Chapter Notes on General Senses and Spinal Cord

Learning Outcomes

  • Describe the structures and functions of the spinal cord

  • Describe the major sensory and motor tracts of the spinal cord

  • Describe the structures and functions of the spinal nerves

  • Describe the functions of the peripheral nerves of the nerve plexuses

  • Define sensation, types of general senses, and their receptors

  • Describe the components of a reflex arc and different types of reflexes

  • Understand spinal cord injury and define different types of neurological exams

Spinal Cord Anatomy

  • The spinal cord and brain together form the Central Nervous System (CNS).

  • Enclosed by three protective layers known as the meninges:

    • Pia mater: Delicate inner layer.

    • Arachnoid mater: Spider-web-like middle layer.

    • Dura mater: Tough outer layer.

  • The spinal cord ends at the conus medullaris, typically at the L1/L2 level in adults, and L3/L4 in infants.

  • The spinal cord is composed of both gray and white matter:

    • Gray matter: Contains neuronal cell bodies, organized in a butterfly shape.

    • White matter: Composed of myelinated axons, organizing into ascending and descending tracts.

Reflex Arcs

  • Reflexes: Involuntary and quick muscle responses to stimuli, essential for protective and postural adjustments.

  • Components of a reflex arc include:

    1. Sensory Receptor: Detects stimulus (e.g., pain).

    2. Sensory Neuron: Transmits signal to spinal cord.

    3. Integration Center: Involves interneurons in the spinal cord.

    4. Motor Neuron: Sends signal to the effector.

    5. Effector: Muscle or gland that responds.

  • Types of reflexes:

    • Monosynaptic reflex: only one synapse (e.g., patellar reflex).

    • Polysynaptic reflex: involves one or more interneurons (e.g., withdrawal reflex).

Sensory and Motor Pathways

  • Ascending tracts: Carry sensory information to the brain (e.g., spinothalamic tract for pain and temperature).

  • Descending tracts: Carry motor commands from the brain to the muscles (e.g., corticospinal tracts).

  • Neurons involved in these pathways include:

    • First-order neurons: Detect stimuli and connect to the spinal cord.

    • Second-order neurons: Synapse with first-order neurons and decussate (cross over);

    • Third-order neurons: Convey information from the thalamus to the sensory cortex.

Spinal Nerves and Plexuses

  • The spinal cord gives rise to 31 pairs of spinal nerves:

    • Cervical (8), Thoracic (12), Lumbar (5), Sacral (5), Coccygeal (1).

  • Each spinal nerve is a mixed nerve (contains both sensory and motor fibers).

  • Major plexuses:

    • Cervical plexus: Innervates head and neck (e.g., phrenic nerve to diaphragm).

    • Brachial plexus: Supplies arms (e.g., radial and median nerves).

    • Lumbar plexus: Supplies anterior leg (e.g., femoral nerve).

    • Sacral plexus: Supplies posterior leg (e.g., sciatic nerve).

Sensation and Perception

  • Sensation: Activation of sensory receptors that transduce stimuli into neural signals.

  • Perception: The brain's interpretation of sensory input.

  • Types of sensations include:

    • Somatosensation: Touch, pain, temperature, proprioception.

    • Special senses: Vision, hearing, taste, smell, balance.

Reflex Testing and Neurological Exams

  • Neurological exams assess sensory and motor functions of the spinal cord.

  • Common tests include:

    • Deep tendon reflexes: e.g., patellar reflex.

    • Two-point discrimination: Tests sensory thresholds.

    • Romberg test: Tests balance and proprioception.

  • Spinal cord injury can lead to varying degrees of paralysis including:

    • Paraplegia: Lower limb paralysis.

    • Quadriplegia: Paralysis of all four limbs.

    • Paresis: Partial weakness of a limb.

Clinical Applications

  • Spina bifida: A developmental defect where the spinal cord may be exposed due to improper closure of the vertebral arch.

  • Shingles: Reactivation of varicella-zoster virus affecting sensory neurons leading to painful rashes.

  • Pain management strategies include medications like aspirin which inhibit pain signaling pathways.

Important Sensory Receptors

  • Mechanoreceptors: Detect mechanical stimuli (touch, pressure).

    • Nociceptors: Detect pain (activated by harmful stimuli).

    • Thermoreceptors: Detect temperature changes.

    • Proprioceptors: Sense body position and movement.

Sensory Adaptation

  • Tonic receptors: Adapt slowly and continue signaling the presence of a stimulus.

  • Phasic receptors: Adapt quickly and signal changes or the start/end of a stimulus.

  • Example: The sensation of water temperature when initially entering cold water vs. after some time.