Spinal Cord & Spinal Nerves
Chapter 13: Spinal Cord & Spinal Nerves
Objectives
Identify and locate the layers of the meninges and the spaces associated with the meninges.
Discuss the location and structure of the spinal cord.
List the functions of the ascending and descending pathways in the spinal cord, using examples.
Discuss the structural and functional components of two and three-neuron reflex arcs.
Explain the structure of the spinal nerves.
Functions of the Spinal Cord
Conduction:
Nerve fibers conduct sensory and motor information up and down the spinal cord.
Neural Integration:
Spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (e.g., bladder control).
Locomotion:
The spinal cord contains groups of neurons that coordinate repetitive sequences of contractions necessary for walking.
Reflexes:
Involuntary responses to stimuli that are vital for posture, coordination, protection.
Surface Anatomy
General Structure:
The spinal cord is a cylinder of nervous tissue that continues from the brainstem at the foramen magnum of the skull.
The inferior margin ends at the L1 vertebra or slightly beyond.
The spinal cord gives rise to 31 pairs of spinal nerves.
Longitudinal Grooves:
Present on anterior and posterior sides, including:
Anterior Median Fissure
Posterior Median Sulcus
Regional Division:
The spinal cord is divided into cervical, thoracic, lumbar, sacral regions, named for where spinal nerves exit vertebrae.
Enlargements:
Two areas of the cord are thicker than others:
Cervical Enlargement: Nerves to the upper limb.
Lumbar Enlargement: Nerves to the pelvic region and lower limbs.
Structures:
Medullary Cone (Conus Medullaris):
The point where the cord tapers below the lumbar enlargement.
Cauda Equina:
A bundle of nerve roots that occupy the vertebral canal from L2 to S5.
Meninges of Spinal Cord
Meninges:
Three fibrous membranes that enclose the brain and spinal cord. From superficial to deep:
Dura Mater: Outer layer.
Arachnoid Mater: Middle layer; adheres to the dura mater and is separated from the pia mater by fibrous strands spanning the subarachnoid space, which is filled with cerebrospinal fluid (CSF).
Pia Mater: Delicate inner layer that follows the contours of the spinal cord.
Dural Sheath:
Surrounds the spinal cord and is separated from the vertebrae by the epidural space.
Lumbar Puncture (Spinal Tap):
A procedure that takes a sample of CSF from the lumbar region.
Cross-Sectional Anatomy
Gray Matter:
Central area shaped like a butterfly, surrounded by white matter in three columns.
Composed of neuron cell bodies with little myelin, serving as a site of information processing and synaptic integration.
White Matter:
Myelinated axons that carry signals between different parts of the central nervous system (CNS).
Columns: Three pairs of white matter bundles on each side of the cord:
Posterior (Dorsal) Column
Lateral Column
Anterior (Ventral) Column
Tracts: Subdivisions of each column that contain fibers with similar origin, destination, and function.
Spinal Tracts
Ascending Tracts:
Carry sensory information up the spinal cord.
Involve three neurons:
First-order Neurons: Detect stimulus and transmit signals to the spinal cord or brainstem.
Second-order Neurons: Continue to the thalamus.
Third-order Neurons: Carry signals to the sensory region of the cerebral cortex.
Descending Tracts:
Carry motor signals down from the brainstem through the spinal cord.
Involve two motor neurons:
Upper Motor Neuron: Originates in the cerebral cortex or brainstem, terminating at a lower motor neuron.
Lower Motor Neuron: Its soma is located in the brainstem or spinal cord, and its axon leads to the target organ.
Decussation:
Refers to tracts that cross at the midline, such that the brain senses and controls the contralateral (opposite) side of the body.
Contralateral: A term describing when the origin and destination of a tract are on opposite sides of the body.
Ipsilateral: Describes when the origin and destination of a tract are on the same side of the body, indicating no decussation has occurred.
General Anatomy of Nerves and Ganglia
Nerve Structure:
A nerve is a cord-like organ composed of numerous nerve fibers (axons) bound together by connective tissue.
Mixed Nerves: Contain both afferent (sensory) and efferent (motor) fibers.
Surrounding Tissues:
Neurilemma: Formed by Schwann cells surrounding nerve fibers of the peripheral nervous system.
Endoneurium: Loose connective tissue external to the neurilemma.
Perineurium: Tissue that wraps fascicles, or bundles of nerve fibers.
Epineurium: Dense irregular connective tissue that wraps the entire nerve.
Blood vessels penetrate these connective tissue coverings to provide necessary blood supply.
General Anatomy of Nerves and Ganglia - Continued
Nerve Types:
Sensory (Afferent) Nerves: Carry signals from sensory receptors to the CNS.
Motor (Efferent) Nerves: Carry signals from the CNS to muscles and glands.
Mixed Nerves: Consist of both afferent and efferent fibers.
Ganglia:
Clusters of neurosomas outside the CNS that are enveloped in endoneurium continuous with that of the nerve.
Bundles of nerve fibers lead into and out of the ganglion.
The posterior root ganglion is associated with spinal nerves.
Spinal Nerves
Total Number: 31 pairs of spinal nerves, classified as mixed nerves:
8 Cervical: C1–C8
12 Thoracic: T1–T12
5 Lumbar: L1–L5
5 Sacral: S1–S5
1 Coccygeal: Co1
Root Structure:
Each spinal nerve is formed from two roots:
Posterior (Dorsal) Root: Responsible for sensory input to the spinal cord.
Posterior (Dorsal) Root Ganglion: Contains neuronal cell bodies of sensory neurons.
Rootlets enter the posterior horn of the cord.
Anterior (Ventral) Root: Responsible for motor output from the spinal cord.
Rootlets leave the spinal cord and converge to form the anterior root.
Cauda Equina: Formed from roots arising from L2 to S5, which occupy the vertebral canal.
Distal Branches of Spinal Nerves
Beyond Vertebrae: The nerve divides into distal branches:
Anterior Ramus:
In the thoracic region, gives rise to intercostal nerve.
In other regions, forms nerve plexuses.
Posterior Ramus: Innervates muscles and joints in the vicinity of the spine and skin of the back.
Nerve Plexuses
Definition: Anterior rami branch and merge repeatedly, forming five nerve webs called nerve plexuses:
Cervical Plexus: C1 to C5; supplies the neck and a nerve to the diaphragm.
Brachial Plexus: C5 to T1; supplies the upper limb and parts of the shoulder and neck.
Lumbar Plexus: L1 to L4; supplies the abdominal wall, anterior thigh, and genitalia.
Sacral Plexus: L4, L5, and S1 to S4; supplies the lower trunk and lower limbs.
Coccygeal Plexus: S4, S5, Co1.
Relay Function:
Nerve plexuses serve as relay stations for directing sensory information to the brain and motor information from the brain to the correct area of the body.
Somatosensory Function: Transmits sensory signals from the bones, joints, muscles, and skin.
Proprioception: Brain receives information about body position and movements from nerve endings in the muscles, tendons, and joints.
Somatic Motor Function: Primarily involved in stimulating muscle contraction.
Cutaneous Innervation & Dermatomes
Dermatome: A specific area of skin that conveys sensory input to a particular spinal nerve.
Dermatome Map: A diagram showing regions of the skin innervated by each spinal nerve, with 50% overlap noted at the edges.
It is necessary to anesthetize three successive spinal nerves to produce a total loss of sensation in one dermatome.
Reflexes
Definition: Quick, involuntary, stereotyped reactions of glands or muscles to stimulation; require external stimulus.
Characteristics:
Not spontaneous actions but responses to sensory input.
Quick in execution, involving few interneurons and minimal synaptic delay.
Involuntary; occur without intent and are difficult to suppress.
Stereotyped; occur essentially the same way each time.
Pathway of a Somatic Reflex Arc
Somatic Receptors: Located in the skin, muscles, tendons.
Afferent (Sensory) Nerve Fibers: Carry information from receptors to the spinal cord’s posterior horn or brainstem.
Integrating Center: A synaptic contact point between neurons in the gray matter of the cord or brainstem, determining whether efferent neurons issue a signal to muscles.
Efferent (Motor) Nerve Fibers: Carry motor impulses to the muscles.
Effectors: Muscles that fulfill the response.
Two vs Three Neuron Reflex Arcs
Two Neuron Reflex Arc (Monosynaptic):
Involves only an afferent (sensory) and efferent (motor) neuron; synapse serves as the integration center.
Three Neuron Reflex Arc (Polysynaptic):
Involves an afferent neuron, an interneuron, and an efferent neuron.
Patellar Reflex Arc (Monosynaptic)
Testing the knee-jerk reflex involves:
Tapping the patellar ligament excites nerve endings of muscle spindle in the quadriceps femoris.
Stretch signals travel to the spinal cord via primary afferent fibers and dorsal roots.
A primary afferent neuron stimulates an alpha motor neuron in the spinal cord.
Efferent signals from the alpha motor nerve fiber stimulate the quadriceps to contract, producing a knee jerk response.
Simultaneously, a branch of the afferent nerve fiber stimulates an inhibitory motor neuron in the spinal cord.
The inhibitory motor neuron inhibits the alpha motor neuron that supplies the hamstring muscles.
Inhibition of hamstring contraction prevents opposing action against the quadriceps’ movement.
Stretch Reflex
Muscle Spindle: Stretch receptors embedded in skeletal muscles that inform the brain of muscle length and body movements, thus preventing overstretching.
Enables the brain to send motor commands back to muscles, controlling coordinated movement, corrective reflexes, muscle tone, posture.
Knee-Jerk (Patellar) Reflex: A monosynaptic reflex with one synapse between the afferent and efferent neurons.
Testing somatic reflexes can aid in diagnosing diseases.
Reciprocal Inhibition: A reflex phenomenon that inhibits antagonist muscles from contracting when the agonist is activated.
Plantar & Babinski Reflex
Plantar Reflex:
A reflex test where stroking the lateral portion of the foot elicits plantar flexion of the toes.
Babinski Reflex:
An abnormal response occurring after 1.5 years indicates a lesion in the corticospinal tract.
Crossed Extension Reflex
Definition: Contraction of extensor muscles in the limb opposite to the one that is withdrawn (flexor reflex) to maintain balance by extending the other leg.
Ipsilateral Reflex Arc: The stimulus and response occur on the same side.
Contralateral Reflex Arc: The input and output occur on opposite sides of the body.
Intersegmental Reflex: Occurs when input and output take place at different levels (segments) of the spinal cord.
Example: Pain in the foot leads to contraction of abdominal muscles.