The Peripheral Nervous System Key Terms

**capsaicin**

molecule that activates nociceptors by interacting with a temperature-sensitive ion channel and is the basis for “hot” sensations in spicy food

**chemoreceptor**

sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain

**encapsulated ending**

configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue

**exteroceptor**

sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin

**free nerve ending**

configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli

**general sense**

any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin

**interoceptor**

sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels

**kinesthesia**

general sensory perception of movement of the body

**mechanoreceptor**

detect stimuli such as touch, pressure, vibration, and sound from the external and internal environments

**nociceptor**

receptor cell that senses pain stimuli

**osmoreceptor**

receptor cell that senses differences in the concentrations of bodily fluids on the basis of osmotic pressure

**photoreceptor**

receptor cell specialized to respond to light stimuli

**proprioception**

general sensory perceptions providing information about location and movement of body parts; the “sense of the self

**proprioceptor**

receptor cell that senses changes in the position and kinesthetic aspects of the body

**receptor cell**

cell that transduces environmental stimuli into neural signals

**sensory modality**

a particular system for interpreting and perceiving environmental stimuli by the nervous system. what is perceived after a stimulus.

**sensory transduction**

the translation of the sensory stimulus into neuronal activity

**somatosensation**

general senses related to the body, usually thought of as the senses of touch, which would include pain, temperature, and proprioception

**special sense**

any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance

**submodality**

specific sense within a broader major sense such as sweet as a part of the sense of taste, or color as a part of vision

**thermoreceptor**

sensory receptor specialized for temperature stimuli

**transduction**

process of changing an environmental stimulus into the electrochemical signals of the nervous system

**visceral sense**

sense associated with the internal organs

**cranial nerve ganglion**

sensory ganglion of cranial nerves

**cranial nerve**

one of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck

**dorsal (posterior) root ganglion**

sensory ganglion attached to the posterior nerve root of a spinal nerve

**endoneurium**

innermost layer of connective tissue that surrounds individual axons within a nerve

**enteric plexus**

neuronal plexus in the wall of the intestines, which is part of the enteric nervous system

**epineurium**

outermost layer of connective tissue that surrounds an entire nerve

**esophageal plexus**

neuronal plexus in the wall of the esophagus that is part of the enteric nervous system

**fascicle**

small bundles of nerve or muscle fibers enclosed by connective tissue

**gastric plexuses**

neuronal networks in the wall of the stomach that are part of the enteric nervous system

**paravertebral ganglia**

autonomic ganglia superior to the sympathetic chain ganglia

**perineurium**

layer of connective tissue surrounding fascicles within a nerve

**plexus**

network of nerves or nervous tissue

**prevertebral ganglia**

autonomic ganglia that are anterior to the vertebral column and functionally related to the sympathetic chain ganglia

**spinal nerve**

one of 31 nerves connected to the spinal cord

**sympathetic chain ganglia**

autonomic ganglia in a chain along the anterolateral aspect of the vertebral column that are responsible for contributing to homeostatic mechanisms of the autonomic nervous system

**trigeminal ganglion**

sensory ganglion that contributes sensory fibers to the trigeminal nerve

**brachial plexus**

nerve plexus associated with the lower cervical spinal nerves and first thoracic spinal nerve

**extraocular muscles**

six skeletal muscles that control eye movement within the orbit

**facial nerve**

seventh cranial nerve; responsible for contraction of the facial muscles and for part of the sense of taste, as well as causing saliva production

**femoral nerve**

systemic nerve of the anterior leg that arises from the lumbar plexus

**glossopharyngeal nerve**

ninth cranial nerve; responsible for contraction of muscles in the tongue and throat and for part of the sense of taste, as well as causing saliva production

**hypoglossal nerve**

twelfth cranial nerve; responsible for contraction of muscles of the tongue

**intercostal nerve**

systemic nerve in the thoracic cavity that is found between two ribs

**lumbar plexus**

nerve plexus associated with the lumbar spinal nerves

**median nerve**

systemic nerve of the arm, located between the ulnar and radial nerves

**nerve plexus**

proximal segments of peripheral nerves emerging from the foramina of the spine with interchanging and intertwining nerves from different spinal levels that then form individual nerves more distally

**olfactory nerve**

first cranial nerve; responsible for the sense of smell

**optic nerve**

second cranial nerve; responsible for visual sensation

**oculomotor nerve**

third cranial ~~nerve~~; allows movement of the eye muscles

**phrenic nerve**

systemic nerve from the cervical plexus that innervates the diaphragm

**radial nerve**

systemic nerve of the arm, the distal component of which is located near the radial bone

**sacral plexus**

nerve plexus associated with the lower lumbar and sacral spinal nerves

**saphenous nerve**

systemic nerve of the lower anterior leg that is a branch from the femoral nerve

**sciatic nerve**

systemic nerve from the sacral plexus that is a combination of the tibial and fibular nerves and extends across the hip joint and gluteal region into the upper posterior leg

**sciatica**

painful condition resulting from inflammation or compression of the sciatic nerve or any of the spinal nerves that contribute to it

**spinal accessory nerve**

eleventh cranial nerve; responsible for contraction of neck muscles

**systemic nerve**

nerve in the periphery distal to a nerve plexus or spinal nerve

**tibial nerve**

systemic nerve of the posterior leg that begins as part of the sciatic nerve

**trigeminal nerve**

fifth cranial nerve; responsible for cutaneous sensation of the face and contraction of the muscles of mastication

**trochlear nerve**

fourth cranial nerve; responsible for contraction of one of the extraocular muscles

**ulnar nerve**

systemic nerve of the arm located close to the ulna, a bone of the forearm

**vagus nerve**

tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities

**vestibulocochlear nerve**

eighth cranial nerve; responsible for the sensations of hearing and balance

**anterior column**

white matter between the anterior horns of the spinal cord composed of many different groups of axons of both ascending and descending tracts

**anterior horn**

gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the ventral horn

**ascending tract**

central nervous system fibers carrying sensory information from the spinal cord or periphery to the brain

**cauda equina**

bundle of spinal nerve roots that descend from the lower spinal cord below the first lumbar vertebra and lie within the vertebral cavity; has the appearance of a horse’s tail

**descending tract**

central nervous system fibers carrying motor commands from the brain to the spinal cord or periphery

**dorsal (posterior) nerve root**

axons entering the posterior horn of the spinal cord

**lateral column**

white matter of the spinal cord between the posterior horn on one side and the axons from the anterior horn on the same side; composed of many different groups of axons, of both ascending and descending tracts, carrying motor commands to and from the brain

**lateral horn**

region of the spinal cord gray matter in the thoracic, upper lumbar, and sacral regions that is the central component of the sympathetic division of the autonomic nervous system

**posterior columns**

white matter of the spinal cord that lies between the posterior horns of the gray matter, sometimes referred to as the dorsal column; composed of axons of ascending tracts that carry sensory information up to the brain

**posterior horn**

gray matter region of the spinal cord in which sensory input arrives, sometimes referred to as the dorsal horn

**posterolateral sulcus**

feature of the posterior spinal cord marking the entry of posterior nerve roots and the separation between the posterior and lateral columns of the white matter

**ventral (anterior) nerve root**

axons emerging from the anterior or lateral horns of the spinal cord

**corneal reflex**

protective response to stimulation of the cornea causing contraction of the orbicularis oculi muscle resulting in blinking of the eye

**crossed extensor reflex**

an innate, polysynaptic, intersegmental reflex arc; in association with withdrawal reflex, activation of contralateral limb muscles enables the other side of the body to respond, often to **maintain** balance

**flexor withdrawal reflex**

an innate, polysynaptic reflex; nociceptor activation triggers ipsilateral muscle activation to immediately withdraw from painful stimulus

**golgi tendon reflex**

an innate, polysynaptic reflex; tendon stretch causes the muscle pulling the tendon to relax (ipsilateral)

**golgi tendon organ**

located in tendon; the specific sensory receptors responsible for providing information about tendon length or the rate of change of the length; a mechanoreceptor & proprioceptor; involved in tendon reflex

**muscle spindle**

located in muscle; the specific sensory receptors responsible for providing information about muscle length or the rate of change of the length; a mechanoreceptor & proprioceptor; involved in stretch reflex

**integration center**

the site of communication between sensory and motor neurons

**monosynaptic reflex**

a reflex arc in which the sensory neuron synapses directly with the motor neuron (does not involve an interneuron)

**polysynaptic reflex**

a reflex that uses one or more interneuron and involves more than one synapse between the sensory neuron and motor neuron

**reciprocal activation**

when the primary response of a reflex inhibits a muscle; refers to activation of the antagonist muscle which occurs synchronously to the primary inhibition

**reciprocal inhibition**

when the primary response of a reflex activates a muscle; refers to inhibition of the antagonist muscle which occurs synchronously to the primary activation

**reflex**

fast, automatic responses to stimuli that send information over a specific neural pathway

**stretch reflex**

response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length

**Babinski sign**

dorsiflexion of the foot with extension and splaying of the toes in response to the plantar reflex, normally suppressed by corticospinal input

**clasp-knife response**

sign of UMN disease when a patient initially resists passive movement of a muscle but will quickly release to a lower state of resistance

**deep tendon reflex**

another term for stretch reflex, based on the elicitation through deep stimulation of the tendon at the insertion

**fasciculation**

small muscle twitch as a result of spontaneous activity from an LMN

**fibrillation**

in motor responses, a spontaneous muscle action potential that occurs in the absence of neuromuscular input, resulting from LMN lesions

**flaccid paralysis**

loss of voluntary muscle control and muscle tone, as the result of LMN disease

**flaccidity**

presentation of a loss of muscle tone, observed as floppy limbs or a lack of resistance to passive movement

**hemisection**

cut through half of a structure, such as the spinal cord

**hyperflexia**

overly flexed joints