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12 The Spinal Cord, Spinal Nerves, and Spinal Reflexes PowerPoint® Lecture Presentations prepared by Alexander G. Cheroske Mesa Community College at Red Mountain © 2011 Pearson Education, Inc. Section 1: Functional Organization of the Spinal Cord • Learning Outcomes • 12.1 Discuss the anatomical features of the spinal cord. • 12.2 Describe the three meningeal layers that surround the spinal cord. • 12.3 Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands. • 12.4 Describe the major components of a spinal nerve. © 2011 Pearson Education, Inc. Section 1: Functional Organization of the Spinal Cord • Learning Outcomes • 12.5 Describe the rami associated with spinal nerves. • 12.6 Relate the distribution pattern of spinal nerves to the region they innervate and describe the cervical plexus. • 12.7 Relate the distribution pattern of the brachial plexus with its function. • 12.8 Relate the distribution patterns of the lumbar plexus and sacral plexus with their functions. © 2011 Pearson Education, Inc. Section 1: Functional Organization of the Spinal Cord • The nervous system is highly organized allowing efficient function • Input pathways routing sensations • Most often outside our awareness • Only a fraction reaches our conscious awareness • Processing centers prioritizing and distributing information • Motor centers directing responses to stimuli © 2011 Pearson Education, Inc. Section 1: Functional Organization of the Spinal Cord • Central Nervous System organization • Brain, cranial nerves, cranial reflexes (Chapter 13) • More complex • Spinal cord, spinal nerves, spinal reflexes (this chapter) • Simpler © 2011 Pearson Education, Inc. A diagram of a functional perspective for studying the CNS The Brain Sensory receptors Sensory input over cranial nerves Reflex centers in brain Motor output over cranial nerves Effectors Muscles The Spinal Cord Glands Sensory receptors Sensory input over spiral nerves Reflex centers in spinal cord Motor output over spinal nerves Adipose tissue Figure 12 Section 1 © 2011 Pearson Education, Inc. Module 12.1: Spinal cord functional anatomy • Adult spinal cord dimensions • Length: ~45 cm (18 in.) • Width: ~14 mm (0.55 in.) maximum • Superficial anatomy • Cervical enlargement • Supplies nerves to shoulder and upper limbs • Lumbar enlargement • Supplies nerves to pelvis and lower limbs • Conus medullaris • Tapered terminal end inferior to lumbar enlargement © 2011 Pearson Education, Inc. Module 12.1: Spinal cord functional anatomy • Superficial anatomy (continued) • Cauda equina (cauda, tail + equus, horse) • Long, inferiorly extending dorsal and ventral roots + filum terminale • Resembles horse’s tail • Filum terminale • Slender thread of connective tissue attaching conus medullaris to 2nd sacral vertebra • Provides longitudinal support to spinal cord © 2011 Pearson Education, Inc. Module 12.1: Spinal cord functional anatomy • Superficial anatomy (continued) • 31 pairs of spinal nerves • Arise from 31 segments of spinal cord • Identified by adjacent vertebrae • Cervical nerves • From vertebrae immediately inferior • Last vertebrae with this number system is C8 • Thoracic nerves • From vertebrae immediately superior © 2011 Pearson Education, Inc. The 31 pairs of spinal nerves Cervical spinal nerves Cervical enlargement Posterior median sulcus Thoracic spinal nerves Lumbar enlargement Conus medullaris Lumbar spinal nerves Interior tip of spinal cord Cauda equina Sacral spinal nerves Filum terminale Coccygeal nerve (Co1) Figure 12.1 © 2011 Pearson Education, Inc. 1 Module 12.1: Spinal cord functional anatomy • Spinal cord anatomy in cross section • Posterior median sulcus • Shallow longitudinal groove on posterior surface • Dorsal root • Contains axons of neurons whose cell bodies are in dorsal root ganglion • Dorsal root ganglion • Contains sensory neuron cell bodies • Each spinal segment contains one on each side © 2011 Pearson Education, Inc. Module 12.1: Spinal cord functional anatomy • Spinal cord anatomy in cross section (continued) • Spinal nerve • Contains axons of both sensory and motor neurons • Sensory enter CNS through dorsal root • Motor exit CNS through ventral root • Ventral root • Contains axons of motor neurons extending into periphery to control somatic and visceral effectors • Anterior median fissure • Deep groove along anterior (ventral) surface © 2011 Pearson Education, Inc. Module 12.1: Spinal cord functional anatomy • Spinal cord anatomy in cross section (continued) • White matter • Superficial • Contains large numbers of myelinated and unmyelinated axons • Gray matter • Surrounds central canal • Forms butterfly or H shape • Dominated by cell bodies of neurons, neuroglia, and unmyelinated axons • Greater amount in spinal cord segments serving limbs © 2011 Pearson Education, Inc. Cross sections of three of the spinal cord’s 31 segments Posterior median sulcus Dorsal root Dorsal root ganglion White matter Spinal nerve Gray matter Segment C3 Ventral root Anterior median fissure White matter Gray matter Segment T3 Central canal Segment L1 Segment S2 © 2011 Pearson Education, Inc. Figure 12.1 2 Module 12.1 Review a. A typical spinal cord has how many pairs of spinal nerves, and where does the spinal cord end? b. Describe the composition of the gray matter of the spinal cord. c. Describe the gross anatomical features of a cross section of spinal cord. © 2011 Pearson Education, Inc. Module 12.2: Spinal meninges • Spinal meninges • Series of specialized membranes that provide physical stability and shock absorption for the spinal cord • Blood vessels branching within deliver oxygen and nutrients to spinal cord • Are continuous with cranial meninges and connective tissues surrounding spinal nerves • Three layers 1.Dura mater 2.Arachnoid mater 3.Pia mater © 2011 Pearson Education, Inc. Module 12.2: Spinal meninges • Three layers 1. Pia mater (pia, delicate + mater, mother) • Innermost meningeal layer • Meshwork of elastic and collagen fibers • Bound to underlying nervous tissue © 2011 Pearson Education, Inc. Module 12.2: Spinal meninges • Three layers (continued) 2. Arachnoid mater (arachine, spider) • Middle meningeal layer • Includes: • Simple squamous epithelium (arachnoid membrane) • Network of collagen and elastin fibers connecting layer to pia mater (arachnoid trabeculae) • Subarachnoid space • Contains cerebrospinal fluid (CSF) that acts as shock absorber and diffusion medium • Lumbar puncture (spinal tap) withdraws CSF © 2011 Pearson Education, Inc. A section demonstrating the procedure called a lumbar puncture or spinal tap Dura mater Epidural space Body of third lumbar vertebra Interspinous ligament Lumbar puncture needle with tip in subarachnoid space Cauda equina in subarachnoid space Figure 12.2 © 2011 Pearson Education, Inc. 5 Module 12.2: Spinal meninges • Three layers (continued) 3. Dura mater (dura, hard) • Outermost covering • Contains dense collagen fibers oriented longitudinally • Has narrow subdural space separating from arachnoid mater • Epidural space • Between dura mater and vertebral canal • Contains areolar connective tissue, blood vessels, and protective adipose tissue © 2011 Pearson Education, Inc. A posterior view of the dissected spinal cord showing the basic relationships among the spinal meninges Gray matter Pia mater White matter Ventral root Spinal nerve Dorsal root Arachnoid mater Dura mater Figure 12.2 © 2011 Pearson Education, Inc. 1 A cross-sectional view showing the structures surrounding the spinal cord and the spaces between the meningeal layers Cerebrospinal fluid (CSF) Anterior Ventral root Spinal meninges Vertebral body Spinal cord Epidural space Dorsal root ganglion Pia mater Arachnoid mater Dura mater Dorsal root Figure 12.2 © 2011 Pearson Education, Inc. 2 Module 12.2: Spinal meninges • Supporting ligaments • Prevent lateral movement of spinal cord • Denticulate ligaments • • Extend from pia mater through arachnoid to dura mater Prevent superior–inferior movement of spinal cord • Dural connections at foramen magnum • Coccygeal ligament © 2011 Pearson Education, Inc. An anterior view of the cervical spinal cord showing the meninges, supporting ligaments, and the roots of the spinal nerves Spinal cord Anterior median fissure Pia mater Denticulate ligaments Dorsal root Blood vessels within the subarachnoid space Ventral root, formed by several “rootlets” from one cervical segment Arachnoid mater (reflected) Dura mater (reflected) Figure 12.2 © 2011 Pearson Education, Inc. 3 Module 12.2 Review a. Identify and describe the three spinal meninges. b. Where is the cerebrospinal fluid that surrounds the spinal cord located? c. Name the structures and spinal coverings that would be penetrated during a lumbar puncture procedure. © 2011 Pearson Education, Inc. Module 12.3: Gray and white matter • Gray matter • Structural organization • Horns (projections toward outer surface of spinal cord) • Posterior gray horn • • • Lateral gray horn • Only in thoracic and lumbar segments • Contains visceral motor nuclei Anterior gray horn • • Contains somatic & visceral sensory nuclei Contains somatic motor nuclei Gray commissures (contain axons that laterally cross spinal cord) © 2011 Pearson Education, Inc. A cross section showing most of the anatomical landmarks of the spinal cord Posterior median sulcus Posterior gray commissure Structural Organization of Gray Matter The projections of gray matter toward the outer surface of the spinal cord are called horns. Anterior view of spinal cord Posterior gray horn Central canal Dura mater Lateral gray horn Arachnoid mater (broken) Anterior gray horn Pia mater Dorsal root ganglion Anterior median fissure Anterior gray commissure Ventral root Figure 12.3 © 2011 Pearson Education, Inc. 1 Module 12.3: Gray and white matter • Gray matter (continued) • Functional organization • Nuclei (groups of cell bodies) • Sensory nuclei • • Receive and relay sensory information from peripheral receptors Motor nuclei • © 2011 Pearson Education, Inc. Issue motor commands to peripheral effectors A diagrammatic view of the organization of the gray matter of the spinal cord Site of a frontal section that separates the sensory (posterior, or dorsal) nuclei from the motor (anterior, or ventral) nuclei Posterior gray horn Dorsal root ganglion Functional Organization of Gray Matter Gray commissures Lateral gray horn Anterior gray horn Somatic The cell bodies of neurons in the gray matter of the spinal cord are organized into functional groups called nuclei. Visceral Sensory nuclei Visceral Motor nuclei Somatic Ventral root Figure 12.3 © 2011 Pearson Education, Inc. 2 Module 12.3: Gray and white matter • White matter • Columns (areas of white matter on spinal cord sides) • Posterior white column • • Lateral white column • • Either side between anterior and posterior columns Anterior white column • • Between posterior gray horns and posterior median sulcus Between anterior gray horns and median fissure Anterior white commissure • Interconnects anterior white columns © 2011 Pearson Education, Inc. Module 12.3: Gray and white matter • White matter (continued) • Tracts (bundle of axons relatively uniform in diameter, myelination, conduction speed, and functional type) • Ascending tracts • • Carry sensory information toward brain Descending tracts • Convey motor commands to spinal cord © 2011 Pearson Education, Inc. Organization of Tracts in the Posterior White Column The organization of the white matter into columns containing tracts The posterior white column contains ascending tracts providing sensations from the trunk and limbs. Leg Hip Trunk Arm Structural and Functional Organization of White Matter Posterior white column Lateral white column Anterior white column Flexors/Extensors Anterior white commissure Trunk Shoulder Arm Forearm Hand In the cervical enlargement, which contains neurons involved with sensations and motor control of the upper limbs, the motor nuclei of the anterior gray horn are grouped by region, with motor neurons controlling flexor muscles medial to those controlling extensor muscles. Figure 12.3 © 2011 Pearson Education, Inc. 3 Module 12.3 Review a. Differentiate between sensory nuclei and motor nuclei. b. A person with polio has lost the use of his leg muscles. In which area of his spinal cord would you expect the virus-infected motor neurons to be? c. A disease that damages myelin sheaths would affect which portion of the spinal cord? © 2011 Pearson Education, Inc. Module 12.4: Spinal nerve structure and distribution • Connective tissue layers of a spinal nerve 1. Epineurium • Outermost covering of nerve • Dense network of collagen fibers 2. Perineurium • Middle layer • Fibers extend inward from epineurium • • Divide nerve into compartments that contain bundles of axons (fascicles) Branching blood vessels from epineurium continue on to form capillaries in endoneurium © 2011 Pearson Education, Inc. Module 12.4: Spinal nerve structure and distribution • Connective tissue layers of a spinal nerve (continued) 3. Endoneurium • Innermost layer • Delicate connective tissues surrounding individual axons • Capillaries here supply axons, Schwann cells, and fibroblasts © 2011 Pearson Education, Inc. A sectional view of a spinal nerve showing its connective tissue layers Connective Tissue Layers of a Spinal Nerve Epineurium Perineurium Endoneurium Artery and vein within the perineurium Fascicle Schwann cell Myelinated axon Figure 12.4 © 2011 Pearson Education, Inc. 1 Module 12.4: Spinal nerve structure and distribution • Spinal nerve branches • Called rami (singular ramus, a branch) • Some carry visceral motor fibers of autonomic nervous system (ANS) • In thoracic and upper lumbar segments, sympathetic division (“fight or flight”) motor fibers © 2011 Pearson Education, Inc. Module 12.4: Spinal nerve structure and distribution • Specific rami • Dorsal ramus • • Innervates muscles, joints, and skin of back Ventral ramus • • Innervates structures of lateral & anterior trunk and limbs Communicating rami • Present in thoracic and superior lumbar segments • Contain axons on sympathetic neurons Animation: Peripheral Distal Spinal Nerves © 2011 Pearson Education, Inc. The branching of a spinal nerve to form rami Dorsal root Dorsal root ganglion Dorsal ramus Ventral ramus Ventral root Communicating rami Autonomic nerve Sympathetic ganglion Figure 12.4 © 2011 Pearson Education, Inc. 2 Module 12.4: Spinal nerve structure and distribution • Dermatome • Specific bilateral region of skin surface monitored by single pair of spinal nerves • C1 usually lacks sensory branch to skin • • When present, helps monitor scalp with C2 and C3 Face is monitored by pair of cranial nerves • Boundaries between dermatomes overlap • Clinically important to determine damage or infection of spinal nerve or dorsal root ganglion • Loss of sensation or signs on skin in dermatome © 2011 Pearson Education, Inc. Dermatomes, the specific bilateral regions of the skin surface monitored by a single pair of spinal nerves Anterior Posterior Figure 12.4 © 2011 Pearson Education, Inc. 3 Module 12.4: Spinal nerve structure and distribution • Shingles • Viral infection of dorsal root ganglia • Caused by varicella-zoster virus • Same herpes virus as chickenpox • Produces painful rash and blisters on dermatome served by infected nerves • Those who have had chickenpox are more at risk • Virus can remain dormant within anterior gray horns • Unknown trigger for reactivation © 2011 Pearson Education, Inc. Figure 12.4 © 2011 Pearson Education, Inc. 4 Module 12.4 Review a. Identify the three layers of connective tissue of a spinal nerve and identify the major peripheral branches of a spinal nerve. b. Describe a dermatome. c. Explain the etiology (cause) of shingles. © 2011 Pearson Education, Inc. Module 12.5: Motor and sensory information in spinal rami • Motor commands • Ventral root (axons of somatic & visceral motor neurons) • Dorsal ramus (somatic & visceral motor fibers to skin and skeletal muscles of back) • Ventral ramus (somatic & visceral motor fibers to ventrolateral body surface, structures of body wall, limbs) © 2011 Pearson Education, Inc. Module 12.5: Motor and sensory information in spinal rami • Motor commands (continued) Rami communicantes (“communicating branches”) • • • • White ramus • Preganglionic fibers that are myelinated • Visceral motor fibers to sympathetic ganglion Gray ramus • Postganglionic fibers that are unmyelinated • Innervate glands and smooth muscles of body wall or limbs Sympathetic nerve • Preganglionic and postganglionic fibers to structures of thoracic cavity © 2011 Pearson Education, Inc. Module 12.5: Motor and sensory information in spinal rami • Sensory information • Sympathetic nerve (sensory information from visceral organs) • Ventral ramus (sensory information from ventrolateral body surface, body wall structures, and limbs) • Dorsal ramus (sensory information from skin and skeletal muscles of back) • Dorsal root (sensory information to spinal cord) © 2011 Pearson Education, Inc. Module 12.5 Review a. Define gray ramus and white ramus. b. Indicate whether the following fibers make up the white rami or gray rami: 1) preganglionic fibers connecting a spinal nerve with a sympathetic ganglion in the thoracic and lumbar region of the spinal cord. 2) postganglionic fibers connecting a sympathetic ganglion in the thoracic or lumbar region with the spinal nerve. c. Which ramus innervates the skin and muscles of the back? © 2011 Pearson Education, Inc. Module 12.6: Spinal nerve plexuses introduction and the cervical plexus • Nerve plexus (plexus, braid) • Complex interwoven network of nerves • Originally occurred during development as different ventral rami innervating small skeletal muscles • Later, muscles fuse to form larger muscles with compound origins • Ventral rami remain and blend fibers to form plexuses © 2011 Pearson Education, Inc. Module 12.6: Spinal nerve plexuses introduction and the cervical plexus • Ventral rami plexuses 1. Cervical plexus 2. Brachial plexus 3. Lumbar plexus 4. Sacral plexus Animation: Peripheral Nerves: Nerve Plexus © 2011 Pearson Education, Inc. The cervical, brachial, lumbar, and sacral plexuses (at left), and the major peripheral nerves of each (at right) Cervical plexus Brachial plexus Lesser occipital nerve Great auricular nerve Transverse cervical nerve Supraclavicular nerve Phrenic nerve Axillary nerve Musculocutaneous nerve Thoracic nerves Radial nerve Lumbar plexus Ulnar nerve Median nerve Iliohypogastric nerve Sacral plexus Ilioinguinal nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Saphenous nerve Sciatic nerve Figure 12.6 © 2011 Pearson Education, Inc. 1 Module 12.6: Spinal nerve plexuses introduction and the cervical plexus • Cervical plexus • Ventral rami of spinal nerves C1–C5 • Branches innervate • Muscles of neck and to control • Diaphragmatic muscles (phrenic nerve) • Extends into thoracic cavity • Skin of neck • Skin of superior part of chest © 2011 Pearson Education, Inc. The cervical plexus, which consists of the ventral rami of spinal nerves C1–C5, and some of the muscles its branches innervate Cranial Nerves Accessory nerve (XI) Hypoglossal nerve (XII) Lesser occipital nerve Great auricular nerve Nerve Roots of Cervical Plexus C1 C2 C3 C4 C5 Geniohyoid muscle Transverse cervical nerve Thyrohyoid muscle Ansa cervicalis Omohyoid muscle Supraclavicular nerves Clavicle Phrenic nerve Sternohyoid muscle Sternothyroid muscle Figure 12.6 © 2011 Pearson Education, Inc. 2 ¯ Figure 12.6 © 2011 Pearson Education, Inc. 2 Module 12.6 Review a. Define nerve plexus, and list the major nerve plexuses. b. An anesthetic blocks the function of the dorsal rami of the cervical spinal nerves. Which areas of the body will be affected? c. Injury to which of the nerve plexuses would interfere with the ability to breathe? © 2011 Pearson Education, Inc. Module 12.7: Brachial plexus • Brachial plexus • Innervates pectoral girdle and upper limb • Contributions from ventral rami of nerves C4–T1 • Nerves originate from trunks and cords • Trunks (large bundles of axons from several spinal nerves) • Cords (smaller branches that originate in trunks) • Clinical importance of cutaneous nerve • Damage or injury can be precisely localized by testing sensory function in hand Animation: Brachial Plexus © 2011 Pearson Education, Inc. The brachial plexus, which innervates the pectoral girdle and upper limbs with contributions from the ventral rami of spinal nerves C4–T1 Trunks of Brachial Plexus Dorsal scapular nerve Suprascapular nerve Spinal Nerves Forming Brachial Plexus C4 nerve C5 nerve C6 nerve C7 nerve C8 nerve T1 nerve Superior Middle Inferior Musculocutaneous nerve Median nerve Ulnar nerve Radial nerve Lateral antebrachial cutaneous nerve Superficial branch of radial nerve Deep radial nerve Ulnar nerve Median nerve Palmar digital nerves Figure 12.7 © 2011 Pearson Education, Inc. 1 The trunks and cords from which the nerves that form the brachial plexus originate Dorsal scapular nerve Roots (ventral rami) Trunks Divisions C5 SUPERIOR TRUNK C6 Suprascapular nerve Cords Peripheral nerves MIDDLE TRUNK Lateral cord C7 Posterior cord C8 Lateral pectoral nerve Medial pectoral nerve Subscapular nerves T1 Axillary nerve INFERIOR TRUNK Medial cord Musculocutaneous nerve First rib Medial antebrachial cutaneous nerve Median nerve Posterior brachial cutaneous nerve © 2011 Pearson Education, Inc. Long thoracic nerve Ulnar nerve Radial nerve Figure 12.7 2 The distribution of the cutaneous nerves of the wrist and hand Posterior Anterior Radial nerve Ulnar nerve Median nerve Figure 12.7 © 2011 Pearson Education, Inc. 3 Figure 12.7 © 2011 Pearson Education, Inc. 3 Module 12.7 Review a. Define a nerve plexus trunk and cord. b. Describe the brachial plexus. c. Name the major nerves associated with the brachial plexus. © 2011 Pearson Education, Inc. Module 12.8: Lumbar and sacral plexuses • Lumbar and sacral plexuses • Arise from lumbar and sacral segments of spinal cord • Innervate pelvic girdle and lower limbs • Lumbar plexus • Innervates mostly anterior and side surfaces • Sacral plexus • Innervates mostly posterior surfaces • Contains sciatic nerve (longest & largest nerve in body) Animation: Lumbar Sacral Plexus © 2011 Pearson Education, Inc. The origins of the spinal nerves of the sacral plexus Spinal Nerves Forming the Sacral Plexus Lumbosacral trunk L4 nerve L5 nerve Nerves of the Sacral Plexus S1 nerve Superior gluteal S2 nerve Inferior gluteal S3 nerve Sciatic Posterior femoral cutaneous Pudendal S4 nerve Co1 Sacral plexus, anterior view Figure 12.8 © 2011 Pearson Education, Inc. 1 A posterior view of the lower limb showing the distribution of the nerves of the sacral plexus Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Tibial nerve Common fibular nerve Sural nerve Figure 12.8 © 2011 Pearson Education, Inc. 2 The dermatomes of the sensory nerves innervating the ankle and foot Saphenous nerve Sural nerve Sural nerve Saphenous nerve Fibular nerve Tibial nerve Fibular nerve Figure 12.8 © 2011 Pearson Education, Inc. 3 An anterior view of the lower trunk and lower limb showing the distribution of the nerves of both 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 Figure 12.8 © 2011 Pearson Education, Inc. 4 Module 12.8 Review a. Describe the lumbar plexus and sacral plexus. b. List the major nerves of the sacral plexus. c. Compression of which nerve produces the sensation that your lower limb has “fallen asleep”? © 2011 Pearson Education, Inc. Section 2: Reflexes and Neural Circuits • Learning Outcomes • 12.9 Describe the steps in a neural reflex. • 12.10 Describe the steps in the stretch reflex. • 12.11 Explain withdrawal reflexes and crossed extensor reflexes and the responses produced by each. • 12.12 CLINICAL MODULE Explain the value of reflex testing and how higher centers control and modify reflex responses. © 2011 Pearson Education, Inc. Section 2: Reflexes and Neural Circuits • Neuronal pools • Functional groups of interconnected neurons • Most cases are interneurons in CNS • May involve several regions of brain • May involve neurons in one specific location in brain or spinal cord • Estimated number of pools ~100s to 1000s • Patterns of neuronal interactions suggest functional classifications • • Neural circuit (“wiring diagram”) Simple circuits in PNS and spinal cord control reflexes • Preprogrammed responses to specific stimuli © 2011 Pearson Education, Inc. Section 2: Reflexes and Neural Circuits • Common neural circuit patterns • Divergence • Spread of information from one neuron to many • Permits broad distribution of a specific input • Example: sensory information coming to CNS • Parallel processing • Several neurons or neural pools process same information simultaneously • Many responses can occur simultaneously © 2011 Pearson Education, Inc. The common types of neural circuits Divergence Divergence is the spread of information from one neuron to several neurons, or from one pool to multiple pools. Divergence permits the broad distribution of a specific input. Considerable divergence occurs when sensory neurons bring information into the CNS: The information is distributed to neuronal pools throughout the spinal cord and brain. Figure 12 Section 2 © 2011 Pearson Education, Inc. The common types of neural circuits Parallel Processing Parallel processing occurs when several neurons or neuronal pools process the same information simultaneously. Many responses can occur simultaneously. Figure 12 Section 2 © 2011 Pearson Education, Inc. Section 2: Reflexes and Neural Circuits • Common neural circuit patterns (continued) • Serial processing • Information relayed from one neuron or neuronal pool to another (stepwise fashion) • Example: sensory relay from one brain part to another • Convergence • Several neurons synapse on single postsynaptic neuron • Several patterns of activity by presynaptic neurons can have same postsynaptic effect • Example: breathing movements of diaphragm and ribs can be controlled subconsciously or consciously © 2011 Pearson Education, Inc. The common types of neural circuits Serial Processing In serial processing, information is relayed in a stepwise fashion, from one neuron to another or from one neuronal pool to the next. This pattern occurs as sensory information is relayed from one part of the brain to another. Figure 12 Section 2 © 2011 Pearson Education, Inc. The common types of neural circuits Convergence In convergence, several neurons synapse on a single postsynaptic neuron. Several patterns of activity in the presynaptic neurons can therefore have the same effect on the postsynaptic neuron. Through convergence, the same motor neurons can be subject to both conscious and subconscious control. For example, the movements of your diaphragm and ribs are now being controlled by your brain at the subconscious level. But the same motor neurons can also be controlled consciously, as when you take a deep breath and hold it. Figure 12 Section 2 © 2011 Pearson Education, Inc. Section 2: Reflexes and Neural Circuits • Reverberation • Collateral axonal branches extend back toward source of impulse to further stimulate presynaptic neurons • • Like positive feedback involving neurons Once activated, reverberating circuits will continue to function until something breaks cycle • Synaptic fatigue • Inhibitory stimuli © 2011 Pearson Education, Inc. The common types of neural circuits Reverberation In reverberation, collateral branches of axons somewhere along the circuit extend back toward the source of an impulse and further stimulate the presynaptic neurons. Reverberation is like a positive feedback loop involving neurons: Once a reverberating circuit has been activated, it will continue to function until the synaptic fatigue or inhibitory stimuli break the cycle. Figure 12 Section 2 © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Reflexes • Are rapid, automatic responses to specific stimuli • Show little variability • Preserve homeostasis by making rapid adjustments in functions of organs or organ systems • In neural reflexes: • Sensory fibers carry information from peripheral receptors to integration center • Motor fibers carry motor commands to peripheral effectors • Reflex arc • “Wiring” of a single reflex from receptor to effector Animation: Components of a Reflexive Arc © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Example: Simple withdrawal reflex 1. Arrival of stimulus and activation of receptor • Receptor is specialized cell or dendrites of sensory neuron • • Sensitive to: • Physical or chemical changes in body • Or changes in external environment Example: pain receptor in hand 2. Activation of sensory neuron • Stimulation of dendrites produces graded polarization leading to action potential • Action potential travels through dorsal root to spinal cord © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Example: Simple withdrawal reflex (continued) 3. Information processing • Sensory neuron releases excitatory neurotransmitters at postsynaptic membrane of interneuron • Neurotransmitter produces EPSP which is integrated with other simultaneous stimuli 4. Activation of motor neuron • Activation of interneuron leads to stimulation of motor neuron to carry action potential to periphery • Axonal collaterals may relay sensation to other centers in brain and spinal cord © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Example: Simple withdrawal reflex (continued) 5. Response of peripheral effector • Release of neurotransmitters by synaptic knobs leads to response by peripheral effector • Generally removes or opposes original stimulus • • An example of negative feedback Example: skeletal muscle contraction moving hand away from painful sensation © 2011 Pearson Education, Inc. STEP 2 The Activation of a Sensory Neuron The steps in a reflex arc: a simple withdrawal reflex STEP 1 The Arrival of a Stimulus and Activation of a Receptor STEP 3 Information Processing Dorsal root ganglion To higher centers REFLEX ARC Receptor Stimulus Effector STEP 5 The Response of a Peripheral Effector STEP 4 The Activation of a Motor Neuron Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) Figure 12.9 © 2011 Pearson Education, Inc. 1 Module 12.9: Reflexes • Reflex classification based on: 1. Their development 2. Nature of resulting motor response 3. Complexity of neural circuit involved 4. Site of information processing • Categories are not mutually exclusive © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Reflex categories 1. Development • Innate reflexes • Connections formed between neurons genetically or developmentally programmed • Generally appear in a predictable sequence • • Example: simplest (withdrawal) to complex (suckling) Acquired reflexes • Learned rather than preestablished • Enhanced by repetition © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Reflex categories (continued) 2. Nature of response • Somatic reflexes • Involuntary control of skeletal muscles • • • Example: withdrawal reflex Rapid response that can later be supplemented voluntarily Visceral reflexes (autonomic reflexes) • Control or adjust activities of smooth & cardiac muscle, glands, and adipose tissues © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Reflex categories (continued) 3. Complexity of circuit • • Polysynaptic reflexes • Involve at least one interneuron, one sensory neuron, and one motor neuron • Longer delay between stimulus and response due to increased number of synapses • Produce more complex reflexes Monosynaptic reflexes • Simplest reflex arc involving one sensory and one motor neuron • Faster response time due to only one synapse © 2011 Pearson Education, Inc. Module 12.9: Reflexes • Reflex categories (continued) 4. Processing site • Spinal reflexes • Occur in nuclei of spinal cord • Two types 1. Single segmental (within one spinal segment) 2. Intersegmental (multiple segments) • Cranial reflexes • Occur in nuclei of brain © 2011 Pearson Education, Inc. Module 12.9 Review a. Define reflex and list the components of a reflex arc. b. What are common characteristics of reflexes? c. Describe the various classifications of reflexes. © 2011 Pearson Education, Inc. Module 12.10: Monosynaptic reflex • Stretch reflex • Best-known monosynaptic reflex • Provides automatic regulation of skeletal muscle length • Example: patellar reflex © 2011 Pearson Education, Inc. Module 12.10: Monosynaptic reflex • Steps of the patellar reflex 1. Arrival of stimulus and activation of receptor • Patellar tendon tapped by physician • Quadriceps tendon receptors stretch 2. Activation of sensory neuron • Receptors stimulate sensory neuron that extends into spinal cord • Sensory neuron synapses with motor neuron 3. Information processing in CNS • Information processing at motor neuron cell body © 2011 Pearson Education, Inc. Module 12.10: Monosynaptic reflex • Steps of the patellar reflex (continued) 4. Activation of motor neuron • Motor neuron activated and action potential is generated and propagated 5. Response of peripheral receptor • Stimulation of skeletal muscle fibers leads to contraction of knee extensors © 2011 Pearson Education, Inc. The patellar reflex, a stretch reflex and the best-known monosynaptic reflex STEP 2 Activation of a Sensory Neuron STEP 1 Arrival of the Stimulus and Activation of a Receptor STEP 3 Information Processing in the CNS Stretch Spinal cord REFLEX ARC Receptor (muscle spindle) Contraction Response Effector STEP 4 Activation of a Motor Neuron STEP 5 Response of a Peripheral Effector KEY Sensory neuron (stimulated) Motor neuron (stimulated) Figure 12.10 © 2011 Pearson Education, Inc. 1 Module 12.10: Monosynaptic reflex • Muscle spindles • Are sensory receptors of stretch reflex • Consist of a bundle of specialized muscle fibers (intrafusal muscle fibers) • Are surrounded by larger skeletal muscle fibers responsible for muscle tone and contraction of entire muscle • Sensory neuron surrounds muscle spindle and is always sending impulses to CNS through dorsal root • Are innervated by gamma motor neurons that alter tension in spindle and control sensitivity of receptor © 2011 Pearson Education, Inc. The structure of a muscle spindle Skeletal muscle fibers Axon of gamma motor neuron in CNS Intrafusal fibers Route of impulses to the CNS Muscle spindle Dendrites of the sensory neuron Axon of gamma motor neuron in CNS Figure 12.10 © 2011 Pearson Education, Inc. 2 The effects of distortion of intrafusal fibers on skeletal muscle tone Sensory Region Action Potential Frequency in Sensory Neuron Effect on Skeletal Muscle Normal muscle tone persists Resting length Muscle tone increases Stretched Muscle tone decreases Compressed Figure 12.10 © 2011 Pearson Education, Inc. 3 Module 12.10: Monosynaptic reflex • Postural reflexes • Many stretch reflexes that help maintain upright posture • Coordinated activities of opposing muscles to keep body’s weight over feet • Example: leaning forward stretches calf muscle receptors which stimulate the muscles to increase tone • • Returns body to upright position Postural muscles generally have firm muscle tone and extremely sensitive stretch receptors • Allow for very fine, subconscious adjustments © 2011 Pearson Education, Inc. Module 12.10 Review a. Define stretch reflex. b. In the patellar reflex, identify the response observed and the effectors involved. c. ln the patellar reflex, in what way does stimulation of the muscle spindle by gamma motor neurons affect the speed of the reflex? © 2011 Pearson Education, Inc. Module 12.11: Polysynaptic reflexes • Polysynaptic reflexes • Responsible for automatic actions involved in complex movements • • Examples: walking and running May involve sensory and motor responses on the same side of body or opposite sides • Same side: ipsilateral reflexes • • Examples: stretch reflex, withdrawal reflex Opposite sides: contralateral reflexes • Example: crossed extensor reflex © 2011 Pearson Education, Inc. Module 12.11: Polysynaptic reflexes • Polysynaptic reflexes (continued) • All share basic characteristics: • Involve pools of interneurons • Are intersegmental in distribution • Involve reciprocal inhibition • Have reverberating circuits that prolong response • Several reflexes may cooperate to produce coordinated, controlled response © 2011 Pearson Education, Inc. Figure 12.11 © 2011 Pearson Education, Inc. 3 Module 12.11: Polysynaptic reflexes • Withdrawal reflexes • Move affected body parts away from stimulus • Strongest are triggered by painful stimuli but other stimuli can initiate • Show tremendous versatility because sensory neurons activate many pools of interneurons • • Intensity and location of stimulus affect: • Distribution of effects • Strength and character of motor responses Example: flexor reflex, crossed extensor reflexes © 2011 Pearson Education, Inc. Module 12.11: Polysynaptic reflexes • Withdrawal reflex example: flexor reflex • Grabbing an unexpectedly hot pan causes pain receptors in hand to be stimulated • Sensory neurons activate interneurons in spinal cord • Interneurons • Activate motor neurons in anterior gray horn to contract flexor muscles • Activated inhibitory interneurons keep extensors relaxed • = Reciprocal inhibition © 2011 Pearson Education, Inc. Module 12.11: Polysynaptic reflexes • Withdrawal reflex example: flexor reflex (continued) • Mild discomfort might cause brief contraction of hand and wrist muscles • More powerful stimuli may produce coordinated muscle contractions of hand, wrist, forearm, and arm • Severe pain may also involve muscles of shoulder, trunk • Contractions may persist due to reverberating circuits © 2011 Pearson Education, Inc. The flexor reflex, a representative withdrawal reflex Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated Sensory neuron (stimulated) Muscles undergoing reciprocal inhibition Extensors inhibited Excitatory interneuron Motor neuron (stimulated) Motor neuron (inhibited) Inhibitory interneuron Figure 12.11 © 2011 Pearson Education, Inc. 1 Module 12.11: Polysynaptic reflexes • Crossed extensor reflexes • Example: stepping on a tack • • Flexor reflex pulls injured foot away • Flexor muscles stimulated • Extensor muscles inhibited Crossed extensor reflex straightens uninjured leg and supports shifting weight • Activated by collaterals of excitatory and inhibitory interneurons • Extensor muscles stimulated • Flexor muscles inhibited © 2011 Pearson Education, Inc. The crossed extensor reflex, which involves a contralateral reflex arc To motor neurons in other segments of the spinal cord Extensors inhibited Flexors stimulated Extensors stimulated Flexors inhibited Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) Painful stimulus Motor neuron (inhibited) Inhibitory interneuron Figure 12.11 © 2011 Pearson Education, Inc. 2 Module 12.11 Review a. Identify the basic characteristics of polysynaptic reflexes. b. Describe the flexor reflex. c. During a withdrawal reflex of the foot, what happens to the limb on the side opposite the stimulus? What is this response called? © 2011 Pearson Education, Inc. CLINICAL MODULE 12.12: Brain influences on spinal reflexes and diagnostics using reflexes • Brain influences on spinal reflexes • Can facilitate or inhibit motor neurons or interneurons involved • • Facilitation = reinforcement Example: voluntary movement to pull apart clasped hands can reinforce stretch reflexes and increase response (example: bigger kick after patellar tap) © 2011 Pearson Education, Inc. CLINICAL MODULE 12.12: Brain influences on spinal reflexes and diagnostics using reflexes • Reflexes used in diagnostic testing • Specific examples • Babinski reflex • Stroking lateral side of sole of foot • Positive response: toes fan due to lack of inhibitory control of reflex response from descending motor pathways • • Normal in infants • Can indicate damaged higher centers or descending tracts in adults Negative response: toes curl due to development and normal reflex response • © 2011 Pearson Education, Inc. = Plantar reflex CLINICAL MODULE 12.12: Brain influences on spinal reflexes and diagnostics using reflexes • Reflexes used in diagnostic testing (continued) • Specific examples (continued) • Abdominal reflex • Depends on descending facilitation • Light stroking of skin of anterior abdomen produces reflexive twitch of abdominal muscles • Absence of response may indicate damage to descending tracts © 2011 Pearson Education, Inc. Biceps reflex Figure 12.12 © 2011 Pearson Education, Inc. 1 Triceps reflex Figure 12.12 © 2011 Pearson Education, Inc. 1 Ankle-jerk reflex Figure 12.12 © 2011 Pearson Education, Inc. 1 Babinski sign Figure 12.12 © 2011 Pearson Education, Inc. 2 Plantar reflex Figure 12.12 © 2011 Pearson Education, Inc. 3 Abdominal reflex Figure 12.12 © 2011 Pearson Education, Inc. 4 CLINICAL MODULE 12.12 Review a. Define reinforcement as it pertains to spinal reflexes. b. What purpose does reflex testing serve? c. After injuring her back, 22-year-old Tina exhibits a positive Babinski reflex. What does this imply about her injury? © 2011 Pearson Education, Inc.