Principles of Human Physiology: Sensory Systems

Presentation Details

Ability to describe the function of sensory receptors and how they perform that function.
Ability to explain the sensory transduction mechanism for each of the special senses.

Afferent Branch of Peripheral Nervous System:
- Comprises sensory neurons that transmit sensory information to the central nervous system (CNS).
Perception:
- Defined as the conscious interpretation of the world based on sensory inputs.
- Influenced by various factors including memory and other neural processes (e.g., thought).
- Key Question: Is perception always accurate or uniform across different individuals?

Receptor Class and Associated Sensations:
- Photoreceptors:
- Function: Vision
- Modality: Photons of light
- Chemoreceptors:
- Function: Taste, Smell, and Pain
- Modality: Chemicals dissolved in saliva, mucus, and extracellular fluid
- Thermoreceptors:
- Function: Temperature sensation
- Modality:
  - Warm receptors: Detect increasing temperatures (30°C to 43°C)
  - Cold receptors: Detect decreasing temperatures (35°C to 20°C)
- Mechanoreceptors:
- Function: Vibration, Sound, Balance, and Equilibrium
- Modality:
  - Pacinian Corpuscle: Detect pressure
  - Hair Cells: Involved in balance and sound

Definition: The process by which sensory receptors convert stimulus energy into electrical signals.
Types of Receptors:
- Afferent Neuron: Specialized receptor end of an afferent neuron which transmits signals to the CNS.
- Action potential direction: From sensory receptors to the CNS.
Sensory Receptor Mechanism:
- Receptor Protein: Interacts with stimuli and activates voltage-gated calcium channels, promoting neurotransmitter release.

Labeled Lines:
- Each specific neural pathway correlates with a specific modality and location in the brain.
Sensory Cortex Locations:
- Vestibular Cortex: Balance
- Somatosensory Cortex: Touch and spatial perception
- Olfactory Cortex: Smell
- Auditory Cortex: Hearing
- Visual Cortex: Sight
- Gustatory Cortex: Taste

Defined as a single afferent neuron and all receptors associated with it.
Receptive Field:
- The area in which stimulation will activate a sensory unit.

Coding for Stimulus Intensity:
- Frequency Coding:
- The frequency of action potentials reflects stimulus intensity.
- Threshold: The minimum stimulus strength required to elicit an action potential.
- Suprathreshold: Refers to stimuli above threshold levels.
- Relative Refractory Period: The period during which a neuron can only fire an action potential if the stimulus is strong enough.
Population Coding:
- Number of receptors activated contributes to perceived stimulus intensity.
- Recruitment: A stronger stimulus activates more receptors and potentially involves more than one afferent neuron.
Location Coding:
- Acuity: The clarity or precision with which a stimulus is perceived.
- Factors Influencing Acuity:
1. Size of receptive field
2. Degree of overlap between receptive fields

Definition: Measures tactile acuity and threshold, the ability to perceive two distinct points of stimulus on the skin.
Two-Point Discrimination Thresholds for Selected Areas of the Body:
- Lips: 1 mm (greatest acuity)
- Index Finger: 2 mm
- Thumb: 3 mm
- Palm of Hand: 10 mm
- Big Toe: 10 mm
- Forehead: 18 mm
- Sole of Foot: 22 mm
- Breast: 31 mm
- Abdomen: 36 mm
- Shoulder: 38 mm
- Back: 42 mm
- Thigh: 46 mm
- Upper Arm: 47 mm
- Calf: 48 mm (least acuity)

Understanding the principles of sensory physiology is crucial for grasping how different sensory modalities interact with the nervous system and contribute to perception.