The Spinal Cord

Overview and Structure

The spinal cord is a cylindrical structure of nervous tissue that extends from the lower end of the brain and is surrounded by the vertebral column. It serves as a major pathway for transmitting information between the brain and the rest of the body.

• Length: Approximately 44 cm in adults

• Diameter: Ranges from 1 cm to 1.4 cm

• Location: Continuous with the lower end of the brain

• Function: Transmits sensory and motor information between the brain and peripheral nerves

Spinal Nerves

Branching and Classification

Spinal nerves branch off the spinal cord at regular intervals, with a total of 31 pairs. Each pair exits the vertebral column between adjacent vertebrae and is named according to its region of emergence.

• Cervical nerves (C1–C8): 8 pairs; emerge from the cervical region of the neck

• Thoracic nerves (T1–T12): 12 pairs; emerge from the thoracic region

• Lumbar nerves (L1–L5): 5 pairs; emerge from the lumbar region

• Sacral nerves (S1–S5): 5 pairs; emerge from the sacral region

• Coccygeal nerve: 1 pair; emerges from the tip of the coccyx

Spinal nerves are classified as mixed nerves because they contain both sensory and motor axons.

Spinal Nerve Roots

Each spinal nerve is formed by the union of two roots:

• Dorsal root: Contains sensory axons entering the spinal cord

• Ventral root: Contains motor axons exiting the spinal cord

These roots join to form the mixed spinal nerve, which then branches into dorsal rami and ventral rami that supply different regions of the body.

Spinal Cord Gray and White Matter

Organization and Function

The spinal cord is organized into gray matter and white matter:

• Gray matter: Concentrated within a butterfly-shaped region in the center of the spinal cord; contains neuron cell bodies, dendrites, and synapses

• White matter: Surrounds the gray matter; consists of myelinated axons organized into tracts

Gray Matter Horns

• Dorsal (posterior) horn: Contains sensory neurons; receives information from the periphery

• Ventral (anterior) horn: Contains motor neurons; sends information to muscles and glands

• Lateral horn: Present in thoracic and upper lumbar regions; contains neurons of the autonomic nervous system

White Matter Tracts

White matter is organized into ascending tracts and descending tracts:

• Ascending tracts: Transmit sensory information from the spinal cord to the brain

• Descending tracts: Transmit motor commands from the brain to the spinal cord

Tracts are further classified based on their function and direction of information flow.

Functional Pathways and Signal Transmission

Signal Transmission in the Spinal Cord

Signals from the brain travel along descending tracts to effectors, while signals from sensory receptors travel along ascending tracts to the brain. For example, to wiggle a finger, the brain sends a motor command down the spinal cord to the appropriate spinal nerve.

Pain Modulation and Analgesic Systems

The body has analgesic systems that block the perception of pain during periods of stress. These systems work by blocking synaptic transmission between pain-transmitting afferent neurons and interneurons in the spinal cord.

• Example: Endorphins released during stress can inhibit pain transmission, preventing the perception of pain.

Contralateral Pathways

Most ascending and descending tracts cross over to the opposite side of the CNS, a phenomenon called contralateral transmission. This means that sensory input from the right side of the body is processed by the left side of the brain, and vice versa for motor control.

Table: Classification of Spinal Nerves

Key Terms and Definitions

• Spinal cord: Main pathway for information between brain and body

• Spinal nerves: Mixed nerves carrying sensory and motor information

• Gray matter: Region containing neuron cell bodies and synapses

• White matter: Region containing myelinated axons organized into tracts

• Dorsal root: Sensory axons entering the spinal cord

• Ventral root: Motor axons exiting the spinal cord

• Contralateral: Pathways crossing to the opposite side of the CNS

• Analgesic systems: Biological mechanisms that block pain perception

Relevant Equations

• Nerve conduction velocity: where is velocity, is distance, and is time.

Examples and Applications

• Example: When you touch a hot object, sensory neurons transmit the signal to the spinal cord, which then relays the information to the brain for perception and initiates a motor response to withdraw your hand.

• Application: Understanding spinal cord pathways is essential for diagnosing and treating neurological disorders such as spinal cord injuries and multiple sclerosis.

Additional info: Academic context and definitions have been expanded for clarity and completeness.