An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes Learning Outcomes 13-1 Describe the basic structural and organizational characteristics of the nervous system. 13-2 Discuss the structure and functions of the spinal cord, and describe the three meningeal layers that surround the central nervous system. 13-3 Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands. 13-4 Describe the major components of a spinal nerve, and relate the distribution pattern of spinal nerves to the regions they innervate.

An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes Learning Outcomes 13-5 Discuss the significance of neuronal pools, and describe the major patterns of interaction among neurons within and among these pools. 13-6 Describe the steps in a neural reflex, and classify the types of reflexes. 13-7 Distinguish among the types of motor responses produced by various reflexes, and explain how reflexes interact to produce complex behaviors. 13-8 Explain how higher centers control and modify reflex responses.

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 Reflex centers in brain Effectors 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

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

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 C8 C3 T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 Posterior median sulcus T9 KEY Spinal cord regions = Cervical T10 T3 = Thoracic T11 Lumbar enlargement = Lumbar T12 = Sacral Conus medullaris L1 L2 Inferior tip of spinal cord Lumbar spinal nerves L3 L4 Cauda equina L5 L1 S1 Sacral spinal nerves S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) S2 b Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. a 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 adult 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.

Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 1 of 4). 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 a 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 adult 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. b Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter.

Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 2 of 4). Thoracic spinal nerves T8 Posterior median sulcus T9 T10 T3 T11 Lumbar enlargement T12 Conus medullaris a 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 adult 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. b Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter.

Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 3 of 4). Conus medullaris L1 L2 Inferior tip of spinal cord Lumbar spinal nerves L3 L4 Cauda equina L5 L1 a 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 adult 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. b Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter.

Figure 13-2 Gross Anatomy of the Adult Spinal Cord (Part 4 of 4). Inferior tip of spinal cord Cauda equina S1 Sacral spinal nerves S2 S3 S4 S5 S2 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) b Inferior views of cross sections through representative segments of the spinal cord, showing the arrangement of gray matter and white matter. a 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 adult 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.

13-2 Spinal Cord 31 Spinal Cord Segments Based on vertebrae where spinal nerves originate Positions of spinal segment and vertebrae change with age Cervical nerves Named for inferior vertebra All other nerves Named for superior vertebra

13-2 Spinal Cord Roots Two branches of spinal nerves Ventral root Contains axons of motor neurons Dorsal root Contains axons of sensory neurons Dorsal root ganglia Contain cell bodies of sensory neurons

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

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 Tough and fibrous Cranially Caudally Fuses with periosteum of occipital bone Is continuous with cranial dura mater Caudally Tapers to dense cord of collagen fibers Joins filum terminale in coccygeal ligament

13-2 Spinal Cord The Dura Mater The epidural space Between spinal dura mater and walls of vertebral canal Contains loose connective and adipose tissue Anesthetic injection site

13-2 Spinal Cord The Arachnoid Mater Middle meningeal layer Arachnoid membrane Simple squamous epithelia Covers arachnoid mater

13-2 Spinal Cord 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 Interlayer Spaces of Arachnoid Mater Cerebrospinal Fluid (CSF) Carries dissolved gases, nutrients, and wastes Lumbar puncture or spinal tap withdraws CSF

13-2 Spinal Cord The Pia Mater Is the innermost meningeal layer Is a mesh of collagen and elastic fibers Is bound to underlying neural tissue

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 the 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 Control and Location Sensory or motor nucleus location within the gray matter determines which body part it controls

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 is 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

of white matter. The right half indicates the Figure 13-5a The Sectional Organization of the Spinal Cord (Part 1 of 2). Posterior white column Posterior gray horn Lateral white column Lateral gray horn Dorsal root ganglion Anterior gray horn Anterior white column a 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.

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

13-3 Gray Matter and White Matter Spinal Cord Summary Gray matter Is covered by a thick layer of white matter White matter Consists of ascending and descending axons Organized in columns Contains axon bundles with specific functions Spinal cord is so highly organized: It is possible to predict results of injuries to specific areas

13-4 Spinal Nerves and Plexuses Anatomy of Spinal Nerves Each spinal cord segment: Is connected to a pair of spinal nerves Each 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 13-6a A Peripheral Nerve. Blood vessels Connective Tissue Layers Epineurium covering peripheral nerve Perineurium (around one fascicle) Endoneurium Schwann cell Myelinated axon Fascicle a A typical peripheral nerve and its connective tissue wrappings

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

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 Dermatomes. N V C2–C3 C2 C3 C3 C4 T2 T2 C4 C5 T3 T1 T4 T2 L1 C6 T10 L2 T11 L4 L3 T1 C6 C7 T12 L5 L1 S4 S3 L2 S2 C8 T1 L3 C8 L1 L1 C7 S5 S1 L5 L4 L2 S2 KEY L5 L3 Spinal cord regions = Cervical = Thoracic S1 = Lumbar = Sacral L4 ANTERIOR POSTERIOR

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

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 13-9 Peripheral Nerves and Nerve Plexuses (Part 1 of 2). C1 Lesser occipital nerve C2 Great auricular nerve Cervical plexus C3 Transverse cervical nerve C4 C5 Supraclavicular nerve C6 Phrenic nerve Brachial plexus C7 C8 T1 T2 T3 Axillary nerve T4 T5 T6 T7 Musculocutaneous nerve T8 T9 Thoracic nerves T10 T11

Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 2 of 2). Radial nerve Lumbar plexus L2 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

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 13-10 The Cervical Plexus (Part 1 of 2). Cranial Nerves Accessory nerve (N XII) Hypoglossal nerve (N XII) Nerve Roots of Cervical Plexus C1 C2 C3 C4 C5 Clavicle

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)

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

Figure 13-12a The Lumbar and Sacral Plexuses.

Figure 13-12a The Lumbar and Sacral Plexuses.

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 13-12b The Lumbar and Sacral Plexuses. Lumbosacral trunk Nerve Roots of Sacral Plexus Sacral Plexus L4 The sacral plexus is formed by a branch from L4 and ventral rami of L5–S4. Spinal Segments Nerve and Distribution Superior Gluteal L5 L4–S2 Gluteus minimus, gluteus medius, and tensor fasciae latae muscles L5 Inferior Gluteal S1 L4–S2 Gluteus maximus muscle S2 Posterior Femoral Cutaneous Sacrum S3 S1–S3 Skin over perineum and posterior thigh and leg S4 Sciatic L4–S3 Semimembranosus, semitendinosus, and adductor magnus muscles; branches into tibial and fibular nerves S5 Pudendal Co1 S2–S4 Muscles of the perineum; skin over external genitalia, bulbospongiosus and ischiocavernosus muscles b Sacral plexus, anterior view

Figure 13-12c 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 c Nerves of the lumbar and sacral plexuses, anterior view

Figure 13-12d 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 d Nerves of the sacral plexus, posterior view

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

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

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

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

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

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 13-14 Spinal Reflexes (Part 1 of 4). 2 Dorsal root Arrival of stimulus and activation of receptor Activation of a sensory neuron Sensation relayed to the brain by axon collaterals Spinal cord 3 Information processing in the CNS REFLEX ARC Receptor Stimulus 5 Response by a peripheral effector Effector Ventral root KEY 4 Sensory neuron (stimulated) Activation of a motor neuron Excitatory interneuron Motor neuron (stimulated)

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 Sites of Information Processing Spinal reflexes Occur in spinal cord Cranial reflexes Occur in brain

Figure 13-14 Spinal Reflexes (Part 2 of 4). Receptor (muscle spindle) Stretch Spinal cord REFLEX ARC Stimulus Effector Contraction KEY Response Sensory neuron (stimulated) Motor neuron (stimulated)

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 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 13-14 Spinal Reflexes (Part 3 of 4). Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated Extensors inhibited

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 The Babinski Reflexes Normal in infants May indicate CNS damage in adults

Figure 13-17a The Babinski Reflexes. The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults.

Figure 13-17b 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.