peripheral nervous system (4)

Peripheral nervous system

sensory (afferent) division

motor (efferent) division

somatic (body)

visceral (internal organs)

sensory division

detect stimuli + bring it to CNS

somatic, visceral

somatic sensory

skin + musculoskeletal system

general senses - touch, temp, pain, muscle stretch, chemical concentration

special senses - vision, equilibrium, taste, smell

somatic visceral

abdominopelvic + thoracic cavity organs, blood pressure + organ stretch

motor division somatic

voluntary movements → lower motor neurons triggering contraction when stimulated by upper motor neurons

motor division visceral

autonomic nervous system

homeostasis through involuntary motor functions

sympathetic NS - fight or flight, maintains homeostasis when body engaged in physical work, mediates visceral response to emotions

parasympathetic NS - maintain homeostasis at rest, digest

peripheral nerves

axons of many neurons bound together by CT sheath

innervate structures of the body

mixed nerves - motor + sensory, most common, can be seperated

spinal + cranial

spinal nerves

from spinal cord, innervate head + neck

anterior root - motor neurons entering from anterior horn

posterior root - motor neurons entering from posterior horn, posterior (dorsal root) ganglion

axons connect PNS w/ spinal cord gray matter

fuse to form spinal nerves, 31 pairs

cranial nerves

from brain, innervate head + neck, 12 pairs of cranial nerves

structure

epineruium - CT sheath wrapping around entire nerve

fascicles - bundled axon groups, have blood vessels

perineurium - CT sheath wrapping up fascicles

endoneurium - CT sheath wrapping each axon in a fascicle

functional overview

sensory receptors detect stimuli at sensory receptors

transmitted along sensory neurons via spinal / cranial nerves to sensory neurons of CNS

signals transmitted to cerebral cortex for interpretation + integration

motor response initiated by commands from motor areas of the brain to upper motor neurons

impulses travel to spinal cord + synapse on interneurons then lower motor neurons

lower motor neurons carry impulse to appropriate muscle fibers via spinal / cranial nerves

contraction is triggered

Cranial nerves

roman numeral - attachment site in the brain, name - function

sensory - olfactory (I), optic (II), vestibulocochlear (VIII)

motor - oculomotor (III), trochlear (IV), abductens (VI), accessory (XI), hypoglossal (XII)

mixed - trigeminal (V), facial (VII), glossopharyngeal (IX), vagus (X)

olfactory (I)

originates from unmyelinated axons in cell bodies of olfactory epithelium

penetrate through cribriform plate + ethmoid bone, olfactory bulb

terminates medial temporal bone

smell

optic (II)

originates myelinated axons in posterior eye

both nerves meet forming X - optic chiasmia

terminate primary visual cortex on occipital lobe

vision

vestibulocochlear (VIII)

originates vestibular + cochlear → share epineruium

fuse together when they leave temporal bone

vestibular terminates cerebellum + medulla oblongata, balance + equilibrum

cochlear terminates auditory cochlear areas in medulla oblongata, hearing

oculomotor (III)

originates midbrain + innervates extrinsic eye muscles + smooth eye muscles

moves the eyeball, opening the eye, constricting the pupil, changing lens shape

trochlear (IV)

originates inferior midbrain

innervates superior oblique eye muscle

abductens (VI)

originates in pons

innervates lateral rectus (eyes)

turns eye laterally

accessory (XI)

originates medulla + cervical + spinal cord

cranial destination - innervates larynx, speech

spinal destination - innervates trapezius + sternocleidomastoid, move head + shoulders

hypoglossal (XII)

originates inferior medulla

innervates intrinsic + extrinsic tongue muscles

moves tongue

trigeminal (V)

originates midbrain, pons, sensory receptors around face

trigeminal ganglia → split into ophthalmic, maxillary, mandibular nerves

all terminate in primary somatosensory cortex, mandibular terminates in mastication muscles

ophthalmic - scalp, forehead, eyes, nose

maxillary - skin of middle of face

mandibular - chin + lateral face, motor - masseter + temporalis muscles

facial (VII)

originates medulla, pons, sensory portion of tongue, inner ear, palate, nasal cavity

geniculate ganglion, more

innervates somatosensory areas of cerebral cortex - sensory

innervates facial expression + neck muscles - motor

• split into 5 branches temporal, zygomatic, buccal, mandibular, cranial nerves

taste + somatic sensation, salivatory glands

glossopharyngeal (IX)

motor originate medulla

sensory originate tongue, pharynx, ear, blood vessels of neck

superior + inferior ganglion

innervate tongue + throat,

swallowing + salivation

vagus (X)

motor originate medulla

sensory originate tongue, pharynx, ear skin, blood vessels of neck

superior + inferior ganglion

innervates throat, neck, thoracic + abdominal viscera

taste, mucus membranes of throat, blood O2 concentration

speaking, swallowing

spinal nerves structure

spinal nerve splits - posterior + anterior ramus (mixed nerves)

rami communicates - small branches stemming from anterior ramus, visceral motor of sympathetic nervous system

nerve plexuses - anterior rami of cervical, lumbar, sacral spinal nerves merge forming complex nerve networks

axons of spinal nerves enter different nerve plexuses preventing complete nerve cutoff

Cervical plexuses

deep lateral neck, anterior rami of C1-4

phrenic nerve - motor branch C3-5, drives ventilation (diaphragm)

supraclavicular - sensory to skin on superior chest + shoulder C3+4

transverse cervical - sensory to skin superior chest + shoulder, C2+3

ansa cervicalis - motor to geniohyoid + infrahyoid muscles C1-3

greater auricular - sensory to ear skin, C2+3

lesser occipital - sensory to scalp skin + posterior ear C2

Brachial plexuses

motor + sensory to upper limbs

anterior rami of C5-T1

trunks - 1st structure formed

• superior C5+6, middle C7, inferior C8-T1

cords - anterior + posterior division of trunks

medial cord - anterior division of inferior trunk

lateral cord - anterior division of superior + middle trunk

posterior cord - posterior divisions

*all descend into arm

brachial plexuses nerves

axillary nerve - branch of posterior cord, near axilla, deltiod, teres minor

radial nerve - posterior cord descends + becomes radial nerve, triceps brachii, forearm, thumb, fingers

musculocutaneous nerve - lateral nerve, forearm, biceps brachii

median nerve - merged medial + lateral cords, forearm, sensory innervation to hand

• flexor retinaculum - CT band that median nerve can get trapped under → carpal tunnel

ulnar nerve - medial cord continuation, superficial forearm, funny bone

Thoracic spinal nerves

posterior rami serve deep back muscles

intercostal nerve - each anterior ramus traveling between 2 ribs

1) intercostal branch of anterior ramus of T1 travels in 1st intercostal space innervating intercostal muscles + axilla skin

2) T2-6 serve intercostal muscles + skin of chest wall

3) T7-12 serve intercostal muscles overlaying skin + abdominal walls

Lumbar plexuses

anterior rami L1-4

anterior to vertebrae + embedded in posterior psoas major muscle

iliohypogastric nerve - L1, motor to transversus abdominus + internal oblique, sensory to skin of lateral gluteal region + suprapubic region

Ilioinguinal nerve - L1, motor to transversus abdominus + internal oblique, sensory to medial thigh, scrotum, labia majoria

Genitofemoral nerve - L1-2, motor to cremaster, sensory to anteromedial thigh, scrotum, labia majoria

femoral nerve - L2-4 posterior division, motor to quadriceps femoris, iliopsoas, sartorus, sensory to anterior + medial thigh, medial leg, foot

lateral femoral cutaneous nerve - L2+3, sensory to anterolateral thigh

obturator nerve - L2-4, anterior division, motor to thigh adductors, gracilis, sensory to superiomedial thigh

Sacral plexuses

anterior rami L4-S4, innervate gluteal region

superior gluteal nerve - L4-S1, motor to gluteus medius + minimus, tensor fasciae latae

inferior gluteal nerve - L5-S2, motor to gluteus maximus

pundendal nerve - L4-S3, motor to pelvic floor muscles, external anal sphincter + urethral sphincter, sensory to skin of genitalia

posterior femoral cutaneous nerve - S2-4, sensory to posterior thigh skin

sciatic nerve - S1-3, sensory to hip joint, axons from anterior + posterior divisions travel together + share epineruium

• splits into common fibular + tibial nerve

common fibular nerve - L4-S2, motor to lateral + anterior leg muscle, foot muscles, sensory to knee joint, anterior + distal leg skin, dorsum of foot

• branches into common peroneal nerve

tibial nerve - L4-S3, motor to hamstring + posterior leg muscles, plantar foot muscles, sensory to knee + ankle joint, posterior + lateral leg skin, plantar surface of foot skin

• branches into sural nerve

sensory receptors

sensory transduction - conversion of sensory stimulus to electrical signal

sensory receptor - region of nerve ending

encapsulated nerve endings - surrounded by specialized + supportive cells

free nerve endings - lack of specialized cells

sensory receptors

1) before any stimulus, ion channels in the axolemma of somatic sensory neurons are closed

2) stimulus applied + mechanically gated Na+ channels open, Na+ enters axoplasm → receptor potential generated

3) enough Na+ enter + membrane potential reaches threshold, voltage gated Na+ channels open + action potential is generated down spinal cord

receptor potential

temporary depolarization from Na+ entering axoplasum

rapidly adapting receptors

respond rapidly w/ high intensity but stop sending stimuli after a certain period - adaptation

slowly adapting receptors

respond w/ constant action potentials that don’t diminish w/ time

classification of sensory receptors

location of stimuli - exoreceptors, interoceptors

type of stimuli they depolarize to - mechanoreceptors, thermoreceptor, chemoreceptor, photoreceptor, nociceptor

exoreceptors

close to surface of body, detect stimuli originating outside of body

interoceptors

within body interior, detect stimuli originating inside body

mechanoreceptors

encapsulated exo/ineroceptors in skin + musculoskeletal system

respond to mechanical touch, mechanically gated ion channels

thermoreceptor

exoreceptor responding to thermal stimuli

slowly adapting receptors

small knobs on ends of free nerve endings

cold - 10-40 C, superficial

hot - 32-48 C, deep

painful - 49+ C

chemoreceptor

exo/interoceptor responding to chemicals in air or bodily fluids

smell + taste

photoreceptor

special sensory exoreceptors responding to light, eye

nociceptor

slowly adapting receptors respond to pain or noxious stimuli

mechanoreceptors

superficial → deep

merkel cell fibers, tactile corpuscles, ruffini endings, lamellated corpuscles, hair follicle receptors, proprioceptors

merkel cell fibers

nerve endings surrounded by merkel capsule

slowly adapting receptors, mechanically gated ion channels

in floors of epidermal ridges

discriminative touch stimuli - fine touches

tactile corpuscles

in dermal papillae

projections of dermis into epidermis

discriminative touch stimuli - genral touches

ruffini endings

spindle shaped, dermis + hypodermis + ligaments

slowly adapting receptors

lamellated corpuscles

onion like layers, deep in dermis, vibrations

layers act as filters for what stimuli activates them

rapidly adapting receptors

hair follicle receptors

free nerve endings wrapped around base of hair follicle in dermis + hypodermis

proprioceptors

musculoskeletal system

detect movement + position of joint / body part

bodies sense of its position in space

sensory neuron structure

cell body of neuron in posterior root ganglion (dorsal), lateral to spinal cord

cell body of cranial nerves in cranial nerve ganglia

peripheral process + central process

peripheral process

long axon splits into nerve endings each associated w/ sensory receptor

terminates near neurons cell body

central process

exits cell body + travels through posterior root of spinal cord to enter posterior horn

somatic sensory neuron structure

1) peripheral process transmits action potential from sensory receptor to neurons other axon, the central process

2) central process transmits action potential from peripheral process to posterior horn eventually synapsing on a neuron

Sensory neuron classification

speed which peripheral axons conduct action potentials

large diameter + myelin sheath → fastest

• proprioceptive, discriminative, nondiscriminative stimuli

small diameter + no myelin sheath → slowest

• pain + temp

receptive fields

area served by a particular neuron

dependent on neuron branching + number

more neurons → smaller receptive field

two point discrimination threshold - 2 stimuli paced close together on skin + moved apart until subject can feel 2 distinct points

dermatomes

skin divided into segments based on spinal nerve that supplies the region w/ somatic sensation

dermatome map - clinically test integrity of sensory pathways to different parts of the body

referred pain

pain that originates in organ being perceived as cutaneous pain

usually along dermatome nerve

many spinal nerves carry 1st order somatic + visceral sensory neurons causing referred pain

motor output

upper motor neurons repay messages to lower motor neurons which releases acetylcholine onto muscle fiber initiating muscle contraction

lower motor neurons

multipolar neurons

cell bodies - CNS large myelinated axons - PNS

α-motor neurons - stimulate muscle fibers to contract by excitation-contraction mechanism

γ-motor neurons - innervate muscle fibers called intrafusal fibers (stretch receptors)

reflex arcs

programmed automatic responses to stimuli

protective, prevent tissue damage

sensory stimulus + rapid motor response

golgi tendon organs

mechanoreceptors in tendons monitoring tension created by muscle contractions

golgi tendon organ consists of encapsulated bundle of collagen fibers attached to 20 extrafusal muscle fibers

• contain 1 somatic sensory axon whose endings are wrapped around its enclosed collagen fibers

greater tension generated → greater rapid firing

muscle spindles

tapered structures embedded among extrafusal muscle fibers (regular contractile muscle fibers)

intrafusal muscle fibers - specialized + have contractile filaments made of actin + myosin

innervated by γ-motor neurons (muscles above)

Classes of sensory neurons innervating intrafusal fibers

primary afferent - responds to stretch when first initiated

secondary afferent - responds to sciatic length of muscle + position of limb

mechanically gated ion channels open when intrafusal fibers are stretched

more spindles → finer movement

reflex types

monosynaptic - involve 1 synapse in spinal cord

polysynaptic - multiple synapses

visceral reflexes - innervate internal organs, connected w/ autonomic nervous system

somatic reflexes - somatic sensory + motor neurons

• simple stretch, flexion + crossed extension, golgi tendon, cranial nerve endings

cranial nerve endings

polysynaptic, 2 nerves (one afferent, one efferent)

gag reflex - somatic motor neurons of vagus nerve trigger pharynx muscle contractions

corneal blink reflex - blink when something touches cornea of eye, somatic sensory receptors

simple stretch

upper motor neurons have optimal length for skeletal muscles, when it deviates from that, simple stretch reflex kicks in

monosynaptic reflex, shortens muscle

sensory afferent synapse on interneurons inhibiting antagonist muscles

tapping on certain tendons - patellar reflex, jaw-jerk

1) external force stretches muscle

2) muscle spindles detect stretch, primary + secondary afferents transmit action potential to spinal cord

3) in spinal cord, sensory afferents synapse on α-motor neurons triggering action potential

4) α-motor neurons stimulate muscle to contract + returns to optimal length

flexion withdrawal + crossed extension

involve rapidly conducting nociceptive afferents + multiple synapses in spinal cord - polysynaptic

pulling hand away from hot stove

crossed extension reflex - preserves balance by triggering opposite limb

1) when stimulated, nociceptive afferents transmit painful stimulus to spinal cord

2) α-motor neurons stimulate muscle to flex the limb receiving painful stimulus

3) α-motor neurons stimulate muscles that extend the opposite limb to maintain balance

golgi tendon

polysynaptic causing muscle relaxation to protect muscle

happens when tension increased dramatically

dropping heavy weight when working out

peripheral neuropathies

disorder effecting sensory + motor neurons

sensory neuron disorders

sensory peripheral neuropathy

dependent on which nerve was injured

decreases / eliminates sensation

“shooting” pain from inflammation + inappropriate firing of nociceptors

proprioceptive damage - damaged ligaments + tendons, difficult monitoring + contracting movement

lower neuron disorders

motor peripheral neuropathies

injury to spinal / cranial nerve or lower motor neurons

muscles can’t contract bc damaged α-motor neuron

paralysis + paresis (muscle weakness)

upper motor neuron disorder

can result from damage or disease anywhere along the pathways from motor cortices to spinal cord

spinal shock - initial response, paralysis, wears off in a few days + spasticity develops

Babinski sign - stroking bottom of foot, healthy adult will cause plantar reflex, patient with an upper motor neuron disorder will extend the hallux + splay out the other toes

spasticity

increase in stretch reflexes + muscle tone,

clonus - the alternating contraction + relaxation of a stretched muscle

caused by a loss of normal inhibition mediated by upper motor neurons

Detection and Interpretation of Somatic Sensation by the Nervous System

cf

Control of Movement by the Nervous System

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