SSP:LEC 1

Lecture 1 Recording 1: Somatic Sensation and Sensory Receptors

Somatic Sensation

  • Somatic sensation refers to the sensory information derived from:

    • Skin, Muscles, Bones, Tendons, Joints

  • These sensations are initiated by a variety of sensory receptors collectively known as somatic receptors.

  • Somatic Receptors respond to:

    • Touch and pressure

    • Sense of posture and movement

    • Temperature

    • Pain

Sensory Receptors

  • Definition: Sensory receptors are specialized cells that generate graded potentials, referred to as receptor potentials, in response to specific stimuli.

  • Different types of stimuli, known as modalities, are sensed by different sensory receptors.

  • Examples of modalities include:

    • Temperature, Pressure, Sound, Light

  • There exists a specialized receptor for each type of stimulus.

Types of Somatic Receptors

  • Afferent Activation of sensory receptors includes:

    1. Meissner's corpuscle - Rapidly adapting mechanoreceptor sensitive to touch and pressure

    2. Merkel's corpuscle - Slowly adapting mechanoreceptor sensitive to touch and pressure

    3. Free neuron ending - Slowly adapting; includes nociceptors and thermoreceptors

    4. Pacinian corpuscle - Rapidly adapting mechanoreceptor sensitive to vibration and deep pressure

    5. Ruffini corpuscle - Slowly adapting mechanoreceptor sensitive to skin stretch

  • The distribution of these receptors can be categorized as follows:

    • Rapidly adapting mechanoreceptors (e.g., Meissner's corpuscle) respond quickly to initial stimuli but then decrease response over time.

    • Slowly adapting mechanoreceptors (e.g., Merkel's corpuscle, Ruffini corpuscle) continue to respond over a sustained period.

Mechanism of Afferent Activation

  • The activation of an afferent neuron occurs due to various stimuli, such as:

    • Muscle stretch, also known as stretch reflex

  • Mechanism:

    • Stretching a muscle leads to an increased opening of specialized sodium (Na+) channels.

    • This allows Na+ to enter the afferent fiber, causing depolarization of the afferent neuron.

    • If sufficient Na+ enters, reaching the depolarization threshold (approximately −50extmV-50 ext{ mV}), voltage-gated Na+ channels open, resulting in an action potential.

Lecture 1 Recording 2: The Receptor Potential and Adaptation

Understanding the Receptor Potential

  • Receptor Potential is generated when a stimulus, such as a poke in the arm, interacts with a receptor membrane:

    • The sequence includes:

    1. Stimulus energy activating receptor membrane

    2. Action potentials being conducted along the afferent neuron to the Central Nervous System (CNS)

    • The process is highlighted by nodes of Ranvier along the axon, where action potentials are regenerated.

Characteristics of Action Potentials

  • Action potentials depend on the intensity of the receptor potential, which is influenced by stimulus intensity.

  • Response dynamics involve the relationship between stimulus intensity and the number of action potentials generated by afferent neurons.

Adaptation of Sensory Receptors

  • Adaptation refers to the decrease in receptor potential over time despite the continuous presence of a stimulus:

    • Rapidly adapting receptors (e.g., Meissner's corpuscle) stop generating action potentials shortly after stimulus onset.

    • Slowly adapting receptors (e.g., Merkel’s corpuscle) continue to generate a response gradually decreasing in magnitude.

  • The amount of stimulation correlates with receptor potential, which remains active but decreases when the stimulus is constant.

Lecture 1 Recording 3: Localizing the Site of a Stimulus

Stimulus Localization

  • The capability to localize stimuli in skin is a vital sensory function.

  • Receptive Fields:

    • Defined as the area where a stimulus can activate the sensory receptors.

    • Different areas of the body have different receptive field sizes:

    • E.g., lips (1 cm receptive field) vs. back of the skin (3 cm receptive field).

    • Two distinct points may be felt where the stimulus is adequately localized, while only one point is registered in larger receptive fields.

Role of Lateral Inhibition

  • Lateral Inhibition is a crucial mechanism for enhancing stimulus localization across some sensory systems.

    • In lateral inhibition:

    • Afferent neurons at the edges of a stimulus are inhibited more strongly compared to those at the center.

    • This inhibition increases contrast between the center and periphery of the stimulated zone, amplifying the brain's capability to pinpoint sensory input.

  • Mechanism:

    • Information from peripheral afferent neurons is suppressed, allowing for enhanced clarity in localization of the center of stimulation.

References

  • Chapter 7, Vander’s Human Physiology

  • Chapter 10, Vander’s Human Physiology

  • Copyright © The McGraw-Hill Companies, Inc.

  • Permitted reproduction or display guidelines apply.

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