بِسْمِ اللهِ الرَّحْمٰنِ الرَّحِيْمِ
PRIMARY SENSORY CODING�VISION
SOMATIC SENSATION
SOMATOSENSORY SYSTEM
SOMATOSENSORY SYSTEM�� “DEFINITION AND TYPES OF SENSATIONS”��Somatosensory system is defined as the sensory system associated with different parts of the body. ��Sensations are of two types:� 1. Somatic sensations
2. Special sensations.
1. Somatic Sensations
Somatic sensations are the sensations arising from skin, muscles, tendons and joints. These sensations have specific receptors, which respond to a particular type of stimulus.
2. Special Sensations
Special sensations are the complex sensations for which the body has some specialized sense organs. These sensations are usually called special senses. Sensations of vision, hearing, taste and smell are the special sensations
TYPES OF SOMATIC SENSATIONS��Generally, somatic sensations are classified into three types:
� 1. Epicritic sensations
2. Protopathic sensations
3. Deep sensations.
�2. Protopathic Sensations��Protopathic sensations are the crude sensations. These sensations are primitive type of sensations. Protopathic sensations are:�i. Pressure sensation�ii. Pain sensation�iii. Temperature sensation with a wider range, i.e. above 40°C and below 25°C.
3. Deep Sensations
Deep sensations are sensations arising from deeper structures beneath the skin and visceral organs. Deep sensations are:
�Kinesthetic sensation is of two types:
VISIO
In physiology, "visio" can refer to various aspects related to vision or sight. This includes the anatomical structures and physiological processes involved in the perception of light, the formation of visual images, and the interpretation of visual stimuli by the brain.
VISIO
ANATOMY OF THE EYE
The ANATOMY OF EYE is quite intricate and comprises several key structures that work together to facilitate vision. Here's a brief overview:
Cornea: The transparent, dome-shaped outermost layer of the eye that covers the iris, pupil, and anterior chamber.
Iris: The colored part of the eye surrounding the pupil. The iris controls the size of the pupil.
Pupil: The black circular opening in the center of the iris. It allows light to enter the eye and reaches the lens.
Lens: A transparent, flexible, and biconvex structure located behind the iris and the pupil.
Retina: The innermost layer of the eye, which contains photoreceptor cells called rods and cones.
Optic Nerve: A bundle of nerve fibers that carries visual information from the retina to the brain. It exits the eye at the optic disc (also known as the blind spot).
Vitreous Humor: A clear, gel-like substance that fills the space between the lens and the retina. It helps maintain the shape of the eye and provides a medium for light to pass through.
Sclera: The tough, white outer layer of the eye, often referred to as the "white of the eye." It provides structural support and protects the inner components of the eye.
Choroid: A layer of blood vessels between the retina and the sclera. It supplies oxygen and nutrients to the retina and helps regulate intraocular pressure.
Ciliary Body: A ring of tissue behind the iris that contains muscles that control the shape of the lens and produce aqueous humor (a clear fluid that helps maintain intraocular pressure).
ANATOMY OF EYE
NEURAL BASIS OF VISUAL PROCESS
Retina contains the visual receptors which are also called light sensitive receptors, photoreceptors or electromagnetic receptors. Visual receptors are rods and cones. There are about 6 million cones and 12 million rods in the human eye. Distribution of the rods and cones varies in different areas of retina. Fovea has only cones and no rods. While proceeding from fovea towards the periphery of retina, the rods increase and the cones decrease in number. At the periphery of the retina, only rods are present and cones are absent.
STRUCTURE OF ROD CELL
Rod cells are cylindrical structures with a length of about 40 to 60 µ and a diameter of about 2 µ. �
Each rod is composed of four structures:
STRUCTURE OF CONE CELL�� Cone cell is the visual receptor with length of 35 µ to40 µ and a diameter of about 5 µ. Generally, the cone cell is flask shaped. Shape and length of the cone vary in different parts of the retina. Cones in the fovea are long, narrow and almost similar to rods. Near the periphery of retina, cones are short and broad.
Like rods, cones are also formed by four parts:
� 1. Outer segment � 2. Inner segment � 3. Cell body � 4. Synaptic terminal
FUNCTIONS OF RODS AND CONES
Functions of Rods� Rods are very sensitive to light and have a low threshold. So, the rods are responsible for dim light vision or night vision or scotopic vision. But, rods do not take part in resolving the details and boundaries of objects (visual acuity) or the color of the objects (color vision). Vision by rod is black, white or in the combination of black and white namely, grey. Therefore, the colored objects appear faded or greyish in twilight.
Functions of Cones� Cones have high threshold for light stimulus. So, the cones are sensitive only to bright light. Therefore, cone cells are called receptors of bright light vision or daylight vision or photopic vision. Cones are also responsible for acuity of vision and the color vision
CHEMICAL BASIS OF VISUAL PROCESS
Photosensitive pigments present in rods and cones are concerned with chemical basis of visual process. Chemical reactions involved in these pigments lead to the development of electrical activity in retina and generation of impulses (action potentials), which are transmitted through optic nerve. Photochemical changes in the visual receptor cells are called Wald visual cycle.
RHODOPSIN
Rhodopsin or visual purple is the photosensitive pigment of rod cells. It is present in membranous disks located in outer segment of rod cells
Photochemical Changes in Rhodopsin
Wald Visual Cycle
When retina is isolated and examined in dark, the rods appear in red because of rhodopsin. During exposure to light, rhodopsin is bleached and the color becomes yellow. When rhodopsin absorbs the light that falls on retina, it is split into retinine and the protein called opsin through various intermediate photochemical reactions
Following changes occur due to absorption of light energy by rhodopsin:
1. First, rhodopsin is decomposed into bathorhodopsin that is very unstable� 2. Bathorhodopsin is converted into lumirhodopsin� 3. Lumirhodopsin decays into metarhodopsin I� 4. Metarhodopsin I is changed to metarhodopsin II� 5. Metarhodopsin II is split into scotopsin and all-trans retinal� 6. All-trans retinal is converted into all-trans retinol (vitamin A) by the enzyme dehydrogenase in the presence of reduced nicotinamide adenine dinucleotide (NADH2).��Metarhodopsin is usually called activated rhodopsin since it is responsible for development of receptor potential in rod cells
PHOTOTRANSDUCTION
• Visual or phototransduction is the process by which light energy is converted into receptor potential in visual receptors. �• Resting membrane potential in other sensory receptor cells is usually between –70 and –90 mV. �• In the visual receptors during darkness, negativity is reduced and resting membrane potential is about –40 mV. It is because of influx of sodium ions. This potential is constant and it is also called dark current.
• Influx of sodium ions into outer segment of rod cell occurs mainly because of cyclic guanosine monophosphate (cGMP) present in the cytoplasm of cell.
• Concentration of sodium ions inside the rod cell is regulated by sodium potassium pump
Photo transduction Cascade of Receptor Potential
Following is the phototransduction cascade of receptor potential� 1. When a photon (the minimum quantum of light energy) is absorbed by rhodopsin, the 11-cisretinal is decomposed into metarhodopsin through few reactions. Metarhodopsin II is considered as the active form of rhodopsin. It plays an important role in the development of receptor potential. ��2. Metarhodopsin II activates a G protein called transducin that is present in rod disks
3. Activated transducin activates the enzyme called cyclic guanosine monophosphate phospho diesterase (cGMP phosphodiesterase), which is also present in rod disks��4. Activated cGMP phosphodiesterase hydrolyzes cGMP to 5’-GMP��5. Now, the concentration of cGMP is reduced in rod cell
6. Reduction in concentration of cGMP immediately causes closure of sodium channels in the membrane of visual receptors��7. Sudden closure of sodium channels prevents entry of sodium ions leading to hyperpolarization. The potential reaches –70 to –80 mV. It is because of sodium-potassium pump.
VISUAL PATHWAY
Visual pathway or optic pathway is the nervous pathway that transmits impulses from retina visual center in cerebral cortex
VISUAL RECEPTORS
Rods and cones which are present in the retina of eye form the visual receptors. Fibers from the visual receptors synapse with dendrites of bipolar cells of inner nuclear layer of the retina
CONNECTIONS OF VISUAL RECEPTORS TO OPTIC NERVE
Two pathways exist between the visual receptors and optic nerve:
1. Private pathway � 2. Diffuse pathway.
PRIVATE PATHWAY
The individual cones in fovea centralis are connected to separate bipolar cells. Each bipolar cell is connected to separate ganglionic cell, namely midget ganglionic cell. Thus, individual cone is connected to an individual optic nerve fiber. This type of private pathway is responsible for visual acuity and intensity discrimination.
DIFFUSE PATHWAY
A number of cones and rods are connected with a polysynaptic bipolar cell. The bipolar cells are connected to diffused ganglionic cells. So, there is great overlapping. This type of pathway is present outside
USER COURSE OF VISUAL PATHWAY
Retina:
The process begins with the photoreceptor cells (rods and cones) in the retina at the back of the eye. These cells convert light signals into electrical signals.
Optic Nerve:
The electrical signals generated by the photoreceptor cells are transmitted via the optic nerve, which exits the back of the eye and carries visual information to the brain.
Optic Chiasm:
At the base of the brain, near the hypothalamus, the optic nerves from each eye partially cross over at a structure called the optic chiasm. This crossover allows for binocular vision, where information from both eyes is combined.
Optic Tract:
After the optic chiasm, the visual information continues as the optic tract, which contains fibers from both eyes. These fibers carry information from the contralateral (opposite) visual field.
Lateral Geniculate Nucleus (LGN):
The optic tract terminates in the lateral geniculate nucleus, a structure within the thalamus. Here, visual information is relayed and processed before being sent to the visual cortex.
Optic Radiation:
From the LGN, visual information is relayed via nerve fibers known as the optic radiation, also called the geniculocalcarine tract. These fibers project to the primary visual cortex (also known as V1 or the striate cortex) located in the occipital lobe of the brain.
Visual Cortex:
The primary visual cortex is the main processing center for visual information.
From the primary visual cortex, visual information can be further processed in higher-order visual areas and other regions of the brain involved in visual perception, memory, and cognition. This pathway is crucial for visual perception.
PRIMARY SENSORY CODING
PRIMARY SENSORY CODING
Primary sensory coding refers to the process by which sensory stimuli from the environment are converted into electrical signals that can be interpreted by the nervous system. Each sensory modality, such as vision, hearing, touch, taste, and smell, has its own specialized receptors and neural pathways for transmitting information to the brain.
Types of Sensory Receptors and the Stimuli They Detect
Modality of Sensation—The “Labeled Line”
Principle:�
“Each of the principal types of sensation that we can experience—pain, touch, sight, sound, and so forth—is called a modality of sensation.� We experience these different modalities of sensation but nerve fibers transmit only impulses.”
The specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle.
Transduction
Definition of Transduction:� Transduction refers to the process by which sensory stimuli are converted into neural signals. It occurs at the level of sensory receptors, which are specialized cells located in sensory organs such as the eyes, ears, skin, and taste buds.
Encoding and Coding
Encoding
Definition:� Encoding refers to the representation of sensory information in the nervous system. It involves the conversion of various features of a sensory stimulus into neural signals.
Coding:
Definition:
Coding refers to the specific patterns of neural activity that represent different features of a sensory stimulus. It involves how sensory information is organized and represented by populations of neurons in the nervous system.
specific sensory systems
2. Auditory System:��Sensory Input: � The auditory system processes information from sound waves received by the ears.�Sensory Receptors: � Hair cells in the cochlea transduce sound vibrations into neural signals.��Coding Mechanisms:�Frequency Coding:� Different frequencies of sound are encoded by the firing rates of auditory nerve fibers, with higher frequencies leading to higher firing rates.�Tonotopic Mapping: � Spatial organization in the cochlea and auditory cortex reflects the frequency content of auditory stimuli�Temporal Coding:� Precise timing of action potentials conveys information about the timing and rhythm of sound stimuli.
3. Somatosensory System:�Sensory Input: �The somatosensory system processes information from touch, pressure, temperature, and pain receptors in the skin and internal organs.�Sensory Receptors:� Mechanoreceptors, thermoreceptors, and nociceptors transduce mechanical, thermal, and painful stimuli into neural signals.��Coding Mechanisms:�Spatial Coding: �Different regions of the body are represented in somatotopic maps in the somatosensory cortex, reflecting the spatial organization of sensory receptors.�Intensity Coding: �The intensity of tactile stimuli is encoded by the firing rates of sensory neurons, with stronger stimuli leading to higher firing rates. �Adaptation and Coding Changes: �Sensory adaptation and plasticity mechanisms modulate the coding of somatosensory information in response to changing environmental conditions or behavioral demands.
4. Olfactory System:��Sensory Input: �The olfactory system processes information from chemical stimuli detected by olfactory receptors in the nasal cavity.�Sensory Receptors: �Olfactory receptor neurons transduce odour molecules into neural signals.��Coding Mechanisms:�Sparse Coding:�Different odours are represented by the selective activation of specific combinations of olfactory receptor neurons, leading to a sparse and combinatorial representation of odor quality.�Adaptation and Discrimination: �Olfactory adaptation and discrimination mechanisms allow for the detection of novel odors and the discrimination between similar odorants.
NEURAL CIRCUIT
Sensory pathways are the routes through which sensory information travels from the peripheral sensory organs to the central nervous system (CNS), where it is processed and interpreted.
General Organization:� Sensory pathways typically consist of three main stages:
Peripheral Stage: � Sensory receptors detect stimuli in the external environment and convert them into neural signals.�Ascending Stage:� Neural signals are transmitted through a series of relay nuclei and pathways to higher brain regions for processing and perception.�Central Stage: � Higher brain regions integrate and interpret sensory information, leading to conscious perception and behavioral responses.
Integration of Sensory Information:
Once sensory information reaches the CNS, it undergoes integration, where it is processed and combined with other sensory inputs and internal cognitive factors. Integration occurs at multiple levels of the CNS, including sensory relay nuclei, association areas, and multimodal integration regions.
Different sensory modalities are integrated to form a unified perception of the external world, allowing us to perceive complex sensory experiences.��1. Processing in Sensory areas:
Sensory information is initially processed in primary sensory areas of the brain, which are specialized for the processing of specific sensory modalities. For example, visual information is processed in the primary visual cortex (V1) in the occipital lobe, auditory information in the primary auditory cortex in the temporal lobe, and somatosensory information in the primary somatosensory cortex (S1) in the parietal lobe. These primary sensory areas analyze basic features of sensory stimuli, such as orientation, frequency, intensity, and location.
2. Higher-Level Processing and Perception:
After initial processing in primary sensory areas, sensory information is further analyzed and integrated in higher-order brain regions, including association cortices and multimodal integration areas. Association areas integrate sensory inputs with memories, emotions, and cognitive processes to form complex perceptions and interpretations of sensory stimuli. Multimodal integration areas combine inputs from multiple sensory modalities to create a unified and coherent representation of the external environment.
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