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14 The Nervous System: The Spinal Cord and Spinal Nerves PowerPoint® Lecture Presentations prepared by Steven Bassett Southeast Community College Lincoln, Nebraska © 2012 Pearson Education, Inc. Introduction • The Central Nervous System (CNS) consists of: • The spinal cord • Integrates and processes information • Can function with the brain • Can function independently of the brain • The brain • Integrates and processes information • Can function with the spinal cord • Can function independently of the spinal cord © 2012 Pearson Education, Inc. Gross Anatomy of the Spinal Cord • Features of the Spinal Cord • • • • 45 cm in length Passes through the foramen magnum Extends from the brain to L1 Consists of: • • • • • Cervical region Thoracic region Lumbar region Sacral region Coccygeal region © 2012 Pearson Education, Inc. Gross Anatomy of the Spinal Cord • Features of the Spinal Cord • Consists of (continued): • • • • • Cervical enlargement Lumbosacral enlargement Conus medullaris Cauda equina Filum terminale: becomes a component of the coccygeal ligament • Posterior and anterior median sulci © 2012 Pearson Education, Inc. Figure 14.1a Gross Anatomy of the Spinal Cord Cervical spinal nerves C1 C2 C3 C4 C5 C6 C7 C8 Cervical enlargement T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 T9 Posterior median sulcus T10 T11 Lumbosacral enlargement T12 L1 Conus medullaris L2 Lumbar spinal nerves L3 L4 Inferior tip of spinal cord Cauda equina L5 Sacral spinal nerves S1 S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) 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 spinal cord, however, extends from the brain only to the level of vertebrae L1–L2. © 2012 Pearson Education, Inc. Gross Anatomy of the Spinal Cord • Features of the Spinal Cord • Transverse view • • • • White matter Gray matter Central canal Dorsal root and ventral root: merge to form a spinal nerve • Dorsal root is sensory: axons extend from the soma within the dorsal root ganglion • Ventral root is motor © 2012 Pearson Education, Inc. Figure 14.1d Gross Anatomy of the Spinal Cord Posterior median sulcus Dorsal root Dorsal root ganglion White matter Central canal Spinal nerve Gray matter Ventral root Anterior median fissure C3 T3 L1 S2 Inferior views of cross sections through representative segments of the spinal cord showing the arrangement of gray and white matter © 2012 Pearson Education, Inc. Gross Anatomy of the Spinal Cord • Features of the Spinal Nerves • Consist of: • Sensory nerves (afferent nerves): transmit impulses toward the spinal cord • Motor nerves (efferent nerves): transmit impulses away from the spinal cord © 2012 Pearson Education, Inc. Figure 14.1d Gross Anatomy of the Spinal Cord Posterior median sulcus Dorsal root Dorsal root ganglion White matter Central canal Spinal nerve Gray matter Ventral root Anterior median fissure C3 T3 L1 S2 Inferior views of cross sections through representative segments of the spinal cord showing the arrangement of gray and white matter © 2012 Pearson Education, Inc. Spinal Meninges • Features of spinal meninges: • Specialized membranes that provide protection, physical stability, and shock absorption • Continuous with the cranial (cerebral) meninges • Denticulate ligaments help anchor the spinal cord in position • Made of three layers • Dura mater: tough, fibrous outermost layer • Arachnoid mater: middle layer • Pia mater: innermost layer © 2012 Pearson Education, Inc. Figure 14.2c The Spinal Cord and Spinal Meninges Gray matter White matter Ventral root Spinal nerve Dorsal root Pia mater Dorsal root ganglion Arachnoid mater Dura mater Posterior view of the spinal cord showing the meningeal layers, superficial landmarks, and distribution of gray and white matter © 2012 Pearson Education, Inc. Figure 14.2a The Spinal Cord and Spinal Meninges Spinal cord Anterior median fissure Pia mater Denticulate ligaments Arachnoid mater (reflected) Dura mater (reflected) Spinal blood vessel Dorsal root of sixth cervical nerve Ventral root of sixth cervical nerve © 2012 Pearson Education, Inc. Anterior view of spinal cord showing meninges and spinal nerves. For this view, the dura and arachnoid membranes have been cut longitudinally and retracted (pulled aside); notice the blood vessels that run in the subarachnoid space, bound to the outer surface of the delicate pia mater. Sectional Anatomy of the Spinal Cord • Gray matter • Central canal • Consists of somas (cell bodies) surrounding the central canal • White matter • Consists of axons • Nerves are organized into tracts or columns • Located outside the gray matter area © 2012 Pearson Education, Inc. Sectional Anatomy of the Spinal Cord • Organization of Gray Matter • Somas are organized into groups called nuclei • Sensory nuclei • Motor nuclei • Transverse view shows: • • • • Posterior gray horns Lateral gray horns Anterior gray horns Gray commissure © 2012 Pearson Education, Inc. Sectional Anatomy of the Spinal Cord • Organization of gray matter • Posterior gray horns: somatic sensory and visceral nuclei • Lateral gray horns: visceral motor nuclei • Anterior gray horns: somatic motor nuclei • Gray commissure • Consists of axons crossing from one side to the other © 2012 Pearson Education, Inc. Figure 14.4b Sectional Organization of the Spinal Cord Posterior median sulcus From dorsal root Posterior gray horn Posterior gray commissure Somatic Visceral Lateral gray horn Visceral Anterior gray horn Somatic Anterior gray commissure Sensory nuclei Motor nuclei To ventral root Anterior median fissure The left half of this sectional view shows important anatomical landmarks; the right half indicates the functional organization of the gray matter in the anterior, lateral, and posterior gray horns. © 2012 Pearson Education, Inc. Sectional Anatomy of the Spinal Cord • Organization of white matter • Consists of columns of nerves (fascicles) • Columns convey either: • Sensory tracts (ascending tracts) • Motor tracts (descending tracts) © 2012 Pearson Education, Inc. Figure 14.4c Sectional Organization of the Spinal Cord Leg Posterior white column (funiculus) Hip Trunk Arm Lateral white column (funiculus) Flexors Extensors Hand Forearm Arm Shoulder Trunk Anterior white column (funiculus) Anterior white commissure The left half of this sectional view shows the major columns of white matter. The right half indicates the anatomical organization of sensory tracts in the posterior white column for comparison with the organization of motor nuclei in the anterior gray horn. Note that both sensory and motor components of the spinal cord have a definite regional organization. © 2012 Pearson Education, Inc. Spinal Nerves • There are 31 pairs of spinal nerves • • • • • 8 cervical nerves 12 thoracic nerves 5 lumbar nerves 5 sacral nerves 1 coccygeal nerve © 2012 Pearson Education, Inc. Figure 14.1a Gross Anatomy of the Spinal Cord Cervical spinal nerves C1 C2 C3 C4 C5 C6 C7 C8 Cervical enlargement T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 T9 Posterior median sulcus T10 T11 Lumbosacral enlargement T12 L1 Conus medullaris L2 Lumbar spinal nerves L3 L4 Inferior tip of spinal cord Cauda equina L5 Sacral spinal nerves S1 S2 S3 S4 S5 Coccygeal nerve (Co1) Filum terminale (in coccygeal ligament) 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 spinal cord, however, extends from the brain only to the level of vertebrae L1–L2. © 2012 Pearson Education, Inc. Spinal Nerves • Spinal nerves • Each peripheral nerve consists of: • Epineurium: outer layer – becomes continuous with the dura mater • Perineurium: layer surrounding a fascicle – a fascicle is a bundle of axons • Endoneurium: layer surrounding a single axon © 2012 Pearson Education, Inc. Figure 14.5a Anatomy of a Peripheral Nerve Blood vessels Connective Tissue Layers Epineurium covering peripheral nerve Perineurium (around one fascicle) Endoneurium Schwann cell Myelinated axon A typical peripheral nerve and its connective tissue wrappings © 2012 Pearson Education, Inc. Fascicle Spinal Nerves • Peripheral Distribution of Spinal Nerves • Four branches of the spinal nerves: • White ramus • Gray ramus • White and gray ramus are collectively called rami communicantes • Dorsal ramus • Ventral ramus © 2012 Pearson Education, Inc. Spinal Nerves • Branches of the spinal nerves (details) • Rami communicantes (white and gray ramus) • Innervates smooth muscles, glands, and organs • Motor impulses leave the spinal cord through the ventral root to the spinal nerves • Dorsal ramus • Innervates skeletal muscles of the neck and back • Ventral ramus • Innervates skeletal muscles of the limbs © 2012 Pearson Education, Inc. Figure 14.6a Peripheral Distribution of Spinal Nerves Motor Commands Postganglionic fibers to smooth muscles, glands, etc., of back To skeletal muscles of back Dorsal Dorsal root ganglion root Visceral Somatic motor motor Dorsal ramus Ventral ramus To skeletal muscles of body wall, limbs Ventral root Postganglionic fibers to smooth muscles, glands, etc., of body wall, limbs Spinal nerve Sympathetic ganglion Gray ramus (postganglionic) Rami communicantes White ramus (preganglionic) Sympathetic nerve Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity KEY Somatic motor commands Visceral motor commands Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera The distribution of motor neurons in the spinal cord and motor fibers within the spinal nerve and its branches. Although the gray ramus is typically proximal to the white ramus, this simplified diagrammatic view makes it easier to follow the relationships between preganglionic and postganglionic fibers. © 2012 Pearson Education, Inc. Spinal Nerves • Sensory impulses associated with the spinal nerves • Sensory impulses travel in the spinal nerve through the dorsal root to the spinal cord © 2012 Pearson Education, Inc. Figure 14.6b Peripheral Distribution of Spinal Nerves Sensory Information From interoceptors of back From exteroceptors, proprioceptors of back Dorsal root Somatic Visceral sensory sensory Dorsal ramus Ventral ramus From exteroceptors, proprioceptors of body wall, limbs Dorsal root ganglion From interoceptors of body wall, limbs Rami communicantes KEY Ventral root Somatic sensations Visceral sensations From interoceptors of visceral organs A comparable view detailing the distribution of sensory neurons and sensory fibers © 2012 Pearson Education, Inc. Spinal Nerves • Dermatomes • Each pair of spinal nerves monitors specific surface areas • These are clinically important areas regarding surgery © 2012 Pearson Education, Inc. Figure 14.7 Dermatomes C2–C3 NV C2–C3 C2 C3 T2 C6 L1 L2 C8 C7 T1 L3 L4 L5 C3 C4 C5 T1 T2 T3 T4 T5 T6 T7 T8 T9 T 10 T 11 T 12 T2 T3 T4 T5 T6 T7 T8 T9 T 10 T 11 T 12 L1 L2 L4 L3 L5 C4 C5 T2 C6 T1 C7 SS S2 4 3 L1 S5 C8 S1 L5 L2 S2 L3 S1 L4 ANTERIOR © 2012 Pearson Education, Inc. POSTERIOR Nerve Plexuses • There are four nerve plexuses • • • • Cervical plexus Brachial plexus Lumbar plexus Sacral plexus • Sometimes the lumbar and sacral are combined to form the lumbosacral plexus © 2012 Pearson Education, Inc. Figure 14.8 Peripheral Nerves and Nerve Plexuses Cervical plexus Brachial plexus C1 C2 C3 C4 C5 C6 C7 C8 T1 T2 T3 T4 T5 T6 T7 T8 Lesser occipital nerve Great auricular nerve Transverse cervical nerve Supraclavicular nerve Phrenic nerve Axillary nerve Musculocutaneous nerve T9 Thoracic nerves T10 T11 T12 Radial nerve L1 Lumbar plexus L2 Ulnar nerve L3 Sacral plexus L4 L5 S1 S2 S3 S4 S5 Co1 Median nerve Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior Inferior Gluteal nerves Pudendal nerve Sciatic nerve Lateral femoral cutaneous nerve Saphenous nerve Common fibular nerve Tibial nerve Medial sural cutaneous nerve © 2012 Pearson Education, Inc. Nerve Plexuses • The Cervical Plexus (C1–C5) • Consists of cutaneous and muscular branches • Cutaneous branch innervates: • Head • Neck • Chest © 2012 Pearson Education, Inc. Nerve Plexus • The Cervical Plexus • Consists of cutaneous and muscular branches • Muscular branch innervates: • • • • • • • Omohyoid, sternohyoid, geniohyoid, thyrohyoid Sternothyroid Scalenes Sternocleidomastoid Levator scapulae Trapezius Diaphragm (controlled by the phrenic nerve of the cervical plexus) © 2012 Pearson Education, Inc. Figure 14.9 The Cervical Plexus Accessory nerve (N XI) Cranial Hypoglossal nerves nerve (N XII) Great auricular nerve Lesser occipital nerve C1 C2 Nerve roots of cervical plexus C3 C4 C5 Supraclavicular nerves Clavicle Geniohyoid muscle Transverse cervical nerve Thyrohyoid muscle Ansa cervicalis Omohyoid muscle Phrenic nerve Sternohyoid muscle Sternothyroid muscle © 2012 Pearson Education, Inc. Nerve Plexus • The Brachial Plexus (C4–T1) • The immediate nerves emerging from C5 to T1 are the: • Superior trunk • Middle trunk • Inferior trunk • These trunks all merge to form the lateral cord © 2012 Pearson Education, Inc. Figure 14.10b The Brachial Plexus Dorsal scapular nerve Suprascapular nerve C4 C5 C6 C7 C8 Superior trunk Middle trunk BRACHIAL PLEXUS Inferior trunk T1 Musculocutaneous nerve Median nerve Ulnar nerve Radial nerve Lateral antebrachial cutaneous nerve Deep radial nerve Superficial branch of radial nerve Ulnar nerve Median nerve Anterior interosseous nerve Radial nerve Deep branch of ulnar nerve Superficial branch of ulnar nerve Ulnar nerve Palmar digital nerves Median nerve Anterior view of the brachial plexus and upper limb showing the peripheral distribution of major nerves © 2012 Pearson Education, Inc. Anterior Distribution of cutaneous nerves Nerve Plexus • The cords of the brachial plexus • Lateral cord: merging of the trunks • Medial cord: an extension of the inferior trunk • Posterior cord: an extension of the middle trunk © 2012 Pearson Education, Inc. Figure 14.10a The Brachial Plexus KEY Dorsal scapular nerve Roots (ventral rami) Nerve to subclavius muscle Trunks C5 Divisions SUPERIOR TRUNK Cords C6 Peripheral nerves Suprascapular nerve MIDDLE TRUNK Lateral cord C7 BRACHIAL PLEXUS Posterior cord C8 Lateral pectoral nerve Medial pectoral nerve Subscapular nerves T1 Axillary nerve Medial cord First rib Musculocutaneous nerve Medial antebrachial cutaneous nerve Median nerve INFERIOR TRUNK Long thoracic nerve Thoracodorsal nerve Ulnar nerve Posterior brachial cutaneous nerve © 2012 Pearson Education, Inc. Radial nerve The trunks and cords of the brachial plexus Nerve Plexus • The cords of the brachial plexus (details) • Lateral cord: extends to form the musculocutaneous nerve • The lateral cord and medial cord extend to form the median nerve • Medial cord extends to form the ulnar nerve • Posterior cord: branches to form the radial nerve and axillary nerve © 2012 Pearson Education, Inc. Figure 14.10a The Brachial Plexus KEY Dorsal scapular nerve Roots (ventral rami) Nerve to subclavius muscle Trunks C5 Divisions SUPERIOR TRUNK Cords C6 Peripheral nerves Suprascapular nerve MIDDLE TRUNK Lateral cord C7 BRACHIAL PLEXUS Posterior cord C8 Lateral pectoral nerve Medial pectoral nerve Subscapular nerves T1 Axillary nerve Medial cord First rib Musculocutaneous nerve Medial antebrachial cutaneous nerve Median nerve INFERIOR TRUNK Long thoracic nerve Thoracodorsal nerve Ulnar nerve Posterior brachial cutaneous nerve © 2012 Pearson Education, Inc. Radial nerve The trunks and cords of the brachial plexus Figure 14.10b The Brachial Plexus Dorsal scapular nerve Suprascapular nerve C4 C5 C6 C7 C8 Superior trunk Middle trunk BRACHIAL PLEXUS Inferior trunk T1 Musculocutaneous nerve Median nerve Ulnar nerve Radial nerve Lateral antebrachial cutaneous nerve Deep radial nerve Superficial branch of radial nerve Ulnar nerve Median nerve Anterior interosseous nerve Radial nerve Deep branch of ulnar nerve Superficial branch of ulnar nerve Ulnar nerve Palmar digital nerves Median nerve Anterior view of the brachial plexus and upper limb showing the peripheral distribution of major nerves © 2012 Pearson Education, Inc. Anterior Distribution of cutaneous nerves Figure 14.10c The Brachial Plexus Musculocutaneous nerve Axillary nerve Branches of axillary nerve Radial nerve Ulnar nerve Median nerve Posterior antebrachial cutaneous nerve Deep branch of radial nerve Superficial branch of radial nerve Radial nerve Dorsal digital nerves Ulnar nerve Median nerve Posterior Distribution of cutaneous nerves © 2012 Pearson Education, Inc. Posterior view of the brachial plexus and the innervation of the upper limb Figure 14.11 The Cervical and Brachial Plexuses Clavicle, cut and removed Deltoid muscle Musculocutaneous nerve Right axillary artery over axillary nerve Median nerve Radial nerve Biceps brachii, long and short heads Cervical plexus Right common carotid artery Brachial plexus (C5–T1) Sternocleidomastoid muscle, sternal head Sternocleidomastoid muscle, clavicular head Right subclavian artery Ulnar nerve Coracobrachialis muscle Skin Right brachial artery Median nerve © 2012 Pearson Education, Inc. Retractor holding pectoralis major muscle (cut and reflected) Table 14.2 The Brachial Plexus © 2012 Pearson Education, Inc. Nerve Plexus • The Lumbar and Sacral Plexuses (T12–S4) • Also called the lumbosacral plexus • Lumbar plexus nerves • Genitofemoral nerve • Lateral femoral cutaneous nerve • Femoral nerve • Sacral plexus nerves • Sciatic nerve (branches to form the common fibular nerve and the tibial nerve) • Pudendal nerve © 2012 Pearson Education, Inc. Figure 14.12a The Lumbar and Sacral Plexuses, Part I T12 T12 subcostal nerve L1 Iliohypogastric nerve L2 LUMBAR PLEXUS Ilioinguinal nerve L3 Genitofemoral nerve Lateral femoral cutaneous nerve Branches of Femoral branch genitofemoral nerve Genital branch Femoral nerve Obturator nerve L4 L5 Lumbosacral trunk The lumbar plexus, anterior view © 2012 Pearson Education, Inc. Figure 14.12b The Lumbar and Sacral Plexuses, Part I L5 Lumbosacral trunk S1 Superior gluteal nerve Inferior gluteal nerve S2 SACRAL PLEXUS S3 S4 Sciatic nerve Posterior femoral cutaneous nerve Pudendal nerve S5 Co1 The sacral plexus, anterior view © 2012 Pearson Education, Inc. Figure 14.12c The Lumbar and Sacral Plexuses, Part I Subcostal nerve Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve (cut) Obturator nerve Sciatic nerve Saphenous nerve Saphenous nerve Sural nerve Fibular nerve Common fibular nerve Tibial nerve Superficial fibular nerve Deep fibular nerve Sural nerve Saphenous nerve The lumbar and sacral plexuses, anterior view © 2012 Pearson Education, Inc. Tibial nerve Saphenous nerve Sural nerve Fibular nerve Table 14.3 The Lumbar and Sacral Plexuses © 2012 Pearson Education, Inc. Figure 14.12d The Lumbar and Sacral Plexuses, Part I Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Saphenous nerve Sural nerve Fibular nerve Tibial nerve Common fibular nerve Medial sural cutaneous nerve Lateral sural cutaneous nerve Tibial nerve Sural nerve Saphenous nerve Sural nerve Saphenous nerve Tibial nerve © 2012 Pearson Education, Inc. Sural nerve Fibular nerve Medial plantar nerve Lateral plantar nerve The sacral plexus, posterior view Figure 14.13c The Lumbar and Sacral Plexuses, Part II (Part 1 of 2) Gluteus maximus (cut) Inferior gluteal nerve Pudendal nerve Perineal branch Hemorrhoidal branch Gluteus medius (cut) Gluteus minimus Superior gluteal nerve Piriformis Posterior femoral cutaneous nerve Perineal branches Sciatic nerve Descending cutaneous branch Semitendinosus A diagrammatic posterior view of the right hip and lower limb detailing the distribution of peripheral nerves © 2012 Pearson Education, Inc. Biceps femoris (cut) Figure 14.13c The Lumbar and Sacral Plexuses, Part II (Part 2 of 2) Tibial nerve Popliteal artery and vein Medial sural cutaneous nerve Common fibular nerve Lateral sural cutaneous nerve Gastrocnemius Small saphenous vein Sural nerve Calcaneal tendon Tibial nerve (medial calcaneal branch) A diagrammatic posterior view of the right hip and lower limb detailing the distribution of peripheral nerves © 2012 Pearson Education, Inc. Figure 14.13a The Lumbar and Sacral Plexuses, Part II Gluteus maximus Superior gluteal nerve Inferior gluteal nerve Gluteus medius Gluteus minimus Internal pudendal artery Pudendal nerve Nerve to gemellus and obturator internus A dissection of the right gluteal region © 2012 Pearson Education, Inc. Tibial branch Common fibular Components of sciatic nerve branch Greater trochanter of femur Posterior femoral cutaneous nerve Gluteus maximus Figure 14.13b The Lumbar and Sacral Plexuses, Part II Biceps femoris Sartorius Gracilis Semimembranosus Popliteal artery Semitendinosus Nerve to medial head of gastrocnemius Gastrocnemius, medial head Medial sural cutaneous nerve A dissection of the popliteal fossa © 2012 Pearson Education, Inc. Tibial nerve Lateral sural cutaneous nerve Common fibular nerve Plantaris Nerve to lateral head of gastrocnemius Gastrocnemius, lateral head Nerve Plexus • Summary of the spinal nerves • • • • • Cervical spinal nerves emerge from C1–C8 Thoracic spinal nerves emerge from T1–T12 Lumbar spinal nerves emerge from L1–L5 Sacral spinal nerves emerge from S1–S5 Coccygeal spinal nerves emerge from Co1 © 2012 Pearson Education, Inc. Nerve Plexus • Summary of the nerve plexuses • • • • • • Cervical plexus nerves emerge from C1–C5 Brachial plexus nerves emerge from C5–T1 There is not a thoracic plexus Lumbar plexus nerves emerge from T12–L4 Sacral plexus nerves emerge from L4–S4 There is not a coccygeal plexus © 2012 Pearson Education, Inc. Figure 14.3 Posterior View of Vertebral Column and Spinal Nerves Occipital bone Spinal cord emerging from foramen magnum Cervical plexus (C1–C5) Cervical spinal nerves (C1–C8 ) Brachial plexus (C5–T1) Thoracic spinal nerves (T1–T12) Lumbar spinal nerves (L1–L5) Sacral plexus (L4–S4) Coccygeal nerves (Co1) © 2012 Pearson Education, Inc. Lumbar plexus (T12–L4) Sciatic nerve Sacral spinal nerves (S1–S5) emerging from sacral foramina Reflexes • Reflex • An immediate involuntary response • Reflex arc • The neural “wiring” of a single reflex • Begins at a sensory receptor and ends at a peripheral receptor © 2012 Pearson Education, Inc. Reflexes • Reflexes are classified according to: • Their development • Innate or acquired • The site where information is processed • Spinal or cranial (cerebral) • The nature of the resulting motor response • Somatic, visceral, or autonomic • The complexity of the neural circuit • Monosynaptic or polysynaptic © 2012 Pearson Education, Inc. Reflexes • Pathway of a reflex arc • • • • • 1. Activation of a sensory receptor 2. Relay of information to the CNS 3. Information processing 4. Activation of a motor neuron 5. Response by the effector ANIMATION Components of a Reflex Arc © 2012 Pearson Education, Inc. Figure 14.14 A Reflex Arc Dorsal root Activation of a sensory neuron Arrival of stimulus and activation of receptor Sensation relayed to the brain by collateral REFLEX ARC Receptor Stimulus Effector Response by effector Ventral root Activation of a motor neuron Information processing in CNS KEY Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) © 2012 Pearson Education, Inc. Figure 14.15 The Classification of Reflexes 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 • Include superficial and stretch reflexes • One synapse • Processing in the spinal cord Acquired Reflexes Visceral (Autonomic) Reflexes Polysynaptic Cranial Reflexes • Learned • Control actions of smooth and cardiac muscles, glands • Multiple synapses (two to several hundred) • Processing in the brain © 2012 Pearson Education, Inc. Reflexes • Spinal reflexes can be: • Monosynaptic • Involves a single segment of the spinal cord • Polysynaptic • Integrates motor output from several spinal segments © 2012 Pearson Education, Inc. Figure 14.16 Neural Organization and Simple Reflexes Sensory receptor Ganglion CENTRAL NERVOUS SYSTEM Sensory neuron CENTRAL NERVOUS SYSTEM Ganglion Sensory neuron Interneurons Circuit 2 Motor neuron Motor neurons Circuit 1 Sensory receptor (muscle spindle) Skeletal muscle 1 Skeletal muscle A monosynaptic reflex circuit involves a peripheral sensory neuron and a central motor neuron. In this example, stimulation of the receptor will lead to a reflexive contraction in a skeletal muscle. © 2012 Pearson Education, Inc. Skeletal muscle 2 A polysynaptic reflex circuit involves a sensory neuron, interneurons, and motor neurons. In this example, the stimulation of the receptor leads to the coordinated contractions of two different skeletal muscles. Reflexes • Stretch reflex • • • • • 1. Stimulus stretches a muscle 2. Activates a sensory neuron 3. Information is processed in the spinal cord 4. Motor neurons are activated 5. Muscle (effector) contracts © 2012 Pearson Education, Inc. Figure 14.17a Stretch Reflexes Activation of a sensory neuron Stimulus. Stretching of muscle stimulates muscle spindles Information processing at motor neuron Response. Contraction of muscle Activation of motor neuron Steps 1–5 are common to all stretch reflexes. © 2012 Pearson Education, Inc. KEY Sensory neuron (stimulated) Motor neuron (stimulated) Figure 14.17b Stretch Reflexes Receptor (muscle spindle) Spinal cord Stretch REFLEX ARC Stimulus Effector KEY Contraction Sensory neuron (stimulated) Motor neuron (stimulated) Response © 2012 Pearson Education, Inc. The patellar reflex is controlled by muscle spindles in the quadriceps group. The stimulus is a reflex hammer striking the muscle tendon, stretching the spindle fibers. This results in a sudden increase in the activity of the sensory neurons, which synapse on spinal motor neurons. The response occurs upon the activation of motor units in the quadriceps group, which produces an immediate increase in muscle tone and a reflexive kick.
