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 the letter 'H' or a butterfly; 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 commands away from the brain.

Example: The spinothalamic tract is an ascending pathway that transmits pain and temperature sensations to 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 such as touch, pressure, vibration, and stretch.

• Thermoreceptors: Respond to changes in temperature.

• Photoreceptors: Detect light (primarily found in the retina of the eye).

• Chemoreceptors: Respond to chemical stimuli (e.g., taste, smell, changes in blood chemistry).

• Nociceptors: Detect pain caused by 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 (found 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

A reflex is a rapid, automatic response to a stimulus. Reflexes help protect the body and maintain homeostasis.

• Components of a Reflex Arc:

  1. Receptor

  2. Sensory neuron

  3. Integration center

  4. Motor neuron

  5. Effector

• Example: The patellar (knee-jerk) reflex helps maintain posture by causing the quadriceps muscle to contract in response to a tap on the patellar tendon.

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.

Additional info: The study of the spinal cord, sensory receptors, and reflexes is essential for understanding how the nervous system integrates and responds to internal and external stimuli, maintaining body function and homeostasis.