human phys ch 7 part 1

Sensation and Perception

• Sensation

  • Initial perception occurs in extremities (fingers and toes).

  • Sensory perception can vary based on damage to sensory receptors, affecting how sensations are experienced.

  • Without sensation, we may experience numbness, touch, pain, etc.

• Action Potentials

  • An action potential is a rapid change in membrane potential that propagates along the neuron.

  • Definition: "An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory."

  • Depolarization: This stage occurs when sodium channels open, allowing Na⁺ ions to enter the neuron, causing the inside to become more positive.

  • Threshold: The depolarization must reach negative 50 millivolts for an action potential to occur.

  • If depolarization does not reach threshold, it returns to resting state.

  • Repolarization: This stage involves the slow closing of potassium channels, which causes hyperpolarization due to excess K⁺ exiting the neuron.

• Transmitting a Signal

  • Action potentials must propagate down the axon to release neurotransmitters at the axon terminal.

  • Neurons send signals quickly through afferent pathways to the central nervous system (CNS), where integration occurs.

  • Signals are transmitted via segments of the axon, resembling a wave motion.

• Neurons and Neurotransmission

  • Neurons communicate through neurotransmitters without initiating direct contact between them.

  • The sensation feeds information into the central nervous system to interpret the stimulus.

Types of Sensory Receptors

• Classification of Receptors

  • Mechanoreceptors

  • Thermoreceptors

  • Photoreceptors

  • Chemoreceptors

  • Nociceptors: These receptors respond to pain and itch sensations.

• Adequate Stimulus

  • Definition: An adequate stimulus must sufficiently activate a receptor for sensation to be perceived.

  • Example of inadequate stimulus: Not feeling a light touch unless it becomes more intense.

Response to Sensory Stimuli

• Perception of Touch and Pain

  • The sensation of a paper cut exemplifies how initial perception may not register pain until inflammation intensifies its perception.

  • Initial sensation of a cut is recognized, but the intensity of pain can escalate due to inflammatory responses.

• Intensity and Sensation Integration

  • Perception not only depends on the stimulus but also on the integration of such stimuli from multiple tissues and pathways.

  • Example of integration: Feeling a draft also leads to auditory sensations (hearing the wind).

• Olfactory Fatigue & Perception Ignorance

  • Fatigue occurs when constant stimulation reduces the ability to perceive the stimulus over time (e.g., familiar fragrances become unnoticed).

  • Central Processing: Signals reach the integration center of the CNS, and responses are generated accordingly.

Chronic Pain and Its Management

• Chronic Pain

  • Chronic pain presents similar mechanisms of action potential conduction but may remain in a more amplified or dulled state due to prolonged receptor activation.

  • Dull Pain Example: Persistent headaches classified as tonic sensations versus immediate pains categorized as phasic.

• Treatment Options

  • Medications (e.g., narcotics, prescribed marijuana) aim to manage chronic pain rather than completely eliminate it, allowing for improved functionality.

Sensory Coding and Modality

• Primary Sensory Coding

  • Definition: The conversion of environmental stimuli into electrical signals for processing by the CNS involves action potentials from receptors to axon terminals.

  • Example Experiment: Touching a cold object initiates rapid activation of thermoreceptors, quickly transmitting signals to the CNS.

• Types of Stimuli

  • Different modalities (e.g., chemical, thermal, mechanical) are perceived variably based on receptor types affected.

  • Intensity and frequency of action potentials correlate to perception strength.

• Differential Sensation Example:

  • Ice cubes versus copper wire on the skin activate distinct types of receptors (thermoreceptors and potentially nociceptors).

Response Variability and Perceptual Changes

• Adaptation of Sensory Receptors

  • Difference between rapidly adapting receptors (e.g., response to quick changes in stimuli) and slowly adapting receptors (e.g., sustained pressure or pain).

  • Example of rapid adaptation: Sudden exposure to bright light; example of slow adaptation: Persistent headache from screen time.

• Frequency and Intensity of Signals

  • Changes in intensity over time reflect variations in receptor stimulation.

  • Continuous, increasing stimulation leads to altered intensity and perception intensity.