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