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13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes
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13-2 Spinal Cord Functions: Link between brain and rest of the body
Conduction: has tracts of nerve fibers for upward and downward travel of sensory and motor information Can work independently of brain Reflexes – quick responses to stimuli
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Location and Gross Anatomy of the Spinal Cord
Located in vertebral canal Arises from the brainstem at the foramen magnum of the skull to between L1 and L2 vertebrae ~ 18 inches (45 cm) long 1/2 inch (14 mm) wide
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Enlargements of the Spinal Cord are caused by greater amounts of gray matter in those regions
Cervical enlargement Nerves of shoulders and upper limbs Lumbar enlargement Nerves of pelvis and lower limbs
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The 31 pairs of spinal nerves
Conus medullaris The tapered terminal end of the spinal cord-Located below the lumbar enlargement (between L1-L2) Cauda equina: Nerve roots extending inferiorly below conus medullaris (L2-S5) that innervates the pelvic organs and lower limbs Filum terminale: Thin thread of fibrous tissue (pia mater) at end of conus medullaris that helps anchor the conus medullaris to the coccyx Cervical spinal nerves Cervical enlargement Posterior median sulcus Thoracic spinal nerves Lumbar enlargement Conus medullaris Lumbar spinal nerves Interior tip of spinal cord Figure The spinal cord contains gray matter and white matter Cauda equina Sacral spinal nerves Filum terminale Coccygeal nerve (Co1) Figure 5
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Cross Section of Spinal Cord
Spinal Cord has a small central canal lined with ependymal cells which help circulate the CSF Spinal cord has bilateral symmetry: Grooves divide the spinal cord into left and right side Posterior median sulcus – shallow depression on posterior side Anterior median fissure – deeper groove on anterior side
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Spinal Tissue divided into:
Inside Gray matter (shaped like H) Location of mostly cell bodies and unmyelinated axons Greater amount in spinal cord segments serving limbs Outside White Matter - Location of mostly myelinated axons
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Figure 13-2 Gross Anatomy of the Adult Spinal Cord
Posterior median sulcus Dorsal root White matter Dorsal root ganglion Gray matter C1 Central canal C2 Cervical spinal nerves C3 C4 C5 Spinal nerve Ventral root C6 C7 Cervical enlargement Anterior median fissure C8 C3 T1 T2 T3 T4 T5 T6 T7 Thoracic spinal nerves T8 Posterior median sulcus T9 T10 Lumbar enlargement T3 T11 T12 Conus medullaris L1 L2 Inferior tip of spinal cord Lumbar spinal nerves L3 L4 Cauda equina L5 L1 S1 Sacral spinal nerves S2 S3 S4 S5 Filum terminale (in coccygeal ligament) S2 Coccygeal nerve (Co1) 8
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Spinal cord gives rise to 31 pairs of Spinal Nerves
Spinal nerves are part of PNS C1-C8, T1-T12, L1-L5, S1-S5, Co1 Spinal cord segment: refers to the part supplied by each pair of spinal nerves - First pair of spinal nerves pass between the skull and C1 - Rest pass through the intervertebral foramina Spinal nerves are mixed nerves: contain sensory and motor nerve fibers
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The Spinal cord is protected by the vertebrae and the spinal meninges
Spinal Meninges are specialized membranes that isolate spinal cord from surroundings Protecting spinal cord Carry blood supply Continuous with cranial meninges Meningitis: Viral or bacterial infection of meninges
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The Three Meningeal Layers of the spinal cord
Dura mater Outer layer, Dense irregular CT Arachnoid mater Middle avascular layer Pia mater Delicate Inner vascularized layer Downward extensions of the pia mater anchor the spinal cord at the coccyx- filum terminale
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Figure 13-3b The Spinal Cord and Spinal Meninges
Dura mater ANTERIOR Arachnoid mater 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 A sectional view through the spinal cord and meninges, showing the peripheral distribution of spinal nerves POSTERIOR 12
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Epidural Space Between spinal dura mater and walls of vertebral canal Contains blood vessels, loose connective and adipose tissue Anesthetic injection site; Epidural shot Subdural Space Between dura mater and arachnoid mater Little or no space Subarachnoid space Between arachnoid mater and pia mater Filled with cerebrospinal fluid (CSF) that carries dissolved gases, nutrients, and wastes Site of lumbar puncture or spinal tap to withdraw CSF for diagnostic purposes L3 & L4 L4 and L5
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Figure 13-4 The Spinal Cord and Associated Structures
Anterior median fissure Pia mater Paired denticulate ligaments are found along length of spinal cord Extend from pia mater to dura mater Help suspend and anchor the spinal cord laterally Denticulate ligaments Dorsal root Ventral root, formed by several “rootlets” from one cervical segment Arachnoid mater (reflected) Dura mater (reflected) Spinal blood vessel 14
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13-3 Gray Matter and White Matter
Centrally located; H-shaped, surrounds central canal Contains neuron cell bodies (organized in nuclei), neuroglia, unmyelinated axons Nuclei are masses of gray matter within the CNS Sensory nuclei receive and relay sensory information from peripheral receptors; Located on posterior (dorsal) side Motor nuclei issue commands to peripheral effectors; Located on anterior (ventral) side
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Gray Horns Projections of gray matter towards the outward surface of the spinal cord Posterior gray horns contain somatic and visceral sensory nuclei Anterior gray horns contain somatic motor nuclei Lateral gray horns-only found in thoracic and upper lumbar segments-; contain visceral motor nuclei Sensory or motor nucleus location within the gray matter determines which body part it controls
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Gray commissures (bridge of “H”)
Found posterior and anterior to central canal Contain axons that laterally cross spinal cord
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Figure 13-5a The Sectional Organization of the Spinal Cord
Posterior median sulcus Posterior white column 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. Posterior gray horn Somatic Sensory nuclei Lateral white column Lateral gray horn Visceral Dorsal root ganglion Visceral Anterior gray horn Motor nuclei Somatic Ventral root Anterior gray commissure Anterior white commissure Anterior white column Anterior white column Anterior median fissure 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. 18
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Figure 13-5b The Sectional Organization of the Spinal Cord
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 commissure Posterior gray horn Dura mater Arachnoid mater (broken) Lateral gray horn Dorsal root Central canal Anterior gray horn Anterior gray commissure Anterior median fissure Dorsal root ganglion Pia mater ANTERIOR Ventral root A micrograph of a section through the spinal cord, showing major landmarks in and surrounding the cord. 19
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Organization of White Matter into three columns
Posterior white columns lie between posterior gray horns and posterior median sulcus Anterior white columns lie between anterior gray horns and anterior median fissure Lateral white columns located on each side of spinal cord between anterior and posterior columns
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Organization of White Matter
Each column contains tracts or fasciculi whose axons share functional and structural characteristics Tract: Bundles of axons with common origin or destination Relay same information in same direction Ascending tracts Carry sensory information to brain Descending tracts Carry motor commands away from brain
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Figure 13-5a The Sectional Organization of the Spinal Cord
Posterior white column Posterior gray horn Lateral white column Lateral gray horn Dorsal root ganglion Anterior gray horn Anterior white column 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. 22
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13-4 Spinal Nerves and Plexuses
Anatomy of Spinal Nerves Every spinal cord segment is connected to a pair of spinal nerves (31 pairs total) Every spinal nerve Is surrounded by three connective tissue layers That support structures and contain blood vessels
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Three Connective Tissue Layers of Spinal Nerves
Epineurium Outer layer Dense network of collagen fibers Perineurium Middle layer Divides nerve into fascicles (axon bundles) Endoneurium Inner layer Surrounds individual axons
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Figure 13-6 A Peripheral Nerve
Blood vessels Connective Tissue Layers Epineurium covering spinal nerve Perineurium (around one fascicle) Endoneurium Myelinated axon Fascicle Schwann cell 25
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Figure 13-6 A Peripheral Nerve
Blood vessels Perineurium (around one fascicle) Endoneurium 26
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The 31 pairs of spinal nerves extend from spinal cord into the periphery and innervate muscles, skin and internal organs Most enter through the intervertebral foramen Spinal Nerves are Mixed Nerves: Carry both afferent (sensory) and efferent (motor) fibers
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Within the vertebral canal-near its attachment to the spinal cord- each spinal nerve forms two branches, a ventral and a dorsal root Ventral (Anterior) root Carries axons of motor neurons which have received motor commands from the anterior horn of spinal cord away from the spinal cord to the periphery to control somatic and visceral effectors Dorsal (Posterior) root Carries axons of sensory neurons which bring sensory information to the posterior horn of spinal cord Dorsal (posterior) root ganglia: Swelling on dorsal root containing cell bodies of sensory neurons The ventral root and the dorsal root come together to form a spinal nerve
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Figure 13-3a The Spinal Cord and Spinal Meninges
Gray matter White matter Dorsal root ganglion Ventral root Spinal nerve Dorsal root Meninges Pia mater Arachnoid mater Dura mater A posterior view of the spinal cord, showing the meningeal layers, superficial landmarks, and distribution of gray matter and white matter 29
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Each spinal nerve has branches that collect sensory information and distributes motor commands
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Spinal Nerves - Branches
Distal to the vertebral canal – after it passes through the intervertebral foramina, each spinal nerve forms branches: Posterior (dorsal) ramus - supplies to the skin & muscles of the back Anterior (ventral) ramus – supplies to the extremities and anterior wall of the trunk Meningeal ramus – supplies to the meninges and vertebrae Rami communicantes (autonomic nerves) – supplies to the internal organs
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Figure Dermatomes C2C3 N V C2C3 Dermatomes: Specific bilateral region of the skin that is monitored by a pair of spinal nerves Each spinal nerve (except C1) services its own dermatome (though there is overlap) Clinically important to determine damage or infection of spinal nerve or dorsal root ganglion Results in loss of sensation or signs on skin in dermatome C2 C3 C3 C4 T2 C4 C5 T3 T1 T2 T4 T5 T3 C5 T6 T4 T7 T5 T8 T6 T9 T2 T2 T7 T10 T8 T11 T12 T9 L1 C6 T10 L2 T11 L4 L3 T1 C6 T12 L5 C7 L1 S S L2 4 S2 3 C8 T1 L3 C8 L1 C7 S5 S1 L5 L4 L2 S2 L5 L3 S1 L4 ANTERIOR POSTERIOR 32
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Viral infection of dorsal root ganglia
Figure Shingles 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 33
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13-4 Spinal Nerves and Plexuses
The ventral ramus branch of each thoracic nerve (T1-T12) goes directly to the intercostal muscles. The ventral ramus of all other spinal nerves form a network by joining with other anterior rami branches….plexus before going to their respective regions The plexus will then give rise to individual nerves that control specific skeletal muscles of the neck, arms and legs.
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13-4 Spinal Nerves and Plexuses
The Four Major Plexuses of Ventral Rami Cervical plexus Brachial plexus Lumbar plexus Sacral plexus
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The Cervical Plexus Includes ventral rami of spinal nerves C1–C5
Innervates skin and muscles of neck, thoracic cavity, diaphragmatic muscles Major nerve: Phrenic nerve (controls diaphragm): Damage to spinal cord above C3 causes breathing difficulties
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Figure 13-10 Peripheral Nerves and Nerve Plexuses
C1 Lesser occipital nerve C2 Great auricular nerve Cervical plexus C3 Transverse cervical nerve C4 C5 Supraclavicular nerve C6 C7 Phrenic nerve Brachial plexus C8 T1 T2 T3 Axillary nerve T4 T5 T6 T7 Musculocutaneous nerve T8 T9 Thoracic nerves T10 T11 37
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The Brachial Plexus Includes ventral rami of spinal nerves C5–C8, & part of T1 Innervates skin and muscles of pectoral girdle and upper limbs Major nerve: Median nerve: Passes through the wrist. Carpal Tunnel Syndrome: Inflammation of the carpal tendons can put pressure on this nerve causing numbness and pain in the fingers
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Figure 13-12a The Brachial Plexus
Spinal Nerves Forming Brachial Plexus Trunks of Brachial Plexus Dorsal scapular nerve C4 Suprascapular nerve C5 Superior C6 Middle C7 Inferior C8 T1 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 Major nerves originating at the right brachial plexus, anterior view 39
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No thoracic plexus! Ventral rami of thoracic nerves do not interconnect Become intercostal nerves to innervate intercostal muscles
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The Lumbar Plexus Includes ventral rami of spinal nerves T12–L4
Supplies genitals, lateral and ventral abdominal wall and anterior/medial thigh Major nerves Femoral nerve: Extension of leg Obturator nerve: Adduction of hip
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Figure 13-10 Peripheral Nerves and Nerve Plexuses
T12 L1 Radial nerve Lumbar plexus 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 42
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Includes ventral rami of spinal nerves L4–S4
The Sacral Plexus Includes ventral rami of spinal nerves L4–S4 Supplies to the buttocks and lower extremities Major nerve: Sciatic nerve, longest & largest nerve in body: Flexion of leg Two branches of the sciatic nerve Tibial nerve Common Fibular nerve Sciatica: Compression of nerve against ishium -results in numbness of lower extremity or pain radiating from lower back to leg
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Figure 13-13b The Lumbar and Sacral Plexuses
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 S4 nerve Sciatic S5 Posterior femoral cutaneous Co1 Pudendal Sacral plexus, anterior view 44
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Figure 13-13e 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 Nerves of the sacral plexus, posterior view 45
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13-6 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-negative feedback Spinal reflexes Occur in spinal cord Typically fast reflexes (ex: flexor reflex) Cranial reflexes Occur in brain Involving special sense organs
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Figure 13-15 Events in a Neural Reflex
Reflex arc: Specific pathway followed by a nerve impulse in a reflex Dorsal root Arrival of stimulus and activation of receptor Activation of a sensory neuron Sensation relayed to the brain by axon collaterals Information processing in the CNS REFLEX ARC Receptor Stimulus Response by effector Effector Ventral root KEY Sensory neuron (stimulated) Activation of a motor neuron Excitatory interneuron Motor neuron (stimulated) 47
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Figure 13-16 The Classification of Reflexes
can be classified by development response response complexity of circuit processing site Innate Reflexes Somatic Reflexes Monosynaptic Spinal Reflexes • Genetically • Control skeletal muscle • One synapse • Processing in determined contractions the spinal cord • Include superficial and stretch reflexes Acquired Reflexes Visceral (Autonomic) Reflexes Polysynaptic Cranial Reflexes • Learned • Control actions of smooth and • Multiple synapse • Processing in cardiac muscles, glands, and adipose tissue (two to several hundred) the brain Somatic Reflexes are often used to diagnose neurological Impairment 48
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Ipsilateral reflex arc
Stimulus and response occur on same side of body Contralateral reflex arc Stimulus and response occur on opposite sides of body
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Stretch reflex are Monosynaptic reflexes involving muscle spindles that regulate muscle length: Example: Patellar Reflex Receptor (muscle spindle) Spinal cord Stretch REFLEX ARC Stimulus Effector Contraction KEY Sensory neuron (stimulated) Motor neuron (stimulated) Response 50
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Withdrawal Reflexes are Polysynaptic Reflexes that move body part away from stimulus (pain or pressure). Example: Flexor reflex Distribution within gray horns to other segments of the spinal cord Painful stimulus Flexors stimulated Extensors inhibited KEY Sensory neuron (stimulated) Motor neuron (inhibited) Excitatory interneuron Inhibitory interneuron Motor neuron (stimulated) 51
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Crossed extensor reflexes: Polysynaptic reflexes
Example: stepping on a sharp stone Flexor reflex pulls injured foot away Flexor muscles stimulated Extensor muscles inhibited Crossed extensor reflex straightens uninjured leg and supports shifting weight Extensor muscles stimulated Flexor muscles inhibited
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To motor neurons in other segments of the spinal cord
The crossed extensor reflex, which involves a contralateral reflex arc To motor neurons in other segments of the spinal cord Extensors inhibited Extensors stimulated Flexors stimulated Flexors inhibited Figure Withdrawal reflexes and crossed extensor reflexes are polysynaptic reflexes Sensory neuron (stimulated) Excitatory interneuron Motor neuron (stimulated) Motor neuron (inhibited) Painful stimulus Inhibitory interneuron Figure 53
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Spinal cord injury Can be due to accidents, fall, violence
Can result in abnormal reflexes and/or paralysis Extent of paralysis dependent on location of injury Monoplegia: paralysis of one limb Paraplegia: paralysis of both lower limbs Quadriplegia: paralysis of all four limbs
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