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Peripheral Nervous System (PNS) PNS – all neural structures outside the brain and spinal cord Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings Provides links to and from the external environment Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Sensory Receptors Structures specialized to respond to stimuli Activation of sensory receptors results in graded potentials that trigger nerve impulses along the afferent PNS fibers to the CNS The realization of these stimuli, sensation and perception, occur in the brain Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Classification by Stimulus Type Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch Thermoreceptors – sensitive to changes in temperature Photoreceptors – respond to light energy (e.g., retina) Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors – sensitive to pain-causing stimuli Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Class by Location: Exteroceptors Respond to stimuli arising outside the body Found near the body surface Sensitive to touch, pressure, pain, and temperature Include the special sense organs Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Class by Location: Interoceptors (Visceroceptos) Respond to stimuli arising within the body Found in internal viscera and blood vessels Sensitive to chemical changes, stretch, and temperature changes Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Class by Location: Proprioceptors Respond to degree of stretch of the organs they occupy Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles Constantly “advise” the brain of one’s movements Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptor Classification by Structural Complexity Receptors are structurally classified as either simple or complex Most receptors are simple: Are modified dendritic endings of sensory neurons encapsulated and unencapsulated varieties Complex receptors are special sense organs associated with special senses (vision, hearing, equilibrium, smell, taste) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Simple Receptors: Unencapsulated Free dendritic nerve endings Respond chiefly to temperature and pain Cold receptors found superficially (10-40°C) Heat receptors found deeper (32-48°C) Outside of these ranges, nociceptors are activated and pain is perceived Nociceptors also respond to pinch and chemicals released from damaged tissue E.g. slap a sunburn or ungloved hands Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Simple Receptors: Unencapsulated Abundant in epithelia and connective tissues Unmyelinated Distal ends have knob-like swellings Merkel Cells (tactile; form Merkel discs) lying in the deeper dermal layers. Function as light touch receptors Hair follicle receptors (free nerve endings wrapped around hair follicles) are light touch receptors that detect bending hair (e.g. detection of a landing mosquito) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Simple Receptors: Encapsulated Dendritic Endings Enclosed in a connective tissue capsule All are mechanoreceptors Meissner’s corpuscles (tactile corpuscles): found beneath the epidermis and are numerous in sensitive, hairless skin, e.g. fingers & soles of feet Pacinian corpuscles (lamellated corpuscles): found deep in the dermis and subcutaneous tissue underlying the skin Mechanoreceptors stimulated by deep pressure Stimulated only wihen pressure is first applied, e.g. sense vibrations (on/off) Huge receptor!! Up to 3 mm long and 1.5 mm wide! Muscle spindles, Golgi tendon organs, and Ruffini’s corpuscles Joint kinesthetic receptors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Simple Receptors: Encapsulated Dendritic Endings Ruffin endings: found in the dermis, subcutaneous tissue, and joint capsules. Respond to deep, continuous pressure Muscle spindles: Proprioceptors found thru out the perimysium of skeletal muscle. They detect muscle stretch and initiate a reflex that resists the stretch Joint kinesthetic receptors: Proprioceptors that monitor stretch in the anterior capsules that enclose the synovial joints Provide info. on joint position and motion Golgi tendon organs: Proprioceptors located in tendons close to the skeletal muscle insertion. Activated by compression when tendon fibers are stretched with muscle contraction. When golgi tendon organs are activated, the contracting muscle is inhibited causing it to relax. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings From Sensation to Perception Survival depends upon sensation and perception Sensation is the awareness of changes in the internal and external environment Perception is the conscious interpretation of those stimuli Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Organization of the Somatosensory System Input comes from exteroceptors, proprioceptors, and interoceptors The three main levels of neural integration in the somatosensory system are: Receptor level – the sensory receptors Circuit level – ascending pathways Perceptual level – neuronal circuits in the cerebral cortex Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.2 Processing at the Receptor Lever In order for a stimulus to excite a receptor: The receptor must have specificity for the stimulus energy (e.g. light, chemicals, touch, etc..) The receptor’s receptive field must be stimulated. The smaller the field, the better the brain can localize it Stimulus energy must be converted into a graded potential (transduction) A generator potential in the associated sensory neuron must reach threshold Voltage gated Na+ channels on the axon are opened and nerve impulse propagates toward CNS Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Adaptation of Sensory Receptors Adaptation occurs when sensory receptors are subjected to an unchanging stimulus Phasic receptors: Fast adapting burst of impulses at onset and offset Report changes in environment Tonic receptors Provide sustained response with no adaptation Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Processing at the Circuit Level The task here is to deliver impulses to the appropriate region of the cerebral cortex for stimulus localization and perception Ascending Sensory Pathways: Chains of three neurons conduct sensory impulses upward to the brain First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from the skin to the spinal cord or brain stem Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and transmit impulses to the thalamus or cerebellum Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex of the cerebrum Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Processing at the Perceptual Level Interpretation of sensory input occurs at the cerebral cortex The ability to identify a sensation depends on the location of the target neurons in the cortex regardless of what stimulated the neurons Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Main Aspects of Sensory Perception Perceptual detection – detecting that a stimulus has occurred Magnitude estimation – how much of a stimulus is acting Spatial discrimination – identifying the site or pattern of the stimulus (e.g. the 2-point descrimination test: back vs. hand) Feature abstraction – used to identify a substance that has specific texture or shape Quality discrimination – the ability to identify submodalities of a sensation (e.g., sweet or sour tastes) Pattern recognition – ability to recognize patterns in stimuli (e.g. familiar face) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Structure of a Nerve Nerve – cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue Connective tissue coverings include: Endoneurium – loose connective tissue that surrounds axons Perineurium – coarse connective tissue that bundles fibers into fascicles Epineurium – tough fibrous sheath around a nerve Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Classification of Nerves Sensory and motor divisions Sensory (afferent) – carry impulse to the CNS Motor (efferent) – carry impulses from CNS Mixed – sensory and motor fibers carry impulses to and from CNS; most common type of nerve Ganglia: a collection of neuron cell bodies associated with nerves in the PNS Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Regeneration of Nerve Fibers Rengeneration is most effective, in general, only over short distances Damage to nerve tissue is serious because mature neurons are amitotic If the soma of a damaged nerve remains intact, damage can be repaired Regeneration involves coordinated activity among: Macrophages – remove debris Schwann cells – form regeneration tube and secrete growth factors Axons – regenerate damaged part Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Regeneration of Nerve Fibers -Axon and myelin sheath distal to injury disintegrate due to lack of nutrients from the cell body (Wallerian degeneration) -Neurilemma remains intact -Schwann cells proliferate and migrate to injury site and form a regeneration tube that guide axon sprouts -Cell body changes; chromatophilic substance breaks up and massive protein synthesis begins (cell body swells) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.4 Spinal Nerves Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head They are named according to their point of issue 8 cervical (C1-C8) (Only 7 cervical vertebrae!) 12 thoracic (T1-T12) 5 Lumbar (L1-L5) 5 Sacral (S1-S5) 1 Coccygeal (C0) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Spinal Nerves Cervical nerves: exit superior to the vertebrae C8 emerges inferior to C7 (between C7 and T1) T, L, S and coccygeal all exit inferior to the same numbered vertebrae Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.6 Spinal Nerves: Roots Ventral roots: motor (efferent) fibers Dorsal roots: sensory (afferent) fibers They unite to form a spinal nerve before emerging from the vertebral column via a intervertebral foramina After emerging from the foramina, the spinal nerve divides into small dorsal & larger ventral rami Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Innervation of Specific Body Regions Spinal nerve rami supply the entire somatic region of the body from the neck down Dorsal rami supply the posterior body trunk Ventral rami supply the anterior body trunk and limbs and are thicker than dorsal rami Roots: each root is either sensory or motor (medial) Rami: each rami is sensory and motor (lateral) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Ventral Rami T2-T12 All ventral rami branch and join one another lateral to the vertebral column forming the nerve plexuses (which primarily serve the limbs) Only ventral rami form plexuses Extreme branching of nerves results in each muscle being supplied by more than one spinal nerve The strategy is to offset damage via redundant innervation and prevent paralysis Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Spinal Nerve Innervation: Back, Anterolateral Thorax, and Abdominal Wall The back is innervated by dorsal rami via several branches The thorax is innervated by ventral rami T1-T12 as intercostal nerves Intercostal nerves supply muscles of the ribs, anterolateral thorax, and abdominal wall. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Cervical Plexus Nerve Plexus combine sets of spinal nerves that serve the same area of the body into one large grouped nerve. The cervical plexus is formed by ventral rami of C1-C4 and is located deep to the sternocleidomastoid muscle Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders The most important nerve of this plexus is the phrenic nerve which innervates the diaphram Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Brachial Plexus and Upper Limb Gives rise to nerves that innervate the upper limbs Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus) There are four major branches of this plexus Roots – five ventral rami (C5-T1) Trunks – upper, middle, and lower, which form divisions Divisions – anterior and posterior serve the front and back of the limb Cords – lateral, medial, and posterior fiber bundles Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Brachial Plexus Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.9a Brachial Plexus: Nerves Axillary – innervates the deltoid and teres minor Musculocutaneous – sends fibers to the biceps brachii and brachialis Median – branches to most of the flexor muscles of arm Ulnar – supplies the flexor carpi ulnaris and part of the flexor digitorum profundus Radial – innervates essentially all extensor muscles Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Brachial Plexus: Distribution of Nerves Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.9c Lumbar Plexus and Lower Limb Arises from L1-L4 Lies within the psoas major muscle Major branches descend and innervate the anterior and medial thigh Femoral nerve: anterior thigh muscles (quads), skin of anterior thigh, surface of leg from knee to foot Obturator nerve: adductor muscles Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Sacral Plexus Arises from L4-S4 and serves the buttock, lower limb, pelvic structures, and the perineum The major nerve is the sciatic, the longest and thickest nerve of the body The sciatic is actually composed of two nerves: the tibial and the common fibular (peroneal) nerves Supplies entire lower limb (except antero-medial thigh) Tibial nerve: calf muscles and skin, sole of foot Sural nerve: posterolateral leg skin Medial/lateral plantar nerves: foot Fibular nerve: knee joint, skin of lateral calf, dorsum of foot, muscles of anterolateral leg Superior/inferior gluteal nerves: gluteal muscles and tensor fascia latae Pudendal nerve: skin of perineum (ano-genital region) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Sacral Plexus Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.11 Innervation of the Skin: Dermatomes A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve All spinal nerves except C1 participate in dermatomes Lumbar (anterior) vs. Sacral (posterior) Cervical (upper limbs) vs. Thoracic (trunk) Innervation of Joints: Question: Which nerves serve which synovial joints? Answer: They obey Hilton’s Law which is… Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Don’t you go changing, baby!” Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.12 Dermatomes Actually, Hilton’s Law states, “Any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint.” Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Motor Endings Motor endings are PNS elements that activate effectors by releasing neurotransmitters at: Neuromuscular junctions Varicosities at smooth muscle and glands Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Innervation of Skeletal Muscle Takes place at a neuromusclular junction Acetylcholine is the neurotransmitter that diffuses across the synaptic cleft ACh binds to receptors resulting in: Movement of Na+ and K+ across the membrane Depolarization of the interior of the muscle cell An end-plate potential that triggers an action potential Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Innervation of Visceral Muscle and Glands Junctions of autonomic motor endings and their effectors (smooth, cardiac muscle and glands) Autonomic motor axons branch repeatedly, each branch forming synapses with effector cells via variocosities (“string of beads”) Contain either Ach or norepinephrine that act on 2nd messenger systems Thus, they have a slower response time Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Levels of Motor Control The cerebellum and the basal nuclei are the ultimate planners and coordinators of complex motor activities The three levels of motor control are Segmental level Projection level Precommand level Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Hierarchy of Motor Control Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.13 Segmental Level Lowest level of motor control Consists of spinal cord circuits Activates ventral horn neurons in a group of cord segments causing them to stimulate specific groups of muscles Central Pattern Generators (CPGs) are circuits that stimulate often repeated activities (walking, breathing, swallowing) without significant input from the cerebral cortex Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Projection Level The projection level consists of: Cortical motor areas that produce the direct (pyramidal) system Brain stem motor areas that oversee the indirect (multineuronal) system Axons here produce discrete voluntary movements of skeletal muscles Projection motor pathways convey info. to lower motor neurons and send feedback to higher command levels Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Precommand Level Cerebellar and basal nuclei systems that: Regulate motor activity Precisely start or stop movements Coordinate movements with posture Block unwanted movements Monitor muscle tone Key center is cerebellum (the target for ascending proprioceptors, tactile, equilibrium, vision) Lacks direct connections with the spinal cord. Acts on motor pathways on the motor cortex via the thalamus Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Basal Nuclei Receive inputs from all cortical areas and send their output back to premotor and prefrontal cortical areas via the thalamus Involved in complex aspects of motor control At rest, the basal nuclei inhibits motor centers of the brain Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Reflexes A reflex is a rapid, predictable motor response to a stimulus Reflexes may: Be inborn (intrinsic) or learned (acquired) Involve only peripheral nerves and the spinal cord Involve higher brain centers as well Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Reflex Arc There are five components of a reflex arc Receptor – site of stimulus Sensory neuron – transmits the afferent impulse to the CNS Integration center – either monosynaptic or polysynaptic region within the CNS Motor neuron – conducts efferent impulses from the integration center to an effector Effector – muscle fiber or gland that responds to the efferent impulse Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Reflexes Reflexes are classified functionally as somatic reflexes if they activate skeletal muscle, or autonomic (visceral) reflexes if they activate visceral effectors of smooth, cardiac muscle or glands Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Spinal Reflexes Do Not Need A Brain! The brain, however, is advised and can facilitate, inhibit, and adapt the reflex Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Stretch and Deep Tendon Reflexes For skeletal muscles to perform normally, they need 2 types of information: 1. Tension in the muscle (and tendon) 2. Length of the muscle Supplied by the golgi tendon organs Supplied by the muscle spindles Both play a role in spinal reflexes, both are proprioceptors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Muscle Spindles Are composed of 3-10 intrafusal muscle fibers that lack myofilaments in their central regions, are noncontractile, and serve as receptive surfaces Muscle spindles are wrapped with two types of afferent endings: primary sensory endings of type 1a fibers and secondary sensory endings of type 2 fibers These regions are innervated by gamma (γ) efferent fibers Note: contractile muscle fibers are extrafusal fibers and are innervated by alpha (α) efferent fibers Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Operation of the Muscle Spindle Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.17 Stretch Reflex Stretching the muscle activates the muscle spindle Excited γ motor neurons of the spindle cause the stretched muscle to contract Afferent impulses from the spindle result in inhibition of the antagonist muscle Example: patellar reflex Tapping the patellar tendon stretches the quadriceps and starts the reflex action The quadriceps contract and the antagonistic hamstrings relax Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Stretch Reflex Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.16 Stretch Reflex The stretch reflex makes sure that the muscle maintains a certain length E.g. knee jerk reflex keeps our legs from buckling! All stretch reflexes are monosynaptic and ipsilateral. Important things to know: 1. KJR proves that the sensory and motor connections between that muscle and the spinal cord are intact 2. The vigor of the response indicates the degree of excitability of the spinal cord Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Golgi Tendon Reflex The opposite of the stretch reflex Contracting the muscle activates the Golgi tendon organs Afferent Golgi tendon neurons are stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated As a result, the contracting muscle relaxes and the antagonist contracts Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Golgi Tendon Reflex Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 13.18 Golgi Tendon Reflex The Golgi Tendon Reflex protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load. First, as a load is placed on the muscle, the afferent neuron from the Golgi tendon organ fires into the CNS Second, the motor neuron from the spinal cord is inhibited via an IPSP (Inhibitory postsynaptic potential) and muscle relaxes. As a result, you drop the load. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Flexor and Crossed Extensor Reflexes The flexor reflex is initiated by a painful stimulus (actual or perceived) that causes automatic withdrawal of the threatened body part The crossed extensor reflex has two parts The stimulated side is withdrawn The contralateral side is extended Let’s demonstrate! Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Crossed Extensor Reflex Interneurons + + – + + – Afferent fiber Efferent fibers Efferent fibers Extensor inhibited Flexor stimulated s xe e l F Flexor inhibited Arm movements Extensor stimulated s end t x E Key: + Excitatory synapse – Inhibitory synapse Right arm (site of stimulus) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Left arm (site of reciprocal activation) Figure 13.19 Superficial Reflexes Initiated by gentle cutaneous stimulation Example: Plantar reflex is initiated by stimulating the lateral aspect of the sole of the foot The response is downward flexion of the toes Indirectly tests for proper corticospinal tract functioning Babinski’s sign: abnormal plantar reflex indicating corticospinal damage where the great toe dorsiflexes and the smaller toes fan laterally (seen in infants up to 1 year of age) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings KU Game Day: The Undefeated Lady Cougars! Wed. 7 pm Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings