Chapter 13 The Peripheral Nervous System and Reflex

  • Slides: 128
Download presentation
Chapter 13: The Peripheral Nervous System and Reflex Activity

Chapter 13: The Peripheral Nervous System and Reflex Activity

Peripheral Nervous System (PNS) • Links outside world and CNS • Includes all neural

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

Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division Motor (efferent) division

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

Sensory Receptors • Classified according to 1. Type of stimulus they detect 2. Body

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

Stimulus Type 1. Mechanoreceptors – mechanical force – Touch pressure (including BP), vibration, and

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

Location 1. Exteroceptors – sensitive to stimuli arising outside of the body – Body

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

Structural Complexity • Simple Receptors of General Senses – – Receptors respond to several

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

Simple Receptors 1. Unencapsualted Dendritic Endings (cont) • Temperature outside range – cold –

Simple Receptors 1. Unencapsualted Dendritic Endings (cont) • Temperature outside range – cold – 10 -40 C and hot – 32 -48 C – 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

Table 13. 1

Table 13. 1

Simple Receptors 2. Encapsulated Dendritic Endings – consist of one or more fiber terminals

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

Simple Receptors • 2. Encapsulated Dendritic Endings (cont) – • Muscle spindles – fusiform

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

Table 13. 1

Table 13. 1

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

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

Sensory Integration • Sensation – awareness of changes in internal and external environment •

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

Somatosensory System • Part of the sensory system serving the body wall and limbs

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

Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular

Perceptual level (processing in cortical sensory centers) 3 Motor cortex Somatosensory cortex Thalamus Reticular formation Pons 2 Circuit level (processing in Spinal ascending pathways) cord Cerebellum Medulla Free nerve endings (pain, cold, warmth) Muscle spindle Receptor level (sensory reception Joint and transmission kinesthetic to CNS) receptor 1 Figure 13. 2

1. Receptor Level • Sensation – stimulus must excite a receptor and APs must

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

1. Receptor Level • Adaptation– sensory receptors can change sensitivity in presence of a

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

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

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

3. Perceptual Level • Interpretation of Sensory input in cerebral cortex • Projection –

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

3. Perceptual Level • Sensory Perception – • Perceptual detection – ability to detect

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

Perception of Pain • Receptors activated by extremes of pressure and temperature, as well

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

Transmission Lines – Nerves & Their Ganglia Structure and Classification – Nerve – cordlike

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 • •

Endoneurium Axon Myelin sheath Perineurium Epineurium Fascicle Blood vessels (b) Figure 13. 3 b

Endoneurium Axon Myelin sheath Perineurium Epineurium Fascicle Blood vessels (b) Figure 13. 3 b

Nerves & Their Ganglia • Classified according to the direction which they transmit impulses

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

Regeneration of Nerve Fibers • Real mature neurons do not divide • Damage severe

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

Regeneration of Nerve Fibers 1. Axon becomes fragmented at the injury site 2. Macrophages

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

Endoneurium Schwann cells Droplets of myelin 1 The axon becomes fragmented at the injury

Endoneurium Schwann cells Droplets of myelin 1 The axon becomes fragmented at the injury site. Fragmented axon Site of nerve damage Figure 13. 4 (1 of 4)

Schwann cell Macrophage 2 Macrophages clean out the dead axon distal to the injury.

Schwann cell Macrophage 2 Macrophages clean out the dead axon distal to the injury. Figure 13. 4 (2 of 4)

Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a

Aligning Schwann cells form regeneration tube 3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. Fine axon sprouts or filaments Figure 13. 4 (3 of 4)

Schwann cell Site of new myelin sheath formation 4 The axon regenerates and a

Schwann cell Site of new myelin sheath formation 4 The axon regenerates and a new myelin sheath forms. Single enlarging axon filament Figure 13. 4 (4 of 4)

Cranial Nerves • • 12 pairs associated with brain 1 st – forebrain Rest

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

Frontal lobe Temporal lobe Infundibulum Facial nerve (VII) Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX)

Frontal lobe Temporal lobe Infundibulum Facial nerve (VII) Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Accessory nerve (XI) Hypoglossal nerve (XII) Filaments of olfactory nerve (I) Olfactory bulb Olfactory tract Optic nerve (II) Optic chiasma Optic tract Oculomotor nerve (III) Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Cerebellum Medulla oblongata (a) Figure 13. 5 (a)

Cranial nerves I – VI I II IV V Olfactory Optic Oculomotor Trochlear Trigeminal

Cranial nerves I – VI I II IV V Olfactory Optic Oculomotor Trochlear Trigeminal VI Abducens Cranial nerves VII – XII VII Facial VIII Vestibulocochlear IX X XI XII (b) Glossopharyngeal Vagus Accessory Hypoglossal Sensory function Motor function PS* fibers Yes (smell) Yes (vision) No No Yes (general sensation) No No Yes Yes No No No Yes No Sensory function Motor function PS* fibers Yes (taste) Yes (hearing and balance) Yes Some Yes No Yes (taste) No No Yes Yes Yes No No *PS = parasympathetic Figure 13. 5 (b)

Cranial Nerves I. Olfactory – tiny sensory nerves of smell • Run from nasal

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

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Cranial Nerves V. Trigeminal – 3 branches, sensory fibers to the face and motor

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

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Cranial Nerves IX. Glossopharyngeal – tongue and Pharynx X. Vagus – only cranial nerve

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

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Table 13. 2

Cranial Nerves • Mixed nerves • Cell bodies located in cranial sensory ganglia except

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

Spinal Nerves • • 31 pairs Each 1000 s of nerve fibers Named according

Spinal Nerves • • 31 pairs Each 1000 s of nerve fibers Named according to their point of issue 8 pairs cranial spinal nerves – C 1 –C 8 only 7 vertebrae – C 8 emerges inferior to 7 th vertebrae 12 pairs of thoracic – T 1 - T 12 5 pairs of lumbar – L 1 -L 5 5 pairs of sacral – S 1 -S 5 1 pair of coccygeal - Co 1

Cervical plexus Brachial plexus Cervical enlargement Intercostal nerves Cervical nerves C 1 – C

Cervical plexus Brachial plexus Cervical enlargement Intercostal nerves Cervical nerves C 1 – C 8 Thoracic nerves T 1 – T 12 Lumbar enlargement Lumbar plexus Sacral plexus Cauda equina Lumbar nerves L 1 – L 5 Sacral nerves S 1 – S 5 Coccygeal nerve Co 1 Figure 13. 6

Spinal Nerves • Connect to spinal cord by a dorsal root and a ventral

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

Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal

Dorsal root ganglion Dorsal ramus of spinal nerve Ventral ramus of spinal nerve Spinal nerve Gray matter White matter Ventral root Dorsal and ventral rootlets of 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)

Spinal Nerves • Short • Immediately after emerging from spinal cord – divides into

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

Dorsal ramus Ventral ramus Spinal nerve Rami communicantes Sympathetic trunk ganglion Intercostal nerve Dorsal

Dorsal ramus Ventral ramus Spinal nerve Rami communicantes Sympathetic trunk ganglion Intercostal nerve 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)

Innervation of Body Regions Supply entire somatic region of body Dorsal rami – posterior

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 • •

Innervation of Body Regions • Back – segmented plan • Each dorsal ramus innervates

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 – T 12 – course anteriorly – deep to each rib – intercostal nerves • T 12 – subcostal nerve

Innervation of Body Regions • Cervical Plexus and Neck – cervical plexus – formed

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

Ventral rami Segmental branches Hypoglossal nerve (XII) Lesser occipital nerve Greater auricular nerve Transverse

Ventral rami Segmental branches Hypoglossal nerve (XII) Lesser occipital nerve Greater auricular nerve Transverse cervical nerve Ansa cervicalis Accessory nerve (XI) Phrenic nerve Ventral rami: C 1 C 2 C 3 C 4 C 5 Supraclavicular nerves Figure 13. 8

Innervation of Body Regions • Brachial Plexus and Upper Limb – situated partially in

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. 2. 3. 4. ventral rami Trunks Divisions Cords

Posterior divisions Cords Roots (ventral rami): C 4 C 5 Dorsal scapular Nerve to

Posterior divisions Cords Roots (ventral rami): C 4 C 5 Dorsal scapular Nerve to subclavius Suprascapular C 6 C 7 Lateral C 8 Posterior T 1 Medial Axillary Musculocutaneous Radial Median Ulnar (a) Roots (rami C 5 – T 1), trunks, divisions, and cords Anterior divisions Posterior divisions Trunks Upper Middle Trunks Lower Long thoracic Medial pectoral Lateral pectoral Upper subscapular Lower subscapular Thoracodorsal Medial cutaneous nerves of the arm and forearm Roots Figure 13. 9 (a)

Innervation of Body Regions - Auxiliary nerve – innervates deltoid, teres minor, skin and

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

Axillary nerve Anterior divisions Posterior divisions Trunks Roots Humerus Radial nerve Musculocutaneous nerve Ulna

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)

Table 13. 4

Table 13. 4

Innervation of Body Regions • Lumbosacral and Lower Limb – lumbosacral plexus • Lumbar

Innervation of Body Regions • Lumbosacral and Lower Limb – lumbosacral plexus • Lumbar plexus – L 1 -L 4 – anterior and medial thigh – • femoral nerve – quads, thigh flexors and knee extensors • Obturator nerve – adductor muscles

Innervation of Body Regions • Lumbosacral and Lower Limb – • Sacral Plexus –

Innervation of Body Regions • Lumbosacral and Lower Limb – • Sacral Plexus – L 4 –S 4 – 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

Ventral rami Iliohypogastric Ilioinguinal Genitofemoral Lateral femoral cutaneous Obturator Femoral Lumbosacral trunk Ventral rami:

Ventral rami Iliohypogastric Ilioinguinal Genitofemoral Lateral femoral cutaneous Obturator Femoral Lumbosacral trunk Ventral rami: Iliohypogastric L 1 Ilioinguinal Femoral Lateral femoral L 2 cutaneous Obturator L 3 Anterior femoral cutaneous Saphenous L 4 L 5 (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

Table 13. 5

Table 13. 5

Innervation of Skin: Dermatomes • Dermatome – area of skin innervated by cutaneous branches

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

C 2 C 3 C 4 C 5 C 6 C 7 C 8

C 2 C 3 C 4 C 5 C 6 C 7 C 8 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 C 2 C 3 C 4 C 5 T 1 T 2 T 3 T 4 T 5 T 6 T 7 T 8 T 9 T 10 T 11 T 2 C 5 C 6 C 7 L 1 C 8 L 2 T 12 S 3 T 2 C 5 C 6 L 1 C 8 L 2 S 1 L 4 S 2 S 3 S 4 S 5 C 6 C 7 S 2 C 6 C 7 C 8 S 1 L 3 L 5 L 4 T 11 T 12 L 1 L 3 L 5 C 7 C 6 S 1 S 2 L 3 C 5 L 2 L 5 L 4 L 3 L 5 L 4 S 1 Anterior view S 1 (b) Posterior view L 4 L 5 S 1 Figure 13. 12

Innervation of Joints • Hilton’s law – any nerve serving a muscle that produces

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

Motor Endings and Motor Activity • Motor Endings – PNS elements that activate effectors

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

Innervations of Skeletal Muscle • Neuromuscular junction- axon reaches target – single muscle fiber

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

Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Nucleus

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 Ca 2+ channels open and Ca 2+ enters the axon terminal. Ca 2+ 3 Ca 2+ entry causes some synaptic vesicles to release their contents (acetylcholine) by exocytosis. Axon terminal of motor neuron ACh Junctional folds of sarcolemma Sarcoplasm of muscle fiber Na+ 6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase. Mitochondrion Synaptic cleft Fusing synaptic vesicles 4 Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma. 5 ACh binding opens ion channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. Synaptic vesicle containing ACh K+ Degraded ACh Na+ Postsynaptic membrane ion channel opens; ions pass. Postsynaptic membrane ion channel closed; ions cannot pass. K+ Acetylcholinesterase Figure 9. 8

Innervation of Visceral Muscle and Glands • Autonomic motor axons – branch – forming

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

Varicosities Autonomic nerve fibers innervate most smooth muscle fibers. Smooth muscle cell Synaptic vesicles

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

Levels of Motor Control • Segmental level • Projection level • Precommand level

Levels of Motor Control • Segmental level • Projection level • Precommand level

Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by

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. 13 a

Segmental Level • The lowest level of the motor hierarchy • Central pattern generators

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

Projection Level • Consists of: – Upper motor neurons that direct the direct (pyramidal)

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 CPGcontrolled motor actions • Projection motor pathways keep higher command levels informed of what is happening

Precommand Level • Neurons in the cerebellum and basal nuclei – Regulate motor activity

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

Precommand Level • Cerebellum – Acts on motor pathways through projection areas of the

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

Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by

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. 13 a

Precommand level • Cerebellum • Basal nuclei Projection level • Primary motor cortex •

Precommand level • Cerebellum • Basal nuclei Projection level • Primary motor cortex • Brain stem nuclei Segmental level • Spinal cord (b) Structures involved Figure 13. 13 b

Reflexes • Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus

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

Reflex Arc • Components of a reflex arc (neural path) 1. Receptor—site of stimulus

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

Stimulus Skin 1 Receptor Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron

Stimulus Skin 1 Receptor Interneuron 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector Spinal cord (in cross section) Figure 13. 14

Spinal Reflexes • Spinal somatic reflexes – Integration center is in the spinal cord

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

Stretch and Golgi Tendon Reflexes • For skeletal muscle activity to be smoothly coordinated,

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

Muscle Spindles • Composed of 3– 10 short intrafusal muscle fibers in a connective

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

Muscle Spindles • Contractile end regions are innervated by gamma ( ) efferent fibers

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

Secondary sensory endings (type II fiber) Primary sensory endings (type Ia fiber) Muscle spindle

Secondary sensory endings (type II fiber) Primary sensory endings (type Ia fiber) Muscle spindle Connective tissue capsule Efferent (motor) fiber to muscle spindle Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers Sensory fiber Golgi tendon organ Tendon Figure 13. 15

Muscle Spindles • Excited in two ways: 1. External stretch of muscle and muscle

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

Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber Time

Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber 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. 16 a, b

Muscle Spindles • Contracting the muscle reduces tension on the muscle spindle • Sensitivity

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

Time (c) Only motor (d) - Coactivation. neurons activated. Both extrafusal and intrafusal muscle

Time (c) Only motor (d) - Coactivation. neurons activated. Both extrafusal and intrafusal muscle Only the extrafusal muscle fibers contract. The muscle spindle Muscle spindle becomes slack and no tension is main. APs are fired. It is tained and it can unable to signal further still signal changes length changes. in length. Figure 13. 16 c, d

Stretch Reflexes • Maintain muscle tone in large postural muscles • Cause muscle contraction

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

Stretch Reflexes • How a stretch reflex works: – Stretch activates the muscle spindle

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

Stretch Reflexes • Reciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit the

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

Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and

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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers by stretch, the associated sensory of the stretched muscle. Afferent fibers also neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor at higher frequency to the spinal cord. neurons (purple) controlling antagonistic muscles. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle 3 a Efferent impulses of alpha motor neurons 3 b Efferent impulses of alpha motor cause the stretched muscle to contract, which resists or reverses the stretch. neurons to antagonist muscles are reduced (reciprocal inhibition). Figure 13. 17 (1 of 2)

Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and

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 13. 17 (1 of 2), step 1

Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and

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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers by stretch, the associated sensory of the stretched muscle. Afferent fibers also neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor at higher frequency to the spinal cord. neurons (purple) controlling antagonistic muscles. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle Figure 13. 17 (1 of 2), step 2

Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and

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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers by stretch, the associated sensory of the stretched muscle. Afferent fibers also neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor at higher frequency to the spinal cord. neurons (purple) controlling antagonistic muscles. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle 3 a Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. Figure 13. 17 (1 of 2), step 3 a

Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and

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 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers by stretch, the associated sensory of the stretched muscle. Afferent fibers also neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor at higher frequency to the spinal cord. neurons (purple) controlling antagonistic muscles. Sensory neuron Cell body of sensory neuron Initial stimulus (muscle stretch) Spinal cord Muscle spindle Antagonist muscle 3 a Efferent impulses of alpha motor neurons 3 b Efferent impulses of alpha motor cause the stretched muscle to contract, which resists or reverses the stretch. neurons to antagonist muscles are reduced (reciprocal inhibition). Figure 13. 17 (1 of 2), step 3 b

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1 Muscle spindle 3 a 3 b 3 b Patella Spinal cord (L 2–L 4) Hamstrings (flexors) Patellar ligament 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 3 a The motor neurons (red) send + – Excitatory synapse Inhibitory synapse activating impulses to the quadriceps causing it to contract, extending the knee. 3 b The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Figure 13. 17 (2 of 2)

The patellar (knee-jerk) reflex—a specific example of a stretch reflex Quadriceps (extensors) 1 Muscle

The patellar (knee-jerk) reflex—a specific example of a stretch reflex Quadriceps (extensors) 1 Muscle spindle Spinal cord (L 2–L 4) Hamstrings (flexors) + – Patellar ligament 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. Excitatory synapse Inhibitory synapse Figure 13. 17 (2 of 2), step 1

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1 Muscle spindle Spinal cord (L 2–L 4) Hamstrings (flexors) + – Patellar ligament 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. Excitatory synapse Inhibitory synapse Figure 13. 17 (2 of 2), step 2

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1 Muscle spindle 3 a Patella Spinal cord (L 2–L 4) Hamstrings (flexors) Patellar ligament 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 3 a The motor neurons (red) send + – Excitatory synapse Inhibitory synapse activating impulses to the quadriceps causing it to contract, extending the knee. Figure 13. 17 (2 of 2), step 3 a

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1

The patellar (knee-jerk) reflex—a specific example of a stretch reflex 2 Quadriceps (extensors) 1 Muscle spindle 3 a 3 b 3 b Patella Spinal cord (L 2–L 4) Hamstrings (flexors) Patellar ligament 1 Tapping the patellar ligament excites muscle spindles in the quadriceps. 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons. 3 a The motor neurons (red) send + – Excitatory synapse Inhibitory synapse activating impulses to the quadriceps causing it to contract, extending the knee. 3 b The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. Figure 13. 17 (2 of 2), step 3 b

Golgi Tendon Reflexes • Polysynaptic reflexes • Help to prevent damage due to excessive

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

Golgi Tendon Reflexes • Produce muscle relaxation (lengthening) in response to tension – Contraction

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

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) 3 a Efferent impulses + Excitatory synapse – Inhibitory synapse to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3 b Efferent impulses to antagonist muscle cause it to contract. Figure 13. 18

1 Quadriceps strongly contracts. Golgi tendon organs are activated. Interneurons Quadriceps (extensors) Golgi tendon

1 Quadriceps strongly contracts. Golgi tendon organs are activated. Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Figure 13. 18, step 1

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) + Excitatory synapse – Inhibitory synapse Figure 13. 18, step 2

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) 3 a Efferent impulses + Excitatory synapse – Inhibitory synapse to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. Figure 13. 18, step 3 a

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with

1 Quadriceps strongly contracts. Golgi tendon organs are activated. 2 Afferent fibers synapse with interneurons in the spinal cord. Interneurons Quadriceps (extensors) Golgi tendon organ Spinal cord Hamstrings (flexors) 3 a Efferent impulses + Excitatory synapse – Inhibitory synapse to muscle with stretched tendon are damped. Muscle relaxes, reducing tension. 3 b Efferent impulses to antagonist muscle cause it to contract. Figure 13. 18, step 3 b

Flexor and Crossed-Extensor Reflexes • Flexor (withdrawal) reflex – Initiated by a painful stimulus

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

Flexor and Crossed-Extensor Reflexes • Crossed extensor reflex – Occurs with flexor reflexes in

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

+ Excitatory synapse – Inhibitory synapse Interneurons Efferent fibers Afferent fiber Efferent fibers Extensor

+ Excitatory synapse – Inhibitory synapse Interneurons Efferent fibers 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. Arm movements Flexor inhibited Extensor stimulated Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated. Figure 13. 19

Superficial Reflexes • Elicited by gentle cutaneous stimulation • Depend on upper motor pathways

Superficial Reflexes • Elicited by gentle cutaneous stimulation • Depend on upper motor pathways and cordlevel reflex arcs

Superficial Reflexes • Plantar reflex – Stimulus: stroking lateral aspect of the sole of

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

Superficial Reflexes • Babinski’s sign – Stimulus: as above – Response: dorsiflexion of hallux

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

Superficial Reflexes • Abdominal reflexes – Cause contraction of abdominal muscles and movement of

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

Developmental Aspects of the PNS • Spinal nerves branch from the developing spinal cord

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

Developmental Aspects of the PNS • Distribution and growth of spinal nerves correlate with

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