Introduction to Anatomy and Physiology: Sensation, Motor Output, and Reflexes

A comprehensive set of vocabulary flashcards covering sensory receptors, motor neurons, reflex arcs, and related neurological disorders as presented in Human Anatomy and Physiology Chapter 13.

Sensory Transduction

The process of converting a stimulus into an electrical signal, which begins at a region of a nerve ending called a sensory receptor.

Sensory Receptor

Nerve endings where sensory transduction begins; classified as either Encapsulated Nerve Endings (surrounded by specialized supporting cells) or Free Nerve Endings (lacking supportive cells).

Axolemma

The membrane of a nerve ending that contains many gated ion channels responding to various stimuli to change ion flow.

Rapidly Adapting Receptors

Receptors that respond rapidly with high intensity but stop sending stimuli after a certain period, a phenomenon known as Adaptation.

Slowly Adapting Receptors

Receptors that respond to stimuli with constant action potentials that do not diminish over time.

Exteroceptors

Sensory receptors located close to the surface of the body that detect stimuli originating from outside the body.

Interoceptors

Sensory receptors located within the body’s interior that detect stimuli originating from within the body.

Mechanoreceptor

Encapsulated receptors in the skin, musculoskeletal system, and organs that respond to mechanically deformed tissue using mechanically gated ion channels.

Thermoreceptor

Receptors that respond to temperature changes: Cold Receptors respond between $10^{\circ}\text{C}$ and $40^{\circ}\text{C}$, while Hot Receptors respond between $32^{\circ}\text{C}$ and $48^{\circ}\text{C}$.

Chemoreceptor

Receptors that depolarize in response to specific chemicals in body fluids or air binding to them.

Photoreceptor

Receptors found only in the eye that respond to light stimuli.

Nociceptor

Receptors that detect noxious stimuli which are perceived by the C N S as pain.

Capsaicin

A chemical found in hot peppers that opens specific ligand-gated ion channels in nociceptors; it is used topically to treat neuropathies like shingles.

Tactile (Merkel) Nerve Endings

Nerve endings associated with Merkel cells in epidermal ridges that provide the finest spatial resolution for detecting form and texture.

Tactile (Meissner) Corpuscles

Receptors in dermal papillae that detect discriminative touch, though with less spatial resolution than Merkel nerve endings.

Bulbous Corpuscles (Ruffini Endings)

Receptors located in the dermis, hypodermis, and ligaments that respond to stretch and movement.

Lamellated Corpuscles (Pacinian Corpuscles)

Deep dermal receptors with a layered, onion-like appearance that respond only to high-frequency vibratory stimuli and deep pressure.

Hair Follicle Receptors

Free nerve endings wrapped around the base of a hair follicle that respond to stimuli causing the hair to bend.

Proprioceptors

Receptors in the musculoskeletal system used to detect movement and the body position of a joint or body part.

First-Order Somatic Sensory Neurons

Pseudounipolar neurons with cell bodies in a posterior root ganglion (or cranial nerve ganglia) that conduct sensory information from the P N S to the C N S.

Peripheral Process

The long axon of a sensory neuron that splits into nerve endings with sensory receptors on one end.

Central Process

The part of a sensory neuron axon that exits the cell body and travels through the posterior root to enter the spinal cord or brainstem.

Receptive Fields

The area served by a single neuron; smaller fields result from less branching and more neurons, providing more sensation.

Two-Point Discrimination

A clinical method used to measure the relative size of receptive fields in different body areas.

Dermatomes

Body segments based on the specific spinal nerve that supplies the region with somatic sensation; used to test the integrity of sensory pathways.

Referred Pain

A phenomenon where pain originating in an organ is perceived as cutaneous pain because spinal nerves carry both somatic and visceral neurons.

Upper Motor Neurons

Neurons in the primary motor cortex of the C N S that initiate movement by relaying messages to lower motor neurons.

Lower Motor Neurons

Multipolar neurons with cell bodies in the C N S and axons in the P N S that release acetylcholine onto muscle fibers to initiate contraction.

Alpha-($\alpha$) Motor Neurons

Lower motor neurons that directly stimulate skeletal muscle fibers to contract.

Gamma-($\gamma$) Motor Neurons

Lower motor neurons that innervate specialized stretch receptors known as intrafusal fibers.

Reflex

A programmed, automatic response to selected sensory input, most of which are protective to prevent tissue damage.

Muscle Spindles

Stretch receptors embedded in regular (extrafusal) muscle fibers that contain $2$ to $12$ intrafusal muscle fibers.

Golgi Tendon Organs

Mechanoreceptors within tendons that monitor the tension generated by muscle contractions; they consist of an encapsulated bundle of collagen fibers.

Monosynaptic Reflex

A reflex that involves a single synapse within the spinal cord between a sensory neuron and a motor neuron.

Polysynaptic Reflex

A reflex involving multiple synapses between neurons in the spinal cord.

Simple Stretch Reflex

A monosynaptic reflex, such as the Patellar (Knee-Jerk) reflex, that shortens a muscle to return it to its optimal length after being stretched.

Flexion (Withdrawal) Reflex

A spinal reflex that involves pulling a limb away from a painful stimulus through the action of nociceptive afferents and $\alpha$-motor neurons.

Crossed-Extension Reflex

A reflex occurring simultaneously with the flexion reflex that stimulates muscles to extend the opposite limb to maintain balance.

Golgi Tendon Reflex

A reflex that protects muscles and tendons by causing them to relax when tension increases dramatically.

Gag Reflex

A visceral reflex where the Glossopharyngeal nerve detects a stimulus in the throat and the Vagus nerve triggers pharyngeal muscle contractions.

Corneal Blink Reflex

A somatic reflex where a stimulus to the cornea via the Trigeminal nerve causes the orbicularis oculi muscle to contract.

Peripheral Neuropathies

Disorders affecting P N S sensory and motor neurons, which can lead to paralysis, paresis, or sensory loss.

Spinal Shock

The initial paralytic response to an Upper Motor Neuron disorder, resulting from the removal of input to spinal cord circuits.

Spasticity

A later development in Upper Motor Neuron disorders characterized by increased stretch reflexes, increased muscle tone, and clonus.

Clonus

A phenomenon consisting of alternating contraction and relaxation of muscles associated with spasticity.

Babinski Sign

An abnormal reflex in adults where stroking the foot causes the hallux to extend and other toes to splay; normal only in infants up to $18$ months.

Amyotrophic Lateral Sclerosis (A L S)

Also known as Lou Gehrig’s Disease; a fatal condition involving the degeneration of $\alpha$-motor neurons in the spinal cord and upper motor neurons in the cerebral cortex.