📌 Professor’s Notes: Quick Revision
High-yield revision notes for rapid review before university examinations.
- The sensory system detects changes in the external and internal environment. A stimulus activates a sensory receptor, which converts it into an electrical signal (sensory transduction). The impulse travels through afferent pathways to the brain, where it is interpreted as a sensation and may produce an appropriate motor response.
- Sensation is the conscious awareness of a stimulus. Normal sensation is esthesia, loss of sensation is anesthesia, abnormal spontaneous sensation is paresthesia, and kinesthesia is the perception of body position and movement. Sensations are classified into somatic, visceral, and special sensations.
- The four dimensions of sensation are:
- Modality – type of sensation.
- Intensity – strength of the stimulus.
- Affect – emotional component.
- Acuity – ability to localize and discriminate stimuli accurately. High acuity depends on a high receptor density, small receptive fields, and greater cortical representation.
- Sensory receptors are specialized transducers that convert stimuli into nerve impulses. They are classified:
- By function: Exteroceptors, interoceptors, proprioceptors, and teleceptors.
- By adequate stimulus: Mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors, and photoreceptors.
- By location: Superficial, deep, and visceral receptors.
- Important mechanoreceptors:
- Pacinian corpuscles – rapidly adapting; detect deep pressure and high-frequency vibration.
- Meissner corpuscles – rapidly adapting; detect light touch and low-frequency vibration.
- Merkel discs – slowly adapting; detect sustained touch, edges, shapes, and texture.
- Ruffini endings – slowly adapting; detect skin stretch and sustained pressure.
- Krause end bulbs – rapidly adapting; detect light touch.
- Hair follicle receptors – detect movement of hairs and light touch.
- A receptor (generator) potential is a graded local depolarization produced when a receptor is stimulated. Its amplitude is proportional to stimulus intensity. When threshold is reached, an action potential is generated at the first node of Ranvier. In Pacinian corpuscles, stretch-sensitive sodium channels open, causing sodium influx and receptor potential formation. Stronger stimuli increase the frequency of action potentials, thereby encoding stimulus intensity.
- Properties of receptors:
- Specificity – each receptor responds best to one type of stimulus.
- Adequate stimulus – stimulus requiring the lowest threshold.
- Adaptation – decline in receptor response during constant stimulation.
- Phasic receptors adapt rapidly.
- Tonic receptors adapt slowly or not at all.
- Acuity depends on receptor density and receptive field size.
- Stimulus intensity is coded by action potential frequency and receptor recruitment.
- Important physiological principles:
- Weber–Fechner law: Perceived sensation increases with the logarithm of stimulus intensity.
- Law of projection: Stimulation anywhere along a sensory pathway is perceived at the receptor site (explains phantom limb).
- Doctrine of specific nerve energies: The quality of sensation depends on the activated sensory pathway, not the type of stimulus.
- A sensory unit consists of one sensory neuron and all its receptors.
📝 Practice MCQs
🎙️ Frequently Asked Viva Questions
High-yield viva questions with concise answers for rapid university examination preparation.
Q1. What is sensory transduction?
Answer: Sensory transduction is the conversion of a physical or chemical stimulus into an electrical signal (receptor potential), which can generate an action potential.
Q2. How does sensation differ from perception?
Answer: Sensation is the conscious awareness of a stimulus, whereas perception is the brain’s interpretation and recognition of that sensory information.
Q3. What is an adequate stimulus?
Answer: An adequate stimulus is the type of energy to which a receptor is most sensitive and responds with the lowest threshold.
Q4. Why are proprioceptors important?
Answer: Proprioceptors provide information about body position, movement, and muscle tension, helping maintain posture, balance, and coordinated movements.
Q5. Which receptor detects high-frequency vibration and deep pressure?
Answer: Pacinian corpuscle. It is a rapidly adapting mechanoreceptor specialized for high-frequency vibration and deep pressure.
Q6. Why does the Pacinian corpuscle adapt rapidly?
Answer: Its concentric lamellae filter sustained pressure, so it mainly responds to the beginning and end of a mechanical stimulus.
Q7. Which receptor is responsible for fine tactile discrimination?
Answer: Meissner corpuscle. It detects light touch, low-frequency vibration, and fine tactile discrimination, especially in glabrous skin.
Q8. Which receptor helps recognize edges, shapes, and texture?
Answer: Merkel disc. It is a slowly adapting receptor that detects sustained touch, edges, shapes, and surface texture.
Q9. What is receptor potential?
Answer: Receptor potential is a local graded depolarization produced when a sensory receptor is stimulated. If it reaches threshold, it triggers an action potential.
Q10. Where is the action potential generated in a Pacinian corpuscle?
Answer: The action potential is generated at the first node of Ranvier, while the receptor potential develops at the unmyelinated nerve terminal.
Q11. Differentiate between phasic and tonic receptors.
Answer: Phasic receptors adapt rapidly and detect changes in stimuli. Tonic receptors adapt slowly and continuously signal sustained stimuli such as pain and muscle stretch.
Q12. How is the intensity of a stimulus encoded?
Answer: Stimulus intensity is coded by increasing the frequency of action potentials and by recruiting additional sensory receptors.
Q13. State the law of projection.
Answer: Stimulation of a sensory pathway at any point is perceived as arising from the location of its peripheral receptor. This explains phantom limb sensations.
Q14. What is the doctrine of specific nerve energies?
Answer: The quality of sensation depends on the specific sensory pathway and cortical area activated, not on the type of stimulus applied.
Q15. Why do fingertips have better sensory acuity than the back?
Answer: Fingertips have a higher receptor density, smaller receptive fields, and larger cortical representation, allowing more precise localization of stimuli.
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