Peripheral Nervous System

Chapter 13

Sensory Receptor Types

• Nociceptors– Respond to excess heat, pressure, or

chemicals– Tissue damage– All parts of the body but brain

• Thermoreceptors– Temperature of skin and blood– Maintains homeostatic control via the

hypothalmus• Photoreceptors

– Light absorbing pigments– Light detection

Sensory Receptor TypesMechanoreceptors• Touch, pressure, & vibrations• Bend or stretch PM of

receptor cell = changing permeability– Stretch receptors – position of

body parts– Hair cells - sound waves and

H2O movements

Chemoreceptors• Chemicals in the internal &

external environment– O2 in arterioles– Osmoreceptors - changes in

[blood solute]– Pheromone detection

Sensory Receptor Locations

• Exteroceptors– Stimuli outside the body– Skin and special sense organs

• Interoceptors– Stimuli within the body– Chemical messengers, tissue stretch, and temperature

• Proprioceptors– Internal stimuli– Monitor position and stretch of joints, tendons, and

muscles

Sensory Receptor StructuresUnencapsulated

• Free nerve endings– Most body tissues– Temperature and painful

stimuli– Capsaicin and itch

• Merkel discs– Deeper epidermal layers– Light touch

• Hair follicle receptors– Shaft of hair follicle– Light touch and hair bending

Encapsulated

• Meissner’s corpuscles– Dermal papillae of sensitive and

hairless skin– Discriminative touch

• Pacinian corpuscles– Deep in the dermis– Deep pressure initially, vibration

• Ruffini endings– Deep dermis and hypodermis– Deep continuous pressure

• Muscle spindles– Perimysium of skeletal muscle– Detect muscle stretch & initiate a reflex

• Golgi tendon– Insertion tendons– Activation inhibits contracting muscle

Sensory Input• All senses trigger the same TYPE of signal

– Distinction occurs in activated brain area– Typically graded response, but AP’s possible

• Sensory receptors detect sensations and carry to the brain– Awareness of environmental change

• Brain constructs perceptions by integrating sensations with other information– Neuronal communication involving multiple brain

areas

The Working Brain

Sensory Adaptation

• Sensory receptors become less responsive– Fewer action potentials

• Limits reactions to normal background stimuli– Shower or hot tub

temperature– Odors over time

Decoding Nerves

• Epineurium – covers nerve• Perineurium – covers fascicle• Endoneurium – cover axon• Direction of transmission

– Sensory afferents go to CNS• Dorsal root ganglia

– Motor efferents come from CNS• Sympathetic & parasympathetic

ganglia– Mixed carry both; most nerves

• Classified as cranial or spinal

Nerve Fiber Regeneration• Mature neurons don’t divide*• Cell body damage = death• Cut/compressed axons regenerate

– Separated ends seal off and swell– Distal end of injury disintegrates

• Lack of nutrients• Neurilemma maintained in endoneurium

– Schwann cells proliferate & encourage axon growth

• Guide ‘sprouting’ axons to original contacts

• Greater distance decreases chances• Regrowth never exact = retraining• Extremely rare in CNS

Cranial Nerves

• Ventral portion of the brain

• Sensory, motor, or mix

• 1st 2 pairs attach to forebrain

• Remainders originate on brainstem

Numbers Cranial Nerve FunctionI OLFACTORY Smell (sensory)

II OPTIC Vision (sensory)

III OCULOMOTOR Eye movement (motor)(medial, inferior, superior rectus muscle & inferior oblique muscle)

IV TROCHLEAR Eye movements (motor)(superior oblique muscle)

V TRIGEMINAL Temperature, pain, crude touch of face (sensory) & mastication (motor)

VI ABDUCENS Eye movement (motor)(lateral rectus muscle)

VII FACIAL Taste (2/3 of anterior tongue) (sensory)Facial expressions (motor)

VIII VESTIBULOCOCHLEAR* Hearing & Equilibrium (sensory)

IX GLOSSOPHRAYNGEAL Taste (1/3 of posterior tongue) (sensory)Pharynx (swallowing & gag reflex) (motor)

X VAGUS Senses blood pressure (sensory)Stimulate heart rate and digestive organs (motor)

XI (SPINAL) ACCESSORY Head and neck movement (motor)e.g. trapezius, levator scapula

XII HYPOGLOSSAL Tongue movement (motor)

Testing Cranial Nerves for Disorders• Olfactory

– Smell substances– Anosima

• Optic– Eye chart– Anopsias

• Oculomotor– Follow object; pupil reflex– Strabismus, double vision, ptosis

• Trochlear– See oculomotor

• Trigeminal – Corneal reflex; close/move jaws; touch

face with objects• Abducens

– See oculomotor

• Facial– Make various faces; tasting substances– Bell’s palsy, loss of taste, can’t close eye

• Vestibulocochlear– Tuning fork; distance of sound– Deafness, vertigo, tinnitus

• Glossopharyngeal– Swallowing & gag reflex; say ‘ah’

• Vagus– See glossopharyngeal– Horseness, swallowing problems, death

• (Spinal) accessory– Move head/shoulders against resistance

• Hypoglossal– Stick out, retract, & move tongue to sides

Spinal Nerve Anatomy• Roots

– Dorsal root: peripheral receptors (sensory) to spinal cord

– Ventral root: ventral horn to skeletal muscles (motor)

• Branches: laterally pass through intervertebral foramen– Dorsal ramus: dorsal trunk– Ventral ramus: limbs & rest of trunk– Meningeal branch: meninges and blood

vessels• Plexus

– Criss cross joining of ventral rami– Excludes T2 – T12

• 8 cervical– Cervical plexus– Brachial plexus

• 12 thoracic– Intercostal nerves & enlargements

• 5 lumbar– Lumbar plexus

• 5 sacral– Sacral plexus

• 1 coccygeal– Tailbone & perineum

31 Pairs of Spinal Nerves

Cervical Nerves • Cervical plexus

– Phrenic nerve: diaphragm• Irritation causes hiccups

• Brachial plexus C5 – C8– Median nerve: flexor muscles of the anterior forearm

and small hand muscles• Carpal tunnel syndrome and suicide attempts

– Radial nerve: extensor muscle of posterior forearm and triceps brachii

• ‘Saturday night paralysis’– Ulnar nerve: similar to median nerve

• ‘Funny bone’ and paralysis/distortion of medial fingers

Lumbosacral Plexus• Innervates lower limbs, buttocks, and pelvic

muscles• Lumbar plexus L1 – L4

– Femoral nerve: quadriceps and sartorius• Branches to saphenous

– Obturator nerve: adductor muscles• Sacral plexus L4 – S4

– Sciatic nerve: entire lower leg (except femoral innervation)

• Tibial: hamstrings• Common fibular nerve: anterior tibialis

• Receptor– Senses stimulus

• Sensory neuron (afferents)– Message to the CNS

• Integration center– Synapses in CNS

• Monosynaptic (single motor or sensory neuron)• Polysynaptic (multiple interneurons)

• Motor neuron (efferents)– Message to effectors

• Effector– Muscle fibers or glands

• Reflexes are rapid, predictable responses to a stimulus– Somatic and autonomic

Reflex Arc

http://a248.e.akamai.net/7/248/430/20080327144023/www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/arc_reflex%20a.(1).jpg

Classifying Reflexes• Somatic: activate skeletal muscle

– Spinal: integration center is spinal cord (brain not required)• Stretch: ensures muscle length maintained (knee-jerk reflex)

– All monosynaptic and ipsilateral• Flexor reflex: withdrawl from painful stimuli (glass or hot stove)

– Polysynaptic and ipsilateral– Common pairing with crossed extensor in weighted limbs (contralateral)

• Superficial: cutaneous stimulation (plantar reflex)– Cranial nerve: integration center is brain stem

• Corneal: stimulation causes blinking

• Autonomic (visceral): activate smooth or cardiac muscle– Pupillary light: controls diameter of pupil (inside/outside)– Ciliospinal: ipsilateral pupil dilation from pain/stimuli