Chapter 14 The Spinal Cord and Spinal Nerves
Spinal Cord • Begins at foramen magnum extends down to L1/L2. • Contiguous with the medulla of the brain stem. • Passes through the vertebral foramen of all vertebrae. • Consists of cervical, thoracic and lumbar divisions same as vertebral column. • PNS - 31 paired spinal nerves one at each level C1-C8, T1-T12 and L1-L5, S1-S5 and Co-1. • Contains gray and white matter, however gray matter is central and white matter surrounds gray matter. • At L1/L2 level cona medullaris tapers into cauda equina
Spinal cord levels: C1-C8 T1-T12 L1-L5 Cauda eqiuna S1-S5
The Meninges • Consists of 3 layers around the brain and spinal cord • Dura mater- tough outer fibrous connective tissue layer; two layers periosteal (periosteum) and meningeal. • Arachnoid mater- inner layer attached to dura mater - subarachnoid space between arachnoid and pia mater filled with CSF. • Pia mater- thin connective tissue layer closely adhered to brain surface and spinal cord. Pia mater goes deep into the sulci (grooves) of the brain.
Gray matter of spinal cord • Shaped like a “butterfly” with central canal. • Wings contain anterior, and posterior horns. • Thoracic and Lumbar levels also have lateral horns. • Contains neuron cell bodies, unmyelinated axons, dendrites and neuroglia. • Gray commissure has fibers that communicate between right and left halves of spinal cord.
White matter of spinal cord • Contains myelinated ascending and descending spinal tracts. • Divided into columns or funiculi; each column consists of tracts or fasciculi. • Also has white commissural tracts that cross thru cord • Ascending tracts carry sensory information from body to brain • Descending tracts carry motor information from brain to motor nerves • Tracts named according to where they originate and terminate e.g. Lateral spinothalamic tract
Ascending spinal tracts • Transmit sensory information from peripheral body. • Receptors relay information to spinal cord via sensory nerve in dorsal root and may synapse in dorsal horn of gray matter, decussate or ascend to a higher level. • Tracts are named according to their origin and destination, e.g. Lateral spinothalamic tracts; anterior spinothalamic; posterior spinalcerebellar tract, etc.
Ascending tracts Three neuron chain: First-order: Receptor to afferent nerve into spinal cord. Second order: Interneuron to relay nucleus in thalamus. Third order: Thalamus projects information to cortex.
Descending spinal tracts • Take information from brain and descend in gray columns to appropriate level to a lower motor neuron. • Motor innervation is contralateral i.e. the left side of the brain controls the right side of the body and vice versa. Cross over at pyramidal decussation in medulla. • Tracts are named according to their origin and destination, e.g. Lateral corticospinal tract; anterior corticospinal tract; reticulospinal tract, etc.
Descending tracts Two neuron chain: Upper motor neuron: From cortex or brain stem to LMN Lower motor neuron: From spinal cord to muscle or target organ
Nerve structure • Nerves are only in the periphery • Cable-like organs in PNS = cranial and spinal nerves • Consists of 100’s to 100,000’s of myelinated and unmyelinated axons (nerve fibers). • Endoneurium surrounds each axon (nerve fiber). • Axons are grouped into bundles of fascicles • Perineurium surrounds each fascicle • Epineurium surrounds each nerve bundle • Conduction is saltatory (i.e. jumps node to node) in myelinated nerves and continuous in nonmyelinated.
Spinal Nerves • There are 31 pairs of spinal nerves. Spinal nerves are all mixed nerves (i.e. sensory and motor). Each forms by combining the dorsal and ventral roots at a segment of the spinal cord. • As spinal nerves exit vertebral foramen they branch into a meningeal nerve and a dorsal and ventral ramus. • The meningeal branch re-enters the spinal column to supply the meninges, spinal ligaments and vertebrae. • After leaving the spinal cord, the ventral ramus of select nerves combine to form nerve plexuses that innervate the appendages and other parts of the body.
Dorsal and ventral ramus nerves • Dorsal ramus nerves innervates the ligaments, muscles and skin of the back. • Ventral ramus nerves innervates the anterior and lateral muscles, skin and ligaments of the trunk. They also give rise to the nerve plexuses that innervate the appendages.
Thoracic level -Dorsal ramus -Ventral ramus -Dorsal root ganglion -Sympathetic chain ganglion -white and gray rami communicantes
Spinal nerve plexuses from ventral ramus - major nerves off of them. • Plexus= network of nerves • Cervical plexus – forms from C1 – C4 -Phrenic nerve is off of cervical plexus C3-C5 • Brachial plexus – forms from C5 – C8, and T1 -Axillary, radial, median, ulnar nerves • Lumbar plexus – forms from L1 - L4 -Femoral and obturator nerves • Sacral plexus – forms from L4 – S4 -Sciatic nerve
Spinal nerve plexuses
Cervical plexus -C1- C5 • Phrenic nerve -C3- C5
Brachial plexus -C5-C8 & T1 • Axillary nerve - C5 & C6 • Median nerve - C5-C8 & T1 • Radial nerve - C5-C8 & T1 • Ulnar nerve - C8 & T1
Lumbar plexus L1- L4 • Femoral nerve -L2-L4 • Obturator nerve -L2-L4
Sacral plexus -L4- S4 • Sciatic nerve -Tibial n. L4-S3 -Comn fibular -L4-S2
DERMATOMES • The area of skin that carries sensory information to the CNS from all of the spinal nerves (except C1) and cranial nerve V (facial). • Entire body surface is supplied by a peripheral nerve. • Knowing the distribution of these nerves is very helpful diagnostically in determining whether a nerve is affected at the central or peripheral level and at what level the injury may be.
Reflex Arc • Consists of 1. sensory receptor, 2. afferent nerve, 3. interneuron, 4. efferent nerve and 5. an effector organ (muscle or gland). • Occurs at the spinal level
Receptors: Classification by location • Exteroceptors- receptors sensitive to changes outside of body. Are found on or near the body surface: pain, pressure, touch, temperature and special sense organs (eye, ear, nose, mouth). • Interoceptors- visceroceptors receive stimuli from viscera; stretch, temperature, chemical, taste. • Proprioceptors- located in skeletal muscles and joints, tendons, ligaments; perceive stretch in these organs
Proprioceptors • Encapsulated receptors that monitor stretch in muscles and tendons. Ex. Muscle spindles Extrafusal fibers Intrafusal
Classification by Stimulus • Mechanoreceptors- respond to mechanical forces: touch, pressure, stretch, vibration, and itch. • Thermoreceptors- respond to temperature changes • Chemoreceptors- respond to chemicals in solution and blood chemistry. • Photoreceptors- respond to changes in light-eye • Nociceptors- respond to pain and harmful stimuli leading to pain.
Classification by Structure • Free nerve endings- in all tissues of body; abundant in epithelial and connective tissue. - respond to pain, temperature, itch and light pressure Ex.- Merckels discs, hair follicle receptors • Encapsulated nerve endings- enclosed in a capsule of connective tissue; vary in shape and distribution. Ex.- Meissner’s, Pacinian and Ruffini’s corpuscles
Reflex Arcs • May be unilateral or contralateral • Unilateral knee jerk: Flexors contract extensor relax
Reflex Arcs • Unilateral: Excess tension on tendon causes inhibition of flexor and stimulation of extensor
Crossed-Extensor reflex arc • Pain to one foot causes retraction of injured foot and extension of opposite foot