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Home , Central nervous system, Motor nerve, Motor neuron, Negative feedback, Nervous system, Reflex arc

Peripheral Nervous

Involuntary (); Organization of : voluntary actions (higher centers)

Nervous system

Integration Peripheral nervous system (CNS) (PNS) Motor Sensory output input

Brain Spinal cord Motor division Sensory division (efferent) (afferent)

Autonomic nervous system nervous system (involuntary; smooth & ) (voluntary; )

Sympathetic division Parasympathetic division

Peripheral Nervous System

Motor Units: : A single motor 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: of motor units to produce smooth, steady muscle tension (multiple fiber summation) Motoneuron Pool: Set of motor 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 fibers (~ 14 µm diameter) • Innervate extrafusal skeletal muscle fibers (generate force) 2) Gamma () motor neurons: • Give rise to small Type A gamma (Aγ) fibers (~ 5 µm diameter) • Innervate intrafusal muscle fibers (small, specialized fibers – )

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 of sensory information from each muscle • Requires specialized receptors: A) Muscle spindle – Detect muscle length B) Golgi – Detect tendon (muscle) tension

Guyton & Hall – Figure 54.2 Peripheral Nervous System

Muscle Spindle – :

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 capsule • Central regions lacking / 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 – :

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

• 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:

• Muscle spindles function as length comparators (intrafusal vs. extrafusal length) • Designed to oppose change in intrafusal length ( 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 :

Step 2: activation Step 1: 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) sensory input and motor output (20 – 40 msec)

# of for Sensory afferent Response in reflex arc reflex fibers of muscles

1 Muscle stretch Ia

Costanzo – Figure 3.31 Peripheral Nervous System

Spinal Cord Reflexes: 2)

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- 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 ; 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 :

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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