Chapter11

Nervous System: Spinal Cord and Peripheral Nerves

Objectives

• Anatomy of the Spinal Cord and its Functions

  • Describe the anatomy of the spinal cord.

  • List the three main functions of the spinal cord:

    • Sensory pathway

    • Motor pathway

    • Reflex center

• Peripheral Nervous System

  • Discuss reflexes and identify components of the reflex arc.

  • List and describe the functions of the 12 pairs of cranial nerves.

Key Terms

• Spinal Nerves: Classification and functions of spinal nerves and plexuses.

  • Major Plexuses: Cervical, brachial, and lumbosacral.

• Dermatome: An area of skin supplied by sensory fibers of a single spinal nerve.

• Functional Classification of the Peripheral Nervous System:

  • Afferent nerves (sensory)

  • Efferent nerves (motor)

Overview of the Nervous System

• The brain, spinal cord, and peripheral nervous system work together to communicate and coordinate bodily functions.

• The spinal cord has three primary functions:

  • Sensory Pathway: Transmits sensory information from the body to the brain.

  • Motor Pathway: Sends motor commands from the brain to the body.

  • Reflex Center: Processes reflex actions that do not involve direct brain activity.

Anatomy of the Spinal Cord

• Location and Size:

  • Continuation of the brain stem, about 17 inches long, occupies the spinal cavity, and ends at the first lumbar vertebra (L1).

  • Protected by vertebrae, meninges, cerebrospinal fluid (CSF), and the blood-brain barrier.

  • Differences in length between infants and adults due to varying growth rates of the vertebral column and spinal cord.

  • Lumbar Puncture: Procedure to extract CSF, performed between L3 and L4 to avoid injury to the spinal cord.

Structure of Spinal Cord

• Gray and White Matter:

  • Gray Matter: Butterfly-shaped central area containing cell bodies, interneurons, and synapses.

    • Dorsal Horn: Processes sensory information.

    • Ventral Horn: Contains motor neurons; integrates reflex responses.

    • Central Canal: Extends the length of the spinal cord, allows CSF flow.

  • White Matter: Composed of myelinated axons forming sensory (ascending tracts) and motor (descending tracts) pathways.

Functions of Spinal Cord Tracts

• Ascending Tracts: Carry sensory information to the brain, e.g., the spinothalamic tract conveys touch, pressure, and pain.

• Descending Tracts: Transmit motor information from the brain, e.g., the corticospinal tract directs voluntary muscle movement.

• Decussation: Many tracts cross over at the brain stem or spinal cord, leading to contralateral control (e.g., left brain controls right body).

Reflexes

• Definition and Types of Reflexes:

  • Reflex: An involuntary response to a stimulus, typically processed at the spinal cord level, such as the withdrawal reflex.

• Patellar Reflex (Knee-Jerk):

  • Example of a reflex action demonstrating the reflex arc components:

    1. Sensory Receptor: Detects stimulus.

    2. Afferent Neuron: Carries impulse to the spinal cord.

    3. Interneuron (if present): Integrates response.

    4. Efferent Neuron: Sends impulse to effector.

    5. Effector Organ: Performs response (muscle contraction).

• Importance of Reflexes: Protects body from harm, aids in balance, and regulates organ functions.

Peripheral Nervous System

• Components: Nerves and ganglia outside the central nervous system (CNS), including sensory (afferent) and motor (efferent) nerves.

• Types of Nerves:

  • Sensory Nerves: Transmit sensory information only.

  • Motor Nerves: Transmit motor commands only.

  • Mixed Nerves: Contain both afferent and efferent fibers (e.g., all spinal nerves are mixed).

Cranial Nerves

• Cranial Nerves (12 pairs): Connect the brain to the head, neck, and shoulders, with various specific functions including sensory, motor, and autonomic roles.

• Naming and Functions: Each cranial nerve has a Roman numeral and a name; functions include sensation (sight, smell, taste), motor control of muscles, and regulating gland secretions.

• Key Reflexes Involving Cranial Nerves: Gag reflex, pupillary reflex, and others demonstrating the CNS's interaction with peripheral receptors.

Clinical Insights

• Spinal and Cranial Nerve Damage: Damage results in specific losses of function, e.g., paralysis (quadriplegia or paraplegia) depending on the level and extent of injury.

• Dermatomes: Clinical significance in assessing nerve injuries based on sensory loss across skin areas.

• Nerve Plexuses: Networks of nerves that distribute and integrate the fibers supplying specific body areas, with major groups being cervical, brachial, and lumbosacral.

Summary

• The spinal cord is central to transmitting information between the brain and body while enabling reflex responses. The peripheral nervous system further facilitates this communication through a network of cranial and spinal nerves. Understanding both the anatomy and function of these systems is crucial for diagnosing and treating neurological disorders.