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 1

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

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

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 5

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 6

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

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 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 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. 14

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. 15

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 19

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 21

Figure 13-8 Dermatomes 22 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 22

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

Figure 13-9 Shingles 24

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

13-6 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) 29

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 35

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 38

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 42

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) 47

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) 48

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 13-21a The Babinski Reflexes The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. 52

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. 53