Spinal Cord and Spinal Nerves Anatomy

Spinal Cord and Spinal Nerves

• Chapter 14 focuses on the spinal cord and spinal nerves.

Introduction

• The Central Nervous System (CNS) includes the spinal cord and brain.
• The spinal cord integrates and processes information.
• It can function with or independently of the brain.
• The brain also integrates and processes information.
• It can function with or independently of the spinal cord

Gross Anatomy of the Spinal Cord

• The spinal cord is approximately 45 cm in length.
• It passes through the foramen magnum.
• The spinal cord stops growing in length around age 4.
• It extends from the brain to the L₁ vertebra in adults.
• The spinal cord consists of cervical, thoracic, lumbar, sacral, and coccygeal regions.
• The cervical enlargement is located in the neck region.
• The lumbosacral enlargement is in the T10-T12 region.
• The conus medullaris is in the T12-L1 region.
• The cauda equina is below L1.
• The filum terminale becomes a component of the coccygeal ligament.
• The spinal cord has a posterior median sulcus and an anterior median fissure.
• There are 31 spinal segments.
• Each spinal segment includes a dorsal root, dorsal root ganglion, ventral root, and spinal nerve.

Gross Anatomy of the Spinal Nerves

• Spinal nerves are classified as mixed nerves.
• They include both sensory and motor neurons.
• Sensory neurons (afferent neurons) transmit impulses toward the spinal cord.
• Motor neurons (efferent neurons) transmit impulses away from the spinal cord.

Spinal Meninges

• There are 3 layers of membrane (meninges) that cover the spinal cord.
• The meninges are the dura mater, arachnoid mater, and pia mater.
• These membranes provide protection, physical stability, and shock absorption.
• The spinal meninges are continuous with the cranial (cerebral) meninges and have the same names.

Spinal Meninges - Dura Mater

• The dura mater is the tough, fibrous outermost layer of the meninges.
• It stabilizes the spinal cord within the vertebral canal.
• Cranial and sacral attachments stabilize the longitudinal axis of the spinal cord.
• The dura mater is translucent, whitish, and resembles milk jug plastic.

Spinal Meninges - Arachnoid Mater

• The arachnoid mater is the middle meningeal layer.
• It has a spiderweb-like appearance and is very fragile.
• It is separated from the pia mater by the subarachnoid space where cerebrospinal fluid flows.
• Arachnoid granulations are small areas of the arachnoid that permeate the dura mater.
• Arachnoid trabeculae are spiderweb-like strands extending from the arachnoid to the outer layer of the pia mater.

Spinal Meninges - Pia Mater

• The pia mater is the deepest meningeal layer.
• Blood vessels are found in this layer.
• It is firmly bound to the brain and spinal cord tissue.
• Looks very thin and clear.

Sectional Anatomy of the Spinal Cord

• Gray matter is in the center of the spinal cord and includes the central canal.
• It includes somas (cell bodies) and glial cells (supporting cells).
• White matter is on the outside of the spinal cord.
• White matter consists of axons wrapped in myelin.
• Nerves in the white matter are organized into tracts or columns.

Sectional Anatomy of the Spinal Cord - Organization of Gray Matter

• Somas in the gray matter are organized into sensory and motor nuclei.
• The transverse view shows posterior gray horns, lateral gray horns, and anterior gray horns.
• Posterior gray horns contain somatic sensory and visceral nuclei.
• Lateral gray horns contain visceral motor nuclei.
• Anterior gray horns contain somatic motor nuclei.
• The gray commissure consists of axons crossing from one side to the other.

Sectional Anatomy of the Spinal Cord - Organization of White Matter

• White matter consists of columns of nerves.
• These columns include the posterior, anterior, and lateral white columns.
• Columns contain sensory tracts (ascending) that carry sensory information toward the brain.
• They also contain motor tracts (descending) that carry motor commands into the spinal cord.

Spinal Cord Injuries

• The effect of spinal injury will differ based on the level of injury.
• Injuries at C1-C2 can cause paralysis of the muscles used for breathing.
• Injuries at C5-C6 can cause paralysis of the legs.
• Injuries at C7-C8 can cause paralysis of legs, elbows, hands and trunk.
• Injuries at all levels can result in loss of bladder and bowel control

Spinal Nerves

• There are 31 pairs of spinal nerves.
• Each pair emerges from one segment of the spinal cord.
• There are 8 cervical nerves.
• The first cervical nerve comes out above C1.
• Cervical nerves are named for the vertebrae below them.
• There is a transition from cervical to thoracic nerves between the C7 and T1 vertebrae where spinal nerve C8 is located.
• The rest of the spinal nerves are named for the vertebra above them.
• There are 12 thoracic nerves, 5 lumbar nerves, and 5 sacral nerves.
• Each peripheral nerve consists of epineurium, perineurium, and endoneurium.
• Epineurium is the outer layer of connective tissue and becomes continuous with the dura mater.
• Perineurium is the layer of CT surrounding a fascicle, which is a bundle of axons.
• Endoneurium is the layer of CT surrounding a single axon.
• Distal to the convergence of the nerve roots, the spinal nerves diverge into two branches (rami).
• These branches are the dorsal ramus and ventral ramus.
• The dorsal ramus innervates muscles of the neck and back.
• The ventral ramus innervates muscles of the limbs.
• Spinal nerves T₁ to L₂ have two additional branches (white ramus).
• Motor impulses leave the spinal cord via the ventral nerve root.
• Sensory information enters the spinal cord via the dorsal nerve root.
• It is important to know the difference between a nerve root and a nerve ramus.

Spinal Nerves - Dermatomes

• Each pair of spinal nerves monitors specific surface areas called dermatomes.
• Dermatomes are clinically important areas regarding surgery.
• Lack of sensation in a dermatome after injury, illness, or surgery indicates damage to a specific spinal segment.
• Shingles (Varicella-zoster virus) tends to follow a single dermatome on one side of the body.

Nerve Plexuses

• There are four nerve plexuses: cervical, brachial, lumbar, and sacral.
• Sometimes the lumbar and sacral plexuses are combined to form the lumbosacral plexus.
• The cervical plexus (C₁-C₅) consists of cutaneous and muscular branches that innervate the head, neck, and diaphragm.
• The brachial plexus (C5-T1) innervates the pectoral girdle and upper limbs.
• The ventral rami emerging from C5 to T₁ form the trunks (superior, middle, and inferior).
• The trunks form the divisions (anterior and posterior).
• The divisions form the cords (posterior, lateral, and medial).
• The cords form the spinal nerves (radial, median, ulnar, etc.).
• The lateral cord extends to form the musculocutaneous nerve.
• The lateral and medial cords extend to form the median nerve.
• The medial cord extends to form the ulnar nerve.
• The posterior cord branches to form the radial and axillary nerves.
• Carpal tunnel syndrome is caused by median nerve entrapment.
• The lumbar and sacral plexuses (T12-S4) are also called the lumbosacral plexus.
• The lumbar plexus arises from L₁ to L4.
• The sacral plexus arises from L₄ to S4.
• Femoral nerve is the largest nerve in the lumbar plexus
• Sciatic nerve is largest of the body, is formed from lumbar and sacral and it Branches to form the common fibular nerve and the tibial nerve

Nerve Plexus Summary

• 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 Co.
• 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.

Reflexes

• Reflexes are an immediate involuntary motor response.
• The Reflex Arc is the neural “wiring” of a single reflex.
• It begins at a sensory receptor and ends at a peripheral receptor.
• Pathway of a Reflex Arc:

1. Activation of a sensory receptor
2. Relay of information to the CNS
3. Information processing (one or more synapses)
4. Activation of a motor neuron
5. Response by the effector

• 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), and the complexity of the neural circuit (monosynaptic or polysynaptic).
• Innate/Primitive/Infantile Reflexes:
• Babinski reflex
• Rooting reflex
• Sucking relex
• Moro or startle reflex
• Stepping reflex
• They are all normal in newborns, but if they appear in adults, they indicate brain damage.
• Spinal reflexes can be monosynaptic or polysynaptic.
• Monosynaptic reflexes involve a single segment of the spinal cord.
• Polysynaptic reflexes integrate motor output from several spinal cord segments.
• Spinal Reflexes example, Stretch reflex (example: patellar 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

Chapter 14 Summary

• The chapter covered the structure and functions of the spinal cord.
• It described the spinal meninges, their structure, and function.
• It discussed the structure and location of gray and white matter and their roles in processing and relaying information.
• The chapter identified the regional groups of spinal nerves.
• It defined the term nerve plexus and located and compared the four main spinal nerve plexuses.