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.

Example:

Damage to the spinal cord at the cervical level can result in quadriplegia, affecting all four limbs.

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: Respond to mechanical forces such as touch, pressure, vibration, and stretch.

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Respond to light (found in the retina of the eye).

• Chemoreceptors: Detect chemicals in solution (e.g., taste, smell, changes in blood chemistry).

• Nociceptors: Respond to potentially damaging stimuli that result in pain.

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: Processing in the brain, leading to conscious awareness of the stimulus.

Adaptation

• 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 plexuses that innervate limbs and other body parts.

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.

Branches of Spinal Nerves

• Each spinal nerve splits into two branches: the dorsal ramus and the ventral ramus.

Dermatomes

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

Reflexes

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

• Five Components of a Reflex Arc:

  1. Receptor

  2. Sensory neuron

  3. Integration center

  4. Motor neuron

  5. Effector

• Stretch Reflex Example: The patellar (knee-jerk) reflex helps maintain posture by causing the quadriceps muscle to contract in response to stretching.

Additional info: Reflexes can be somatic (involving skeletal muscle) or autonomic (involving smooth muscle, cardiac muscle, or glands).