Study Guide: The Spinal Cord, Sensory Receptors, and Reflexes

The Spinal Cord

Structure and Function of the Spinal Cord

The spinal cord is a vital part of the central nervous system, serving as a conduit for information between the brain and the rest of the body. It is protected by several layers and contains both gray and white matter, each with distinct roles.

• Spinal Cord Extent: Begins at the foramen magnum of the skull and ends around the level of the first or second lumbar vertebra (L1 or L2).

• Function: Provides two-way communication between the brain and the peripheral nervous system.

• Protective Layers: The meninges (dura mater, arachnoid mater, pia mater) surround and protect the spinal cord.

• Subarachnoid Space: Filled with cerebrospinal fluid (CSF), which cushions and nourishes the spinal cord.

• Conus Medullaris: The cone-shaped structure marking the end of the spinal cord; the filum terminale anchors the cord to the coccyx.

• Denticulate Ligaments: Extensions of the pia mater that secure the spinal cord laterally.

• Cauda Equina: A bundle of nerve roots at the lower end of the spinal cord, resembling a horse's tail.

• Central Canal: Runs the length of the spinal cord and contains cerebrospinal fluid.

• Gray Matter: Shaped like a butterfly or the letter 'H', contains neuron cell bodies, dendrites, and unmyelinated axons.

• Dorsal Horns: Contain interneurons that receive sensory input.

• White Matter: Composed mainly of myelinated and unmyelinated fibers organized into ascending (sensory) and descending (motor) tracts.

• Ascending Tracts: Carry sensory information to the brain; descending tracts carry motor signals away from the brain.

Sensory Receptors & Processing

Types of Sensory Receptors

Sensory receptors are specialized to detect specific types of stimuli and are classified based on the nature of the stimulus they detect.

• Mechanoreceptors: Detect mechanical forces (e.g., touch, pressure, vibration).

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Detect light (primarily in the eyes).

• Chemoreceptors: Detect chemical changes (e.g., taste, smell, blood chemistry).

• Nociceptors: Detect pain from potentially damaging stimuli.

Receptor Location and Function

• Exteroceptors: Detect stimuli from the external environment (e.g., skin, special senses).

• Interoceptors: Detect stimuli from internal organs (e.g., stretch, chemical changes).

• Proprioceptors: Provide information about body position and movement (e.g., in muscles, tendons, joints).

Levels of Sensory Processing

• Receptor Level: Sensory receptors transduce stimuli into electrical signals.

• Circuit Level: Processing in ascending pathways to the brain.

• Perceptual Level: Interpretation of sensory input in the cerebral cortex.

Adaptation of Sensory Receptors

• Phasic Receptors: Adapt quickly to stimuli (e.g., pressure, touch).

• Tonic Receptors: Adapt slowly or not at all (e.g., pain, proprioceptors).

Nerves, Plexuses, and Reflexes

Spinal Nerves and Plexuses

There are 31 pairs of spinal nerves, each serving a specific region of the body. These nerves are grouped into regions and form complex networks called plexuses.

Region	Number of Pairs
Cervical	8
Thoracic	12
Lumbar	5
Sacral	5
Coccygeal	1

• Plexuses: Networks of nerves formed by the ventral rami of spinal nerves. Major plexuses include cervical, brachial, lumbar, and sacral.

• Phrenic Nerve: Originates from the cervical plexus (C3–C5) and controls the diaphragm, essential for breathing.

Reflexes

Reflexes are rapid, automatic responses to stimuli. They are essential for protection and maintaining homeostasis.

• Components of a Reflex Arc:

  1. Receptor

  2. Sensory neuron

  3. Integration center

  4. Motor neuron

  5. Effector

• Example: The stretch reflex (e.g., patellar reflex) helps maintain muscle tone and posture by causing a muscle to contract in response to being stretched.

Dermatomes

A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve. Mapping dermatomes is clinically important for diagnosing nerve injuries.

Structure of a Spinal Nerve

• Composed of two branches: dorsal root (sensory) and ventral root (motor).

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