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

4 Sensory Receptors Classified according to Type of stimulus they detect
Body location 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 Perceptual level (processing in cortical sensory centers)
3 Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Reticular formation Cerebellum Pons Medulla 2 Circuit level (processing in ascending pathways) Spinal cord Free nerve endings (pain, cold, warmth) Muscle spindle 1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor Figure 13.2

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 2nd 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 Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood
vessels (b) Figure 13.3b

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
Axon becomes fragmented at the injury site Macrophages clean out the dead axon distal to injury Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells The axon regenerated and a new myelin sheath forms

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

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

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

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

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

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

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

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

37 Table 13.2

38 Table 13.2

39 Table 13.2

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

42 Table 13.2

43 Table 13.2

44 Table 13.2

45 Table 13.2

46 Table 13.2

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

49 Table 13.2

50 Table 13.2

51 Table 13.2

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 7th 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 Cervical plexus Cervical nerves C1 – C8 Brachial plexus Cervical
enlargement Thoracic nerves T1 – T12 Intercostal nerves Lumbar enlargement Lumbar nerves L1 – L5 Lumbar plexus Sacral plexus Sacral nerves S1 – S5 Cauda equina Coccygeal nerve Co1 Figure 13.6

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 Gray matter White matter Dorsal and Ventral root ventral rootlets
of spinal nerve Ventral root Dorsal root Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Rami communicantes Sympathetic trunk ganglion 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. Figure 13.7 (a)

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

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 1st 4 cervical nerves Cutaneous nerves supply the skin Phernic nerve - diaphragm

62 Segmental branches Hypoglossal Ventral nerve (XII) rami:
Ventral rami Segmental branches Hypoglossal nerve (XII) Ventral rami: Lesser occipital nerve C1 Greater auricular nerve C2 Transverse cervical nerve C3 Ansa cervicalis C4 Accessory nerve (XI) Phrenic nerve C5 Supraclavicular nerves Figure 13.8

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 – ventral rami Trunks Divisions Cords

64 (a) Roots (rami C5 – T1), trunks, divisions, and cords
Roots (ventral rami): C4 Dorsal scapular C5 Nerve to subclavius C6 Suprascapular Upper Posterior divisions C7 Middle Trunks Lateral C8 Lower Cords Posterior T1 Long thoracic Medial Medial pectoral Lateral pectoral Axillary Upper subscapular Musculo- cutaneous Lower subscapular Thoracodorsal Radial Medial cutaneous nerves of the arm and forearm Median Ulnar (a) Roots (rami C5 – T1), trunks, divisions, and cords Anterior divisions Posterior divisions Trunks Roots Figure 13.9 (a)

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

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 Ventral rami: Iliohypogastric L1 Ilioinguinal Femoral L2
Lateral femoral cutaneous Iliohypogastric Ilioinguinal Obturator L3 Genitofemoral Anterior femoral cutaneous Lateral femoral cutaneous Saphenous L4 Obturator Femoral L5 Lumbosacral trunk (a) Ventral rami and major branches of the lumbar plexus (b) Distribution of the major nerves from the lumbar plexus to the lower limb Figure 13.10

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 Anterior view (b) Posterior view Figure 13.12 C2 C3 C4 C5 T1 T2 T3 T2
L2 S1 L1 L3 C8 L4 S2 T12 S3 L5 C6 L1 L1 C6 S4 C7 S2 C7 S5 C8 S3 C8 L2 L2 S1 S2 S2 S1 L3 L3 L1 L5 L2 L5 L4 L4 L3 L5 L5 L4 S1 S1 Anterior view (b) Posterior view L4 L4 L5 L5 S1 Figure 13.12

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 Axon terminal of neuromuscular junction Myelinated axon
of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Nucleus 1 Action potential arrives at axon terminal of motor neuron. Sarcolemma of the muscle fiber 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. Ca2+ Synaptic vesicle containing ACh Ca2+ 3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Axon terminal of motor neuron Mitochondrion Synaptic cleft Fusing synaptic vesicles 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. ACh Junctional folds of sarcolemma Sarcoplasm of muscle fiber Na+ K+ Postsynaptic membrane ion channel opens; ions pass. 5 ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. ACh Degraded ACh Na+ Postsynaptic membrane ion channel closed; ions cannot pass. 6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. K+ Acetylcholinesterase Figure 9.8

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

80 Levels of Motor Control
Segmental level Projection level Precommand level

81 • Programs and instructions (modified by feedback)
Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by feedback) Internal feedback 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) Sensory input Reflex activity Motor output (a) Levels of motor control and their interactions Figure 13.13a

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 Basal nuclei
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 • Programs and instructions (modified by feedback)
Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by feedback) Internal feedback 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) Sensory input Reflex activity Motor output (a) Levels of motor control and their interactions Figure 13.13a

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

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)
Receptor—site of stimulus action Sensory neuron—transmits afferent impulses 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 organ Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

90 1 2 3 4 5 Stimulus Skin Interneuron Receptor Sensory neuron
Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Figure 13.14

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 endings (type II fiber) Efferent (motor) fiber to muscle spindle
Secondary sensory endings (type II fiber) Efferent (motor) fiber to muscle spindle  Efferent (motor) fiber to extrafusal muscle fibers Primary sensory endings (type Ia fiber) Extrafusal muscle fiber Muscle spindle Intrafusal muscle fibers Connective tissue capsule Sensory fiber Golgi tendon organ Tendon Figure 13.15

96 Muscle Spindles Excited in two ways:
External stretch of muscle and muscle spindle 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 Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time Time (a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber. (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs. Figure 13.16a, b

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 Time 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. (d) - Coactivation. Both extrafusal and intrafusal muscle fibers contract. Muscle spindle tension is main- tained and it can still signal changes in length. Figure 13.16c, d

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 Cell body of sensory neuron
Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. The events by which muscle stretch is damped 2 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. 1 When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. 3a 3b Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition). Figure (1 of 2)

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

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

106 Cell body of sensory neuron
Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist. The events by which muscle stretch is damped 2 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. 1 When muscle spindles are activated by stretch, the associated sensory neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle 3a Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Figure (1 of 2), step 3a

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

108 The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + – Excitatory synapse Inhibitory synapse The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b Figure (2 of 2)

109 The patellar (knee-jerk) reflex—a specific example of a stretch reflex
Quadriceps (extensors) 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament + – Excitatory synapse Inhibitory synapse Figure (2 of 2), step 1

110 The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2 Quadriceps (extensors) 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 + – Excitatory synapse Inhibitory synapse Figure (2 of 2), step 2

111 The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2 Quadriceps (extensors) 3a 1 Patella Muscle spindle Spinal cord (L2–L4) Tapping the patellar ligament excites muscle spindles in the quadriceps. 1 Hamstrings (flexors) Patellar ligament Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 2 3a The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. + – Excitatory synapse Inhibitory synapse Figure (2 of 2), step 3a

112 The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2 Quadriceps (extensors) 3a 3b 3b 1 Patella Muscle spindle Spinal cord (L2–L4) 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Hamstrings (flexors) Patellar ligament 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a + – Excitatory synapse Inhibitory synapse The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b Figure (2 of 2), step 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 Quadriceps (extensors) Hamstrings (flexors)
1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) 3a Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3b Efferent impulses to antagonist muscle cause it to contract. + Excitatory synapse Inhibitory synapse Figure 13.18

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

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

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

119 Quadriceps (extensors) Hamstrings (flexors)
1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Spinal cord Golgi tendon organ Hamstrings (flexors) 3a Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3b Efferent impulses to antagonist muscle cause it to contract. + Excitatory synapse Inhibitory synapse Figure 13.18, step 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
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 + Excitatory synapse – Inhibitory synapse Interneurons Efferent fibers
Afferent fiber Efferent fibers Extensor inhibited Flexor inhibited Arm movements Flexor stimulated Extensor stimulated Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb. Figure 13.19

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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