In-Depth Notes on Neuroscience and Sensory Systems

Neurons Involved in Sensory and Motor Function

• Three Neuron Types: Sensory, Upper Motor, and Lower Motor
• Sensory neurons transmit information from receptors to the central nervous system (CNS).
• Upper Motor Neurons (UMN): Originate in the primary motor cortex and influence lower motor neurons.
• Lower Motor Neurons (LMN): Located in the spinal cord, they directly innervate muscles.

Pathways of Motor Control

• Motor Pathway Composition: Comprised of two main neurons (UMN and LMN) for motor function.
• Tracts:
• Lateral corticospinal tract primarily crosses at the medulla oblongata (90% of fibers).
• Ventral corticospinal tract crosses over at lower spinal cord levels.
• Cortical Control: Cerebral cortex sends motor commands via these tracts to muscle fibers.

Spinal Cord Trauma and Paralysis Types

• Trauma Effects: Damage cuts through the spinal cord, leading to loss of motor and sensory function below the injury level.
• Types of Paralysis:
• Flaccid Paralysis: Loss of muscle tone; associated with lower motor neuron damage (e.g., ventral root damage).
• Spastic Paralysis: Muscle tone increased, reflexes intact; results from upper motor neuron damage (e.g., stroke).

Muscle Atrophy and Reflex Action

• Muscle atrophy refers to the shrinkage of muscle cells due to lack of stimulation.
• Atrophy occurs significantly in LMN damage, while UMN damage leads to preserved reflexes but potentially spastic movements.
• Reflex Actions: Intact reflexes in UMN damage due to retained spinal reflex arcs.

Symptoms of Spinal Cord Damage

• Quadriplegia: Loss of use in all four limbs due to cervical injury.
• Paraplegia: Loss of use in the lower limbs due to thoracic/lumbar injury.
• Risk associated with vertebral fractures includes respiratory failure if cervical vertebrae are damaged.

Spinal Shock

• Transient Condition: Mimics complete spinal cord transection; may recover function as inflammation subsides.
• Rehabilitation Focus: Physical therapy to stimulate remaining functional muscles and maintain mobility.

Neurological Conditions:

• Amyotrophic Lateral Sclerosis (ALS): Affects the ventral horn, leading to loss of motor control and eventual respiratory failure as diaphragm muscles weaken.

Receptors and Sensory Processes

• Function of Receptors: Structures that convert environmental stimuli into electrical signals (graded potentials).
• Depolarization: A decrease in potential difference across membrane leading to action potential generation.
• Sensation vs Perception: Sensation involves the detection of stimuli; perception involves the interpretation of sensory information.

Types of Receptors

• Mechanoreceptors: Respond to mechanical changes (touch, pressure, vibrations).
• Photoreceptors: Sensitive to light (e.g., rods and cones in the retina).
• Chemoreceptors: Detect chemical changes (senses of taste and smell).
• Thermoreceptors: Detect temperature variations.
• Nociceptors: Pain receptors alerting to tissue damage.

Stimulus Classification

• Exteroceptors: Respond to external stimuli (general and special senses).
• Interoceptors: Respond to internal stimuli (visceral sensations).
• Proprioceptors: Provide information about body position and movement.

Perceptual Processing Levels

• Receptor Level: Sensory receptors transduce stimuli into signals.
• Circuit Level: Information travels through pathways to the brain.
• Perceptual Level: Higher processing occurs in the cerebral cortex for awareness and interpretation.

Neural Pathways and Pain:

• Phantom Limb Pain: Occurs in amputees due to retained neuronal activity in the brain.
• Visceral Pain: Difficult to localize, often arising from organ issues (e.g., kidney stones).
• Referred vs Radiating Pain:
• Referred Pain: Perceived pain at a location different from the source (e.g., heart attack referred to arm).
• Radiating Pain: Pain that travels along a nerve path (e.g., sciatica).

Connective Tissues in Nerve Structure

• Epineurium: Surrounds the entire nerve.
• Perineurium: Encloses bundles of fibers (fascicles).
• Endoneurium: Surrounds individual axons, found on myelinated fibers.

Neuron Classification

• Sensory Neurons: Afferent (incoming signals to the CNS).
• Motor Neurons: Efferent (outgoing signals from the CNS to muscles).
• Mixed Nerves: Contain both sensory and motor fibers (most spinal nerves).

Pain Tolerance and Threshold

• Threshold: The minimal stimulus needed to perceive pain.
• Tolerance: The maximum level of pain discomfort a person can endure before they seek relief.
• Influences on Pain Tolerance: Genetic predisposition, conditioning, and circumstance can affect individual pain responses.