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Chapter 13: The Peripheral Nervous System and Reflex Activity.

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1 Chapter 13: The Peripheral Nervous System and Reflex Activity

2 Peripheral Nervous System (PNS) Links outside world and CNS Includes all neural structures outside the brain and spinal cord – Sensory receptors – Peripheral nerves and their ganglia – Motor endings Sensory receptors – respond to changes in environment – stimuli – Activated  graded potential  nerve impulse Sensation – awareness of stimuli Perception – interpretation of meaning – Both occur in brain

3 Figure 13.1 Central nervous system (CNS)Peripheral nervous system (PNS) Motor (efferent) divisionSensory (afferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division

4 Sensory Receptors Classified according to 1.Type of stimulus they detect 2.Body location 3.Structural complexity

5 Stimulus Type 1. Mechanoreceptors – mechanical force – Touch pressure (including BP), vibration, and stretch 2. Thermoreceptors – temperature changes 3. Photoreceptors – light energy – retina of eye 4. Chemoreceptors – chemicals in solution – Molecules tasted or smelled, changes in blood or intestinal chemistry 5. Nocieptors – potentially damaging stimuli that result in pain – Searing heat, extreme cold, pressure, inflammatory chemicals

6 Location 1. Exteroceptors – sensitive to stimuli arising outside of the body – Body surface – Touch, pressure, pain, temperature – Senses – vision, hearing, equilibrium, taste, smell 2. Interoceptors – visceroceptors – stimuli with in the body – Internal viscera and blood vessels – Chemical changes, tissue stretch, temp 3. Proprioceptors – internal stimuli – skeletal muscles, tendons, joints, ligaments, and CT coverings – Advise the brain of body movements

7 Structural Complexity Simple Receptors of General Senses – – Receptors respond to several stimuli 2 types 1. Unencapsualted Dendritic Endings – free or naked nerve endings – Present nearly everywhere – Abundant in CT and epithelia – Unmyelinated, small diameter C fibers – Distal endings – small knoblike swellings – Respond to temperature and painful stimuli

8 Simple Receptors 1. Unencapsualted Dendritic Endings (cont) Temperature outside range – cold – 10-40C and hot – 32-48C – perceived as painful Also respond to pinch and chemicals released by damaged tissue Itch Tactile (Merkle discs) – free nerve endings associated with enlarged disc shaped epidermyal cells Also wrap around hair follicles

9 Table 13.1

10 Simple Receptors 2. Encapsulated Dendritic Endings – consist of one or more fiber terminals of sensory neurons enclosed in a CT capsule – Most are mechanoreceptors – vary in size, shape, and distribution Meissner’s corpuscles – small receptors surrounded by Schwann cells and thin CT capsule – touch receptors Pacinian Corpuscles – lamellated corpuscles – scattered deep in epidermis – pressure Ruffini Endings – lie in dermis – flattened capsule – deep and continuous pressure

11 Simple Receptors 2. Encapsulated Dendritic Endings (cont) – Muscle spindles – fusiform proprioceptors – perimysium of skeletal muscle – muscle stretch and reflex that resists stretch Golgi tendon organs – proprioceptors in tendons – tendon fibers stretched – nerve endings are activated Joint Kinesthetic Receptors – proprioceptors – monitor articular capsules of synovial joints – info on joint position and motion

12 Table 13.1

13 Complex Receptors Sense organs Localized collections of cells associated with the special senses

14 Sensory Integration Sensation – awareness of changes in internal and external environment Perception – conscious interpretation of these stimuli We depend on both to survive

15 Somatosensory System Part of the sensory system serving the body wall and limbs Receives input from exteroceptors, proprioceptors, and interoceptors 3 main levels of neural integration – – 1. receptor level – sensory receptors – 2. Circuit level – ascending pathways – 3. Perceptual level – neuronal circuits in cerebral cortex

16 Figure Receptor level (sensory reception and transmission to CNS) Circuit level (processing in ascending pathways) Spinal cord Cerebellum Reticular formation Pons Muscle spindle Joint kinesthetic receptor Free nerve endings (pain, cold, warmth) Medulla Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus

17 1. Receptor Level Sensation – stimulus must excite a receptor and APs must reach the CNS For this to happen – stimulus – energy must match specifically to receptor must be applied within the receptive field Energy must be converted into a graded potential (receptor potential) by transduction Generator potential in the associated neuron must reach a threshold

18 1. Receptor Level Adaptation– sensory receptors can change sensitivity in presence of a constant stimulus Phasic receptors – fast adapting – bursts of impulses at the begging and end of stimulus Tonic Receptors – sustained response – little or no adaptation

19 2. Circuit Level Delivers impulses to the cerebral cortex for stimulus localization and perception

20 3. Perceptual Level Interpretation of Sensory input in cerebral cortex Projection – exact point in cortex that is activated is always the same “where” regardless of how it is activated

21 3. Perceptual Level Sensory Perception – Perceptual detection – ability to detect that a stimulus has occurred Magnitude Estimation – ability to detect how intense the stimulus is Spatial discrimination – identify the site or pattern of stimulation Feature Abstraction – mechanism by which one neuron or circuit is turned to one feature in the presence of another Quality discrimination – ability to differentiate submodalities (qualities) of a sensation Pattern recognition – ability to take in the scene around us and recognize a familiar pattern

22 Perception of Pain Receptors activated by extremes of pressure and temperature, as well as, chemicals release by damaged tissue Sharp pain – small myelinated A delta fibers Burning pain – small unmyelinated C fibers Both release glutamate and substance P  activate 2 nd order neurons Hyperalgesia – pain amplification Phantom Limb pain – pain in tissue that is no longer present

23 Transmission Lines – Nerves & Their Ganglia Structure and Classification – Nerve – cordlike organ Vary in size Consists of parallel bundles of peripheral axons enclosed by CT Axon – surrounded by endoneurium – CT layer Groups of fibers (fascicles) bound together by perineurium Finally fascicles are enclosed by - epineurium

24 Figure 13.3b Blood vessels Fascicle Epineurium Perineurium Endoneurium Axon Myelin sheath (b)

25 Nerves & Their Ganglia Classified according to the direction which they transmit impulses Mixed nerves – both ways Sensory (afferent) nerves – carry impulses towards the CNS Motor (efferent) nerves – carry impulses away from CNS Ganglia – collections of neuron cell bodies associated with nerves in the PNS

26 Regeneration of Nerve Fibers Real mature neurons do not divide Damage severe or close to cell body – entire neuron may die Other neurons attached to that neuron may also die Cell body intact – cut or compressed nerves can regenerate successfully

27 Regeneration of Nerve Fibers 1.Axon becomes fragmented at the injury site 2.Macrophages clean out the dead axon distal to injury 3.Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells 4.The axon regenerated and a new myelin sheath forms

28 Figure 13.4 (1 of 4) Endoneurium Droplets of myelin Fragmented axon Schwann cells Site of nerve damage The axon becomes fragmented at the injury site. 1

29 Figure 13.4 (2 of 4) Schwann cellMacrophage Macrophages clean out the dead axon distal to the injury. 2

30 Figure 13.4 (3 of 4) Fine axon sprouts or filaments Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.

31 Figure 13.4 (4 of 4) Schwann cell Site of new myelin sheath formation 4 The axon regenerates and a new myelin sheath forms. Single enlarging axon filament

32 Cranial Nerves 12 pairs associated with brain 1 st – forebrain Rest – brain stem Only head and neck structures

33 Figure 13.5 (a) Frontal lobe Temporal lobe Infundibulum Facial nerve (VII) Vestibulo- cochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Hypoglossal nerve (XII) (a) Filaments of olfactory nerve (I) Olfactory bulb Olfactory tract Optic chiasma Optic nerve (II) Optic tract Oculomotor nerve (III) Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Cerebellum Medulla oblongata

34 Figure 13.5 (b) *PS = parasympathetic (b) Cranial nerves I – VI I II III IV V VI Olfactory Optic Oculomotor Trochlear Trigeminal Abducens Yes (smell) Yes (vision) No Yes (general sensation) No Yes No Yes No Cranial nerves VII – XII Sensory function Motor function PS* fibers Sensory function Motor function PS* fibers VII VIII IX X XI XII Facial Vestibulocochlear Glossopharyngeal Vagus Accessory Hypoglossal Yes (taste) Yes (hearing and balance) Yes (taste) No Yes Some Yes No Yes No

35 Cranial Nerves I. Olfactory – tiny sensory nerves of smell Run from nasal mucosa to synapse with the olfactory bulb II. Optic – sensory nerve of vision – brain tract III. Oculomotor – “eye mover” – 6 extrinsic muscles that move the eye IV. Trochlear – “pulley” innervates extrinsic eye muscle through a pully shaped ligament

36 Table 13.2

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40 Cranial Nerves V. Trigeminal – 3 branches, sensory fibers to the face and motor fibers to the chewing muscles VI. Abducens – controls extrinsic eye muscle that abducts the eyeball VII. Facial – large nerve – innervates muscles of facial expression VIII. Vestibulocochlear – auditory nerve – hearing and balance

41 Table 13.2

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47 Cranial Nerves IX. Glossopharyngeal – tongue and Pharynx X. Vagus – only cranial nerve that extends beyond the head into the thorax and abdomen XI. Accessory – accessory part of the vagus nerve XII. Hypoglossal – under the tongue, innervates the tongue muscles

48 Table 13.2

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52 Cranial Nerves Mixed nerves Cell bodies located in cranial sensory ganglia except – olfactory and optic Somatic and autonomic motor fibers Serve skeletal muscle and visceral organs Primary functions: Sensory, Motor or both

53 Spinal Nerves 31 pairs Each 1000s of nerve fibers Named according to their point of issue 8 pairs cranial spinal nerves – C1 –C8 only 7 vertebrae – C8 emerges inferior to 7 th vertebrae 12 pairs of thoracic – T1- T12 5 pairs of lumbar – L1-L5 5 pairs of sacral – S1-S5 1 pair of coccygeal - Co1

54 Figure 13.6 Cervical nerves C 1 – C 8 Thoracic nerves T 1 – T 12 Lumbar nerves L 1 – L 5 Sacral nerves S 1 – S 5 Coccygeal nerve Co 1 Cervical plexus Intercostal nerves Cervical enlargement Lumbar enlargement Cauda equina Brachial plexus Lumbar plexus Sacral plexus

55 Spinal Nerves Connect to spinal cord by a dorsal root and a ventral root Each root forms a series of rootlets that attach along the length of spinal cord segment Ventral Roots – motor (efferent) fibers – arise from ventral horn motor neurons – impulses to CNS Dorsal Roots – sensory (afferents) fibers – arise from the sensory neurons in dorsal root ganglia – impulse to spinal cord

56 Figure 13.7 (a) Dorsal root ganglion Gray matter White matter Ventral root Dorsal root Dorsal and ventral rootlets of spinal nerve Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Sympathetic trunk ganglion Spinal nerve Rami communicantes Anterior view showing spinal cord, associated nerves, and vertebrae. The dorsal and ventral roots arise medially as rootlets and join laterally to form the spinal nerve.

57 Spinal Nerves Short Immediately after emerging from spinal cord – divides into dorsal ramus, ventral ramus, and a meningeal branch Meningral branch reenters the canal to innervate the meninges Based to the ventral rami – special rami communicantes – autonomic (visceral) nerve fibers

58 Figure 13.7 (b) Dorsal ramus Ventral ramus Intercostal nerve Spinal nerve Rami communicantes Dorsal root ganglion Dorsal root Ventral root Sympathetic trunk ganglion Sternum (b) Cross section of thorax showing the main roots and branches of a spinal nerve. Branches of intercostal nerve Lateral cutaneous Anterior cutaneous

59 Innervation of Body Regions Supply entire somatic region of body Dorsal rami – posterior trunk Ventral rami – rest of trunk and limbs Nerve Plexuses – complicated interlacing networks formed by ventral rami Fibers from the various rami crisscross one another and become redistributed so that – each branch contains fibers from several spinal nerves and fibers travel via several roots – damage to one segment cannot completely paralyze any muscle limb

60 Innervation of Body Regions Back – segmented plan Each dorsal ramus innervates a narrow strip of muscle and skin in line with which it emerges from the spinal column Anterolateral thorax and Abdominal Wall – T1 – T12 – course anteriorly – deep to each rib – intercostal nerves T12 – subcostal nerve

61 Innervation of Body Regions Cervical Plexus and Neck – cervical plexus – formed by ventral rami of the 1 st 4 cervical nerves Cutaneous nerves supply the skin Phernic nerve - diaphragm

62 Figure 13.8 Hypoglossal nerve (XII) C1C1 C2C2 C3C3 C4C4 C5C5 Segmental branches Lesser occipital nerve Greater auricular nerve Ansa cervicalis Phrenic nerve Supraclavicular nerves Accessory nerve (XI) Transverse cervical nerve Ventral rami: Ventral rami

63 Innervation of Body Regions Brachial Plexus and Upper Limb – situated partially in the next – gives rise to all nerves in upper limb 4 major branches – 1.ventral rami 2.Trunks 3.Divisions 4.Cords

64 Figure 13.9 (a) Upper Middle Trunks Lower Roots (ventral rami): Upper subscapular Lower subscapular Thoracodorsal Medial cutaneous nerves of the arm and forearm Long thoracic Medial pectoral Lateral pectoral Nerve to subclavius Suprascapular Dorsal scapular Posterior divisions Anterior divisions Lateral Posterior Cords Medial Axillary Musculo- cutaneous Radial Median Ulnar Posterior divisions Trunks Roots C4C4 C5C5 C6C6 C7C7 C8C8 T1T1 (a) Roots (rami C 5 – T 1 ), trunks, divisions, and cords

65 Innervation of Body Regions -Auxiliary nerve – innervates deltoid, teres minor, skin and joint capsule -Musculocutaneous nerve – biceps brachial, brachialis muscle -lateral forearm -Median nerve – anterior forearm – skin and flexor muscles -Ulnar nerve – flexors not supported by median nerve -Radial nerve – continuation of posterior cord, posterior skin of limb, extensor muscles – elbow extension, forearm supination. Wrist and finger extension, and thumb abduction

66 Figure 13.9 (c) Median nerve Musculocutaneous nerve Radial nerve Humerus Ulna Ulnar nerve Median nerve Radius Radial nerve (superficial branch) Superficial branch of ulnar nerve Dorsal branch of ulnar nerve Digital branch of ulnar nerve Muscular branch Digital branch (c) The major nerves of the upper limb Axillary nerve Anterior divisions Posterior divisions TrunksRoots

67 Table 13.4

68 Innervation of Body Regions Lumbosacral and Lower Limb – lumbosacral plexus Lumbar plexus – L1-L4 – anterior and medial thigh – femoral nerve – quads, thigh flexors and knee extensors Obturator nerve – adductor muscles

69 Innervation of Body Regions Lumbosacral and Lower Limb – Sacral Plexus – L4 –S4 – buttock and lower limb Sciatic nerve – entire lower limb Tibial nerve – posterior compartments of leg Superior and inferior gluteal nerves – buttock and tensor fascia lata

70 Figure (a) Ventral rami and major branches of the lumbar plexus Iliohypogastric L1L1 L2L2 L3L3 L4L4 L5L5 Ilioinguinal Genitofemoral Lateral femoral cutaneous Obturator Femoral Lumbosacral trunk Lateral femoral cutaneous Anterior femoral cutaneous Saphenous Obturator Iliohypogastric Ilioinguinal Femoral Ventral rami Ventral rami: (b) Distribution of the major nerves from the lumbar plexus to the lower limb

71 Table 13.5

72 Innervation of Skin: Dermatomes Dermatome – area of skin innervated by cutaneous branches of single spinal cord Uniform in width, almost horizontal, and in a direct line with their spinal nerves

73 Figure C2 C3 C4 C5 T1 T2 T3 T4 T5 C6 C8 C7 C6 T6 T7 T8 T9 T10 T11 T12 L1 S2 S3 L1 L2 L3 L4 L5 L2 L3 L4 L5 S1 C5 C6 C8 T2 C5 C6 S1 Anterior view C2 C3 C4 C5 C6 C7 C8 C7 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 L1 L2 L3 S1 (b) Posterior view L5 S2 S1 S3 S2S1S2 S4 S5 L5 L4 L5 L4 C6 C5 L4 L3 L2 L1 L4

74 Innervation of Joints Hilton’s law – any nerve serving a muscle that produces a movement at the joint also innervates the joint and the skin over the joint

75 Motor Endings and Motor Activity Motor Endings – PNS elements that activate effectors by releasing neurotransmitters

76 Innervations of Skeletal Muscle Neuromuscular junction- axon reaches target – single muscle fiber Ending splits into axon terminals that branch over folds of sarcolemma Terminal – acetylcholine – diffuses across synaptic cleft ACh – binds – opens ion channels  propagation of AP

77 Figure 9.8 Nucleus Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Ca 2+ Axon terminal of motor neuron Synaptic vesicle containing ACh Mitochondrion Synaptic cleft Junctional folds of sarcolemma Fusing synaptic vesicles ACh Sarcoplasm of muscle fiber Postsynaptic membrane ion channel opens; ions pass. Na + K+K+ ACh Na + K+K+ Degraded ACh Acetylcholinesterase Postsynaptic membrane ion channel closed; ions cannot pass. Action potential arrives at axon terminal of motor neuron. Voltage-gated Ca 2+ channels open and Ca 2+ enters the axon terminal. Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. ACh binding opens ion channels that allow simultaneous passage of Na + into the muscle fiber and K + out of the muscle fiber. ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase

78 Innervation of Visceral Muscle and Glands Autonomic motor axons – branch – forming synapse en passant Series of varicosities – knoblike swellings – mitochondria and synaptic vessels ACh or norepinephrine

79 Figure 9.27 Smooth muscle cell Varicosities release their neurotransmitters into a wide synaptic cleft (a diffuse junction). Synaptic vesicles Mitochondrion Autonomic nerve fibers innervate most smooth muscle fibers. Varicosities

80 Levels of Motor Control Segmental level Projection level Precommand level

81 Figure 13.13a Feedback Reflex activity Motor output Sensory input (a) Levels of motor control and their interactions Precommand Level (highest) Cerebellum and basal nuclei Programs and instructions (modified by feedback) Projection Level (middle) Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.) Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels Segmental Level (lowest) Spinal cord Contains central pattern generators (CPGs) Internal feedback

82 Segmental Level The lowest level of the motor hierarchy Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles Controls locomotion and specific, oft-repeated motor activity

83 Projection Level Consists of: – Upper motor neurons that direct the direct (pyramidal) system to produce voluntary skeletal muscle movements – Brain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG- controlled motor actions Projection motor pathways keep higher command levels informed of what is happening

84 Precommand Level Neurons in the 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

85 Precommand Level Cerebellum – Acts on motor pathways through projection areas of the brain stem – Acts on the motor cortex via the thalamus Basal nuclei – Inhibit various motor centers under resting conditions

86 Figure 13.13a Feedback Reflex activity Motor output Sensory input (a) Levels of motor control and their interactions Precommand Level (highest) Cerebellum and basal nuclei Programs and instructions (modified by feedback) Projection Level (middle) Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.) Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels Segmental Level (lowest) Spinal cord Contains central pattern generators (CPGs) Internal feedback

87 Figure 13.13b (b) Structures involved Precommand level Cerebellum Basal nuclei Projection level Primary motor cortex Brain stem nuclei Segmental level Spinal cord

88 Reflexes Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes result from practice or repetition, – Example: driving skills

89 Reflex Arc Components of a reflex arc (neural path) 1.Receptor—site of stimulus action 2.Sensory neuron—transmits afferent impulses to the CNS 3.Integration center—either monosynaptic or polysynaptic region within the CNS 4.Motor neuron—conducts efferent impulses from the integration center to an effector organ 5.Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

90 Figure Receptor Sensory neuron Integration center Motor neuron Effector Spinal cord (in cross section) Interneuron Stimulus Skin

91 Spinal Reflexes Spinal somatic reflexes – Integration center is in the spinal cord – Effectors are skeletal muscle Testing of somatic reflexes is important clinically to assess the condition of the nervous system

92 Stretch and Golgi Tendon Reflexes For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary – Muscle spindles inform the nervous system of the length of the muscle – Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

93 Muscle Spindles Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule Intrafusal fibers – Noncontractile in their central regions (lack myofilaments) – Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

94 Muscle Spindles Contractile end regions are innervated by gamma (  ) efferent fibers that maintain spindle sensitivity Note: extrafusal fibers (contractile muscle fibers) are innervated by alpha (  ) efferent fibers

95 Figure Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle Primary sensory endings (type Ia fiber) Connective tissue capsule Muscle spindle Tendon Sensory fiber Golgi tendon organ  Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers

96 Muscle Spindles Excited in two ways: 1.External stretch of muscle and muscle spindle 2.Internal stretch of muscle spindle: Activating the  motor neurons stimulates the ends to contract, thereby stretching the spindle Stretch causes an increased rate of impulses in Ia fibers

97 Figure 13.16a, b (a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber. Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. Time

98 Muscle Spindles Contracting the muscle reduces tension on the muscle spindle Sensitivity would be lost unless the muscle spindle is shortened by impulses in the  motor neurons  –  coactivation maintains the tension and sensitivity of the spindle during muscle contraction

99 Figure 13.16c, d (d) - Coactivation. Both extrafusal and intrafusal muscle fibers contract. Muscle spindle tension is main- tained and it can still signal changes in length. Time (c) Only motor neurons activated. Only the extrafusal muscle fibers contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes.  Time

100 Stretch Reflexes Maintain muscle tone in large postural muscles Cause muscle contraction in response to increased muscle length (stretch)

101 Stretch Reflexes How a stretch reflex works: – Stretch activates the muscle spindle – IIa sensory neurons synapse directly with  motor neurons in the spinal cord –  motor neurons cause the stretched muscle to contract All stretch reflexes are monosynaptic and ipsilateral

102 Stretch Reflexes Reciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit the  motor neurons of antagonistic muscles Example: In the patellar reflex, the stretched muscle (quadriceps) contracts and the antagonists (hamstrings) relax

103 Figure (1 of 2) Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a3b

104 Figure (1 of 2), step1 Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The events by which muscle stretch is damped Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1

105 Figure (1 of 2), step 2 Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2

106 Figure (1 of 2), step 3a Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a

107 Figure (1 of 2), step 3b Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. The events by which muscle stretch is damped Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Initial stimulus (muscle stretch) Cell body of sensory neuron Sensory neuron Muscle spindle Antagonist muscle Spinal cord 1 2 3a3b

108 Figure (2 of 2) The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 3b 1 2 3a 3b

109 Figure (2 of 2), step 1 The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 1

110 Figure (2 of 2), step 2 The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. Excitatory synapse Inhibitory synapse +–+–

111 Figure (2 of 2), step 3a The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 1 2

112 Figure (2 of 2), step 3b The patellar (knee-jerk) reflex—a specific example of a stretch reflex Muscle spindle Quadriceps (extensors) Hamstrings (flexors) Patella Patellar ligament Spinal cord (L 2 –L 4 ) Tapping the patellar ligament excites muscle spindles in the quadriceps. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Excitatory synapse Inhibitory synapse +–+– 1 2 3a 3b 1 2 3a 3b

113 Golgi Tendon Reflexes Polysynaptic reflexes Help to prevent damage due to excessive stretch Important for smooth onset and termination of muscle contraction

114 Golgi Tendon Reflexes Produce muscle relaxation (lengthening) in response to tension – Contraction or passive stretch activates Golgi tendon organs – Afferent impulses are transmitted to spinal cord – Contracting muscle relaxes and the antagonist contracts (reciprocal activation) – Information transmitted simultaneously to the cerebellum is used to adjust muscle tension

115 Figure Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a 3b

116 Figure 13.18, step 1 + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1

117 Figure 13.18, step 2 + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2

118 Figure 13.18, step 3a + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a

119 Figure 13.18, step 3b + Excitatory synapse – Inhibitory synapse Quadriceps strongly contracts. Golgi tendon organs are activated. Afferent fibers synapse with interneurons in the spinal cord. Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Efferent impulses to antagonist muscle cause it to contract. Interneurons Spinal cord Quadriceps (extensors) Golgi tendon organ Hamstrings (flexors) 1 2 3a 3b

120 Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex – Initiated by a painful stimulus – Causes automatic withdrawal of the threatened body part – Ipsilateral and polysynaptic

121 Flexor and Crossed-Extensor Reflexes Crossed extensor reflex – Occurs with flexor reflexes in weight-bearing limbs to maintain balance – Consists of an ipsilateral flexor reflex and a contralateral extensor reflex The stimulated side is withdrawn (flexed) The contralateral side is extended

122 Figure Afferent fiber Efferent fibers Extensor inhibited Flexor stimulated Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Arm movements Interneurons Efferent fibers Flexor inhibited Extensor stimulated + Excitatory synapse – Inhibitory synapse

123 Superficial Reflexes Elicited by gentle cutaneous stimulation Depend on upper motor pathways and cord- level reflex arcs

124 Superficial Reflexes Plantar reflex – Stimulus: stroking lateral aspect of the sole of the foot – Response: downward flexion of the toes – Tests for function of corticospinal tracts

125 Superficial Reflexes Babinski’s sign – Stimulus: as above – Response: dorsiflexion of hallux and fanning of toes – Present in infants due to incomplete myelination – In adults, indicates corticospinal or motor cortex damage

126 Superficial Reflexes Abdominal reflexes – Cause contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skin – Vary in intensity from one person to another – Absent when corticospinal tract lesions are present

127 Developmental Aspects of the PNS Spinal nerves branch from the developing spinal cord and neural crest cells – Supply both motor and sensory fibers to developing muscles to help direct their maturation – Cranial nerves innervate muscles of the head

128 Developmental Aspects of the PNS Distribution and growth of spinal nerves correlate with the segmented body plan Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased numbers of synapses per neuron, and slower central processing Peripheral nerves remain viable throughout life unless subjected to trauma


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