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BIOL 2401 Fundamentals of Anatomy and Physiology Mrs. Willie Grant
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13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes
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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.
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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 4
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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
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13-2 Spinal Cord Enlargements of the Spinal Cord (expanded amount of gray matter in segments of the spinal cord and motor control of the limbs) Cervical enlargement Nerves of shoulders and upper limbs Lumbar enlargement Nerves of pelvis and lower limbs 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
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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 C7 Cervical enlargement Anterior median fissure C8 C3 T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 Posterior median sulcus T9 T10 Lumbar enlargement T3 T11 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 S4 S5 Filum terminale (in coccygeal ligament) S2 Coccygeal nerve (Co1) 7
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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 Are named for inferior vertebra All other nerves Are named for superior vertebra
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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
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13-2 Spinal Cord The Spinal Nerve On 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
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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
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BIOL 2401 Meningitis is an inflammation of the meninges or the three membranes that surround and protect the brain and spinal cord. In most cases, the condition is caused by a viral or bacterial infection. Meningitis can afflict children and adults. Today vaccinations are offered by most of the educational institues to curtail the spread of the condition through teens and young adults. In addition, vaccines are available for travelers visiting foreign countries where the prevalence of meningitis may be higher. Meningitis is extremely contagious and can spread from person to person through coughing and sneezing. Symptoms of meningitis: severe headache, back ache, neck ache, pain in the eyes, especially around light, nausea, fever, vomiting, confusion, sleepiness, lethargy and fatigue.
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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
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13-2 Spinal Cord 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 Joins filum terminale in coccygeal ligament
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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
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13-2 Spinal Cord The Arachnoid Mater Middle meningeal layer
Arachnoid membrane Simple squamous epithelia Covers arachnoid mater
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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) Cerebrospinal Fluid (CSF) Carries dissolved gases, nutrients, and wastes Lumbar puncture or spinal tap withdraws CSF
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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
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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
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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)
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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
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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
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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
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A HORN is an area of gray matter.
BIOL 2401 What is the difference between a horn and a column in the spinal cord? A HORN is an area of gray matter. A COLUMN is a region of white matter.
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13-3 Gray Matter and White Matter
Organization of White Matter 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
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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
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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
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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
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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 32
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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
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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 Dorsal and ventral rami Dorsal ramus Contains somatic and visceral motor fibers Innervates the back Ventral ramus Larger branch Innervates ventrolateral structures and limbs
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Sensory input is conveyed from sensory receptors to the posterior gray horns of the spinal cord.
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Motor output is conveyed from the anterior and lateral gray horns of the spinal cord to effectors (muscles and glands).
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Bilateral regions of skin monitored by specific pairs of nerves.
Figure Dermatomes C2C3 N V Bilateral regions of skin monitored by specific pairs of nerves. C2C3 C2 C3 C3 C4 T2 C4 C5 T3 T1 T2 T4 T5 T3 C5 T6 T4 T7 T5 T8 T6 T9 T2 T2 T7 T10 T8 T11 T12 T9 L1 C6 T10 L2 T11 L4 L3 T1 C6 C7 T12 L5 L1 Peripheral Neuropathy Regional loss of sensory or motor function Due to trauma or compression Examples: when your leg falls asleep. S S L2 4 S2 3 C8 C8 T1 L3 L1 C7 S5 S1 L5 L4 L2 S2 L5 L3 S1 L4 ANTERIOR POSTERIOR 37
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Why is shingles probably NOT a problem for most people in this class?
Figure Shingles Shingles is caused by the varicell-zoster virus (which also causes chickenpox). The herpes virus attacks neurons within the dorsal roots of spinal nerves and sensory ganglia of cranial nerves. This causes a painful rash and blisters whose distribution corresponds to that of the affected sensory nerves and follows its dermatome. Why is shingles probably NOT a problem for most people in this class? 38
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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 The Four Major Plexuses of Ventral Rami Cervical plexus Brachial plexus Lumbar plexus Sacral plexus
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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)
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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 Major nerves Musculocutaneous nerve (lateral cord) Median nerve (lateral and medial cords) Ulnar nerve (medial cord) Axillary nerve (posterior cord) Radial nerve (posterior cord)
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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 The Sacral Plexus Includes ventral rami of spinal nerves L4–S4 Pudendal nerve Sciatic nerve (Two branches: Fibular nerve and Tibial nerve)
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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
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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
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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 Parallel processing Moves same information along several paths simultaneously Reverberation Positive feedback mechanism Functions until inhibited
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13-6 Reflexes Reflexes Automatic responses coordinated within spinal cord Through interconnected sensory neurons, motor neurons, and interneurons Produce simple and complex reflexes
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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)
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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
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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) 53
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Figure 13-16 The Classification of Reflexes
Four classes: 1 Development, 2 Response, 3 Complexity of Circuit, 4 Processing Site Reflexes can be classified by 2 1 3 4 development response complexity of circuit processing site Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes • Control skeletal muscle • One synapse • Genetically • Processing in contractions determined the spinal cord • Include superficial and stretch reflexes Acquired Reflexes Visceral (Autonomic) Reflexes Polysynaptic Cranial Reflexes • Control actions of smooth and • Learned cardiac muscles, glands, and adipose tissue • Multiple synapse • Processing in (two to several hundred) the brain 54
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13-7 Spinal Reflexes Spinal Reflexes
Range in increasing order of complexity Monosynaptic reflexes Polysynaptic reflexes Intersegmental reflex arcs Many segments interact to produce highly variable motor response
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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
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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 57
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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
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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) 59
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13-7 Spinal Reflexes Postural reflexes
Stretch reflexes that help maintain normal upright posture. Stretched muscle responds by contracting Automatically maintain balance
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13-7 Spinal Reflexes Polysynaptic Reflexes
More complicated than monosynaptic reflexes Interneurons control more than one muscle group
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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
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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
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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) 64
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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
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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
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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) 67
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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
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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
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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
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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
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13-8 The Brain Can Alter Spinal Reflexes
The Babinski Reflexes Normal in infants May indicate CNS damage in adults
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Figure 13-21a The Babinski Reflexes
The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. 73
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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. 74
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BIOL 2401 What initiates a nerve impulse in a sensory neuron? Which beranch of the nervsous system includes all integrating centers for reflexes? A sensory receptor produces a generator potential which triggers a nerve impulse. Reflex integrating centers are in the CNS.
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