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Nervous System Central Nervous System Peripheral Nervous System Peripheral Nervous System Involuntary reflexes (spinal cord); Organization of Nervous System: voluntary actions (higher brain centers) Nervous system Integration Central nervous system Peripheral nervous system (CNS) (PNS) Motor Sensory output input Brain Spinal cord Motor division Sensory division (efferent) (afferent) Autonomic nervous system Somatic nervous system (involuntary; smooth & cardiac muscle) (voluntary; skeletal muscle) Sympathetic division Parasympathetic division Peripheral Nervous System Motor Units: Motor Unit: A single motor neuron and all the muscle fibers innervated by it (motor unit = all-or-none) Motor unit size dictates control: Fine Control / Rapid Reaction: 1-10 fibers / MU (e.g., ocular muscles) Gross Control / Slow Reaction: 1000’s fibers / MU (e.g., quadriceps) Recruitment: Addition of motor units to produce smooth, steady muscle tension (multiple fiber summation) Motoneuron Pool: Set of motor neurons innervating Small large motor units activated… muscle fibers within the same muscle • Varying thresholds Motor units overlap; provides coordination Marieb & Hoehn – Figure 9.13 1 Peripheral Nervous System 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) 2) Gamma () motor neurons: • Give rise to small Type A gamma (Aγ) motor nerve fibers (~ 5 µm diameter) • Innervate intrafusal muscle fibers (small, specialized fibers – muscle spindle) 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 • Requires specialized receptors: A) Muscle spindle – Detect muscle length B) Golgi tendon organ – Detect tendon (muscle) tension Guyton & Hall – Figure 54.2 Peripheral Nervous System Muscle Spindle – Anatomy: Sensory Innervation: Primary Ending: Large sensory fiber (Ia) encircling central portion of intrafusal fibers Secondary Ending: Smaller sensory fiber(s) (II) encircling / branched along intrafusal fiber • 3 – 12 intrafusal muscle fibers enclosed in connective tissue capsule • Central regions lacking actin / myosin (non-contractile); serve as sensor regions • Contractile ends; innervated by Aγ motor fibers 2 Costanzo – Figure 3.29 Peripheral Nervous System Muscle Spindle – Anatomy: Nuclear Nuclear chain bag 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 • 1 – 3 fibers / muscle spindle • Innervated by type Ia and type II afferent • Innervated by type Ia afferent fibers fibers (primary / secondary endings) (primary endings) Costanzo – Figure 3.29 Peripheral Nervous System Muscle Spindle – Physiology: 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 • 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) 3 Peripheral Nervous System Muscle Spindle – Physiology: Reflex • 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 A (Type Ia) Type II: Delayed signals; Relay information Guyton & Hall – Figure 54.4 Peripheral Nervous System Reflex: Reflex: Rapid, automatic response to a specific stimuli Reflex Arc: Step 2: Sensory neuron activation Step 1: Receptor activation Step 3: Information Step 4: processing Motor neuron Step 5: activation Effector activation 4 Costanzo – Figure 3.30 Peripheral Nervous System Spinal Cord Reflexes: Limited delay between 1) Stretch reflex sensory input and motor output (20 – 40 msec) # of synapses Stimulus for Sensory afferent Response in reflex arc reflex fibers of muscles 1 Muscle stretch Ia Muscle contraction Costanzo – Figure 3.31 Peripheral Nervous System Spinal Cord Reflexes: 2) Golgi tendon reflex Interneurons # of synapses Stimulus for Sensory afferent Response in reflex arc reflex fibers of muscle(s) 2 Muscle contraction Ib Muscle relaxation 5 Costanzo – Figure 3.32 Peripheral Nervous System Spinal Cord Reflexes: Afterdischarge: 3) Flexor-Withdrawal reflex Persistent neural discharge occurring in polysynaptic reflex circuits Interneurons # of synapses Stimulus for Sensory afferent Response in reflex arc reflex fibers of muscle(s) Many Pain; temperature II, III, and IV Flexion (ipsilateral) Extension (contralateral) Peripheral Nervous System Muscle Spindle – Physiology: Reflex • 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 A (Type Ia) Why don’t we inhibit stretch Type II: reflexes when we voluntarily Delayed signals; Relay information activate our muscles? Answer: Gamma system Guyton & Hall – Figure 54.4 6 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 7 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 8 .
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