CHAPTER 14 - Peripheral Nervous System - Part 1

What does the Peripheral Nervous System (PNS) do?

Provides links to and from the external environment and the body.

What structures make up the PNS?

All neural structures outside the brain and spinal cord.

What are the four main parts of the PNS?

Sensory receptors, nerves & nerve repair, motor endings & motor activity, and reflex activity.

What are sensory receptors?

Specialized structures that respond to changes in the environment (stimuli).

What does activation of sensory receptors cause?

Graded potentials that can trigger nerve impulses.

Where do sensation and perception occur?

In the brain.

What are the three ways receptors are classified?

By stimulus type, location, and structural complexity.

What do mechanoreceptors respond to?

Touch, pressure, vibration, and stretch.

What do thermoreceptors respond to?

Temperature changes.

What do photoreceptors respond to?

Light energy (e.g., in the retina).

What do chemoreceptors respond to?

Chemicals such as smell, taste, and blood chemistry.

What do nociceptors respond to?

Painful stimuli such as extreme heat/cold, pressure, or chemicals.

What determines classification by location?

The location where the stimulus originates, not where the receptor sits.

What are exteroceptors?

Receptors responding to stimuli outside the body.

Where are exteroceptors located?

Skin and special sense organs.

What sensations do exteroceptors detect?

Touch, pressure, pain, temperature, and special senses.

What are interoceptors (visceroceptors)?

Receptors responding to stimuli within internal organs and blood vessels.

What stimuli do interoceptors detect?

Chemical changes, tissue stretch, and temperature changes.

Are we usually aware of interoceptor activity?

No, they often operate without conscious awareness.

What are proprioceptors?

Receptors responding to stretch in muscles, tendons, joints, and ligaments.

What do proprioceptors inform the brain of?

Body position and movement.

What are the two structural categories of sensory receptors?

Simple general sense receptors and special sense receptors.

What are the simple receptors of the general senses?

Modified dendritic endings of sensory neurons.

Where are simple general sense receptors found?

Throughout the body.

What sensations do simple general sense receptors detect?

Touch, pressure, stretch, vibration, temperature, pain, and muscle sense.

Do receptors usually respond to only one stimulus?

No, receptors can respond to multiple types.

What are the two structural types of simple general receptors?

Nonencapsulated (free) nerve endings and encapsulated nerve endings.

Where are nonencapsulated nerve endings found?

Epithelia and connective tissue.

What do nonencapsulated nerve endings respond to?

Temperature, pain, and light touch.

What temperature activates cold receptors?

10–40°C.

What temperature activates heat receptors?

32–48°C.

What happens outside these temperature ranges?

Nociceptors activate and signal pain.

What triggers nociceptors?

Extreme temperature, pinch, or chemicals from damaged tissue.

What is the vanilloid receptor?

A protein in nociceptor membranes is important in pain detection.

What do tactile (Merkel) discs detect?

Light touch.

Where are Merkel discs located?

Deeper layers of the epidermis.

What do hair follicle receptors detect?

Bending of hairs/light touch.

Where are hair follicle receptors found?

Around hair follicles.

What covers encapsulated nerve endings?

A connective tissue capsule.

What does the capsule do to sensitivity?

Decreases sensitivity.

What do Meissner’s (tactile) corpuscles detect?

Discriminative touch.

Where are Meissner’s corpuscles located?

Just beneath the skin in sensitive, hairless areas (such as fingertips).

What do Pacinian (lamellar) corpuscles detect?

Deep pressure and vibration.

Where are Pacinian corpuscles located?

Deep dermis.

Do Pacinian corpuscles adapt quickly?

Yes, they turn off shortly after the stimulus begins.

What do Ruffini endings detect?

Deep and continuous pressure.

Where are Ruffini endings located?

Dermis.

What are the main proprioceptors?

Muscle spindles, tendon organs, and joint kinesthetic receptors.

What produces discriminatory touch sensation?

The combination of free and encapsulated touch receptors at different skin depths.

Nonencapsulated sensory receptors are located where?

Epithelia and connective tissues

Nonencapsulated receptors respond mostly to what?

Pain, temperature, light touch, hair movement

Nonencapsulated free nerve endings detect what types of stimuli?

Temperature, pain, pressure, itch

Thermoreceptors respond to what changes?

Temperature changes

Cold receptors are located where?

In the superficial dermis

Heat receptors are located where?

Deeper in the dermis

What activates nociceptors?

Extreme temperature, pinch, chemicals from damaged tissue

What triggers nociceptors?

Extreme temperature changes, pinch, or chemicals released from damaged tissue

What is the role of nociceptors?

Signal pain by releasing neurotransmitters

Tactile discs are also called what?

Merkel discs

Where are tactile discs found?

Deep in the epidermis

What do tactile discs detect?

Light touch

Where are hair follicle receptors wrapped?

Around hair follicles

Hair follicle receptors respond to what?

Light touch and hair bending

Encapsulated receptors are surrounded by what?

Connective tissue capsule

Encapsulated receptors detect what?

Pressure, stretch, discriminative touch, proprioception

Tactile corpuscles are also called what?

Meissner’s corpuscles

Where are tactile corpuscles located?

Dermal papillae (especially hairless skin)

What do tactile corpuscles detect?

Light touch and texture

Lamellar corpuscles are also called what?

Pacinian corpuscles

Where are lamellar corpuscles located?

Deep dermis and hypodermis

What type of receptors are lamellar corpuscles?

Mechanoreceptors

What do lamellar corpuscles detect?

Deep pressure and vibration

Why are lamellar corpuscles rapidly adapting?

They respond only to the beginning or end of pressure

Bulbous corpuscles are also called what?

Ruffini endings

Where are bulbous corpuscles located?

Dermis, hypodermis, joint capsules

What do bulbous corpuscles detect?

Deep continuous pressure and stretch

Muscle spindles are found where?

Skeletal muscles

Muscle spindles detect what?

Muscle stretch and length

Muscle spindle activation results in what?

Reflexes that resist muscle stretch

Tendon organs are found where?

Tendons

What do tendon organs detect?

Tendon stretch and tension

What reflex do tendon organs help regulate?

Relaxation of overly contracted muscles

Joint kinesthetic receptors are found where?

Joint capsules of synovial joints

What do joint kinesthetic receptors detect?

Joint position and movement

What receptors contribute to proprioception?

Muscle spindles, tendon organs, joint receptors

What are the three levels of sensory integration?

Receptor level, circuit level, perceptual level

At the receptor level, what must occur first?

A stimulus must excite a receptor

What two conditions must a stimulus meet to activate a receptor?

Match the receptor specificity and be applied within the receptive field

What is the receptive field?

The area in which a sensory neuron is activated

What does smaller receptive field size allow?

Greater discriminatory ability and more precise localization

What is transduction?

Conversion of stimulus energy into a graded potential

What is a generator potential?

Graded potential in unipolar sensory neurons that can trigger an action potential

What is a receptor potential?

Graded potential in separate receptor cells that changes neurotransmitter release

What determines whether a generator potential produces an action potential?

Whether it reaches threshold at the axon

What does adaptation refer to?

A change in receptor sensitivity or action potential frequency with a constant stimulus

Phasic receptors are best suited for detecting what?

Changes in stimulus (fast adapting)

Tonic receptors provide what type of information?

Sustained response without much adaptation

At the circuit level, where do first-order sensory neurons synapse?

In the spinal cord or medulla

Second-order neurons transmit signals where?

To the thalamus or cerebellum

Third-order neurons transmit signals to where?

The somatosensory cortex