An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes

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.

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.

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

Spinal Reflexes are rapid, automatic nerve responses triggered by specific stimuli and controlled by spinal cord alone, not the brain. Enlargements of the Spinal Cord caused by: Amount of gray matter in segment Involvement with sensory and motor nerves of 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 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 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 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 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.

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.

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.

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 is based on vertebrae where spinal nerves originate and positions of spinal segment and vertebrae change with age. 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: 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 Ventral rootlets of spinal nerve Dorsal root ganglion Ventral root Spinal nerve Dorsal root Meninges Dorsal rootlets of spinal nerve Pia mater Dura mater: Outer layer of spinal cord is tough and fibrous Arachnoid mater: Middle meningeal layer, simple squamous epithelia and covers arachnoid mater Pia mater: Inner meningeal layer is a mesh of collagen and elastic fibers 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 a

Figure 13-3b The Spinal Cord and Spinal Meninges.
Dura mater Arachnoid mater ANTERIOR 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 POSTERIOR b A sectional view through the spinal cord and meninges, showing the relationship of the meninges, spinal cord, and spinal nerves

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 Dura Mater is tough and fibrous
Cranially: fuses with periosteum of occipital bone, is continuous with cranial 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 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) and filled with cerebrospinal fluid (CSF) which carries dissolved gases, nutrients, and wastes

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

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

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

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

Control and Location Sensory or motor nucleus location within the gray matter determines which body part it controls 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 (Part 1 of 2).
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 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.

Sensory nuclei Motor nuclei Functional Organization of Gray Matter
Figure 13-5a The Sectional Organization of the Spinal Cord (Part 2 of 2). 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 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.

Spinal Cord Summary Spinal cord has a narrow central canal which surrounded by gray matter and containing sensory and motor nuclei Sensory nuclei are dorsal Motor nuclei are ventral

Spinal Cord Summary Gray matter: Is covered by a thick layer of white matter White matter: Consists of ascending and descending axons very organized in columns and contains axon bundles

13-4 Spinal Nerves and Plexuses
Anatomy of Spinal Nerves Each spinal cord segment: is connected to a pair of spinal nerves which is surrounded by3 connective tissue that support structures and contain blood vessels

Figure 13-6a A Peripheral Nerve.
Epineurium: Outer layer is dense network of collagen fibers Perineurium: Middle layer is divides nerve into fascicles (axon bundles) Endoneurium: Inner layer is surrounds individual axons Blood vessels Connective Tissue Layers Epineurium covering peripheral nerve Perineurium (around one fascicle) Endoneurium Schwann cell Myelinated axon Fascicle A typical peripheral nerve and its connective tissue wrappings a

Figure 13-6b A Peripheral Nerve.
Blood vessels Interconnecting branches of spinal nerves which surrounded by connective tissue sheaths Connective Tissue Layers Perineurium (around one fascicle) Endoneurium b A scanning electron micrograph showing the various layers in great detail (SEM × 340)

Peripheral Distribution of Spinal Nerves Motor nerves 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 ramus: Contains somatic and visceral motor fibers and innervates the back Ventral ramus: Larger branch and innervates ventrolateral structures and limbs

Figure 13-8 Peripheral Distribution of Spinal Nerves (Part 2 of 2).
To skeletal muscles of back Postganglionic fibers to smooth muscles, glands, etc., of back 2 The spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. 3 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 4 The axons in the relatively large ventral ramus supply the ventrolateral body surface, structures in the body wall, and the limbs. 1 The ventral root of each spinal nerve contains the axons of somatic motor and visceral motor neurons. Visceral motor nuclei To skeletal muscles of body wall, limbs Somatic motor nuclei Rami communicantes KEY Postganglionic fibers to smooth muscles, and glands of body wall, limbs = Somatic motor commands Sympathetic ganglion = Visceral motor commands 5 The white ramus communicans 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. White rami are only found between T1 and L2. Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity 7 A sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity. 6 The gray ramus communicans contains preganglionic fibers that innervate glands and smooth muscles in the body wall or limbs. These fibers are unmyelinated and have a dark gray color. Gray rami are associated with each spinal nerve. Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera 1

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 and monitored by specific pair of spinal nerves

Figure 13-8 Peripheral Distribution of Spinal Nerves (Part 1 of 2).
From interoceptors of back From exteroceptors, proprioceptors of back 4 The dorsal root of each spinal nerve carriers sensory information to the spinal cord. 3 The dorsal ramus carries sensory information from the skin and skeletal muscles of the back. Somatic sensory nuclei 2 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 KEY = Somatic sensations 1 = Visceral sensations The sympathetic nerve carriers sensory information from the visceral organs. From interceptors of visceral organs

Figure 13-7 Dermatomes. N V Dermatomes is the bilateral region of skin and monitored by specific pair of spinal nerves C2–C3 C2 C3 C3 C4 T2 T2 C4 C5 T3 T1 T4 T2 T5 T3 C5 T6 T4 T7 T5 T8 T6 T9 T2 T2 T7 T10 T8 T11 T9 T12 L1 C6 T10 L2 T11 L4 L3 T1 C6 C7 T12 L5 L1 S4 S3 L2 S2 C8 C8 T1 L3 L1 L1 C7 S5 S1 L5 L4 L2 S2 KEY L5 L3 Spinal cord regions = Cervical = Thoracic S1 = Lumbar = Sacral L4 ANTERIOR POSTERIOR

Peripheral Neuropathy: Regional loss of sensory or motor function due to trauma or compression Nerve Plexuses: Complex, interwoven networks of nerve fibers formed from blended fibers of ventral rami of adjacent spinal nerves and control skeletal muscles of the neck and limbs Cervical plexus Brachial plexus Lumbar plexus Sacral plexus

Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 1 of 2).
Cervical Plexus: Includes ventral rami of spinal nerves C1–C5 Innervates neck, thoracic cavity, diaphragmatic muscles Major nerve is Phrenic nerve (controls diaphragm) C1 Lesser occipital nerve C2 Great auricular nerve Cervical plexus C3 Includes ventral rami of spinal nerves C5–T1 Innervates pectoral girdle and upper limbs 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).
Major nerves Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve Figure 13-9 Peripheral Nerves and Nerve Plexuses (Part 2 of 2). T12 L1 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 Major nerves Pudendal nerve Sciatic nerve Two branches of the sciatic nerve Fibular nerve Tibial nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior Gluteal nerves Inferior Pudendal nerve Saphenous nerve Sciatic nerve

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

Figure 13-10 The Cervical Plexus (Part 2 of 2).

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

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 13-11a The Brachial Plexus.

Figure 13-11b The Brachial Plexus.

Figure 13-12a The Lumbar and Sacral Plexuses.

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 are about 10 million deliver information to CNS Motor neurons about 1/2 million deliver commands to peripheral effectors Interneurons about 20 billion which 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 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

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 Neural Reflexes
Automatic responses coordinated within spinal cord through interconnected sensory neurons, motor neurons, and interneurons and produce simple and complex reflexes Neural Reflexes Rapid, automatic responses to specific stimuli Basic building blocks of neural function One neural reflex produces one motor respons

13-6 Reflexes Reflex arc The wiring of a single reflex
Beginning at receptor Ending at peripheral effector Generally opposes original stimulus (negative feedback)

Figure 13-14 Steps in Spinal Reflexes
1: Arrival of stimulus, activation of receptor Physical or chemical changes 2: Activation of sensory neuron Graded depolarization 3: Information processing by postsynaptic cell Triggered by neurotransmitters 4: Activation of motor neuron Action potential 5: Response of peripheral effector Triggered by neurotransmitters 1 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 Four Classifications of Reflexes Development of Reflexes
By early development By type of motor response By complexity of neural circuit By site of information processing 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

Figure 13-15 The Classification of Reflexes.
development can be classified by complexity of circuit processing site response Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes Genetically determined Control skeletal muscle contractions One synapse Processing in the spinal cord Include superficial and stretch reflexes Acquired Reflexes Visceral (Autonomic) Reflexes Polysynaptic Cranial Reflexes Learned Control actions of smooth and cardiac muscles, glands, and adipose tissue Multiple synapses (two to several hundred) Processing in the brain

13-7 Spinal Reflexes Spinal Reflexes
Range in increasing order of complexity Monosynaptic reflexes Polysynaptic reflexes

Figure 13-14 Spinal Reflexes (Part 2 of 4).
Receptor (muscle spindle) Stretch Spinal cord REFLEX ARC Stimulus Intersegmental reflex arcs: Many segments interact Produce highly variable motor response A stretch reflex: Have least delay between sensory input and motor output. For example, stretch reflex (such as patellar reflex) Completed in 20–40 msecReceptor is muscle spindle Effector Contraction KEY Response Sensory neuron (stimulated) Motor neuron (stimulated)

13-7 Spinal Reflexes Muscle Spindles: The receptors in stretch reflexes. Bundles of small, specialized intrafusal muscle fibers: Innervated by sensory and motor neuron, surrounded by extrafusal muscle fibers which maintain tone and contract muscle Gamma Efferents: Axons of the motor neurons. Complete reflex arc which synapse back onto intrafusal fibers and is important in voluntary muscle contractions, allow CNS to adjust sensitivity of muscle spindles

Figure 13-16 A Muscle Spindle.
Gamma efferent from CNS Extrafusal fiber maintain tone and contract muscle To CNS Sensory region Intrafusal fiber Innervated by sensory and motor neuron Muscle spindle Gamma efferent from CNS Axons of the motor neurons

13-7 Spinal Reflexes Postural Reflexes: Stretch reflexes which maintain normal upright posture, stretched muscle responds by contracting. Automatically maintains balance Tendon Reflex: Prevents skeletal muscles from: Developing too much tension and tearing or breaking tendons Withdrawal Reflexes: Move body part away from stimulus (pain or pressure) For example, flexor reflex pulls hand away from hot stove

Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated pulls hand away from hot stove Extensors inhibited

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

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 Motor neuron (stimulated) Painful stimulus

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 Automatic reflexes Can be activated by brain as needed Use few nerve impulses to control complex motor functions like walking, running, jumping

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

Figure 13-17a The Babinski Reflexes.
The Babinski Reflexes is normal in infants and may indicate CNS damage in adults The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. a

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.

An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes

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An Introduction to the Spinal Cord, Spinal Nerves, and Spinal Reflexes

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