13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes

An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes Rapid, automatic nerve responses triggered by specific stimuli Controlled by spinal cord alone, not the brain

Figure 13-1 An Overview of Chapters 13 and 14 CHAPTER 14: The Brain Sensory input over cranial nerves Motor output over cranial nerves Effectors Reflex centers in brain Sensory receptors Muscles CHAPTER 13: The Spinal Cord Glands Sensory input over spinal nerves Motor output over spinal nerves Reflex centers in spinal cord Sensory receptors Adipose tissue 3

13-2 Spinal Cord Gross Anatomy of the Spinal Cord About 18 inches (45 cm) long 1/2 inch (14 mm) wide Ends between vertebrae L1 and L2 Bilateral symmetry Grooves divide the spinal cord into left and right Posterior median sulcus – on posterior side Anterior median fissure – deeper groove on anterior side

13-2 Spinal Cord Enlargements of the Spinal Cord Caused by: Amount of gray matter in segment Involvement with sensory and motor nerves of limbs Cervical enlargement Nerves of shoulders and upper limbs Lumbar enlargement Nerves of pelvis and lower limbs

13-2 Spinal Cord Gross Anatomy of the Spinal Cord The distal end Conus medullaris Thin, conical spinal cord below lumbar enlargement Filum terminale Thin thread of fibrous tissue at end of conus medullaris Attaches to coccygeal ligament Cauda equina Nerve roots extending below conus medullaris

13-2 Spinal Cord 31 Spinal Cord Segments Cervical nerves Are named for inferior vertebra All other nerves Are named for superior vertebra

Figure 13-2 Gross Anatomy of the Adult Spinal Cord Posterior median sulcus Dorsal root Dorsal root ganglion White matter Central canal Gray matter C1 C2 Cervical spinal nerves C3 C4 C5 Spinal nerve Ventral root C6 Cervical enlargement Anterior median fissure C7 C3 C8 Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. The superficial anatomy and orientation of the adult spinal cord. The numbers to the left identify the spinal nerves and indicate where the nerve roots leave the vertebral canal. The spinal cord extends from the brain only to the level of vertebrae L1L2; the spinal segments found at representative locations are indicated in the cross sections. 8

Figure 13-2 Gross Anatomy of the Adult Spinal Cord Thoracic spinal nerves T8 Posterior median sulcus T9 T10 Lumbar enlargement T3 T11 T12 Conus medullaris 9

Figure 13-2 Gross Anatomy of the Adult Spinal Cord Conus medullaris L1 L2 Inferior tip of spinal cord Lumbar spinal nerves L3 L4 Cauda equina L5 L1 10

Figure 13-2 Gross Anatomy of the Adult Spinal Cord Inferior tip of spinal cord Cauda equina S1 Sacral spinal nerves S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) L2 11

13-2 Spinal Cord The Spinal Nerve Each side of spine Mixed Nerves Dorsal and ventral roots join To form a spinal nerve Mixed Nerves Carry both afferent (sensory) and efferent (motor) fibers

Figure 13-3a The Spinal Cord and Spinal Meninges Gray matter White matter Dorsal root ganglion Ventral root Spinal nerve Dorsal root Meninges Pia mater Arachnoid mater Dura mater A posterior view of the spinal cord, showing the meningeal layers, superficial landmarks, and distribution of gray matter and white matter 13

Figure 13-3b The Spinal Cord and Spinal Meninges Dura mater ANTERIOR Arachnoid mater Pia mater Subarachnoid space Vertebral body Autonomic (sympathetic) ganglion Rami communicantes Ventral root of spinal nerve Ventral ramus Dorsal ramus Spinal cord Adipose tissue in epidural space Denticulate ligament Dorsal root ganglion A sectional view through the spinal cord and meninges, showing the peripheral distribution of spinal nerves POSTERIOR 14

13-2 Spinal Cord The Spinal Meninges Specialized membranes isolate spinal cord from surroundings Functions of the spinal meninges include: Protecting spinal cord Carrying blood supply Continuous with cranial meninges Meningitis Viral or bacterial infection of meninges

13-2 Spinal Cord The Three Meningeal Layers Dura mater Arachnoid mater Outer layer of spinal cord Arachnoid mater Middle meningeal layer Pia mater Inner meningeal layer

13-2 Spinal Cord The Dura Mater The Epidural Space Between spinal dura mater and walls of vertebral canal The Interlayer Spaces of Arachnoid Mater Subdural space Between arachnoid mater and dura mater Subarachnoid space Between arachnoid mater and pia mater Contains collagen/elastin fiber network (arachnoid trabeculae) Filled with cerebrospinal fluid (CSF)

13-2 Spinal Cord The Pia Mater Cerebrospinal Fluid (CSF) Carries dissolved gases, nutrients, and wastes Lumbar puncture or spinal tap withdraws CSF. The Pia Mater Is the innermost meningeal layer Is a mesh of collagen and elastic fibers Is bound to underlying neural tissue

13-2 Spinal Cord Structures of the Spinal Cord Paired denticulate ligaments Extend from pia mater to dura mater Stabilize side-to-side movement Blood vessels Along surface of spinal pia mater Within subarachnoid space

Figure 13-4 The Spinal Cord and Associated Structures Anterior median fissure Pia mater Denticulate ligaments Dorsal root Ventral root, formed by several “rootlets” from one cervical segment Arachnoid mater (reflected) Dura mater (reflected) Spinal blood vessel 20

13-3 Gray Matter and White Matter Sectional Anatomy of the Spinal Cord White matter Is superficial Contains myelinated and unmyelinated axons Gray matter Surrounds central canal of spinal cord Contains neuron cell bodies, neuroglia, unmyelinated axons Has projections (gray horns)

13-3 Gray Matter and White Matter Organization of Gray Matter The gray horns Posterior gray horns contain somatic and visceral sensory nuclei Anterior gray horns contain somatic motor nuclei Lateral gray horns are in thoracic and lumbar segments; contain visceral motor nuclei Gray commissures Axons that cross from one side of cord to the other before reaching gray matter

13-3 Gray Matter and White Matter Organization of Gray Matter The cell bodies of neurons form functional groups called nuclei Sensory nuclei Dorsal (posterior) Connect to peripheral receptors Motor nuclei Ventral (anterior) Connect to peripheral effectors

13-3 Gray Matter and White Matter Organization of White Matter Posterior white columns lie between posterior gray horns and posterior median sulcus Anterior white columns lie between anterior gray horns and anterior median fissure Anterior white commissure area where axons cross from one side of spinal cord to the other Lateral white columns located on each side of spinal cord between anterior and posterior columns

13-3 Gray Matter and White Matter Organization of White Matter Tracts or fasciculi In white columns Bundles of axons Relay same information in same direction Ascending tracts Carry information to brain Descending tracts Carry motor commands to spinal cord

Figure 13-5a The Sectional Organization of the Spinal Cord Posterior white column Posterior gray horn Lateral white column Lateral gray horn Dorsal root ganglion Anterior gray horn Anterior white column The left half of this sectional view shows important anatomical landmarks, including the three columns of white matter. The right half indicates the functional organization of the nuclei in the anterior, lateral, and posterior gray horns. 26

Figure 13-5a The Sectional Organization of the Spinal Cord Posterior median sulcus Functional Organization of Gray Matter Posterior gray commissure The cell bodies of neurons in the gray matter of the spinal cord are organized into functional groups called nuclei. Somatic Sensory nuclei Visceral Visceral Motor nuclei Somatic Ventral root Anterior gray commissure Anterior white commissure Anterior median fissure The left half of this sectional view shows important anatomical landmarks, including the three columns of white matter. The right half indicates the functional organization of the nuclei in the anterior, lateral, and posterior gray horns. 27

13-3 Gray Matter and White Matter Spinal Cord Summary Spinal cord has a narrow central canal Surrounded by gray matter - Containing sensory and motor nuclei Sensory nuclei are dorsal Motor nuclei are ventral

13-4 Spinal Nerves and Plexuses Anatomy of Spinal Nerves Every spinal cord segment Is connected to a pair of spinal nerves Every spinal nerve Is surrounded by three connective tissue layers That support structures and contain blood vessels

13-4 Spinal Nerves and Plexuses Three Connective Tissue Layers of Spinal Nerves Epineurium Outer layer Dense network of collagen fibers Perineurium Middle layer Divides nerve into fascicles (axon bundles) Endoneurium Inner layer Surrounds individual axons

Figure 13-6 A Peripheral Nerve Blood vessels Connective Tissue Layers Epineurium covering spinal nerve Perineurium (around one fascicle) Endoneurium Myelinated axon Fascicle Schwann cell 31

13-4 Spinal Nerves and Plexuses Peripheral Nerves Interconnecting branches of spinal nerves Surrounded by connective tissue sheaths

13-4 Spinal Nerves and Plexuses Peripheral Distribution of Spinal Nerves Spinal nerves Form lateral to intervertebral foramen Where dorsal and ventral roots unite Then branch and form pathways to destination

13-4 Spinal Nerves and Plexuses Peripheral Distribution of Spinal Nerves Motor nerves The first branch White ramus Carries visceral motor fibers to sympathetic ganglion of autonomic nervous system Gray ramus Unmyelinated nerves Return from sympathetic ganglion to rejoin spinal nerve

13-4 Spinal Nerves and Plexuses Peripheral Distribution of Spinal Nerves Motor nerves Dorsal and ventral rami Dorsal ramus Contains somatic and visceral motor fibers Innervates the back Ventral ramus Larger branch Innervates ventrolateral structures and limbs

Figure 13-7 Peripheral Distribution of Spinal Nerves To skeletal muscles of back Postganglionic fibers to smooth muscles, glands, etc., of back The spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. The dorsal ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles of the back. Dorsal root ganglion Dorsal root The axons in the relatively large ventral ramus supply the ventrolateral body surface, structures in the body wall, and the limbs. The ventral root of each spinal nerve contains the axons of somatic motor and visceral motor neurons. To skeletal muscles of body wall, limbs Visceral motor nuclei Somatic motor nuclei Rami communicantes Postganglionic fibers to smooth muscles, glands, etc., of body wall, limbs  Somatic motor commands Sympathetic ganglion  Visceral motor commands The white ramus is the first branch from the spinal nerve and carries visceral motor fibers to a nearby sympathetic ganglion. Because these preganglionic axons are myelinated, this branch has a light color and is therefore known as the white ramus. Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity A sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity. The gray ramus contains postganglionic fibers that innervate glands and smooth muscles in the body wall or limbs. These fibers are unmyelinated and have a dark gray color. Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera 36

13-4 Spinal Nerves and Plexuses Peripheral Distribution of Spinal Nerves Sensory nerves In addition to motor impulses Dorsal, ventral, and white rami also carry sensory information Dermatomes Bilateral region of skin Monitored by specific pair of spinal nerves

Figure 13-7 Peripheral Distribution of Spinal Nerves From interoceptors of back From exteroceptors, proprioceptors of back The dorsal root of each spinal nerve carries sensory information to the spinal cord. The dorsal ramus carries sensory information from the skin and skeletal muscles of the back. Somatic sensory nuclei The ventral ramus carries sensory information from the ventrolateral body surface, structures in the body wall, and the limbs. Dorsal root ganglion From exteroceptors, proprioceptors of body wall, limbs From interoceptors of body wall, limbs Rami communicantes Visceral sensory nuclei Ventral root  Somatic sensations  Visceral sensations The sympathetic nerve carries sensory information from the visceral organs. From interoceptors of visceral organs 38

Figure 13-8 Dermatomes 39 ANTERIOR POSTERIOR C2C3 N V C2C3 C2 C3 C3 L1 C6 T10 L2 T11 L4 L3 T1 C6 T12 L5 C7 L1 S S L2 4 S2 3 C8 T1 L3 C8 L1 C7 S5 S1 L5 L4 L2 S2 L5 L3 S1 L4 ANTERIOR POSTERIOR 39

13-4 Spinal Nerves and Plexuses Peripheral Neuropathy Regional loss of sensory or motor function Due to trauma or compression

Figure 13-9 Shingles 41

13-4 Spinal Nerves and Plexuses Nerve Plexuses Complex, interwoven networks of nerve fibers Formed from blended fibers of ventral rami of adjacent spinal nerves Control skeletal muscles of the neck and limbs

13-4 Spinal Nerves and Plexuses The Four Major Plexuses of Ventral Rami Cervical plexus Brachial plexus Lumbar plexus Sacral plexus

Figure 13-10 Peripheral Nerves and Nerve Plexuses C1 Lesser occipital nerve C2 Great auricular nerve Cervical plexus C3 Transverse cervical nerve C4 C5 Supraclavicular nerve C6 C7 Phrenic nerve Brachial plexus C8 T1 T2 T3 Axillary nerve T4 T5 T6 T7 Musculocutaneous nerve T8 T9 Thoracic nerves T10 T11 44

Figure 13-10 Peripheral Nerves and Nerve Plexuses T12 L1 Radial nerve Lumbar plexus L2 Ulnar nerve L3 Median nerve L4 L5 S1 Iliohypogastric nerve Sacral plexus S2 S3 S4 Ilioinguinal nerve S5 Co1 Lateral femoral cutaneous nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior Gluteal nerves Inferior Pudendal nerve Saphenous nerve Sciatic nerve 45

13-4 Spinal Nerves and Plexuses The Cervical Plexus Includes ventral rami of spinal nerves C1–C5 Innervates neck, thoracic cavity, diaphragmatic muscles Major nerve Phrenic nerve (controls diaphragm)

Figure 13-11 The Cervical Plexus Cranial Nerves Accessory nerve (XI) Hypoglossal nerve (XII) Lesser occipital nerve Nerve Roots of Cervical Plexus C1 C2 C3 C4 C5 Supraclavicular nerves Clavicle 47

Figure 13-11 The Cervical Plexus Great auricular nerve Geniohyoid muscle Transverse cervical nerve Thyrohyoid muscle Ansa cervicalis Omohyoid muscle Phrenic nerve Sternohyoid muscle Sternothyroid muscle 48

Table 13-1 The Cervical Plexus 49

13-4 Spinal Nerves and Plexuses The Brachial Plexus Includes ventral rami of spinal nerves C5–T1 Innervates pectoral girdle and upper limbs Nerves that form brachial plexus originate from: Superior, middle, and inferior trunks Large bundles of axons from several spinal nerves Lateral, medial, and posterior cords Smaller branches that originate at trunks

13-4 Spinal Nerves and Plexuses The Brachial Plexus Major nerves Musculocutaneous nerve (lateral cord) Median nerve (lateral and medial cords) Ulnar nerve (medial cord) Axillary nerve (posterior cord) Radial nerve (posterior cord)

Figure 13-12a The Brachial Plexus Spinal Nerves Forming Brachial Plexus Trunks of Brachial Plexus Dorsal scapular nerve C4 Suprascapular nerve C5 Superior C6 Middle C7 Inferior C8 T1 Musculocutaneous nerve Median nerve Ulnar nerve Radial nerve Lateral antebrachial cutaneous nerve Superficial branch of radial nerve Deep radial nerve Ulnar nerve Median nerve Palmar digital nerves Major nerves originating at the right brachial plexus, anterior view 52

Figure 13-12b The Brachial Plexus Anterior Posterior Radial nerve Ulnar nerve Median nerve Areas of the hands serviced by nerves of the right brachial plexus 53

Figure 13-12c The Brachial Plexus Dorsal scapular nerve C5 SUPERIOR TRUNK C6 Suprascapular nerve MIDDLE TRUNK Lateral cord C7 Posterior cord Lateral pectoral nerve C8 Medial pectoral nerve Subscapular nerves T1 Axillary nerve INFERIOR TRUNK Medial cord Musculocutaneous nerve First rib Long thoracic nerve Medial antebrachial cutaneous nerve KEY Median nerve Ulnar nerve  Roots (ventral rami) Posterior brachial cutaneous nerve Radial nerve  Trunks Right brachial plexus, anterior view  Divisions  Cords  Peripheral nerves 54

Table 13-2 The Brachial Plexus 55

13-4 Spinal Nerves and Plexuses The Lumbar Plexus Includes ventral rami of spinal nerves T12–L4 Major nerves Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve

13-4 Spinal Nerves and Plexuses The Sacral Plexus Includes ventral rami of spinal nerves L4–S4 Major nerves Pudendal nerve Sciatic nerve Two branches of the sciatic nerve Fibular nerve Tibial nerve

Figure 13-13a The Lumbar and Sacral Plexuses Spinal Nerves Forming the Lumbar Plexus T12 T12 nerve Nerves of the Lumbar Plexus L1 L1 nerve L2 Iliohypogastric L2 nerve Ilioinguinal L3 L3 nerve Genitofemoral L4 Lateral femoral cutaneous L4 nerve L5 Femoral Lumbosacral trunk Obturator Lumbar plexus, anterior view 58

Figure 13-13b The Lumbar and Sacral Plexuses Spinal Nerves Forming the Sacral Plexus Lumbosacral trunk L4 nerve L5 nerve Nerves of the Sacral Plexus S1 nerve Superior gluteal S2 nerve Inferior gluteal S3 nerve S4 nerve Sciatic S5 Posterior femoral cutaneous Co1 Pudendal Sacral plexus, anterior view 59

Figure 13-13c The Lumbar and Sacral Plexuses Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve Obturator nerve Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve (cut) Sciatic nerve Saphenous nerve Common fibular nerve Superficial fibular nerve Deep fibular nerve Nerves of the lumbar and sacral plexuses, anterior view 60

Figure 13-13d The Lumbar and Sacral Plexuses Saphenous nerve Sural nerve Sural nerve Saphenous nerve Sural nerve Fibular nerve Tibial nerve Sural nerve Saphenous nerve Fibular nerve Tibial nerve Cutaneous distribution of the nerves in the foot and ankle 61

Figure 13-13e The Lumbar and Sacral Plexuses Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Tibial nerve Common fibular nerve Sural nerve Nerves of the sacral plexus, posterior view 62

13-5 Neuronal Pools Functional Organization of Neurons Sensory neurons About 10 million Deliver information to CNS Motor neurons About 1/2 million Deliver commands to peripheral effectors Interneurons About 20 billion Interpret, plan, and coordinate signals in and out

13-5 Neuronal Pools Neuronal Pools Functional groups of interconnected neurons (interneurons) Each with limited input sources and output destinations May stimulate or depress parts of brain or spinal cord

13-5 Neuronal Pools Five Patterns of Neural Circuits in Neuronal Pools Divergence Spreads stimulation to many neurons or neuronal pools in CNS Convergence Brings input from many sources to single neuron Serial processing Moves information in single line

13-5 Neuronal Pools Five Patterns of Neural Circuits in Neuronal Pools Parallel processing Moves same information along several paths simultaneously Reverberation Positive feedback mechanism Functions until inhibited

Figure 13-14a Neural Circuits: The Organization of Neuronal Pools Divergence A mechanism for spreading stimulation to multiple neurons or neuronal pools in the CNS 67

Figure 13-14b Neural Circuits: The Organization of Neuronal Pools Convergence A mechanism for providing input to a single neuron from multiple sources 68

Figure 13-14c Neural Circuits: The Organization of Neuronal Pools Serial processing A mechanism in which neurons or pools work sequentially 69

Figure 13-14d Neural Circuits: The Organization of Neuronal Pools Parallel processing A mechanism in which neurons or pools process the same information simultaneously 70

Figure 13-14e Neural Circuits: The Organization of Neuronal Pools Reverberation A positive feedback mechanism 71

13-6 Reflexes Reflexes Automatic responses coordinated within spinal cord Through interconnected sensory neurons, motor neurons, and interneurons Produce simple and complex reflexes

13-6 Reflexes Neural Reflexes Rapid, automatic responses to specific stimuli Basic building blocks of neural function One neural reflex produces one motor response Reflex arc The wiring of a single reflex Beginning at receptor Ending at peripheral effector Generally opposes original stimulus (negative feedback)

13-6 Reflexes Five Steps in a Neural Reflex Step 1: Arrival of stimulus, activation of receptor Physical or chemical changes Step 2: Activation of sensory neuron Graded depolarization Step 3: Information processing by postsynaptic cell Triggered by neurotransmitters Step 4: Activation of motor neuron Action potential Step 5: Response of peripheral effector

Figure 13-15 Events in a Neural Reflex Dorsal root Arrival of stimulus and activation of receptor Activation of a sensory neuron Sensation relayed to the brain by axon collaterals Information processing in the CNS REFLEX ARC Receptor Stimulus Response by effector Effector Ventral root KEY Sensory neuron (stimulated) Activation of a motor neuron Excitatory interneuron Motor neuron (stimulated) 75

13-6 Reflexes Four Classifications of Reflexes By early development By type of motor response By complexity of neural circuit By site of information processing

13-6 Reflexes Development of Reflexes Innate reflexes Basic neural reflexes Formed before birth Acquired reflexes Rapid, automatic Learned motor patterns

13-6 Reflexes Motor Response Nature of resulting motor response Somatic reflexes Involuntary control of nervous system Superficial reflexes of skin, mucous membranes Stretch or deep tendon reflexes (e.g., patellar, or “knee-jerk,” reflex) Visceral reflexes (autonomic reflexes) Control systems other than muscular system

13-6 Reflexes Complexity of Neural Circuit Monosynaptic reflex Sensory neuron synapses directly onto motor neuron Polysynaptic reflex At least one interneuron between sensory neuron and motor neuron

13-6 Reflexes Site of Information Processing Spinal reflexes Occur in spinal cord Cranial reflexes Occur in brain

Figure 13-16 The Classification of Reflexes can be classified by development response response complexity of circuit processing site Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes • Genetically • Control skeletal muscle • One synapse • Processing in determined contractions the spinal cord • Include superficial and stretch reflexes Acquired Reflexes Visceral (Autonomic) Reflexes Polysynaptic Cranial Reflexes • Learned • Control actions of smooth and • Multiple synapse • Processing in cardiac muscles, glands, and adipose tissue (two to several hundred) the brain 81

13-7 Spinal Reflexes Spinal Reflexes Range in increasing order of complexity Monosynaptic reflexes Polysynaptic reflexes Intersegmental reflex arcs Many segments interact Produce highly variable motor response

13-7 Spinal Reflexes Monosynaptic Reflexes A stretch reflex Have least delay between sensory input and motor output For example, stretch reflex (such as patellar reflex) Completed in 20–40 msec Receptor is muscle spindle

Figure 13-17 A Stretch Reflex Receptor (muscle spindle) Spinal cord Stretch REFLEX ARC Stimulus Effector Contraction KEY Sensory neuron (stimulated) Motor neuron (stimulated) Response 84

13-7 Spinal Reflexes Muscle Spindles The receptors in stretch reflexes Bundles of small, specialized intrafusal muscle fibers Innervated by sensory and motor neurons Surrounded by extrafusal muscle fibers Which maintain tone and contract muscle

13-7 Spinal Reflexes The Sensory Region Central region of intrafusal fibers Wound with dendrites of sensory neurons Sensory neuron axon enters CNS in dorsal root Synapses onto motor neurons (gamma motor neurons) In anterior gray horn of spinal cord

13-7 Spinal Reflexes Gamma Efferents Axons of the motor neurons Complete reflex arc Synapse back onto intrafusal fibers Important in voluntary muscle contractions Allow CNS to adjust sensitivity of muscle spindles

Figure 13-18 A Muscle Spindle Gamma efferent from CNS Extrafusal fiber To CNS Sensory region Intrafusal fiber Muscle spindle Gamma efferent from CNS 88

13-7 Spinal Reflexes Postural reflexes Stretch reflexes Maintain normal upright posture Stretched muscle responds by contracting Automatically maintain balance

13-7 Spinal Reflexes Polysynaptic Reflexes More complicated than monosynaptic reflexes Interneurons control more than one muscle group Produce either EPSPs or IPSPs

13-7 Spinal Reflexes The Tendon Reflex Prevents skeletal muscles from: Developing too much tension Tearing or breaking tendons Sensory receptors unlike muscle spindles or proprioceptors

13-7 Spinal Reflexes Withdrawal Reflexes Move body part away from stimulus (pain or pressure) For example, flexor reflex Pulls hand away from hot stove Strength and extent of response Depend on intensity and location of stimulus

Figure 13-19 A Flexor Reflex Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated Extensors inhibited KEY Sensory neuron (stimulated) Motor neuron (inhibited) Excitatory interneuron Inhibitory interneuron Motor neuron (stimulated) 93

13-7 Spinal Reflexes Reciprocal Inhibition For flexor reflex to work The stretch reflex of antagonistic (extensor) muscle must be inhibited (reciprocal inhibition) by interneurons in spinal cord

13-7 Spinal Reflexes Reflex Arcs Ipsilateral reflex arcs Occur on same side of body as stimulus Stretch, tendon, and withdrawal reflexes Crossed extensor reflexes Involve a contralateral reflex arc Occur on side opposite stimulus

13-7 Spinal Reflexes Crossed Extensor Reflexes Occur simultaneously, coordinated with flexor reflex For example, flexor reflex causes leg to pull up Crossed extensor reflex straightens other leg To receive body weight Maintained by reverberating circuits

Figure 13-20 The Crossed Extensor Reflex To motor neurons in other segments of the spinal cord Extensors inhibited Flexors stimulated Extensors stimulated Flexors inhibited KEY Sensory neuron (stimulated) Motor neuron (inhibited) Excitatory interneuron Inhibitory interneuron Painful stimulus Motor neuron (stimulated) 97

13-7 Spinal Reflexes Five General Characteristics of Polysynaptic Reflexes 1. Involve pools of interneurons 2. Are intersegmental in distribution 3. Involve reciprocal inhibition 4. Have reverberating circuits Which prolong reflexive motor response 5. Several reflexes cooperate To produce coordinated, controlled response

13-8 The Brain Can Alter Spinal Reflexes Integration and Control of Spinal Reflexes Reflex behaviors are automatic But processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord

13-8 The Brain Can Alter Spinal Reflexes Voluntary Movements and Reflex Motor Patterns Higher centers of brain incorporate lower, reflexive motor patterns Automatic reflexes Can be activated by brain as needed Use few nerve impulses to control complex motor functions Walking, running, jumping

13-8 The Brain Can Alter Spinal Reflexes Reinforcement of Spinal Reflexes Higher centers reinforce spinal reflexes By stimulating excitatory neurons in brain stem or spinal cord Creating EPSPs at reflex motor neurons Facilitating postsynaptic neurons

13-8 The Brain Can Alter Spinal Reflexes Inhibition of Spinal Reflexes Higher centers inhibit spinal reflexes by: Stimulating inhibitory neurons Creating IPSPs at reflex motor neurons Suppressing postsynaptic neurons

13-8 The Brain Can Alter Spinal Reflexes The Babinski Reflexes Normal in infants May indicate CNS damage in adults

Figure 13-21a The Babinski Reflexes The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. 104

Figure 13-21b The Babinski Reflexes The Babinski sign (positive Babinski reflex) occurs in the absence of descending inhibition. It is normal in infants, but pathological in adults. 105