Human Anatomy and Physiology Eleventh Edition Chapter 13 PDF

Human Anatomy and Physiology Eleventh Edition Chapter 13 Peripheral Nervous System PowerPoint® Lectures Slides prepa...

Human Anatomy and Physiology Eleventh Edition Chapter 13 Peripheral Nervous System PowerPoint® Lectures Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Why This Matters Understanding the peripheral nervous system help you to recognize and treat nerve damage Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Video: Why This Matters (Career Connection) Click here to view ADA compliant video: Why This Matters (Career Connection) https://mediaplayer.pearsoncmg.com/assets/secs_wtm_ch_13_christian_v2 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Peripheral Nervous System PNS provides links from and to world outside our body Consists of all neural structures outside brain and spinal cord that can be broken down into four parts: Part 1 – Sensory Receptors Part 2 – Transmission Lines: Nerves and Their Structure and Repair Part 3 – Motor Endings and Motor Activity Part 4 – Reflex Activity Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Place of the PNS in the Structural Organization of the Nervous System Figure 13.1 Place of the PNS in the structural organization of the nervous system. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Part 1 – Sensory Receptors and Sensation Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.1 Sensory Receptors Sensory receptors: specialized to respond to changes in environment (stimuli) – Activation results in graded potentials that trigger nerve impulses Awareness of stimulus (sensation) and interpretation of meaning of stimulus (perception) occur in brain Three ways to classify receptors: by type of stimulus, body location, and structural complexity Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Stimulus Type (1 of 2) Mechanoreceptors—respond to touch, pressure, vibration, and stretch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (example: retina) Chemoreceptors—respond to chemicals (examples: smell, taste, changes in blood chemistry) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Stimulus Type (2 of 2) Nociceptors—sensitive to pain-causing stimuli (examples: extreme heat or cold, excessive pressure, inflammatory chemicals) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Location (1 of 3) Exteroceptors – Respond to stimuli arising outside body – Receptors in skin for touch, pressure, pain, and temperature – Most special sense organs Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Location (2 of 3) Interoceptors (visceroceptors) – Respond to stimuli arising in internal viscera and blood vessels – Sensitive to chemical changes, tissue stretch, and temperature changes – Sometimes cause discomfort but usually person is unaware of their workings Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Location (3 of 3) Proprioceptors – Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles – Inform brain of one's movements Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (1 of 8) Majority of sensory receptors belong to one of two categories: – Simple receptors of the general senses  Modified dendritic endings of sensory neurons  Are found throughout body and monitor most types of general sensory information – Receptors for special senses  Vision, hearing, equilibrium, smell, and taste  All are housed in complex sense organs  Covered in Chapter 15 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (2 of 8) Simple receptors of the general senses – General senses include tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense  No “one-receptor-one-function” relationship – Receptors can respond to multiple stimuli – Receptors have either:  Nonencapsulated (free) nerve endings or  Encapsulated nerve endings Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (3 of 8) – Nonencapsulated (free) nerve endings  Abundant in epithelia and connective tissues  Most are nonmyelinated, small-diameter group C fibers; distal terminals have knoblike swellings  Respond mostly to temperature, pain, or light touch Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (4 of 8) – Nonencapsulated (free) nerve endings (cont.)  Thermoreceptors – Cold receptors are activated by temps from 10 to 40ºC – Located in superficial dermis – Heat receptors are activated from 32 to 48ºC located in in deeper dermis – Outside those temperature ranges, nociceptors are activated and interpreted as pain Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (5 of 8) – Nonencapsulated (free) nerve endings (cont.)  Nociceptors: pain receptors triggered by extreme temperature changes, pinch, or release of chemicals from damaged tissue – Vanilloid receptor: protein in nerve membrane is main player Acts as ion channel that is opened by heat, low pH, chemicals (example: capsaicin in red peppers) Itch receptors in dermis: can be triggered by chemicals such as histamine Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (6 of 8) – Nonencapsulated (free) nerve endings (cont.)  Tactile (Merkel) discs: function as light touch receptors – Located in deeper layers of epidermis  Hair follicle receptors: free nerve endings that wrap around hair follicles – Act as light touch receptors that detect bending of hairs Example: Allows you to feel a mosquito landing on your skin Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.1-1 General Sensory Receptors Classified by Structure and Function (1 of 2) Table 13.1 General Sensory Receptors Classified by Structure and Function. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (7 of 8) – Encapsulated dendritic endings  Almost all are mechanoreceptors whose terminal endings are encased in connective tissue capsule  Vary greatly in shape and include: – Tactile (Meissner’s) corpuscles: small receptors involved in discriminative touch Found just below skin, mostly in sensitive and hairless areas (fingertips) – Lamellar (Pacinian) corpuscles: large receptors respond to deep pressure and vibration when first applied (then turn off) Located in deep dermis Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Classification by Receptor Structure (8 of 8) – Bulbous corpuscles (Ruffini endings): respond to deep and continuous pressure Located in dermis – Muscle spindles: spindle-shaped proprioceptors that respond to muscle stretch – Tendon organ: proprioceptors located in tendons that detect stretch – Joint kinesthetic receptors: proprioceptors that monitor joint position and motion Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.1-2 General Sensory Receptors Classified by Structure and Function (2 of 2) Table 13.1 General Sensory Receptors Classified by Structure and Function. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.2 Sensory Processing Survival depends upon: – Sensation: the awareness of changes in the internal and external environment – Perception: the conscious interpretation of those stimuli Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (1 of 7) Somatosensory system: part of sensory system serving body wall and limbs Receives inputs from: – Exteroceptors, proprioceptors, and interoceptors Input is relayed toward head, but processed along the way Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (2 of 7) Levels of neural integration in sensory systems: 1. Receptor level: sensory receptors 2. Circuit level: processing in ascending pathways 3. Perceptual level: processing in cortical sensory areas Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Three Basic Levels of Neural Integration in Sensory Systems Figure 13.2 Three basic levels of neural integration in sensory systems. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (3 of 7) Processing at the receptor level – Generating a signal: For sensation to occur, the stimulus must excite a receptor, and the AP must reach CNS  Stimulus energy must match receptor specificity (touch receptors do not respond to light)  Stimulus must be applied within receptive field  Transduction must occur—energy of stimulus is converted into graded potential called generator potential (in general receptors) or receptor potential (in special sense receptors)  Graded potentials must reach threshold → AP Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (4 of 7) – Adaptation: Change in sensitivity in presence of constant stimulus  Receptor membranes become less responsive  Receptor potentials decline in frequency or stop  Phasic receptors: (fast-adapting) send signals at beginning or end of stimulus – Examples: receptors for pressure, touch, and smell  Tonic receptors: adapt slowly or not at all – Examples: nociceptors and most proprioceptors Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (5 of 7) Processing at the circuit level – Pathways of three neurons conduct sensory impulses received from receptors upward to appropriate cortical regions – First-order sensory neurons  Conduct impulses from receptor level to spinal reflexes or second-order neurons in CNS – Second-order sensory neurons  Transmit impulses to third-order sensory neurons – Third-order sensory neurons  Conduct impulses from thalamus to the somatosensory cortex (perceptual level) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (6 of 7) Processing at the perceptual level – Interpretation of sensory input depends on specific location of target neurons in sensory cortex – Aspects of sensory perception:  Perceptual detection: ability to detect a stimulus (requires summation of impulses)  Magnitude estimation: intensity coded in frequency of impulses  Spatial discrimination: identifying site or pattern of stimulus (studied by two- point discrimination test) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved General Organization of the Somatosensory System (7 of 7) Processing at the perceptual level (cont.) – Feature abstraction: identification of more complex aspects and several stimulus properties – Quality discrimination: ability to identify submodalities of a sensation (e.g., sweet or sour tastes) – Pattern recognition: recognition of familiar or significant patterns in stimuli (e.g., melody in piece of music) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Perception of Pain (1 of 3) Warns of actual or impending tissue damage so protective action can be taken Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin Impulses travel on fibers that release neurotransmitters glutamate and substance P Some pain impulses are blocked by inhibitory endogenous opioids (example: endorphins) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Perception of Pain (2 of 3) Pain tolerance – All perceive pain at same stimulus intensity – Pain tolerance varies – “Sensitive to pain” means low pain tolerance, not low pain threshold – Genes help determine pain tolerance as well as response to pain medications  Research in use of genetics to determine best pain treatment is ongoing Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Perception of Pain (3 of 3) Visceral and referred pain – Visceral pain results from stimulation of visceral organ receptors  Felt as vague aching, gnawing, burning  Activated by tissue stretching, ischemia, chemicals, muscle spasms – Referred pain: pain from one body region perceived as coming from different region  Visceral and somatic pain fibers travel along same nerves, so brain assumes stimulus comes from common (somatic) region – Example: left arm pain during heart attack Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Map of Referred Pain Figure 13.3 Map of referred pain. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.1 Long-lasting or intense pain, such as limb amputation, can lead to hyperalgesia (pain amplification), chronic pain, and phantom limb pain – NMDA receptors are activated by long-lasting or intense pain  Allow spinal cord to “learn” hyperalgesia  Early pain management critical to prevent Phantom limb pain: pain felt in limb that has been amputated – Now use epidural anesthesia during surgery to reduce phantom pain Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Part 2 – Transmission Lines: Nerves and Their Structure and Repair Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.3 Nerves and Associated Ganglia Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure and Classification (1 of 5) Nerve: cordlike organ of PNS Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue Two types of nerves: spinal or cranial, depending on where they originate Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure and Classification (2 of 5) Connective tissue coverings include: – Endoneurium: loose connective tissue that encloses axons and their myelin sheaths (Schwann cells) – Perineurium: coarse connective tissue that bundles fibers into fascicles – Epineurium: tough fibrous sheath around all fascicles to form the nerve Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure of a Nerve (1 of 2) Figure 13.4a Structure of a nerve. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure of a Nerve (2 of 2) Figure 13.4b Structure of a nerve. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure and Classification (3 of 5) Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Nerves are classified according to the direction they transmit impulses – Mixed nerves: contain both sensory and motor fibers  Impulses travel both to and from CNS – Sensory (afferent) nerves: impulses only toward CNS – Motor (efferent) nerves: impulses only away from CNS Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure and Classification (4 of 5) Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed Types of fibers in mixed nerves: – Somatic afferent (sensory from muscle to brain) – Somatic efferent (motor from brain to muscle) – Visceral afferent (sensory from organs to brain) – Visceral efferent (motor from brain to organs) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Structure and Classification (5 of 5) Ganglia: contain neuron cell bodies associated with nerves in PNS – Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons  Dorsal root ganglia (sensory, somatic) (Chapter 12) – Ganglia associated with efferent nerve fibers contain autonomic motor neurons  Autonomic ganglia (motor, visceral) (Chapter 14) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of Nerve Fibers (1 of 3) Mature neurons are amitotic, but if the soma (cell body) of the damaged nerve is intact, the peripheral axon may regenerate in PNS; does not occur in CNS Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of Nerve Fibers (2 of 3) CNS axons – Most CNS fibers never regenerate – CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration – Astrocytes at injury site form scar tissue – Treatment: neutralizing growth inhibitors, blocking receptors for inhibitory proteins, destroying scar tissue components Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of Nerve Fibers (3 of 3) PNS axons – PNS axons can regenerate if damage is not severe 1. Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down axon 2. Macrophages clean dead axon debris; Schwann cells are stimulated to divide 3. Axon filaments grow through regeneration tube 4. Axon regenerates, and new myelin sheath forms Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of a Nerve Fiber in a Peripheral Nerve (1 of 4) Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of a Nerve Fiber in a Peripheral Nerve (2 of 4) Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of a Nerve Fiber in a Peripheral Nerve (3 of 4) Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regeneration of a Nerve Fiber in a Peripheral Nerve (4 of 4) Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.4 Cranial Nerves 12 pairs of cranial nerves are associated with brain – Two attach to forebrain, rest with brain stem Most are mixed nerves, but two pairs purely sensory Each numbered (I through XII) and named from rostral to caudal “On occasion, our trusty truck acts funny—very good vehicle anyhow” “Oh once one takes the anatomy final, very good vacations are heavenly” Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Location and Function of Cranial Nerves (1 of 4) Figure 13.6a Location and function of cranial nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Location and Function of Cranial Nerves (2 of 4) Figure 13.6b Location and function of cranial nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (1 of 12) I: Olfactory nerves – Sensory nerves of smell – Run from nasal mucosa to olfactory bulbs – Pass through cribriform plate of ethmoid bone – Fibers synapse in olfactory bulbs – Pathway terminates in primary olfactory cortex – Purely sensory (olfactory) function Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-1 Cranial Nerves Table 13.2-1 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (2 of 12) II: Optic nerves – Arise from retinas; really a brain tract – Pass through optic canals, converge, and partially cross over at optic chiasma – Optic tracts continue to thalamus, where they synapse – Optic radiation fibers run to occipital (visual) cortex – Purely sensory (visual) function Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-2 Cranial Nerves Table 13.2-2 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (3 of 12) III: Oculomotor nerves – Fibers extend from ventral midbrain through superior orbital fissures to four of six extrinsic eye muscles – Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and controlling lens shape Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-3 Cranial Nerves Table 13.2-3 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (4 of 12) IV: Trochlear nerves – Fibers from dorsal midbrain enter orbits via superior orbital fissures to innervate superior oblique muscle – Primarily motor nerve that directs eyeball Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-4 Cranial Nerves Table 13.2-4 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (5 of 12) V: Trigeminal nerves – Largest cranial nerves; fibers extend from pons to face – Three divisions  Ophthalmic (V1) passes through superior orbital fissure  Maxillary (V2) passes through foramen rotundum  Mandibular (V3) passes through the foramen ovale – Convey sensory impulses from various areas of face (V1 and V2) – Supply motor fibers (V3) for mastication Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-5 Cranial Nerves Table 13.2-5 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2- 6 Cranial Nerves Table 13.2-6 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (6 of 12) VI: Abducens nerves – Fibers from inferior pons enter orbits via superior orbital fissures – Primarily a motor, innervating lateral rectus muscle Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-7 Cranial Nerves Table 13.2-7 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (7 of 12) VII: Facial nerves – Fibers from pons travel through internal acoustic meatuses and emerge through stylomastoid foramina to lateral aspect of face – Chief motor nerves of face with five major branches – Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands – Sensory function (taste) from anterior two-thirds of tongue Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-8 Cranial Nerves Table 13.2-8 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-9 Cranial Nerves Table 13.2-9 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (8 of 12) VIII: Vestibulocochlear nerves – Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from inner ear through internal acoustic meatuses and enter brain stem at pons-medulla border – Mostly sensory function; small motor component for adjustment of sensitivity of receptors – Formerly auditory nerve Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-10 Cranial Nerves Table 13.2-10 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (9 of 12) IX: Glossopharyngeal nerves – Fibers from medulla leave skull via jugular foramen and run to throat – Motor functions: innervate part of tongue and pharynx for swallowing and provide parasympathetic fibers to parotid salivary glands – Sensory functions: fibers conduct taste and general sensory impulses from pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-11 Cranial Nerves Table 13.2-11 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (10 of 12) X: Vagus nerves – Only cranial nerves that extend beyond head and neck region – Fibers from medulla exit skull via jugular foramen – Most motor fibers are parasympathetic fibers that help regulate activities of heart, lungs, and abdominal viscera – Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-12 Cranial Nerves Table 13.2-12 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (11 of 12) XI: Accessory nerves – Formed from ventral rootlets from C1 to C5 region of spinal cord (not brain) – Rootlets pass into cranium via each foramen magnum – Accessory nerves exit skull via jugular foramina to innervate trapezius and sternocleidomastoid muscles – Formerly spinal accessory nerve Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-13 Cranial Nerves Table 13.2-13 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Cranial Nerves (12 of 12) XII: Hypoglossal nerves – Fibers from medulla exit skull via hypoglossal canal – Innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and speech Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.2-14 Cranial Nerves Table 13.2-14 Cranial Nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Location and Function of Cranial Nerves (3 of 4) Figure 13.6a Location and function of cranial nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Location and Function of Cranial Nerves (4 of 4) Figure 13.6b Location and function of cranial nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Cranial Nerves (1 of 2) Olfactory and optic nerves – Neuron cell bodies located within special sense organs Other nerves with sensory information (V, VII, IX, and X) – Neuron cell bodies located in cranial sensory ganglia Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Cranial Nerves (2 of 2) Some mixed nerves contain both somatic and autonomic fibers – Most motor neuron cell bodies in ventral gray matter of brain stem – Some autonomic motor neurons in ganglia To remember primary functions of cranial nerves as sensory, motor, both: – “Some say marry money, but my brother believes (it’s) bad business (to) marry money.” Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.5 Spinal Nerves (1 of 5) 31 pairs of spinal nerves All are mixed nerves named for point of issue from spinal cord Supply all body parts except head and part of neck – 8 pairs of cervical nerves (C1–C8) – 12 pairs of thoracic nerves (T1–T12) – 5 pairs of lumbar nerves (L1–L5) – 5 pairs of sacral nerves (S1–S5) – 1 pair of tiny coccygeal nerves (C0) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.5 Spinal Nerves (2 of 5) 7 cervical vertebrae give rise to 8 pairs of cervical spinal nerves because: – Each of the first 7 pairs (C1 to C7) exits the vertebral canal superior to vertebra for which it is named – Last spinal nerve (C8) exits canal inferior to C7  So vertebra C7 has a nerve that leaves above it and one that leaves below it Each of the other spinal nerves exits inferior to vertebra for which it is named Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Spinal Nerves Figure 13.7 Spinal nerves. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.5 Spinal Nerves (3 of 5) Each spinal nerve is connected to spinal cord via two roots: – Ventral roots  Contain motor (efferent) fibers from ventral horn motor neurons that innervate skeletal muscles – Dorsal roots  Contain sensory (afferent) fibers from sensory neurons in dorsal root ganglia that conduct impulses from peripheral receptors Both ventral and dorsal roots are branched medially as rootlets that then join laterally to form spinal nerve Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Formation of Spinal Nerves and Rami Distribution (1 of 2) Figure 13.8a Formation of spinal nerves and rami distribution. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.5 Spinal Nerves (4 of 5) Spinal nerves emerge from vertebral column via their respective intervertebral foramina Spinal roots become progressively longer superiorly to inferiorly down cord – Lumbar and sacral roots are very long and extend through lower vertebral canal as cauda equina Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.5 Spinal Nerves (5 of 5) Spinal nerves are quite short (~1–2 cm) Almost immediately after exiting foramen, spinal nerves divide into three branches: – Dorsal ramus: smaller branch – Ventral ramus: larger branch – Meningeal branch: tiny branch that reenters vertebral canal to innervate meninges and blood vessels Rami communicantes contain autonomic nerve fibers that join ventral rami in thoracic region Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Formation of Spinal Nerves and Rami Distribution (2 of 2) Figure 13.8b Formation of spinal nerves and rami distribution. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (1 of 14) Spinal nerve rami and their main branches supply entire somatic region of body from neck down – Dorsal rami supply posterior body trunk – Ventral rami supply rest of trunk and limbs Difference between roots and rami: – Roots lie medial to and form spinal nerves  Each root is purely sensory or motor – Rami lie distal to and are lateral branches of spinal nerves  Can carry both sensory and motor Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (2 of 14) All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses – Found in cervical, brachial, lumbar, and sacral areas – Only ventral rami form plexuses Within plexus, fibers crisscross so that: 1. Each branch contains fibers from several different spinal nerves Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (3 of 14) 2. Fibers from ventral ramus go to body periphery via several routes  Means each limb muscle is innervated by more than one spinal nerve, so damage to one does not cause paralysis Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (4 of 14) Cervical plexus and the neck – First four ventral rami (C1–C4) form looping cervical plexus  Most branches of this form cutaneous nerves – Innervate skin of neck, ear, back of head, and shoulders – Other branches innervate neck muscles – Phrenic nerve  Major motor and sensory nerve of diaphragm, major muscles for breathing  Phrenic nerve receives fibers from C3 to C5 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Cervical Plexus Figure 13.9 The cervical plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.3 Branches of the Cervical Plexus Table 13.3 Branches of the Cervical Plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.2 Irritation of the phrenic nerve causes spasms of the diaphragm, also called hiccups If both phrenic nerves are severed, or if C3–C5 region of spinal cord is destroyed, diaphragm becomes paralyzed – Respiratory arrest occurs – Victim requires mechanical respirators to stay alive  Air is mechanically forced into the lungs—literally breathing for them Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (5 of 14) Brachial plexus and upper limb – Formed by ventral rami of C5–C8 and T1 (and often C4 and/or T2) – Gives rise to nerves that innervate upper limb – Four major branches of this plexus:  Roots—five ventral rami (C5–T1) unite to form…  Trunks—upper, middle, and lower, which unite to form…  Divisions—anterior and posterior, which unite to form …  Cords—lateral, medial, and posterior Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Brachial Plexus (1 of 4) Figure 13.10a The brachial plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Brachial Plexus (2 of 4) Figure 13.10b The brachial plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (6 of 14) Cords of brachial plexus give rise to nerves of the upper limb, five of most important being: – Axillary: innervates deltoid, teres minor, and skin and joint capsule of shoulder – Musculocutaneous: innervates biceps brachii and brachialis, coracobrachialis, and skin of lateral forearm – Median: innervates skin, most flexors, forearm pronators, wrist and finger flexors, thumb opposition muscles Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (7 of 14) Brachial plexus and upper limb (cont.) – Ulnar: supplies flexor carpi ulnaris, part of flexor digitorum profundus, most intrinsic hand muscles, skin of medial aspect of hand, wrist/finger flexion – Radial: innervates essentially all extensor muscles, supinators, and posterior skin of limb Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Brachial Plexus (3 of 4) Figure 13.10c The brachial plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Brachial Plexus (4 of 4) Figure 13.10d The brachial plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.4 Branches of the Brachial Plexus Table 13.4 Branches of the Brachial Plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.3 Injuries to brachial plexus are common Severe injuries can weaken or paralyze entire upper limb Injuries may occur if upper limb is pulled too hard, stretching plexus – Example: when football tackler yanks arm of running back – Blows to top of shoulder can force humerus inferiorly Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.4 Injury to median nerve makes it difficult to use pincer grasp (opposed thumb and index finger) to pick up small objects Seen in carpal tunnel syndrome, when median nerve is compressed – Also, frequent casualty of wrist-slashing suicide attempts Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.5 Ulnar nerve is very vulnerable to injury Severe or chronic damage to ulnar nerve can lead to sensory loss, paralysis, and muscle atrophy – Affected individuals have trouble making a fist and gripping objects – Little and ring fingers become hyperextended at the knuckles and flexed at distal interphalangeal joints – Causes hand to contort into a clawhand Striking the “funny bone,” the spot where this nerve rests against medial epicondyle, can make the little finger tingle. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Homeostatic Imbalance 13.5 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.6 Trauma to the radial nerve results in wrist drop, inability to extend the hand at the wrist Improper use of a crutch can compress radial nerve and impair its blood supply – “Saturday night paralysis”: An intoxicated person falls asleep with an arm draped over the back of a chair or sofa edge, cutting off blood supply to radial nerve Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Homeostatic Imbalance 13.6 Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (8 of 14) Lumbosacral plexus and lower limb – Lumbar and sacral plexuses have significant overlap  Fibers of lumbar plexus contribute to sacral plexus via lumbosacral trunk  Lumbosacral plexus serves mostly lower limb, but also sends some branches to abdomen, pelvis, and buttocks Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (9 of 14) – Lumbar plexus  Arises from L1 to L4  Innervates thigh, abdominal wall, and psoas muscle  Femoral nerve: innervates quadriceps and skin of anterior thigh and medial surface of leg  Obturator nerve: passes through obturator foramen to innervate adductor muscles Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Lumbar Plexus (1 of 2) Figure 13.11a The lumbar plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Lumbar Plexus (2 of 2) Figure 13.11b The lumbar plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (10 of 14) – Sacral plexus  Arises from L4 to S4  Serves the buttock, lower limb, pelvic structures, and perineum  Sciatic nerve – Longest and thickest nerve of body – Innervates hamstring muscles, adductor magnus, and most muscles in leg and foot – Composed of two nerves: tibial and common fibular Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Sacral Plexus (1 of 3) Figure 13.12a The sacral plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Sacral Plexus (2 of 3) Figure 13.12b The sacral plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Sacral Plexus (3 of 3) Figure 13.12c The sacral plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.5 Branches of the Lumbar Plexus Table 13.5 Branches of the Lumbar Plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.6 Branches of the Sacral Plexus Table 13.6 Branches of the Sacral Plexus. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 13.7 Summary of Major Spinal Nerve Plexuses Table 13.7 Summary of Major Spinal Nerve Plexuses. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.7 When spinal roots of lumbar plexus are compressed, gait problems occur Other symptoms are pain or numbness of the anterior thigh Femoral nerve serves prime movers that flex hip and extend knee – Damage can be caused by a herniated disc If obturator nerve is impaired, person experiences pain in the medial thigh Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.8 (1 of 2) Sciatica, a common problem, is characterized by stabbing pain radiating over course of the sciatic nerve – Injury could be caused by a fall, disc herniation, or badly placed injection into the buttock If the nerve is transected, leg is nearly useless and cannot be flexed because hamstrings are paralyzed Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.8 (2 of 2) Foot and ankle cannot move at all, so foot drops into plantar flexion, a condition called footdrop Recovery from sciatic nerve injury is usually slow and incomplete – For lesions below knee, thigh muscles are spared – If tibial nerve is injured, paralyzed calf muscles cannot plantar flex foot, and a shuffling gait develops Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (11 of 14) Anterolateral thorax and abdominal wall – Ventral rami of T12–T12 are intercostal nerves that supply muscles of ribs, anterolateral thorax, and abdominal wall – Give off cutaneous branches to skin along course – Two special thoracic nerves:  Tiny T1  T12, which is a subcostal nerve Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (12 of 14) – Back  Back is innervated by dorsal rami via several branches – Each branch innervates a strip of muscle and skin in line with where it emerges from spinal cord Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (13 of 14) Innervation of skin: dermatomes – Dermatome: area of skin innervated by cutaneous branches of single spinal nerve – All spinal nerves except C1 participate in dermatomes – Extent of spinal cord injuries ascertained by affected dermatomes – Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Map of Dermatomes Figure 13.13 Map of dermatomes. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Specific Body Regions (14 of 14) Innervation of joints – To remember which nerves serve which synovial joint, use:  Hilton’s law: Any nerve serving a muscle that produces movement at a joint also innervates that joint and skin over that joint Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Part 3 – Motor Endings and Motor Activity Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.6 Peripheral Motor Endings (1 of 2) Motor endings: PNS elements that activate effectors by releasing neurotransmitters These element innervate skeletal muscle, visceral muscle, and glands Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Skeletal Muscle (1 of 2) Takes place at neuromuscular junction Neurotransmitter acetylcholine (ACh) is released when nerve impulse reaches axon terminal Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Skeletal Muscle (2 of 2) ACh binds to receptors, resulting in: – Movement of Na+and K+ across membrane – Depolarization of muscle cell – An end plate potential, spreads to adjacent areas of sarcolemma, which triggers opening ofNa+voltage-gated channels – Results in an action potential, which leads to muscle contraction Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Events at the Neuromuscular Junction FOCUS FIGURE 9.1 Events at the Neuromuscular Junction. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Visceral Muscle and Glands Autonomic motor endings and visceral effectors are simpler than somatic junctions Branches form synapses en passant (“synapses in passing”) with effector cells via varicosities Acetylcholine and norepinephrine act indirectly via second messengers Visceral motor responses are slower than somatic responses Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Innervation of Smooth Muscle Figure 9.23 Innervation of smooth muscle. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.7 Levels of Motor Control Cerebellum and basal nuclei are the ultimate planners and coordinators of complex motor activities Complex motor behavior depends on complex patterns of control – Segmental level – Projection level – Precommand level Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Hierarchy of Motor Control (1 of 2) Figure 13.14a Hierarchy of motor control. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Segmental Level Lowest level of motor hierarchy – Consists of reflexes and automatic movements Segmental circuits activate networks of ventral horn neurons to stimulate specific groups of muscles Central pattern generators (CPGs): circuits that control locomotion and specific, often- repeated motor activity – Consist of networks of oscillating inhibitory and excitatory neurons, which set crude rhythms and patterns of movement Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Projection Level Consists of: – Upper motor neurons that initiate direct (pyramidal) system to produce voluntary skeletal muscle movements – Brain stem motor areas that oversee indirect (extrapyramidal) system to control reflex and CPG-controlled motor actions Projection motor pathways send information to lower motor neurons and keep higher command levels informed of what is happening Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Precommand Level (1 of 2) Neurons in cerebellum and basal nuclei – Regulate motor activity – Precisely start or stop movements – Coordinate movements with posture – Block unwanted movements – Monitor muscle tone – Perform unconscious planning and discharge in advance of willed movements Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Precommand Level (2 of 2) Cerebellum – Acts on motor pathways through projection areas of brain stem – Acts on motor cortex via thalamus to fine-tune motor activity Basal nuclei – Inhibit various motor centers under resting conditions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Hierarchy of Motor Control (2 of 2) Figure 13.14b Hierarchy of motor control. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Part 4 – Reflex Activity Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.6 Peripheral Motor Endings (2 of 2) Inborn (intrinsic) reflex: rapid, involuntary, predictable motor response to stimulus – Examples: maintain posture, control visceral activities – Can be modified by learning and conscious effort Learned (acquired) reflexes result from practice or repetition – Example: driving skills Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Components of a Reflex Arc (1 of 2) Components of a reflex arc (neural path) 1. Receptor: site of stimulus action 2. Sensory neuron: transmits afferent impulses to CNS 3. Integration center: either monosynaptic or polysynaptic region within CNS 4. Motor neuron: conducts efferent impulses from integration center to effector organ 5. Effector: muscle fiber or gland cell that responds to efferent impulses by contracting or secreting Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Components of a Reflex Arc (2 of 2) Reflexes are classified functionally as: – Somatic reflexes  Activate skeletal muscle – Autonomic (visceral) reflexes  Activate visceral effectors (smooth or cardiac muscle or glands) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Five Basic Components of all Reflex Arcs Figure 13.15 The five basic components of all reflex arcs. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 13.9 Spinal Reflexes Spinal reflexes occur without direct involvement of higher brain centers – Brain is still advised of spinal reflex activity and may have an effect on the reflex Testing of somatic reflexes important clinically to assess condition of nervous system – If exaggerated, distorted, or absent, may indicate degeneration or pathology of specific nervous system regions – Most commonly assessed reflexes are stretch, flexor, and superficial reflexes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (1 of 13) To smoothly coordinate skeletal muscle, nervous system must receive proprioceptor input regarding: – Length of muscle  Information sent from muscle spindles – Amount of tension in muscle  Information sent from tendon organs Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (2 of 13) Functional anatomy of muscle spindles – Composed of 3–10 modified skeletal muscle fibers called intrafusal muscle fibers that are enclosed in a connective tissue capsule  Central regions of intrafusal fibers lack myofilaments and are noncontractile  End regions contain actin and myosin myofilaments and can contract – Regular effector fibers of muscle referred to as extrafusal muscle fibers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (3 of 13) Functional anatomy of muscle spindles (cont.) Two types of afferent endings in muscle spindle send sensory inputs to CNS: – Anulospiral endings (primary sensory endings)  Endings wrap around spindle – Stimulated by rate and degree of stretch – Flower spray endings (secondary sensory endings)  Small axons at spindle ends – Stimulated by degree of stretch only Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (4 of 13) Functional anatomy of muscle spindles (cont.) Contractile end regions of spindle are innervated by gamma (γ) efferent fibers – Help maintain spindle sensitivity – Note: Extrafusal fibers (contractile muscle fibers) are innervated by alpha (α) efferent fibers of large alpha (α) motor neurons Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Anatomy of the Muscle Spindle and Tendon Organ Figure 13.16 Anatomy of the muscle spindle and tendon organ. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (5 of 13) Functional anatomy of muscle spindles (cont.) Muscle spindles are stretched (and excited) in two ways – External stretch: external force lengthens entire muscle – Internal stretch: γ motor neurons stimulate spindle ends to contract, thereby stretching spindle Stretching results in increased rate of impulses to spinal cord Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Operation of the Muscle Spindle (1 of 2) Figure 13.17a Operation of the muscle spindle. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (6 of 13) Functional anatomy of muscle spindles (cont.) Contracting muscle could reduce tension on muscle spindle, and sensitivity would be lost Situation avoided by muscle spindle also shortening by impulses from γ motor neurons that fire when α neurons fire αγ coactivation maintains tension and sensitivity of spindle during muscle contraction Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Operation of the Muscle Spindle (2 of 2) Figure 13.17b Operation of the muscle spindle. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (7 of 13) Stretch reflex – Brain sets muscle’s length via stretch reflex – Example: knee-jerk reflex is a stretch reflex that keeps knees from buckling when you stand upright – Stretch reflexes maintain muscle tone in large postural muscles and adjust it reflexively  Causes muscle contraction on side of spine in response to increased muscle length (stretch) on other side of spine Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (8 of 13) Stretch reflex (cont.) – How stretch reflex works:  Stretch activates muscle spindle  Sensory neurons synapse directly with α motor neurons in spinal cord   motor neurons cause extrafusal muscles of stretched muscle to contract Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (9 of 13) Stretch reflex (cont.) – Reciprocal inhibition also occurs—afferent fibers synapse with interneurons that inhibit  motor neurons of antagonistic muscles  Example: In patellar reflex, stretched muscle (quadriceps) contracts, and antagonists (hamstrings) relax Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (10 of 13) – All stretch reflexes are monosynaptic and ipsilateral (motor activity is on same side of body) – Positive reflex reactions provide two pieces of info:  Proves that sensory and motor connections between muscle and spinal cord are intact  Strength of response indicates degree of spinal cord excitability Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.9 Stretch reflexes can be hypoactive or absent if peripheral nerve damage or ventral horn injury has occurred – Reflexes are absent in people with chronic diabetes mellitus or neurosyphilis and during coma Stretch reflexes can be hyperactive if lesions of corticospinal tract reduce inhibitory effect of brain on spinal cord Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretched Muscle Spindles Initiate a Stretch Reflex, Causing Contraction of the Stretched Muscle and Inhibition of its Antagonist (1 of 2) FOCUS FIGURE 13.1 Stretch Reflex. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretched Muscle Spindles Initiate a Stretch Reflex, Causing Contraction of the Stretched Muscle and Inhibition of its Antagonist (2 of 2) FOCUS FIGURE 13.1 Stretch Reflex. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (11 of 13) Adjusting muscle spindle sensitivity – When γ neurons are stimulated by brain, spindle is stretched, and contraction force is maintained or increased – If γ neurons are inhibited, spindle becomes nonresponsive, and muscle relaxes – Important as speed and difficulty increase  Example: gymnast on balance beam Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (12 of 13) Tendon reflex – Involves polysynaptic reflexes – Helps prevent damage due to excessive stretch – Important for smooth onset and termination of muscle contraction Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Stretch and Tendon Reflexes (13 of 13) Tendon reflex (cont.) – Produces muscle relaxation (lengthening) in response to tension  Contraction or passive stretch activates tendon reflex  Afferent impulses transmitted to spinal cord – Contracting muscle relaxes; antagonist contracts (reciprocal activation)  Information transmitted simultaneously to cerebellum and used to adjust muscle tension Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Tendon Reflex Figure 13.18 The tendon reflex. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Flexor and Crossed-Extensor Reflexes (1 of 2) Flexor (withdrawal) reflex is initiated by painful stimulus – Causes automatic withdrawal of threatened body part – Ipsilateral and polysynaptic  Many different muscles may be called into play, so needs to be polysynaptic – Protective and important to survival – Brain can override  Example: Knowing a finger stick for blood test is coming, brain overrides pulling arm away Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Flexor and Crossed-Extensor Reflexes (2 of 2) Crossed extensor reflex occurs with flexor reflexes in weight-bearing limbs to maintain balance – Consists of ipsilateral withdrawal reflex and contralateral extensor reflex  Stimulated side withdrawn (flexed)  Contralateral side extended – Examples: Stepping barefoot on broken glass causes damaged leg to withdraw and opposite leg to extend to support weight shift – Someone grabbing your arm causes that arm to flex and opposite arm to extend to pull body away Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Crossed-Extensor Reflex Figure 13.19 The crossed-extensor reflex. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Superficial Reflexes (1 of 3) Superficial reflexes are elicited by gentle cutaneous stimulation of area Clinically important reflexes signal problems in upper motor pathways or cord-level reflex arcs Best known: – Plantar reflex – Abdominal reflex Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Superficial Reflexes (2 of 3) – Plantar reflex  Tests integrity of cord from L4 to S2  Stimulus: stroke lateral aspect of sole of foot  Response: downward flexion of toes  Damage to motor cortex or corticospinal tracts causes abnormal response known as Babinski’s sign – Hallux dorsiflexes; smaller toes fan laterally – Normal in infancy to age of ~1 year because myelination is still incomplete Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Superficial Reflexes (3 of 3) – Abdominal reflexes  Tests integrity of cord from T8 to T12  Stimulus: stroking skin of lateral abdomen above, below, or to side of umbilicus  Response: contraction of abdominal muscles and movement of umbilicus toward stimulus  Vary in intensity from one person to another  Absent when corticospinal tract lesions are present Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 13.10 If primary motor cortex or corticospinal tract is damaged, plantar reflex is replaced by an abnormal reflex called Babinski’s sign The great toe dorsiflexes and smaller toes fan laterally Infants exhibit Babinski’s sign until they are about a year old because nervous systems are not completely myelinated Although clinical relevant, physiological mechanism of Babinski’s sign is not understood Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Developmental Aspects of the Peripheral Nervous System (1 of 2) Spinal nerves branch from developing spinal cord and neural crest cells Exit between forming vertebrae – Supply both motor and sensory fibers to developing muscles to help direct their maturation – Cranial nerves innervate muscles of head Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Developmental Aspects of the Peripheral Nervous System (2 of 2) Distribution and growth of spinal nerves correlate with segmented body plan With age, sensory receptors atrophy, muscle tone decreases in face and neck, reflexes slow – Decreased numbers of synapses per neuron, and slower central processing Peripheral nerves viable throughout life unless subjected to trauma Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Copyright Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved