Download Nervous system Central nervous system Peripheral nervous system

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