Human Anatomy & Physiology I: Peripheral Nervous System PDF

BIOL243 – HUMAN ANATOMY & PHYSIOLOGY I Charles Smith, PhD CSCS HUMAN ANATOMY & PHYSIOLOGY Ch. 13 – Peripheral Nervous System & Reflex Activity PERIPHERAL NERVOUS SYSTEM Links stimuli travelling to and from world outside the body All neural structures outside the brain & spinal cord SE...

BIOL243 – HUMAN ANATOMY & PHYSIOLOGY I Charles Smith, PhD CSCS HUMAN ANATOMY & PHYSIOLOGY Ch. 13 – Peripheral Nervous System & Reflex Activity PERIPHERAL NERVOUS SYSTEM Links stimuli travelling to and from world outside the body All neural structures outside the brain & spinal cord SENSORY RECEPTORS Specialized to respond to changes in the environment (external stimuli) Provide awareness of a stimulus (sensation) which the brain then interprets the meaning (perception) Receptors are classed by Stimulus Type Body Location Structural Complexity RECEPTOR CLASSES (TYPE & LOCATION) Stimulus Type: Body Location: Mechanoreceptors: Exteroceptors: Tactile stimuli Stimuli come from outside the body i.e., touch, pressure, vibrations, stretch i.e., touch, pressure, pain, temperature via Thermoreceptors: skin Temperature-related stimuli Most of our special sense organs i.e., hot, cold, changes therein Interoceptors (visceroceptors): Photoreceptors: Stimuli from viscera and blood vessels Light energy Unaware of these stimuli except when they i.e., eye retinas generate discomfort Chemoreceptors: Proprioceptors: Chemically-related stimuli i.e., taste, smell, cellular/blood chemistry Allow us to perceive body position in space Nociceptors: – kinesthetic awareness Stretch receptors Pain stimuli Can be generated by extremes in any of the other stimuli RECEPTOR CLASSES (STRUCTURE) 2 Major Categories: 1. General Receptors: modified dendritic endings of sensory neurons Pressure: Merkel Cells & Discs; Pacinian Corpuscles; Ruffini Endings Vibration: Pacinian Corpuscles Touch: Hair Follicle Receptors; Meissner’s Corpuscles Stretch: Muscle Spindles; Golgi Tendon Organs; Kinesthetic Receptors; Ruffini Endings Pain: Free Nerve Endings Note: ALL receptors are capable of transmitting pain stimuli brought on by extremes in their sensory responsibilities, but Free Nerve Endings are more commonly associated with nociception 2. Special Receptors: complex sense organs specialized for special senses i.e., hearing, vision, balance (equilibrium), smell, taste PERIPHERAL NERVES Nerve: bundle of myelinated & unmyelinated peripheral axons Surrounded by connective tissue layers Epineurium: surrounds all neuronal fascicles Perineurium: surrounds each neuronal fascicle (bundle of neurons) Endoneurium: surrounds each axon (and its myelin sheath, where applicable) Most nerves contain a mix of afferent & efferent, somatic & autonomic neurons Can be classed individually based upon the direction and type of information they transmit 2 Types of Peripheral Nerve: Cranial Spinal CRANIAL NERVES 12 pairs of cranial nerves which originate & directly transmit information to/from brain/brain stem Most are mixed nerves (afferent & efferent) Numbered I – XII & named from Anterior to Posterior/Caudal Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Aducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal  Some mnemonics: Oh Once One Takes The Anatomy Final, Very Good Vacations Are Heavenly On Old Olympus Towering Tops, A Fine Vested German Viewed A Hop Oh Oh Oh, To Touch And Feel Very Green Vegetables, AH! Some Say Marry Money, But My Brother Says Big Brains Matter More SPINAL NERVES All spinal nerves are mixed nerves 31 pairs supplying all body parts outside head and part of neck 8 cervical pairs (C1 – C8) 12 thoracic pairs (T1 – T12) 5 lumbar pairs (L1 – L5) 5 sacral pairs (S1 – S5) 1 coccygeal pair (Co1) SPINAL NERVES – ROOTS VS. RAMI Roots lie medial to and form the spinal nerves themselves Each root purely sensory OR motor Rami lie distal to and make lateral branches of spinal nerves Carry both sensory AND motor information Rami & their branches supply entire somatic region of body from neck down Dorsal Rami: posterior trunk Ventral Rami: anterior trunk & limbs Only ventral rami form nerve networks (plexuses) All except T2 – T12 Found in cervical, brachial, lumbar, & sacral areas CERVICAL PLEXUS 1st four ventral rami (C1 – C4) form cervical plexus Mostly cutaneous nerves innervating skin of neck, ear, back of head, and shoulders Phrenic Nerve receives fibers from C3 – C5 MAJOR nerve for diaphragm regulating breathing Irritation of nerve causes diaphragmatic spasming (hiccups) If both nerves severed, diaphragm becomes paralyzed Result: respiratory arrest; can’t breathe without respirators Hangman’s Fracture BRACHIAL PLEXUS Remaining cervical rami (C5 – C8) plus T1 Gives rise to nerves innervating upper limb Structure is very complex Roots—five ventral rami (C5 – T1) unite to form… Trunks—upper, middle, and lower, each of which immediately split into… Divisions—anterior and posterior, which give rise to… Cords—lateral, medial, and posterior which split into… Branches: axillary, musculocutaneous, radial, median, ulnar Randy Travis Drinks Cold Beer Really Tired? Drink Cold Brew BRANCHES OF THE BRACHIAL PLEXUS Cords of the brachial plexus split (give rise to) nerves of the upper limb Most Importantly: Axillary Nerve: innervates deltoid & teres minor muscles; cutaneous reception for skin & joint capsule of shoulder Musculocutaneous Nerve: innervates elbow flexor muscles (i.e., biceps brachii, brachialis & coracobrachialis); cutaneous reception for skin of lateral forearm Median Nerve: innervates most wrist flexors & pronators and finger flexors; cutaneous reception for skin of lateral 3 digits (thumb, index, & middle fingers) Ulnar Nerve: innervates flexor carpi ulnaris & most intrinsic hand muscles; cutaneous reception for skin of medial 2 digits (ring & pinky fingers) Radial Nerve: innervates (essentially) all wrist extensors & supinators; cutaneous reception for skin of posterior hand CLINICAL SIGNIFICANCE Stinger: occurs if neck is excessively stretched in lateral flexion (esp. if shoulder depressed) Common with tackling & wrestling moves Injury to median nerve makes “pincer grasp” (thumb and index finger opposition) difficult Difficult to lift small objects Also seen when median nerve is compressed (carpal tunnel syndrome) Common in activities/professions where wrist spends a lot of time in extension (i.e., typing, pianists, weightlifters) Median nerve also commonly severed during wrist-cutting suicide attempts Severe or chronic damage to ulnar nerve can lead to sensory loss, paralysis, muscle atrophy Creates a claw hand Can’t make a fist or grip You strike this nerve at the medial epicondyle when you hit your “funny bone” Trauma to radial nerve restricts ability to extend wrist creating wrist drop Nerve also negatively affected by compression & impaired blood supply Improper crutch usage “Saturday night paralysis”: fall asleep drunk with arm draped over back of chair/sofa LUMBAR PLEXUS Lumbosacral plexus serves mostly the lower limb Also has some branches supplying the abdomen, pelvis, and buttocks Lumbar plexus arises from roots from L1 – L4 lying within the psoas major muscle 6 Branches, but 2 Major ones are: Femoral Nerve: innervates quadriceps, sartorius, iliacus, & pectineus muscles; cutaneous reception for anterior thigh & medial shank Obturator Nerve: innervates adductor muscles; cutaneous reception for medial thigh as well as hip & knee joints Other nerves innervate the deep abdominal wall muscles & provide cutaneous reception for buttock, pubic, and genital areas Compression of lumbar plexus roots can result in impaired gait patterns as well as pain/numbness of anterior & medial thigh SACRAL PLEXUS Arises from nerves L4 – S4 and is located caudally to lumbar plexus Has roughly a dozen branches Half serve buttock & lower limb; rest innervate pelvic structures & perineum Major Branch: Sciatic Nerve Longest & thickest nerve in body Comprises to 2 Nerves (both innervate hamstring muscles) Tibial Nerve: innervates posterior shank muscles, toe flexor muscles & intrinsic foot muscles; cutaneous reception for posterior thigh/shank & sole of foot Common Fibular Nerve: innervates anterior shank muscles & toe extensor muscles; cutaneous reception for anterior & lateral thigh/shank as well as dorsum of foot Sciatica: characterized by stabbing, radiating pain down back of leg (which lessens or disappears upon standing) Often caused by compression (impingement) of nerve from disc herniation, sitting too long, or tightness of psoas muscle Can also result in footdrop where foot hangs in plantar flexion DERMATOMES Area of the skin innervated by cutaneous branches of a single spinal nerve Every spinal nerve (except C1) has a dermatome Knowing map crucial Extent of spinal cord injuries are ascertained by affected dermatomes Existence or loss of different sensations can also indicate how a nerve is being affected Need to know so that you know what nerve(s) need to be blocked for anesthesia Most dermatomes for body/trunk are uniform in width falling in line with their spinal nerves Limbs can be more complicated and variable Dermatomes also overlap a little SPINAL REFLEXES Occur without direct involvement from higher brain centers Brain still knows what’s going on and may have an effect on reflex, but reflex happens even without brain involvement 2 Types: Intrinsic (inborn): rapid, predictable motor response to a stimulus Completely involuntary & built into neural anatomy Testing these reflexes important to clinically assess condition of nervous system Exaggerated, distorted, or absence of reflex may indicate a nerve degeneration or pathology Acquired (learned): result from practice & repetition i.e., driving actions 5 main components: Receptor: site of stimulus action (see earlier types) Sensory Neuron: communicates afferent impulses to CNS Integration Center: often within CNS may be a single synapse between afferent & efferent neurons (monosynaptic) May involve network of synapses and interneurons (polysnaptic) Motor Neuron: conducts efferent impulse from CNS to effector organ Effector: muscle fiber or gland cell responding to efferent stimulus STRETCH REFLEX Makes sure that the muscle remains at the predetermined length Muscle “recoil” after it’s been lengthened Example: Patellar (Knee-jerk) reflex Typically keeps knees from buckling when standing upright Common test: 1. Mallet strikes patellar tendon of flexed, relaxed knee 2. Patellar tendon stretches thereby stretching quadriceps 3. Afferent signal travels to dorsal root ganglion and through dorsal horn of spinal cord 4. Relays with efferent neuron which stimulates quadriceps to contract & hamstrings to relax Reciprocal Inhibition 5. Quadriceps contract & knee extends GOLGI TENDON REFLEX Makes sure we don’t strain muscles so much that they become overly damaged i.e., extreme stretching or contractions Works is relative opposite manner of stretch reflex 1. Tendon senses an extreme amount of tension 2. Afferent information sent to spinal cord then cerebellum Works to adjust muscle tension 3. Motor neurons in spinal cord send efferent signal to relax contracting muscle and activate antagonist muscle Reciprocal activation i.e., getting “stuck” and “failing” during a lift; crumpling to the ground during a hard landing WITHDRAWAL REFLEX Aka “Flexor reflex” Initiated by painful stimuli Protective & important for survival Causes an automatic withdrawal of the threatened body part i.e., hand on a hot burner Often accompanied by the crossed-extensor reflex Withdrawal movement on side ipsilateral to painful stimulus Side contralateral to painful stimulus performs an “extensor” movement Important for maintaining balance i.e., stepping on a LEGO & putting weight onto opposite foot; hopping on opposite foot after stubbing your toe The brain can override this reflex If you know the pain is coming, you’ll automatically withdraw from it SUPERFICIAL REFLEXES Elicited by gentle, cutaneous stimulation i.e., stroking the skin Depend on both upper motor neuron function and spinal cord level reflex arcs Example: Babinski’s Test (Plantar Reflex) Indirectly determines if corticospinal tracts are functioning properly Run a blunt object from heel to toe along lateral aspect of plantar fascia (sole of foot) Toes should curl In infants, the great toe will dorsiflex & toes fan laterally until ~1 yr PERIPHERAL NERVOUS SYSTEM SUMMARY PNS is comprised of all nervous tissue not the brain and spinal cord 12 pairs of cranial nerves + 31 pairs of spinal nerves corresponding to all vertebrae (except C 1) Receive stimuli from a variety of different afferent receptors to CNS & transmit efferent responses from CNS to effectors Nerve Roots then branch and distribute throughout the body by way of the nerve plexuses (Cervical, Brachial, Lumbar & Sacral) The nerves of these plexuses branch out to innervate specific muscles and return cutaneous information from specific body areas These dermatomes often fall in-line with their spinal nerves for the body & trunk but can be more variable on the limbs Damage to these nerves can lead to various symptoms often relating to the function of the individual nerve Why sensation and reflex tests can be so important in diagnosing not only the extent of injury but also in determining the location of nerve injury SAMPLE QUESTIONS 1. Which type of receptor is primarily associated with pain sensation? 2. What nerve of the cervical plexus is extremely important due to its role in controlling respiration? 3. Impingement (or compression) of the median nerve in the brachial plexus is also called what? 4. The central nervous system represents what component of most spinal reflex arcs? 5. During what reflex test do you run a blunt object along the lateral aspect of the sole of the foot and expect to see the toes curl?  COPYRIGHT © Pearson Edited by Charles Smith, PhD CSCS 2024