BIOL 2401 Fundamentals of Anatomy and Physiology Mrs. Willie Grant wgrant4@alamo.edu 210-486-2870

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. 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.

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 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) 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

Figure 13-2 Gross Anatomy of the Adult Spinal Cord Posterior median sulcus Dorsal root Dorsal root ganglion White matter C1 Central canal Gray matter C2 Cervical spinal nerves C3 C4 C5 Spinal nerve Ventral root C6 Cervical enlargement C7 Anterior median fissure C8 C3 T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 Posterior median sulcus T9 T10 T3 T11 Lumbar enlargement T12 Conus medullaris L1 L2 Inferior tip of spinal cord Lumbar spinal nerves L3 L4 Cauda equina L5 L1 S1 Sacral spinal nerves S2 S3 1 What portion of the spinal cord connects with nerves of the upper limbs? S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) S2 7

13-2 Spinal Cord The Spinal Nerve On each side of spine Dorsal and ventral roots join to form a spinal nerve Mixed Nerves Carry both afferent (sensory) and efferent (motor) fibers 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, Pia Mater) The Dura Mater Tough and fibrous Cranially fuses with periosteum of occipital bone Is continuous with cranial dura mater Caudally tapers to dense cord of collagen fibers and joins filum terminale in coccygeal ligament 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) Simple squamous epithelia covering arachnoid mater 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) Carries dissolved gases, nutrients, and wastes Lumbar puncture or spinal tap withdraws CSF The Pia Mater (the innermost meningeal layer) Is a mesh of collagen and elastic fibers 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

13-3 Gray Matter and White Matter Sectional Anatomy of the Spinal Cord White matter (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) Organization of Gray Matter The gray horns Posterior gray horns contain somatic and visceralsensory 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 other before reacing gray matter) 2 What is the difference between a horn and a column in the spinal cord?

13-3 Gray Matter and White Matter Organization of Gray Matter The cell bodies of neurons form functional groups called nuclei (masses of gray matter within the CNS) Sensory nuclei (Dorsal—posterior) Connect to peripheral receptors Motor nuclei (ventral—anterior) Connect to peripheral effectors 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 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 Tracts or fasciculi (a bundle of axons in the white columns that relay the same information in the same direction). Ascending tracts—carry information to brain Descending tracts—carry motor commands to spinal cord

Figure 13-5 The Sectional Organization of the Spinal Cord

13-3 Gray Matter and White Matter Spinal Cord Summary Spinal cord has a narrow central canal surrounded by gray matter Containing sensory (dorsal) and motor nuclei (ventral) 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 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 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 19

13-4 Spinal Nerves and Plexuses Peripheral Distribution of Spinal Nerves Spinal nerves that form lateral to intervertebral foramen where the dorsal and ventral roots unite then branch to form pathways to destination 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 Dorsal and ventral rami Dorsal ramus Contains somatic and visceral motor fibers. Innervates the back. Ventral ramus Larger branch. Innervates ventrolateral structures and limbs 3 Why are all spinal nerves classified as mixed nerves?

Sensory input is conveyed from sensory receptors to the posterior gray horns of the spinal cord.

Motor output is conveyed from the anterior and lateral gray horns of the spinal cord to effectors (muscles and glands).

Dermatome is the specific bilateral region of the skin surface monitored by a single pair of spinal nerves. Peripheral Neuropathy Regional loss of sensory or motor function Due to trauma or compression Examples: when your leg falls asleep.

Includes ventral rami of spinal nerves C1-C5 Includes ventral rami of spinal nerves C1-C5. Innervates neck, thoracic cavity, diaphragm. PHRENIC NERVE Spinal Nerves C5-T1. Innervates pectoral girdle/upper limbs. RADIAL NERVE, ULNAR NERVE, MEDIAN NERVE. Spinal Nerves T12-L4. FEMORAL NERVE Spinal Nerves L4-S4. SCIATIC NERVE branches into FIBULAR NERVE AND TIBIAL NERVE 4 Why does complete severing of the spinal cord at level C2 cause respiratory arrest? 5 What three important nerves arise from the brachial plexus?

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

Figure 13-14 Neural Circuits: The Organization of Neuronal Pools

13-6 Reflexes Reflexes—automatic responses coordinated within spinal cord Through interconnected sensory neurons, motor neurons, and interneurons, producing simple and complex 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

13-7 Spinal Reflexes Spinal Reflexes Monosynaptic reflexes (a stretch reflex) and Polysynaptic reflexes Stretch reflex Have least delay between sensory input and motor output For example, stretch reflex (such as patellar reflex) Completed in 20–40 msec. The receptor is muscle spindle

Polysynaptic Reflexes Produce more complicated responses because the interneurons can control motor neurons that activate several muscle groups simultaneously. A withdrawal reflex moves affected parts of the body away from a stimulus. An example of a withdrawal reflex is the flexor reflex.

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

Ipsilateral reflex arcs occurs on same side of body as stimulus 13-7 Spinal Reflexes Reflex Arcs Ipsilateral reflex arcs occurs 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) 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

Stretch reflexes that help maintain normal upright posture. Spinal Reflexes Polysynaptic Reflexes More complicated than monosynaptic reflexes Interneurons control more than one muscle group Withdrawal Reflex is an example of a Polysynaptic Reflex Postural reflexes Stretch reflexes that help maintain normal upright posture. Stretched muscle responds by contracting Automatically maintain balance

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

Figure 13-18 A Muscle Spindle Gamma efferent from CNS Extrafusal fiber To CNS Sensory region (central, enters CNS at dorsal root, synapses with gamma motor neurons Intrafusal fiber Muscle spindle Gabba efferent from CNS (synapses back Into intrafusalfibers) 38

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 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 Facilitating postsynaptic neurons Inhibition of Spinal Reflexes Higher centers inhibit spinal reflexes by: Stimulating inhibitory neurons Suppressing postsynaptic neurons

Figure 13-21a The Babinski Reflexes Normal in infants The plantar reflex (negative Babinski relfex), A curling of the toes, is seen in healthy adults. 41

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. 42

Clinical Case—Prom Night Do you think Joe will regain motor or sensory activity in his lower extremities? Why or why not? Do you think Joe’s Babinski relfexes are normal (toes plantar flexed) or abnormal (toes go up, Dorsiflexed)? Why?