Sensory Physiology: Mechanisms of Operation and Information Encoding

Introduction

Senses are complex systems that enable our bodies to receive information from both internal and external environments. This allows us to perceive and interact with the world around us. This article explores the mechanisms of sensory physiology, including structure, function, information encoding, and their roles.

Classification of Senses

From a physiological perspective, senses are categorized into two main groups:

• General senses: These involve sensory receptors distributed throughout the body, allowing us to perceive bodily sensations (such as hot, cold, pain, pressure) and visceral sensations (such as fullness, hunger, nausea).

• Special senses: These have unique structures located in specific areas of the body and include vision, hearing, smell, touch, and taste.

Types of Stimuli and Sensory Transduction

Senses respond to stimuli from the environment, which can be classified into three primary categories:

• Electromagnetic stimuli: These include light and heat.

• Mechanical stimuli: These include pressure, sound waves, and vibrations.

• Chemical stimuli: These include acidity, molecular shape and size.

Sensory transduction is the process by which senses convert stimuli into electrical signals. This process occurs in the following steps:

1. Stimulus acts on the receptor: The stimulus from the environment acts on the specialized sensory receptors of each sense.

2. Stimulus converted to electrical signal: The receptor converts the stimulus into an electrical signal called the receptor potential.

3. Information transmission: The receptor potential travels along sensory nerves to the central nervous system.

4. Information processing: The central nervous system receives and processes the information, resulting in a sensation.

Types of Sensory Receptors

The human body has five primary types of sensory receptors:

• Mechanoreceptors: Detect compression or stretch of surrounding tissues. Examples include receptors in the skin, muscles, and joints.

• Thermoreceptors: Detect temperature changes, including hot and cold receptors.

• Photoreceptors: Detect light striking the retina of the eye, such as the rods and cones in the retina.

• Nociceptors: Detect tissue damage and are activated by mechanical, thermal, or chemical agents.

• Chemoreceptors: Detect tastes on the tongue, smells in the nose, CO2 and O2 levels in arteries, and osmolarity of body fluids.

Generating Receptor Potentials

Receptor potentials are generated through four primary mechanisms:

• Mechanical: This occurs through mechanical stimulation of the receptor, such as touching a hard surface.

• Chemical: This occurs through contact with chemical substances, such as smelling a flower.

• Thermal: This occurs through changes in temperature, such as touching a hot or cold object.

• Electromagnetic: This occurs through light striking the receptor, such as seeing light.

Note: Receptors exhibit adaptation, meaning that with continuous stimulation, the intensity of the signal transmitted decreases over time.

Sensory Coding

Information from the senses is encoded as electrical signals and transmitted to the central nervous system, where it is decoded to create sensations.

There are six key characteristics used in sensory coding:

• Sensory modality: Sensations are classified based on the type of sensory receptor activated. For example, touch, pressure, vibration, hot, cold, pain, vision, taste, and smell.

• Stimulus location: The location of the stimulus is encoded by the activation of specific sensory neurons. For instance, touching two distinct points on the skin activates two separate neuron groups.

• Stimulus threshold: This refers to the minimum intensity of a stimulus required to generate a receptor potential strong enough to activate a sensory neuron.

• Intensity: The intensity of the stimulus is encoded by the average frequency of firing or the number of receptors activated.

• Stimulus frequency: The frequency of the stimulus is encoded by the spacing between nerve impulses.

• Stimulus duration: The duration of the stimulus is encoded by the duration of nerve impulse firing.

Sensory Nerves

Sensory nerves are typically divided into two types:

• Type A fibers: These have large and medium diameters, are myelinated, and conduct sensations quickly. Type A fibers generally transmit touch, pressure, temperature, and rapid pain.

• Type C fibers: These have small diameters, are unmyelinated, and conduct sensations slowly. Type C fibers usually transmit slow pain, temperature, and itch.

Sensory Neurons

Information from the senses is transmitted to the central nervous system by sensory neurons. This process involves four neurons:

• First-order neuron: The cell body is located in the dorsal root ganglion or cranial nerve ganglion. This neuron receives information from receptors or peripheral nerve endings.

• Second-order neuron: The cell body is located in the spinal cord or brainstem. This neuron processes information, crosses over, and transmits the sensation to the contralateral thalamus.

• Third-order neuron: The cell body is located in the thalamus. This neuron processes information before relaying it to the cerebral cortex.

• Fourth-order neuron: Located in the cerebral cortex, this neuron processes higher-level information and creates a sensation.

Types of Somatic Sensations

Somatic sensations are categorized into four main types:

• Touch: Generated by mechanoreceptors, this sensation allows us to perceive contact, pressure, and vibrations.

• Temperature: Generated by thermoreceptors, this sensation allows us to perceive changes in temperature.

• Proprioception: Generated by proprioceptors located in the skin, joints, ligaments, muscle spindles, and tendons, this sensation allows us to perceive the body’s position in space and the movement of limbs.

• Pain: Generated by nociceptors, this sensation allows us to perceive tissue damage and warns the body of danger.

Note: Pain is a protective sensation that helps us avoid severe damage.

Vision

Vision is the sense that allows humans to perceive images, colors, light, and depth.

• Eye structure: The eye consists of three layers: the outer layer (including the cornea, sclera, and choroid), the middle layer (including the iris, ciliary body, and lens), and the inner layer (including the retina).

• Retina: This layer contains photoreceptor cells that detect light, including rods and cones. Each eye has about 120 million photoreceptor cells.

• Fovea centralis: The central region of the retina containing only cones, allowing for sharp central vision.

• Mechanism of vision: Light enters the eye, passes through the cornea and lens, and strikes the retina.

• Accommodation: The lens can change its curvature to adjust the focus of light from different distances, allowing the eye to focus on near or far objects.

• Color perception: Cone cells perceive color. Each cone cell is linked to one bipolar cell, and each bipolar cell is linked to one ganglion cell.

• Mechanism of light detection: Rods and cones detect light by changing their membrane potential through the process of converting light energy into chemical energy. Rhodopsin is the pigment found in rods, while cones have three types of pigments, each corresponding to a primary color: red, green, and blue.

Hearing

Hearing is the sense that allows humans to perceive sound, including its intensity, frequency, pitch, and location.

• Ear structure: The ear is divided into three parts: the outer ear, middle ear, and inner ear.

• Outer ear: This includes the pinna and external auditory canal, which conduct sound into the ear.

• Middle ear: This includes the tympanic membrane (eardrum), malleus, incus, stapes, and Eustachian tube. It transmits sound waves from the tympanic membrane to the oval window.

• Inner ear: This is also known as the labyrinth and includes the bony labyrinth and membranous labyrinth. The membranous labyrinth is divided into two parts: the organ of hearing (cochlea) and the organ of balance (semicircular canals, utricle, and saccule).

• Organ of Corti: This organ, located on the basilar membrane within the cochlea, is responsible for sound detection.

• Mechanism of hearing: Sound waves enter the outer ear, causing the tympanic membrane to vibrate. The tympanic membrane vibrates, causing the malleus, incus, and stapes to move, vibrating the oval window. Sound waves then travel into the cochlea, vibrating the basilar membrane and activating hair cells in the organ of Corti.

• Frequency encoding: The frequency of sound is encoded by the location of the hair cell on the basilar membrane. High-frequency sound waves maximally stimulate the basilar membrane near the base of the cochlea, while low-frequency sounds maximally stimulate the basilar membrane near the apex of the cochlea.

Vestibular System

The vestibular system is a part of the inner ear that includes the semicircular canals, utricle, and saccule. It is responsible for maintaining balance, controlling body position, and detecting movement.

• Mechanism of function: The semicircular canals sense rotational acceleration of the head, while the utricle and saccule sense linear acceleration of the head.

• Otoconia: Located in the utricle and saccule, these crystals are twice as dense as the endolymph, helping to sense body movement related to gravity.

Smell

Smell, also known as olfaction, is the sense that allows humans to perceive odors.

• Nose structure: The olfactory epithelium contains olfactory receptor cells. Each olfactory receptor cell is also called an olfactory rod.

• Mechanism of smelling: Odorant molecules in the air dissolve in the mucous layer in the nose and come into contact with the olfactory receptor cells. Olfactory receptor cells convert the information into electrical signals, which are transmitted to the central nervous system.

• Odor discrimination: The olfactory bulb is responsible for differentiating different odors. Each odor is encoded by a unique pattern of activation of olfactory neurons.

Taste

Taste, also known as gustation, is the sense that allows humans to perceive flavors.

• Tongue structure: The tongue contains taste buds, each of which contains two types of cells: taste receptor cells and supporting cells.

• Mechanism of tasting: Substances dissolved in saliva activate taste receptors on taste receptor cells. These cells generate electrical signals that are transmitted to the central nervous system.

• Basic tastes: There are five basic tastes: sweet, sour, bitter, salty, and umami.

• Mechanism of taste perception: The salty taste is due to Na+ entering the cell, the sour taste is due to H+ blocking K+ channels or entering the cell, the sweet taste is due to sweet substances binding to receptors coupled with G proteins, and the bitter taste is due to bitter substances blocking K+ channels.

Notes

• The senses are complex systems involving numerous factors, such as structure, function, information encoding, and processing.

• Sensory systems can be affected by aging, diseases, and the environment.

• Understanding sensory physiology enhances our knowledge of the body’s mechanisms, promotes health maintenance, and facilitates the treatment of sensory-related diseases.

This article provides basic information on sensory physiology. For a more in-depth understanding of this field, refer to specialized resources on physiology.