Peripheral Nervous System Part I Flashcards
How is the peripheral nervous system essential to our survival?
The PNS provides the links between our brains and the outside world
Consistent sensory inputs and motor outputs are essential to survival
What anatomical structures make up the peripheral nervous system?
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
What are sensory receptors specialized to do?
Sensory Receptors - specialized to respond to changes in their environment (stimuli)
What happens when a sensory receptor is activated? What happens when the generated nerve impulses reach the brain?
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
What are the 5 TYPES of receptors we discussed in class? What kind of stimulus does each type respond to?
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
What is an exteroreceptor? What does it respond to? Where could you find one?
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)
What is an interoreceptor? What does it respond to? Where could you find one?
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
What is a proprioceptor? What does it respond to? Where could you find one?
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
What are the 2 structural classifications for receptors?
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
General sensory receptors are either ___________ or _____________.
Nonencapsulated (Free) Nerve Endings
Encapsulated Nerve Endings
Where can you find nonencapsulated nerve endings? What type of fiber are they made of? What kinds of stimuli do they typically respond to?
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
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?
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
What makes a nerve fiber “encapsulated”?
One or more fiber terminals of sensory neurons enclosed in connective tissue capsules
Almost all encapsulated dendritic endings are ___________________.
mechanoreceptors
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?
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
Anatomically, what is a nerve? What are the 2 types of nerve?
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
List the connective tissue coverings for the different layers of the nerve. What does each layer contain?
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
Most nerves contain a mixture of what 4 things?
Most nerves contain a mixture of afferent + efferent fibers and somatic + autonomic (visceral) fibers
Define what it means to be a mixed nerve, a sensory nerve, or a motor nerve.
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
What happens if damage to a neuron is close to the cell body? What about if the damage is further
from the cell body?
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
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.
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
Define Wallerian Degeneration and review the step-by-step process for nerve regeneration in the PNS on slide 15.
Axon fragments and myelin sheaths distal to the injury degenerate – Wallerian Degeneration
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What is a peripheral motor ending? What are some examples of effectors activated by the PNS?
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!
What are the 3 levels of motor control in the PNS?
Segmental Level
Projection Level
Precommand Level
What types of movements are controlled by the segmental level? What is a CPG?
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
What happens at the projection level of control? Reminder this is your middle manager role!
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
What 2 anatomical areas are responsible for the precommand level of control?
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
List the specific functions of the precommand level of control.
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
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!
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
Draw out a reflex arc. Be sure to include a receptor, sensory neuron, integration center, motor neuron, and effector.
What are the effectors in a somatic reflex? What about in an autonomic reflex?
Somatic Reflexes - activate skeletal muscles
Autonomic (Visceral) Reflexes - activate visceral effectors (smooth/cardiac muscle, glands)
