Chapter 13

The Peripheral Nervous System and Reflex Activity

J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

Overview of PNS• Modality

each input is a specific type of sensation: temperature, pain, pressure, touch, body position, equilibrium, hearing, vision, smell, taste

individual sensory neurons generally carry only one modality

• Selectivity of Receptors sensory neurons respond strongly to one type of

stimulus and weakly or not at all to other types some respond accidentally to other types of

stimuli – rubbing one’s eyes mechanically stimulates the eyes’ light receptors in the retina

Components of Sensation• stimulation - stimulus or change in the

environment• transduction

requires a sensory receptor cell or organ which responds to specific stimuli and converts them into

receptor/generator potentials• impulse generation and conduction

if a receptor/generator potential (graded potential) reaches a threshold, then

the neuron’s action potential will be sent to the CNS

• integration some region of the CNS must receive and translate

nerve impulses into sensations and perceptions this generally occurs in the cerebral cortex

Sensory Receptor Classification1. Classified by the type of stimulus

MechanoreceptorsMechanoreceptors mechanical pressure or stretching generate action potentials when deformed

ThermoreceptorsThermoreceptors - changes in temperature NociceptorsNociceptors - pain due to physical or chemical

damage to nearby tissue PhotoreceptorsPhotoreceptors

light strikes retinal receptor cells generates action potentials in response to light

energy ChemoreceptorsChemoreceptors – certain specific chemical

molecules are detected in the mucous fluids of the GI & respiratory tracts, or in the blood or other body fluids

Sensory Receptor Classification2. Classified by location

ExteroceptorsExteroceptors located at or near the body’s surface provide information about the external environment

InteroceptorsInteroceptors (visceroceptors)visceroceptors) found in blood vessels, connective tissues, and

organs provide information about the internal environment

ProprioceptorsProprioceptors located in muscles, tendons, joints and the internal

ear provide information about gravity, body and limb

positions and skeletal muscle movements

Adaptation by Sensory Receptors• a change in sensitivity to a long-lasting stimulus

• primarily by rapidly-adapting phasicphasic receptors pressure, touch, hearing, smell adapt very quickly, i.e., respond less if the

stimulus remains constant allows us to shut out background “noise”

• little adaptation by slowly-adapting tonictonic receptors pain, body position, chemicals in the blood or CSF adapt slowly, continue to respond even when the

stimulus remains constant continuous input is useful for some modalities

because the body needs to make continuous responses to that kind of information

Anesthesia• a partial or complete loss of sensation• General anesthesiaGeneral anesthesia – gas agents act in the CNS

through poorly understood mechanisms• Local anesthesiaLocal anesthesia – drugs injected near peripheral

nerves inhibit the opening of gated sodium channels, preventing local transmission of action potentials

• AnalgesiaAnalgesia – reduced perception of pain without loss of other sensory information or loss of consciousness: biochemical interference with local stimulus (NSAIDs) or mimic endogenous endorphins in CNS (opiates)

• ParesthesiasParesthesias – abnormal sensations (burning, tingling, numbness) not related to normal stimulation, e.g., mechanical pressure on nerves in your leg puts your foot “to sleep”

Functional Types of Nerves• mixed nerves – contain

both sensory and motor fibers

• motor (efferent) nerves [Note: so-called pure

“motor” nerves do also carry proprioceptive sensory signals back to the CNS from the skeletal muscles, joints, and tendons being served by that nerve.]

• sensory (afferent) nerves

Anatomy of Nerves• bundles/fascicles of

axons & dendrites endoneurium – around

individual processes

perineurium around fascicles individual nerve fibers

with their endoneurium

epineurium - outermost covering around entire peripheral nerve

Cranial Nerves• Twelve pairs of nerves which originate from the brain

and exit through foramina of the skull

• First 2 pairs originate from the forebrain (olfactory, optic)

• Remaining 10 pairs originate from the brain stem

What You Should KnowAbout the Cranial Nerves

•name and number

•general region(s) served

•main functional roles

•modality (sensory, motor*, mixed (m/s))(* motor nerves carry proprioception sensory

information back to the CNS)

•See Table 13.2 pp. 501- 507 and slides after end slide in this PPT for details

Spinal Nerves• 31 pairs of spinal nerves

originate from the spinal cord All are mixed (m/s) nerves

Thousands of fibers per spinal nerve

Each pair serves a particular region of the body

Each pair also provides some service to the region supplied by the spinal nerve above it and the spinal nerve below it (redundancy)

Spinal Nerve Anatomy• Spinal nerves are

very short, they divide almost immediately dorsal ramus -

supplies posterior body trunk

ventral ramus - supplies the rest of body trunk and the limbs

meningeal branch - supplies the meninges and blood vessels within meninges

Dorsal and Ventral Rami of a Typical Spinal Nerve

Dermatomes• Areas of skin

innervated by the cutaneous branches of each pair of spinal nerves

• Each pair also provides some service to the region of the spinal nerve above and the spinal nerve below (redundancy)

Reflex Activity• a reflex is a rapid, predictable, automatic response

to a stimulus

• a reflex is unlearned, unpremeditated, and involuntary

• one is conscious of somatic reflexes only after they occur

• reflexes are involved in homeostasis

• two fundamental types of reflexes somatic reflexes - produce contraction of skeletal

muscle autonomic (visceral) reflexes

generally, they are not perceived consciously produce responses by smooth muscle, cardiac muscle,

glands to adjust conditions of the internal environment

Components of a Reflex Arc

• 5 Functional Components1) receptor - dendrites or other sensory structures respond to

changes in the environment2) sensory neuron - conducts an impulse from a receptor to

its axon terminals3) integrating center (some region within the CNS)

simple - monosynaptic (2 cells only: sensory and motor neurons)

complex – polysynaptic (> 2 cells: interneurons involved)4) motor neuron - impulses from integrating center to an

effector5) effector - body part (muscle or gland) which responds to

the motor nerve impulse

Stretch Reflexes• receptors - muscle spindles

and Golgi tendon organs sensory

mechanoreceptors which respond to stretching

increased tension (stretching) stimulates the receptors

sends proprioceptive inputs to spinal cord

• contraction of the skeletal muscle reduces tension on the muscle spindle lowers the rate of action

potential generation decreases input to the

spinal cord and higher centers: cortex and cerebellum

Stretch Reflexes• Remember that

if a muscle is being stretched, the stretch is caused by the contraction of its antagonist.

• This sensory proprioception information contributes to maintaining proper muscle tone.

Patellar Reflex

• monosynaptic

• ipsilateral (same side)

• segmental (at one level of the spinal cord)

• polysynaptic component – for reciprocal inhibition of the antagonist

Golgi (Deep) Tendon Reflex• an increase in muscle tension

activates receptors (Golgi tendon organ) in the tendon

• the muscle relaxes and lengthens in response to its antagonist’s contraction D-T reflex inhibits the

agonist D-T reflex excites the

antagonist

• helps to regulate a smooth start and stop for a contraction

• input from the Golgi tendon organs are sent to the cerebellum and the cortex

• polysynaptic, ipsilateral, and segmental

Flexor Reflex• a pull on the limb,

extending it, will trigger the reflex

• also a painful stimulus – a burn, pin prick, toe stub, etc.

• F-R causes an automatic withdrawal from the (dangerous) stimulus

• polysynaptic, ipsilateral, and segmental

Crossed Extensor Reflex• flexion of a body part is often balanced by extension of

the same body part on the opposite side of the body

• polysynaptic

• contralateral

• segmental

End Chapter 13[Note: Summary slides for the cranial nerves appear after this slide for your exam 4 review.]

Cranial Nerve I: Olfactory• Passes through the cribriform

plate of the ethmoid bone

• Fibers run through the olfactory bulb and terminate in the primary olfactory cortex

• Functions solely by carrying afferent impulses for the sense of smell

Cranial Nerve II: Optic• Arises from the retina of the

eye

• Optic nerves pass through the optic canals and converge at the optic chiasm

• They continue to the thalamus where they synapse

• From there, the optic radiation fibers run to the visual cortex

• Functions solely by carrying afferent impulses for vision

Cranial Nerve III: Oculomotor• Fibers extend from the

ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic eye muscles

• Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape

• Parasympathetic cell bodies are in the ciliary ganglia

• Proprioceptor afferents from extrinsic eye muscles

Cranial Nerve IV: Trochlear• Fibers emerge from

the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle

• Primarily a motor nerve that directs the eyeball

• Proprioceptor afferents from extrinsic eye muscles

Cranial Nerve V: Trigeminal• Composed of three

divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3)

• Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3)

• Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

Cranial Nerve VI: Abdcuens• Fibers leave the inferior pons and enter the

orbit via the superior orbital fissure

• Primarily a motor nerve innervating the lateral rectus muscle

Cranial Nerve VII: Facial• Fibers leave the pons, travel

through the internal acoustic meatus, and emerge through the stylomastoid foramen to the lateral aspect of the face

• Mixed nerve with five major branches

• Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands

• Sensory function is taste from the anterior two-thirds of the tongue

Cranial Nerve VIII: Vestibulocochlear• Fibers arise from the

hearing and equilibrium apparatus of the inner ear, pass through the internal acoustic meatus, and enter the brainstem at the pons-medulla border

• Two divisions – cochlear (hearing) and vestibular (balance)

• Functions are solely sensory – equilibrium and hearing

Cranial Nerve IX: Glossopharyngeal• Fibers emerge from the

medulla, leave the skull via the jugular foramen, and run to the throat

• Nerve IX is a mixed nerve with motor and sensory functions

• Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland

• Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx

Cranial Nerve X: Vagus• The only cranial nerve

that extends beyond the head and neck

• Fibers emerge from the medulla via the jugular foramen

• The Vagus is a mixed nerve

• Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs

• Its sensory function is in taste

Cranial Nerve XI: Accessory• Formed from a cranial root

emerging from the medulla and a spinal root arising from the superior region of the spinal cord

• The spinal root passes upward into the cranium via the foramen magnum

• The accessory nerve leaves the cranium via the jugular foramen

• Primarily a motor nerve Supplies fibers to the

larynx, pharynx, and soft palate

Innervates the trapezius and sternocleidomastoid, which move the head and neck

Cranial Nerve XII: Hypoglossal

• Fibers arise from the medulla and exit the skull via the hypoglossal canal

• Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech

End Chapter 13

Cranial Nerve Slides