Peripheral Peripheral Nervous System

Organization of Nervous System: Involuntary (); Motor Units: voluntary actions (higher centers) : Nervous system A single motor and all the muscle fibers innervated by it (motor unit = all-or-none) Integration Motor unit size dictates control: 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 (involuntary; smooth & ) (voluntary; ) muscle tension (multiple fiber summation) Motoneuron Pool: Set of motor 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: – : 1) Alpha () motor neurons: • Give rise to large Type A alpha (A) motor 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 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 2) Continuous feedback of sensory information from each muscle • 3 – 12 intrafusal muscle fibers enclosed in connective capsule • Requires specialized receptors: • Central regions lacking / (non-contractile); serve as sensor regions A) Muscle spindle – Detect muscle length • Contractile ends; innervated by Aγ motor fibers B) Golgi 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 – : chain bag Nuclear Nuclear chain bag

Muscle spindles emit 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: 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) : • Returns intrafusal fibers to original length by activating extrafusal fibers Step 2: activation Step 1: 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) sensory input and motor 2) output (20 – 40 msec)

Interneurons

# of 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 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- 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 ; 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 (feedback) Control output of cortex / brain stem Basal nuclei • Start / stop movements • Coordinate movements with posture • block unwanted movements

Projection Level Direct () 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

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