Peripheral Nervous System Part I

Question 1

How is the peripheral nervous system essential to our survival?

Answer 1

The PNS provides the links between our brains and the outside world

Consistent sensory inputs and motor outputs are essential to survival

Question 2

What anatomical structures make up the peripheral nervous system?

Answer 2

The PNS consists of all the neural structures outside of the brain and spinal cord

Sensory receptors, afferent nerves + efferent nerves and their associated ganglia, and motor endings

Functionally, the PNS can be viewed as 4 parts:

• Sensory Receptors
• Transmission Lines: the nerves
• Motor Ending and Motor Activity
• Reflex Activity

Question 3

What are sensory receptors specialized to do?

Answer 3

Sensory Receptors - specialized to respond to changes in their environment (stimuli)

Question 4

What happens when a sensory receptor is activated? What happens when the generated nerve impulses reach the brain?

Answer 4

Activation of sensory receptors results in graded potentials that trigger nerve impulses

Nerve impulses are sent along the afferent PNS fibers to the CNS

Awareness of the stimulus (sensation) and interpretation of the meaning of the stimulus (perception) occurs in the brain

Question 5

What are the 5 TYPES of receptors we discussed in class? What kind of stimulus does each type respond to?

Answer 5

Mechanoreceptors: respond to mechanical forces such as touch, pressure, vibration, and stretch

Thermoreceptors: respond to changes in temperature

Photoreceptors: respond to light energy

Ex: the retina

Chemoreceptors: response to chemicals in solution

Ex: smell, taste, changes in blood chemistry

Nociceptors: respond to pain-causing stimuli; these stimuli are potentially harmful

Ex: extreme heat, cold, pressure, or inflammatory chemicals

Question 6

What is an exteroreceptor? What does it respond to? Where could you find one?

Answer 6

Exteroceptors:

Respond to stimuli arising outside the body

Exteroceptors are in the body’s surface

Ex: receptors in the skin for touch, pressure, pain, and temperature and most receptors for the special senses (vision, hearing, equilibrium, smell, and taste)

Question 7

What is an interoreceptor? What does it respond to? Where could you find one?

Answer 7

Interoceptors (Visceroceptors):

Respond to internal stimuli such as from the internal viscera and blood vessels

Sensitive to chemical changes, tissue stretch, and temperature changes

Sometimes cause discomfort (hunger, pain, thirst), but usually we are unaware of their workings

Question 8

What is a proprioceptor? What does it respond to? Where could you find one?

Answer 8

Also respond to internal stimuli, but their locations are more restricted

Occur in skeletal muscles, tendons, joints, ligaments, and the connective tissue coverings of the bones and muscles

Respond to changes in stretch and body position -inform the brain of one’s movements

Question 9

What are the 2 structural classifications for receptors?

Answer 9

Simple Receptors of the General Senses:

• Modified dendritic endings of sensory neurons
• Found throughout the body to monitor most types of general sensory information

Receptors of the Special Senses:

• Vision, hearing, equilibrium, smell, and taste
• Housed within complex sense organs

Question 10

General sensory receptors are either ___________ or _____________.

Answer 10

Nonencapsulated (Free) Nerve Endings

Encapsulated Nerve Endings

Question 11

Where can you find nonencapsulated nerve endings? What type of fiber are they made of? What kinds of stimuli do they typically respond to?

Answer 11

Nonencapsulated (Free) Nerve Endings

• Abundant in epithelial and connective tissues
• Most are nonmyelinated, small diameter, group C fibers; distal terminals have knoblike swellings
• Respond mostly to temperature, pain, or light touch

Question 12

Be ready to define: thermoreceptor, nociceptor, itch receptor, tactile (merkel) discs, and hair follicle receptor. Where could you find each of those types? What kind of stimulus are each of those responding to?

Answer 12

Thermoreceptors:

Cold receptors are activated by temps from 50-104°F

Located in the superficial dermis

Average skin temperature is about 93°F

Heat receptors are activated by temps from 89-118°F

Located in the deeper dermis

Outside of these temperature ranges, heat or cold trigger nociceptors and are interpreted as pain

Nociceptors: pain receptors that respond to extreme temperatures, pinch, or the release of chemicals from damaged tissue
\Itch Receptors: located in the dermis, activated by the chemical histamine

Tactile (Merkel) Discs: lie at the junction of the epidermis and dermis; function as light touch receptors

Hair Follicle Receptors: free nerve endings the wrap around hair follicles – act as light touch receptors that detect bending of the hairs

Question 13

What makes a nerve fiber “encapsulated”?

Answer 13

One or more fiber terminals of sensory neurons enclosed in connective tissue capsules

Question 14

Almost all encapsulated dendritic endings are ___________________.

Answer 14

mechanoreceptors

Question 15

Be ready to define: tactile corpuscle, lamellar corpuscle, bulbous corpuscle, muscle spindle, tendon organ, and joint kinesthetic receptor. Where could you find each of those types? What kind of stimulus do each of those respond to?

Answer 15

Tactile (Meissner’s) Corpuscles: small receptors involved in discriminative touch – found just below the epidermis, mostly in sensitive and hairless areas

Lamellar (Pacinian) Corpuscles: large receptors that respond to 1st application of deep pressure and vibration – located deep in the dermis and subcutaneous tissue

Bulbous Corpuscles (Ruffini Endings): a spray of nerve endings, respond to deep and continuous pressure – located in the dermis, subcutaneous tissue, and joint capsules

Muscle Spindles: spindle-shaped proprioceptors that respond to muscle stretch; found in the perimysium ; initiate the stretch reflex

Tendon Organ: proprioceptors located in tendons; detect stretch; initiate the stretch/relax reflex

Joint Kinesthetic Receptors: proprioceptors that monitor stretch in the articular capsules; provide information on joint position and motion

Question 16

Anatomically, what is a nerve? What are the 2 types of nerve?

Answer 16

Nerve: cordlike organ of the PNS

A bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue

There are 2 types of nerves – cranial and spinal – depending on where they originate

Question 17

List the connective tissue coverings for the different layers of the nerve. What does each layer contain?

Answer 17

Endoneurium: loose connective tissue that encloses individual axons + their myelin sheaths

Perineurium: coarse connective tissue that bundles fibers into fascicles

Epineurium: tough, fibrous sheath that encloses all fascicles and blood + lymphatic vessels to form a nerve

Question 18

Most nerves contain a mixture of what 4 things?

Answer 18

Most nerves contain a mixture of afferent + efferent fibers and somatic + autonomic (visceral) fibers

Question 19

Define what it means to be a mixed nerve, a sensory nerve, or a motor nerve.

Answer 19

Mixed Nerves: contain both sensory and motor fibers – impulses travel both to and from the CNS

Sensory (Afferent) Nerves: impulses only move toward the CNS

Motor (Efferent) Nerves: impulses only move away from the CNS

Question 20

What happens if damage to a neuron is close to the cell body? What about if the damage is further
from the cell body?

Answer 20

If damage to a neuron is severe or close to the cell body, the whole neuron may die – other neurons typically stimulated by its axons may also die

Question 21

Compare/contrast nerve regeneration in the central and peripheral nervous systems. Which system has more regeneration capacity? Be sure to understand the respective roles played by oligodendrocytes and Schwann cells.

Answer 21

Axons of the CNS

• Most axons of the CNS never regenerate following injury
• Oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration
• At the point of injury, astrocytes form scar tissue

Axons of the PNS

• If damage is not severe, axons of the PNS can slowly regenerate (1.5mm/day)
• Cell body swells as protein synthesis revs up
• Axon fragments and myelin sheaths distal to the injury degenerate – Wallerian Degeneration
• Macrophages clean dead axon debris; Schwann cells are stimulated to divide
• Axon filaments grow through regeneration tube
• Axon regenerates and a new myelin sheath forms

Question 22

Define Wallerian Degeneration and review the step-by-step process for nerve regeneration in the PNS on slide 15.

Answer 22

Axon fragments and myelin sheaths distal to the injury degenerate – Wallerian Degeneration

…

Question 23

What is a peripheral motor ending? What are some examples of effectors activated by the PNS?

Answer 23

Peripheral Motor Endings - PNS elements that activate effectors by releasing neurotransmitters

These elements innervate skeletal muscles, visceral muscles, and glands

Recall that the terminals of somatic motor fibers innervate voluntary skeletal muscles by forming neuromuscular junctions!

Question 24

What are the 3 levels of motor control in the PNS?

Answer 24

Segmental Level
Projection Level
Precommand Level

Question 25

What types of movements are controlled by the segmental level? What is a CPG?

Answer 25

Segmental level is the lowest level of motor hierarchy – consists of reflexes and automatic movements

Segmental circuits activate networks of ventral horn neurons to stimulate specific groups of muscles

Central Pattern Generators (CPGs): circuits that control locomotion and specific, often repeated motor activities

Question 26

What happens at the projection level of control? Reminder this is your middle manager role!

Answer 26

Projection level consists of neurons acting through direct and indirect motor pathways

Upper motor neurons of the motor cortex initiate direct (pyramidal) pathways to produce voluntary skeletal muscle movements

Brain stem motor nuclei oversee indirect (extrapyramidal) pathways to control and modify reflex and CPG-controlled motor areas

Projection motor pathways send information to lower motor neurons and keep the higher command levels informed

Question 27

What 2 anatomical areas are responsible for the precommand level of control?

Answer 27

The cerebellum acts on motor pathways via the projection areas of the brain stem and on the motor cortex via the thalamus

The basal nuclei receive sensory inputs all cortical areas and send their outputs back via the thalamus

Question 28

List the specific functions of the precommand level of control.

Answer 28

Regulating motor activity

Precisely start and stop movements

Coordinating movements with posture

Blocking unwanted movements

Monitoring muscle tone

Performing unconscious planning and discharge in advance of willed movements

Question 29

What the difference between an inborn (intrinsic) reflex and a learned reflex? Where are inborn reflexes regulated? Can inborn reflexes be modified? If so, give an example!

Answer 29

Inborn (Intrinsic) Reflex: rapid, involuntary, predictable motor response to a stimulus

Regulated by the subconscious lower regions of the CNS – the brain stem and spinal cord

Ex: maintain posture, control visceral activities

Inborn reflexes are unlearned, but they can be modified by learning and conscious effort

Learned (Acquired) Reflex: results from practice and repetition

Ex: driving a car

Question 30

Draw out a reflex arc. Be sure to include a receptor, sensory neuron, integration center, motor neuron, and effector.

Question 31

What are the effectors in a somatic reflex? What about in an autonomic reflex?

Answer 31

Somatic Reflexes - activate skeletal muscles

Autonomic (Visceral) Reflexes - activate visceral effectors (smooth/cardiac muscle, glands)