The Spinal Cord, Spinal Nerves, and Sensory Pathways

THE SPINAL CORD, SPINAL NERVES, AND SENSORY PATHWAYS PART 1

PROTECTIVE STRUCTURES

• Vertebral Column: Provides hard, protective casing for the spinal cord.
• Meninges: Three protective membranes that cover, enclose, and protect the CNS and its blood supply. They also contain cerebrospinal fluid (CSF).
• Cerebrospinal Fluid (CSF): Acts as a watery cushion, providing buoyancy and protection to the brain and spinal cord.

THE MENINGES

• Spinal Meninges: Enclose the spinal cord; consist of several layers:
  • Epidural Space: Area between the vertebral column and dura mater, serves as a cushion.
  • Dura Mater: The outermost layer made of dense irregular connective tissue, continuous with the epineurium of cranial and spinal nerves starting at the foramen magnum.
  • Arachnoid Mater: Middle layer with a webbed appearance due to collagen and elastin, contains the subdural space above it.
  • Subarachnoid Space: Contains cerebrospinal fluid that flows around the spinal cord.
  • Pia Mater: The thin inner layer, allows blood vessels to reach spinal cord tissues and contains denticulate ligaments that suspend the spinal cord and protect it from displacement.

EXTERNAL ANATOMY

• Functions:

  • Main site for motor/sensory innervation through spinal nerves.
  • Acts as a two-way conduction pathway connecting body and brain.
  • Major integration center for reflexes in the gray matter.

• Location: Runs through the vertebral canal, from the foramen magnum to L1 or L2.

• Structures:

  • Conus Medullaris: The tapered inferior end of the spinal cord.
  • Filum Terminale: A long filament that anchors the spinal cord to the coccyx.
  • Cervical and Lumbar Enlargements: Areas where nerves for upper and lower limbs arise.
  • Cauda Equina: A collection of spinal nerve roots below the spinal cord.
  • Median Sulci: Posterior and anterior median sulci divide the spinal cord into right and left sections.

WHITE MATTER – SPINAL CORD

• Composition: The outer region made up of myelinated and nonmyelinated axons, facilitating communication between the brain and spinal cord.
• Funiculi: Continuous tracts of white matter divided by the medial gray matter;
  • Anterior and lateral are continuous,
  • Dorsal is separated by the dorsal horn.

CELLS IN WHITE MATTER

• Ascending Fibers: Carry sensory information to the brain.
• Descending Fibers: Carry motor commands to effectors from the brain.
• Commissural Fibers: Transmit information between the left and right sides of the spinal cord.

GRAY MATTER – SPINAL CORD

• Contains neuron cell bodies, nonmyelinated axons, and neuroglia; major components include:
  • Gray Commissure: Unmyelinated axons crossing from right to left.
  • Central Canal: Narrow cavity that runs through the center of the spinal cord.
  • Dorsal (Posterior) Horns: Contain interneurons receiving sensory input from dorsal root ganglion neurons.
  • Ventral (Anterior) Horns: Contain interneurons and motor neuron cell bodies sending axons via ventral roots.

CONCEPT CHECK

• What are the main protective mechanisms of the spinal cord?
• Identify the main structures of the spinal cord.
• Relate the organization of both white and gray matter to a cross-section of the spinal cord.

SOMATIC SENSORY AND MOTOR MAP

• Frontal Plane through Postcentral Gyrus: Maps sensory input from different body parts to the primary somatosensory cortex.
• Frontal Plane through Precentral Gyrus: Maps motor output from different body parts to the primary motor cortex.

SOMATIC SENSORY PATHWAYS

• Posterior Column – Medial Lemnicus Pathway:

  • First Order Neurons: Extend from receptors to the medulla oblongata.
  • Second Order Neurons: Decussate before reaching the thalamus, transmitting fine touch, proprioception, and kinesthesia.

• Anterolateral – Spinothalamic Pathway:

  • First Order Neurons: Reside in dorsal root ganglion.
  • Second Order Neurons: Via posterior gray horns; lateral pathway carries pain/temperature, anterior is for touch/pressure.
  • Third Order Neurons: Project to somatosensory cortex.

SOMATIC MOTOR PATHWAYS

• Lower Motor Neurons (Final Common Pathway):
  • Converge signals from all motor neurons, residing in the brainstem and spinal cord, sending axons to effectors.
• Direct Motor Pathways (Pyramidal Pathways):
  • Information goes directly from cerebrum to lower motor neurons.
• Indirect Motor Pathways:
  • Information relayed from brain stem to lower motor neurons, involved in more integrative control.

INTEGUMENT INNERVATION

• Dermatome: Skin area innervated by a single spinal nerve.
• Patterns vary in trunk (horizontal) and limbs (brachial plexus for upper limbs, lumbar/sacral nerves for lower limbs).

CONCEPT CHECK

• Describe the pathway of efferent and afferent information through dorsal and ventral spinal nerves.
• Outline posterior column-medial lemniscus and anteriolateral pathways.
• Identify and define neuron types in the somatic motor pathway.
• Compare direct and indirect motor pathways in terms of location and function.

THE SPINAL CORD, SPINAL NERVES, AND SENSORY PATHWAYS 3

NERVE STRUCTURE

• Nerve Fiber: Axon and surrounding glial cells.
• Endoneurium: Loose connective tissue surrounding each nerve fiber.
• Perineurium: Thicker connective tissue encompassing nerve fibers, forming fascicles.
• Epineurium: Outermost layer encapsulating entire nerve, offering strength.

NERVE PLEXUSES

• Nerve Plexus: Networks formed by ventral rami (excluding T2-T12).
• Hilton’s Law: A nerve innervating a muscle that moves a joint also innervates the joint and skin around it.

SPINAL NERVES

• 31 Pairs of Spinal Nerves: Connect to the spinal cord, named based on their exit point.
  • 8 cervical (C1-C8)
  • 12 thoracic (T1-T12)
  • 5 lumbar (L1-L5)
  • 5 sacral (S1-S5)
  • 1 coccygeal (Co1)

CERVICAL PLEXUS

• Formed from C1-C4, deep to sternocleidomastoid muscle.
• Phrenic Nerve: Key nerve innervating the diaphragm, essential for breathing.

BRACHIAL PLEXUS

• Formed by ventral rami of C5-C8, located in neck and axilla.
• Major Nerves:
  • Musculocutaneous, Median, Ulnar, Radial, Axillary:
  • Innervate various muscles in the upper limb and provide sensory innervation to the skin.

LUMBAR PLEXUS

• Arises from L1 to L4, primarily supplying anterior thigh:
  • Femoral Nerve: Main branch; innervates quadriceps and medial leg surface.
  • Obturator Nerve: Passes through the obturator foramen, innervating adductor muscles.

SACRAL PLEXUS

• Arises from L4-S4, often considered with lumbar plexus.
• Major Nerves:
  • Sciatic Nerve: Largest nerve (comprises tibial and common fibular nerves), innervating the posterior lower limb.

CONCEPT CHECK

• Describe pathways for afferent/efferent info in spinal nerves.
• Identify major nerves in each of the nerve plexuses.
• Effects of injuries to each major nerve.
• Role of dermatomes in diagnosing neuropathy.

THE SPINAL CORD, SPINAL NERVES, AND SENSORY PATHWAYS PART 4

PATHWAYS IN THE CNS

• Ascending Pathways: Carry sensory info to the brain.
• Descending Pathways: Carry motor commands from the brain.

ASCENDING PATHWAYS

1. Spinocerebellar Pathway:
   • Carries proprioceptive input; terminates at the cerebellum.
2. Dorsal Column–Medial Lemniscal Pathway:
   • Carries fine touch and proprioceptive sensations to thalamus and cerebral cortex.
3. Spinothalamic Tract:
   • Hedonistic sensations (pain, temperature) are transmitted to the somatosensory cortex.

DESCENDING PATHWAYS

1. Direct (Pyramidal) Pathways:
   • Connect directly from pyramidal cells to lower motor neurons; responsible for precise voluntary movements.
2. Indirect Pathways:
   • Arise from subcortical motor nuclei; primarily control subconscious movements.

REFLEX ARCS

• Definition: Chains of neurons that mediate reflex behaviors, fundamental for understanding the nervous system.
• Reflexes can be simple (spinal) or complex (cranial), and can involve somatic or autonomic responses.
• Components:
  • Sensory receptor, sensory neuron, integration center, motor neuron, effector.

CONCEPT CHECK

• Define a stereotypical reflex arc.
• Differentiate between types of reflexes (e.g., monosynaptic, polysynaptic).
• Compare ascending vs. descending pathways.

REFLEXES

MONOSYNAPTIC REFLEX

• Simplest type, involves a single synapse.
• Example: Knee-jerk reflex.

POLYSYNAPTIC REFLEX

• Involves one or more interneurons in the pathway.
• Example: Withdrawal reflex.

DISORDERS OF THE PNS

• Conditions such as shingles, migraines, and peripheral neuropathy affect sensory and motor functions.
• Symptoms: Vary between sensations (numbness, tingling) and muscle weakness.
• Causes: Include trauma, systemic disorders (HIV, diabetes), and vitamin deficiencies.

DISORDERS ASSOCIATED WITH SPINAL CORD DAMAGE

• Paraplegia: Loss of function in lower limbs.
• Quadriplegia: Loss of function in all four limbs.

CONGENITAL MALFORMATIONS

• Examples include hydrocephalus, spina bifida, and cerebral palsy, which result in various functional deficits.

EMBRYOLOGY OF THE PNS

• Spinal nerves develop in the fourth week of gestation; structural and functional changes continue into later development stages.

CONCEPT CHECK

• Name major PNS disorders.
• Explain how the PNS evolves over time.