • Innervate extrafusal skeletal muscle Sensory Innervation: fibers (generate force) Primary Ending: 2) Gamma () motor neurons: Large sensory fiber (Ia) encircling • Give rise to small Type A gamma (Aγ) central portion of intrafusal fibers motor nerve fibers (~ 5 µm diameter) Secondary Ending: • Innervate intrafusal muscle fibers Smaller sensory fiber(s) (II) encircling (small, specialized fibers – muscle spindle) / branched along intrafusal fiber What is the length of the muscle? Proper control of muscle function requires: What is the instantaneous tension? How rapidly is the length / tension changing? 1) Excitation of muscle by motor neuron 2) Continuous feedback of sensory information from each muscle • 3 – 12 intrafusal muscle fibers enclosed in connective tissue capsule • Requires specialized receptors: • Central regions lacking actin / myosin (non-contractile); serve as sensor regions A) Muscle spindle – Detect muscle length • Contractile ends; innervated by Aγ motor fibers B) Golgi tendon organ – Detect tendon (muscle) tension Costanzo – Figure 3.29 Costanzo – Figure 3.29 Peripheral Nervous System Peripheral Nervous System Muscle Spindle – Anatomy: Nuclear Nuclear Muscle Spindle – Physiology: chain bag Nuclear Nuclear chain bag Muscle spindles emit sensory nerve impulses continuously • Stretching increases rate; shortening decreases rate Sensory region excited via lengthening of muscle which stretches intrafusal fibers Types of Intrafusal Fibers: 1) Nuclear Chain 2) Nuclear Bag • Group II afferent fibers detect the length of a muscle fiber (nuclear chain) • Small fibers; nuclei arranged in a row • large fibers; nuclei grouped in central region • Number of impulses proportional to degree of stretch (tonic reception) • 3 – 9 fibers / muscle spindle • 1 – 3 fibers / muscle spindle • Group Ia afferent fibers detect the velocity of length change (nuclear chain / bag) • Innervated by type Ia and type II afferent • Innervated by type Ia afferent fibers • Number of impulses proportional to rate of length change (phasic reception) fibers (primary / secondary endings) (primary endings) 1 Peripheral Nervous System Peripheral Nervous System Muscle Spindle – Physiology: Reflex Reflex: Reflex: Rapid, automatic response to • Muscle spindles function as length comparators (intrafusal vs. extrafusal length) a specific stimuli • Designed to oppose change in intrafusal length (negative feedback system) Reflex Arc: • Returns intrafusal fibers to original length by activating extrafusal fibers Step 2: Sensory neuron activation Step 1: Receptor activation A (Type Ia) Step 3: Information processing Type II: Step 4: Motor neuron Delayed signals; Step 5: Relay information activation Effector activation Guyton & Hall – Figure 54.4 Costanzo – Figure 3.30 Costanzo – Figure 3.31 Peripheral Nervous System Peripheral Nervous System Spinal Cord Reflexes: Limited delay between Spinal Cord Reflexes: 1) Stretch reflex sensory input and motor 2) Golgi tendon reflex output (20 – 40 msec) Interneurons # of synapses Stimulus for Sensory afferent Response # of synapses Stimulus for Sensory afferent Response in reflex arc reflex fibers of muscles in reflex arc reflex fibers of muscle(s) 1 Muscle stretch Ia Muscle contraction 2 Muscle contraction Ib Muscle relaxation Costanzo – Figure 3.32 Peripheral Nervous System Peripheral Nervous System Spinal Cord Reflexes: Afterdischarge: Muscle Spindle – Physiology: Reflex 3) Flexor-Withdrawal reflex Persistent neural discharge occurring in polysynaptic • Muscle spindles function as length comparators (intrafusal vs. extrafusal length) reflex circuits • Designed to oppose change in intrafusal length (negative feedback system) • Returns intrafusal fibers to original length by activating extrafusal fibers Interneurons A (Type Ia) Why don’t we inhibit stretch # of synapses Stimulus for Sensory afferent Response Type II: reflexes when we voluntarily in reflex arc reflex fibers of muscle(s) Delayed signals; Relay information activate our muscles? Many Pain; temperature II, III, and IV Flexion (ipsilateral) Answer: Gamma system Extension (contralateral) Guyton & Hall – Figure 54.4 2 Peripheral Nervous System Gamma Efferent System: Higher order signals muscle to contract (+) A motor neuron (+) • Elicits tonic signaling (constant intrafusal stretch) by keeping the length of the intrafusal fibers in proportion to the length of the extrafusal fibers • A motor neurons coactivated with Aα motor neurons Figure 54.3 Peripheral Nervous System Levels of Motor Control: Precommand Level Cerebellum (feedback) Control output of cortex / brain stem Basal nuclei • Start / stop movements • Coordinate movements with posture • block unwanted movements Projection Level Motor cortex Direct (cerebrum) system Convey instructions to spinal cord motor neurons Brain stem Indirect (send copy of instructions to higher levels) nuclei system Segmental Level Spinal Central pattern generators (CPGs): cord Circuits that control specific, oft-repeated Sensory motor activities (e.g., locomotion) Motor input output Spinal cord reflex 3 .
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