Sensory System

Introduction to the Sensory System

  • The sensory system is vital, acting as a bridge to our environment.

  • The essence of identity: Who would you be without it?

Types of Receptors

I. Mechanoreceptors

  • Detect mechanical forces such as touch and pressure.
    II. Nociceptors

  • Detect painful stimuli.
    III. Thermoreceptors

  • Sensitive to temperature changes.
    IV. Chemoreceptors

  • Respond to chemical stimuli (e.g., taste, smell).
    V. Photoreceptors

  • Responsible for detecting light, crucial for vision.

Proprioception

  • Refers to the body's ability to sense its position, movement, and action.

  • Distinction between: I. General senses

    • Receptors are distributed throughout the body.
      II. Special senses

    • Receptors are localized within specific organs, such as those in the skin, muscles, and joints.

  • Somatic vs. Visceral:

    • Somatic: Touch, Pressure, Temperature, Pain.

    • Visceral: Pain, Pressure, Smell, Balance, Taste, Hearing, and Vision.

Receptors Overview

I. Sensory Receptors as Biological Transducers

  • Transform information or stimulus into neural signals.

  • Example of channel operation:

    • Information/Stimulus → Neural Signal

  • Important note: Stimuli can be sensed and processed but not always perceived (e.g., visceral sensory).

II. Sensory Adaptation

  • Phenomenon where the perception of a stimulus decreases over time, despite continued exposure.

    • Can be either central or peripheral and can occur rapidly or slowly.

Examples of Sensory Adaptation

  1. Touch

  2. Olfaction (smell)

  3. Vision

  4. Heat (and cold)

Receptive Fields

  • Understanding receptive fields is vital:

    • Large vs. small receptive fields represent different sensory areas, e.g., 2 mm vs. up to 64 mm.

    • Two-point discrimination template used to measure perceptual capacity regarding touch.

Types of Mechanoreceptors

I. Mechanoreceptors Explained

  • Variety of receptors responsive to touch:

    • Meissner corpuscles

      • Role: Involved in two-point discrimination.

    • Ruffini end organs

      • Detect continuous touch, pressure, and skin stretch.

    • Pacinian corpuscles

      • Responsible for detecting deep pressure, vibration, and proprioception.

    • Hair follicle receptors

      • Respond to light touch through slight hair bending.

    • Merkel disks

      • Detects light touch and superficial pressure.

    • Free nerve endings

      • Responsive to painful stimuli, temperature, itch, joint movement, and proprioception.
        II. Proprioceptors

  • Do not adapt to stimuli.

  • Include Muscle Spindles and Golgi Tendon Organs.
    III. Baroreceptors

  • Respond to changes in pressure, particularly in blood vessels and organs.

Nociceptors

  • Free Nerve Endings

  • Activated by:

    • Physical trauma

    • Temperature extremes

    • Noxious chemicals

  • Receptive fields can be small or large, influencing pain perception.

  • Debate: Is life without pain advantageous or disadvantageous?

Thermoreceptors

  • Free Nerve Endings

  • Sensitive to temperature changes.

  • Showing a ratio of approximately 3:1 Cold to Warm receptors.

  • Typically possess large receptive fields.

  • Adaptation: Yes or No?

Chemoreceptors

  • General Visceral chemotransduction includes:

    • Monitoring pH, ion, and gas concentrations (e.g., blood chemistry).

  • Special Chemoreceptors encompass:

    • Taste & Smell.

  • Relationship highlighted: Taste heavily influenced by olfaction.

  • Example: Carotid body monitors pH and carbon dioxide concentrations.

Photoreceptors

  • Correspond to the sensory structure responsible for Vision.

  • Includes different parts of the retina, namely:

    • Pigmented Part of Retina

    • Neural Part of Retina.

  • Important Structures:

    • Central retinal vein, optic disc, central retinal artery, sclera, optic nerve, choroid.

The Special Senses Overview

I. Olfaction

  • Approximately 10 million bipolar neurons.

  • Structure is simplistic but has hundreds of various (and often nonspecific) chemoreceptors to detect thousands of odors.

  • Pathway summary: Olfactory neuron → olfactory bulb → cerebrum (note: does not pass through thalamus).

    • Historical structures involved include the cribriform plate of ethmoid bone, olfactory bulb, and olfactory tract.

II. Taste (Gustation)

  • Difference between papillae and taste buds.

  • Classification of receptors is nonselective in nature.

  • Five categories of taste perception: Sweet, Sour, Salty, Bitter, Umami.

    • Bitter taste noted for highest physiological sensitivity.

  • Perception influenced by other senses, such as olfaction, texture, and temperature.

  • Pathway: Taste cells → sensory neurons of cranial nerves VII, IX, or X → medulla → thalamus → cortex.

III. Visual System

  • Accessory Structures:

    • Eyebrows, eyelids, eyelashes, conjunctiva (mucous membrane of inner eyelid & exposed eye).

    • Function: Protection and lubrication.

    • Lacrimal Apparatus: Source of tear production from lacrimal gland, about 1 ml/day.

- Additional key structures include extrinsic eye muscles that enhance vision.

  • Aqueous & Vitreous Humors play roles in eye function.

Eye Anatomy

  • Layers of the Eyeball:

    • Fibrous layer: Sclera, Cornea.

    • Vascular layer: Iris, Ciliary body, Choroid.

    • Neural layer: Retina (including central artery/vein and optic nerve).

  • Intrinsic Eye Muscles (Smooth Muscles) including:

    • Ciliary Muscles: Change the shape of the lens for focusing.

    • Pupillary Muscles: Adjust the size of the iris (and pupil).

Iris & Pupillary Muscles

  • Pupillary Dilator Muscles: Radial construction to dilate pupils under low-light conditions through sympathetic stimulation.

  • Pupillary Constrictor Muscles: Circular construction for constricted pupils in high-light conditions via parasympathetic stimulation.

Visual Pathway

  • Understanding the visual pathway sequence from visual fields through optic chiasm to visual cortex (located in the occipital lobe):

    1. Temporal part of the right visual field reaches the nasal retina of the left eye.

    2. The nasal retina connects via optic nerves leading to optic chiasm.

    3. Information tracts through the thalamus, finally arriving at the visual cortex.

The Physiology of Vision

  • Each synapse in the retinal structure allows for processing, vitally impacting visual sensitivity.

    • Discussion of phototransduction involving rods and cones, showing how light manipulation impacts retinal receptor functionality.

Rhodopsin Mechanism

  • In dark conditions, the channel remains open due to the presence of cGMP, leading to polarization of rod cells.

  • A photon of light results in a sequence:

    • cGMP bound to Na+ channel is released, closing the channel and resulting in hyperpolarization.

    • This switch leads to a decreased glutamate release, inhibiting the bipolar cell and allowing AP generation in the ganglion cells.

Vision Spectrum Analysis

  • Notable wavelengths (in nanometers) impacting receptor pigment absorption lead to distinctions among red, green, and blue pigments:

    • Graphical representation of absorption rates relative to varying wavelengths from ultraviolet to infrared.

Conclusion

  • Connectivity of vision perception to broader theories of light, color, and their implications on human experience is emphasized.