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.

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.

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
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

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 Arachnoid mater Dura mater a A posterior view of the spinal cord, showing the meningeal layers, superficial landmarks, and distribution of gray matter and white matter

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 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

13-2 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.
Anterior median fissure Pia mater Denticulate ligaments 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 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)

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 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

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.

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.

Figure 13-5b The Sectional Organization of the Spinal Cord (Part 1 of 2).
POSTERIOR Posterior gray commissure Dura mater Arachnoid mater (broken) Central canal Anterior gray commissure Anterior median fissure Pia mater ANTERIOR b A micrograph of a transverse section through the spinal cord, showing major landmarks in and surrounding the cord.

Structural Organization of Gray Matter
Figure 13-5b The Sectional Organization of the Spinal Cord (Part 2 of 2). POSTERIOR Structural Organization of Gray Matter Posterior median sulcus The projections of gray matter toward the outer surface of the spinal cord are called horns. Posterior gray horn Lateral gray horn Dorsal root Anterior gray horn Dorsal root ganglion ANTERIOR Ventral root b A micrograph of a transverse section through the spinal cord, showing major landmarks in and surrounding the cord.

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

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

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

Figure 13-6b A Peripheral Nerve.
Blood vessels Connective Tissue Layers Perineurium (around one fascicle) Endoneurium b A scanning electron micrograph showing the various layers in great detail (SEM × 340)

Peripheral Nerves Interconnecting branches of spinal nerves Surrounded by connective tissue sheaths

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

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

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 13-8 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 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 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

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 Control skeletal muscles of the neck and limbs

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

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

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.

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.

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

Figure 13-15 The Classification of Reflexes.
can be classified by development response complexity of circuit processing site 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 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

Receptor (muscle spindle) Stretch Spinal cord REFLEX ARC Stimulus 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 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 13-16 A Muscle Spindle.
Gamma efferent from CNS Extrafusal fiber To CNS Sensory region Intrafusal fiber Muscle spindle Gamma efferent from CNS

13-7 Spinal Reflexes Postural Reflexes Stretch reflexes
Maintain normal upright posture Stretched muscle responds by contracting Automatically maintains 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

Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated 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

13-7 Spinal Reflexes Five General Characteristics of Polysynaptic Reflexes Involve pools of interneurons Are intersegmental in distribution Involve reciprocal inhibition Have reverberating circuits Which prolong reflexive motor response Several reflexes cooperate To produce coordinated, controlled response

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

Reinforcement of Spinal Reflexes Higher centers reinforce spinal reflexes By stimulating excitatory neurons in brain stem or spinal cord Creating EPSPs at reflex motor neurons Facilitating postsynaptic neurons

Inhibition of Spinal Reflexes Higher centers inhibit spinal reflexes by: Stimulating inhibitory neurons Creating IPSPs at reflex motor neurons Suppressing postsynaptic neurons

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.

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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