Spinal Cord and Spinal Reflexes (Chapter 12 & 13) - PDF

Spinal Cord and Spinal Reflexes (Chapter 12 &13) Central Nervous System: I. Spinal Cord: provides a two-way conduction pathway to and from the brain A. Gross Anatomy: About 18 inches (45 cm) long 1/2 inch (14 mm) wide Spinal cord ends between vertebrae L 1 and L2...

Spinal Cord and Spinal Reflexes (Chapter 12 &13) Central Nervous System: I. Spinal Cord: provides a two-way conduction pathway to and from the brain A. Gross Anatomy: About 18 inches (45 cm) long 1/2 inch (14 mm) wide Spinal cord ends between vertebrae L 1 and L2 Bilaterally symmetrical:  Grooves divide the spinal cord into left and right Posterior (Dorsal) Median Sulcus: groove on posterior/dorsal side Anterior (Ventral) Median Fissure: deeper groove on anterior/ventral side Enlargements: Caused by: – Amount of gray matter in segment – Involvement with sensory and motor nerves of limbs  Cervical enlargement – Spinal nerves that innervate (supplies/controls) the shoulders and upper limbs  Lumbar enlargement – Spinal nerves that innervate the pelvis and lower limbs The Distal End: Conus medullaris – Thin, conical spinal cord below lumbar enlargement Filum terminale – Thin thread of fibrous tissue at end of conus medullaris – Fusion of the spinal meninges beyond sacral vertebra S 2 – Anchors spinal cord to the coccyx Cauda equina – Long ventral and dorsal roots that extend below conus medullaris + filum terminale 31 Spinal Cord Segments (Spinal Segments): Based on vertebra where spinal nerve is associated with Position of spinal segment and vertebra change with age – Cervical nerves Are named for inferior vertebra – All other nerves Are named for superior vertebra  L2 L2 L2 L2 L3 L4 S2 L5 L4 S1 Spinal Nerve S2 S1 L5 Dorsal Root Ganglion Spinal Nerve S2 S1 S2 Spinal Nerve 8 weeks 24 weeks Newborn Adult Attachment of the Coccygeal Ligament Vertebral Body: Spinal Nerves: Roots of Spinal Nerve: Spinal Nerve  Ventral (Anterior) Root – Each side of spinal cord Contains axons of motor neurons Dorsal and ventral roots join to form spinal nerve  Dorsal (Posterior) Root – Contain mixed nerves Contains axons of sensory neurons Carry both sensory & motor axons Dorsal Root Ganglion: Contain cell bodies of sensory neurons (unipolar neurons)  B. 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 3 Spinal Meningeal Layers: 1. Dura mater – Outer layer of spinal cord 2. Arachnoid mater – Middle meningeal layer 3. Pia mater – Inner meningeal layer 1. Dura Mater Tough and fibrous outer spinal meningeal layer Superiorly – Continuous with cranial dura mater Inferiorly – Tapers to dense cord of collagen fibers: filum terminale in coccygeal ligament Laterally - Fuses with the epineurium of spinal nerve 2. Arachnoid Mater – Middle spinal meningeal layer  Arachnoid membrane  Arachnoid trabeculae  Epidural space Between dura mater and periosteum of the vertebra Contains adipose tissue and blood vessels Epidural: a procedure that injects a local anesthetic in the epidural space  Subdural space Between arachnoid mater and dura mater  Subarachnoid space Between arachnoid mater and pia mater Contains arachnoid trabeculae Filled with cerebrospinal fluid (CSF) Lumbar puncture or spinal tap: withdraws CSF  3. Pia Mater innermost spinal meningeal layer Consist of collagen and elastic fibers attached to the surface of 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  C. Sectional Anatomy: White matter (White columns) – Contains mostly myelinated axons Gray matter – Surrounds central canal of spinal cord – Contain cell bodies, neuroglia & unmyelinated axons – Projections called gray horns a.) Organization of Gray Matter  Gray Horns Posterior gray horns: contain somatic and visceral sensory nuclei Anterior gray horns: contain somatic motor nuclei Lateral gray horns: contain visceral motor nuclei  thoracic & upper lumbar segments (T1 – L2)  Gray Commissures (Transverse Tracts) Commissural fibers cross from one side of cord to the other  The cluster of cell bodies forming functional groups called nuclei  Structural Organization of White Matter: b.) Organization of White Matter (White Columns) Tracts or fasciculi – Found in white columns – Bundles of mostly myelinated axons – Relay information in same direction  Ascending tracts Carry information to brain  Descending tracts Carry motor commands to spinal cord (from brain to spinal cord or within the cord to lower levels)  White Commissures (Transverse Tracts) Commissural fibers Functional Organization of White Matter:  Spinal Cord Summary – Spinal cord has a narrow central canal – Gray matter 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 c.) Variations in Spinal Cord Segments:  Various spinal cord segments vary in: Size Shape Relative amounts of gray and white matter Distribution and shape of gray matter  II. Spinal Nerves: A. Anatomy of Spinal Nerves Every spinal cord segment – connected to a pair of spinal nerves Every spinal nerve – surrounded by three connective tissue layers that support structures and contain blood vessels 3 Connective Tissue Layers: 1. Epineurium – Outer layer – Dense network of collagen fibers (dense collagenous C.T.) – Continuous with the dura mater 2. Perineurium – Middle layer – Divides nerve into fascicles or fasciculi (bundles of axon) 3. Endoneurium – Inner layer – Surrounds individual axons  B. Branches of Spinal Nerves Spinal Nerves: form lateral to intervertebral foramen dorsal and ventral roots unite branch and form pathways to body organs and tissues called Peripheral Nerves contain both sensory fibers (afferent fibers) and motor fibers (efferent fibers):  Rami Communicantes “communicating branches”: – White Ramus Communicans (White Ramus): found only at spinal cord segments: T1 – L2 – Gray Ramus Communicans (Gray Ramus)  Dorsal (Posterior) Ramus and Ventral (Anterior) Ramus: – Dorsal (Posterior) Ramus Smaller branch (smaller in diameter) Generally, innervates (serves/supplies) the posterior region of the body: deep muscles, skin and structures of the upper and lower limbs, and the trunk – Ventral (Anterior) Ramus Larger branch (larger in diameter) Generally, innervates the anterior and lateral regions of the body: muscles, skin and structures of upper and lower limbs, and the trunk  Meningeal Branch: - re-enters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessel of the spinal cord, spinal meninges  C. Spinal Nerve Plexuses and Peripheral Nerves  Spinal Nerve Plexuses: Complex, interwoven networks of nerve fibers Spinal Formed from anterior (ventral) rami of spinal nerves Nerve Plexuses: Peripheral Nerves:  Peripheral Nerves: Terminal branches of spinal nerves Innervate body tissues and organs Surrounded by same connective tissue sheaths 4 Principal / Major Spinal Nerve Plexuses: 1. Cervical Plexus: - consists of ventral rami of spinal nerves C1 - C5 2. Brachial Plexus: - consists of ventral rami of spinal nerves C5 – T1 3. Lumbar Plexus: - consists of ventral rami of spinal nervesT12 – L4 4. Sacral Plexus: - consists of ventral rami of spinal nerves L4 – S4 Intercostal (Thoracic) Nerves: Spinal Nerves T2-T12 - Anterior (ventral) rami: do not form plexuses nerves directly innervate intercostal muscles and the anterior and lateral skin of the thorax - Posterior (dorsal) rami: innervate the deep back muscles and posterior skin of the thorax Intercostal Nerves (Thoracic Nerves) 1. Cervical Plexus – Ventral rami of spinal nerves C1–C5 – Innervates skin and skeletal muscles of head, neck, superior part of thoracic cavity (shoulders & chest), diaphragm  Major peripheral nerve to know: Phrenic nerve (controls muscle of diaphragm) Ventral Rami = Roots  2. Brachial Plexus – Ventral rami of spinal nerves C5–T1 – Innervates pectoral girdle and upper limbs (skin and skeletal muscle) 5 Major Groups of Brachial Plexus: 1. Ventral Rami (Roots) 2. Trunks 3. Divisions 4. Cords 5. Peripheral Nerves (Terminal Branches)   Major peripheral nerves to know: Musculocutaneous nerve Median nerve Ulnar nerve Axillary nerve Radial nerve  3. Lumbar Plexus – Ventral rami of spinal nerves T12–L4 – Supplies the anterolateral abdominal wall, external genitalia, and part of the lower limbs  Major peripheral nerves Genitofemoral nerve Lateral femoral cutaneous nerve Femoral nerve Obturator 4. Sacral Plexus – Ventral rami of spinal nerves L4–S4 – Supplies the buttocks, perineum, lower limbs  Major peripheral nerve to know Sciatic nerve (largest nerve in body) Pudendal nerve Two branches of the sciatic nerve  Common Fibular nerve  Tibial nerve Dermatome: Dermatome Map: - an area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion - loss of sensation in a dermatome can indicate the level of spinal cord damage in the clinical assessment of injury - if one dorsal root is severed or cut, there will be partial loss of sensation from that dermatome called hypoesthesia - there is some overlap between neighboring dermatomes, therefore, for complete loss of sensation called dermatomal anesthesia, at least three dorsal roots are damaged Innervation of skin by three spinal nerves combined into one peripheral nerve through plexus formation Shingles - Caused by the Varicella-Zoster virus (VZV) - Herpes virus that attacks sensory neurons in dorsal roots of spinal nerves and sensory ganglia of cranial nerves (initial exposure) - Remains dormant within the dorsal root ganglia - Skin eruptions follow the distribution of the dermatomal innervation Dermatome Map:  III. Reflexes: rapid, involuntary, automatic response(s) to stimulus(i): Occur through interconnected sensory neurons, motor neurons, and interneurons They can be simple or complex  Reflex arc The wiring of a single reflex Begins at sensory receptor Ends at effector Generally, opposes original stimulus (negative feedback) A. Five Steps in a Reflex Arc (Reflex Circuit): Step 1: Stimulus  Activation of sensory receptor – Physical or chemical changes: graded potential Step 2: Activation of sensory neuron (unipolar neuron) – Action potential at trigger zone of sensory neuron Step 3: Integration at CNS - Sensory neuron synapses with interneuron Step 4: Activation of motor neuron – Interneuron(s) synapse(s) with motor neuron Step 5: Response of effector – Motor neuron synapses with effector Somatic Reflex: Effector: skeletal muscle Visceral (Autonomic) Reflex: Effector: smooth muscle, cardiac muscle, or a gland Somatic Reflex: Visceral Reflex: B. Classification of Reflexes: C. Spinal Reflexes: – Range in increasing order of complexity Monosynaptic reflexes - involves a single synapse Polysynaptic reflexes - involves more than one synapse Monosynaptic Reflex: - the best known is the stretch reflex - least delay in response between sensory input and motor output Examples of stretch reflex: patellar reflex and postural reflex - completed in 20–40 msec  Proprioceptors involved in the stretch reflex are called muscle spindles Muscle Spindles: Alpha  Primarily detect changes in the length of skeletal muscle: Motor Neurons contribute to fine motor control and provide body position information to the central nervous system  Consists of bundles of small, specialized skeletal muscle fibers called intrafusal muscle fibers (intrafusal fibers) Gamma Innervated by somatic sensory neurons and somatic Motor Neurons motor neurons called gamma motor neurons From  Surrounded by extrafusal muscle fibers (extrafusal fibers) CNS Maintain muscle tone and contract muscle Innervated by somatic motor neurons called alpha motor neurons  Gamma motor neuron: synapses with intrafusal fibers From  Alpha motor neurons: synapses with extrafusal fibers CNS Sensory Region of the Muscle Spindle: Central region of intrafusal fibers Dendrites of sensory neurons spiral around each intrafusal fiber Gamma Motor Neurons  Patellar Reflex: Monosynaptic Reflex Arc: Somatic sensory neuron axon enters CNS in dorsal root Synapses onto somatic motor neurons (alpha motor neurons) in anterior/ventral gray horn of spinal cord  Patellar Reflex (Continued): Reciprocal Inhibition (Reciprocal Innervation) - contraction of prime mover (agonist) and relaxation of its antagonist - polysynaptic reflex - prevents conflict between opposing muscles - vital in coordinating body movements  Postural Reflexes – Many stretch reflexes are postural reflexes – Maintain normal upright posture – Stretched muscle responds by contracting automatically to maintain balance  Polysynaptic Reflexes - More complicated than monosynaptic reflexes - Produce either EPSPs or IPSPs at CNS somatic motor nuclei (anterior gray horn) Examples: Tendon reflex, Withdrawal (Flexor) reflex, Crossed Extensor reflex  Tendon Reflex – Prevents skeletal muscles from: Developing too much tension Tearing tendons or breaking bone Proprioceptors (sensory receptors) involved in the tendon reflex are called: Tendon Organs (Golgi Tendon Organs)  Withdrawal (Flexor) 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  Crossed Extensor Reflex Occurs on side opposite to stimulus Complements the withdrawal reflex (flexor reflex) and occurs simultaneously For example: stepping on a sharp object - Flexor reflex causes leg to pull up (flexes leg) - Crossed extensor reflex straightens other leg (extends leg) which receives the body weight Right arm: Left arm: Flexor Reflex Crossed Extensor Reflex (site of stimulus) Ipsilateral Reflexes vs. Contralateral Reflexes:  Ipsilateral reflexes: Occur on same side of body as stimulus Stretch, tendon, and withdrawal (flexor) reflexes  Contralateral reflexes: Occur on side opposite to stimulus Crossed extensor reflex Superficial Reflexes: - Elicited by cutaneous stimulation (e.g. stroking skin with tongue depressor) - Clinically important reflexes because depends both on functional upper motor pathways and on spinal reflex arcs Example: Plantar Reflex  Plantar Reflex (Babinski Reflex): - Tests integrity of spinal cord: L4 – S2 - Determines if primary motor cortex or corticospinal tracts (an example of a descending tract) are functioning properly - Elicited by drawing a blunt object upward along the lateral aspect of the plantar surface (sole) of foot - If primary motor cortex or corticospinal tract is damaged, the plantar reflex is replaced by an abnormal reflex called Babinski sign In infants (until ~ 1yr old), exhibit positive Babinski sign because their nervous system is incompletely myelinated and the corticospinal tract is not fully developed In children (2 yrs. and older) and adults, negative Babinski sign (normal response if no damage to corticospinal tract or primary motor cortex) The End