Download Nervous system Central nervous system Peripheral nervous system

Peripheral Nervous System
Involuntary reflexes (spinal cord);
voluntary actions (higher brain centers)
Organization of Nervous System:
Nervous system
Integration
Central nervous system
Peripheral nervous system
(CNS)
(PNS)
Motor
output
Brain
Spinal cord
Sensory
input
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
muscle fibers within the same muscle
Marieb & Hoehn – Figure 9.13
Small  large motor units activated…
• Varying thresholds
Motor units overlap; provides coordination
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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?
What is the instantaneous tension?
Proper control of muscle function requires:
1) Excitation of muscle by motor neuron
How rapidly is the length / tension changing?
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
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Costanzo – Figure 3.29
Peripheral Nervous System
Muscle Spindle – Anatomy:
Nuclear
chain
Nuclear
bag
Types of Intrafusal Fibers:
2) Nuclear Bag
1) Nuclear Chain
• Small fibers; nuclei arranged in a row
• 3 – 9 fibers / muscle spindle
• Innervated by type Ia and type II afferent
fibers (primary / secondary endings)
• large fibers; nuclei grouped in central region
• 1 – 3 fibers / muscle spindle
• Innervated by type Ia afferent fibers
(primary endings)
Peripheral Nervous System
Costanzo – Figure 3.29
Muscle Spindle – Physiology:
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
• 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)
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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
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 5:
Effector
activation
Step 4:
Motor neuron
activation
Step 3:
Information
processing
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Costanzo – Figure 3.30
Peripheral Nervous System
Limited delay between
sensory input and motor
output (20 – 40 msec)
Spinal Cord Reflexes:
1) Stretch reflex
# of synapses
in reflex arc
Stimulus for
reflex
Sensory afferent
fibers
Response
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
in reflex arc
Stimulus for
reflex
Sensory afferent
fibers
Response
of muscle(s)
2
Muscle contraction
Ib
Muscle relaxation
5
Costanzo – Figure 3.32
Peripheral Nervous System
Spinal Cord Reflexes:
3) Flexor-Withdrawal reflex
Afterdischarge:
Persistent neural discharge
occurring in polysynaptic
reflex circuits
Interneurons
# 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)
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
A
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
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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
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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
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