Human Sensory System PDF

Sense: Sense: A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. Senses used by non-human organisms are even greater in variety and number. In organisms, a sensory organ consists of a group of inte...

Sense: Sense: A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. Senses used by non-human organisms are even greater in variety and number. In organisms, a sensory organ consists of a group of interrelated sensory cells that respond to a specific type of physical stimulus. Via cranial and spinal nerves (nerves of the Central and Peripheral nervous systems), the different types of sensory receptor cells in sensory organs transduce sensory information from these organs towards the central nervous system, finally arriving at the sensory cortices in the brain, where sensory signals are processed and interpreted. Sense: Sensory systems, or senses, are often divided into: 1- external (exteroception) sensory system: Human external senses are based on the sensory organs of the eyes, ears, skin, nose, and mouth. 2- internal (interoception) sensory systems: Internal sensation detects stimuli from internal organs and tissues. Internal senses possessed by humans include the vestibular system (sense of balance) sensed by the inner ear, as well as others such as nociception (pain). Further internal senses lead to signals such as hunger, thirst, suffocation, and nausea. Receptors: Sensory receptors are the cells or structures that detect sensations. Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. During transduction, physical stimulus is converted into action potential by receptors and transmitted towards the central nervous system for processing. Different types of stimuli are sensed by different types of receptor cells. - Receptor cells can be classified into types on the basis of three different criteria: cell type, location, and function. The different types of functional receptor cell types are: 1- mechanoreceptors: Mechanoreceptors are sensory receptors that respond to mechanical forces, such as pressure and vibration, as well as the sensation of sound and body position (balance) 2- photoreceptors: Photoreceptors convert light (visible electromagnetic radiation) into signals. The primary types of photoreceptors are: 1- Cones: They are photoreceptors that respond significantly to color. 2- Rods: They are photoreceptors that are very sensitive to the intensity of light, allowing for vision in dim lighting. The different types of functional receptor cell types are: 3- chemoreceptors: Chemoreceptors interpret Chemical stimuli, such as an object's taste or smell. Chemoreceptors can be subdivided into two main types: direct and distant. 1- Direct chemoreceptors interact directly with the stimulus e.g. the taste chemoreceptors on your taste buds bind chemical compounds allowing you to taste your coffee. 2- Distance or distant chemoreceptors allow us to sense something from a distance and detect stimuli. e.g. when olfactory receptors are bound by odor molecules allowing you to smell your delicious coffee. 4- thermoreceptors: Thermoreceptors are sensory receptors that respond to varying temperatures, it's either sensitive to temperatures above (heat) or below (cold) normal body temperature. They're Located in the skin and respond to heat and cold to assist in body temperature awareness. These receptors will fire off a warning if you miss your cup and pour hot water on your hand instead and trigger motor neurons to get you to move. The different types of functional receptor cell types are: 5- nociceptors: pain receptors interpret the presence of tissue damage, from -sensory information from mechano-, chemo-, and thermoreceptors. Thermal nociceptors are activated by noxious heat or cold at various temperatures. Mechanical nociceptors respond to excess pressure or mechanical deformation. Chemical nociceptors respond to a wide variety of chemicals, some of which are signs of tissue damage. They are involved in the detection of some spices in food. Nociceptors can be subdivided into two main types: 1- somatic (more peripheral: skin, muscles, bones, and joints) 2- visceral (internal organs and their supporting tissues). Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds. Human sensory system: The human sensory system consists of the following subsystems: 1. Visual system 2. Auditory system 3. The somatosensory system 4. Gustatory system 5. Olfactory system 6. Vestibular system 7. Interoceptive system Human sensory system: 1- Visual system (vision): The visual system, or sense of sight, is based on the transduction of light stimuli received through the eyes and contributes to visual perception. The iris in the eye is the colored part that controls the size and diameter of the pupil, which directly affects the amount of light entering the eyes. Behind the lens of the eye lies the vitreous body. It is filled with a gelatinous material called vitreous humor. This substance gives shape to the eyeball and also transmits light to the very back of the eyeball, where the retina is found. This retina contains photoreceptors, which detect light. The visual system detects light on photoreceptors in the retina of each eye that generate electrical nerve impulses for the perception of varying colors and brightness. Human sensory system: There are two types of photoreceptors: 1- rods: These sensors function in low light and are found at the edges of the retina. They also aid in peripheral vision. Rods are very sensitive to light but do not distinguish colors 2- cones: These types of retinal cells work best in bright light, detecting fine details and color. There are three types of cones for detecting three primary colors of light, namely: blue, red, and green. Typically, color blindness occurs when any one of these types of cones is not present. The three types of cones, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low- light vision is in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory. Human sensory system: 2- Auditory system (hearing): Hearing is the transduction of sound waves into a neural signal that is made possible by the structures of the ear. The large, fleshy structure on the lateral aspect of the head is known as the auricle. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes are then attached to the inner ear, where the sound waves will be transduced into a neural signal. Human sensory system: Mechanoreceptors turn motion into electrical nerve pulses, which are located in the inner ear. Since sound is vibration, propagating through a medium such as air, the detection of these vibrations, that is the sense of hearing, is a mechanical sense because these vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear, which detect mechanical motion of the fibers within a range of about 20 to 20,000 hertz, with substantial variation between individuals. Hearing at high frequencies declines with an increase in age. Human sensory system: 3- Somatosensory system (touch): The sense of touch is referred to as tactile perception. The modalities of somatosensation include pressure, vibration, light touch, tickle, itch, temperature, and pain. Somatosensation, also called tactician is a perception resulting from the activation of neural receptors, generally in the skin including hair follicles, but also in the tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure. The touch sense of itching caused by insect bites or allergies involves special itch-specific neurons in the skin and spinal cord. The skin contains general receptors that can detect touch, pain, pressure, and temperature. They are present throughout the skin. Skin receptors generate an impulse, and when activated, is carried to the spinal cord and then to the brain. Human sensory system: Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli. - Low-frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. - Merkel cells are located in the stratum basale of the epidermis. - Deep pressure and vibration are transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. - Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. - Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin Human sensory system: 4- Gustatory system (taste): The gustatory system or the sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). A few recognized submodalities exist within taste: sweet, salty, sour, bitter, and umami. The taste buds contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, and glossopharyngeal. - Salty and sour taste submodalities are triggered by the cations Na+ and H+, respectively. - The other taste modalities result from food molecules binding to a G protein–coupled receptor. - The sweet taste is the sensitivity of gustatory cells to the presence of glucose (or sugar substitutes) dissolved in the saliva. - The bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. Human sensory system: 5- Olfactory system (smell): The nose is an olfactory organ. The olfactory receptor neurons are located in a small region within the superior nasal cavity. Olfactory cells have cilia that project into the nasal cavity, and on the other end of the cell, are the olfactory nerve fibers. The olfactory cells are chemoreceptors, which means that the olfactory cells have protein receptors that can detect subtle differences in chemicals. These chemicals bind to the cilia, which conduct a nerve impulse that is carried to the brain. The brain then translates these impulses into a meaningful smell. In the brain, olfaction is processed by the olfactory cortex. Olfactory receptor neurons in the nose differ from most other neurons in that they die and regenerate on a regular basis. - Loss of the sense of smell can result in food tasting bland Other Sense Organs: Besides The five sense organs, there are another two that help to orient us to the world. They are: - Vestibular System: The vestibular system acts as a sensory system of the body and is responsible for transmitting information to our brain about the motions, head position, and spatial orientation. This system is also involved with motor functions and helps in: 1. Maintain our body posture. 2. Maintaining our body balance. 3. Stabilize our head and body during movement. Thus, the vestibular system is essential for normal movement and equilibrium. - Proprioception system: The proprioception system is described as the conscious or unconscious awareness of joint position. This system helps the body to identify the muscles, joints, and limbs located in 3D space and the direction it is moving in relation to the body. Walking or kicking without looking at our feet, balancing on one leg, touching the nose with eyes closed, and the ability to sense the surface on which we are standing, are a few examples of the proprioception system.  The human sensory and perceptual system Primary Physical stimulus Sensory organ Sensory system Cranial nerves Cerebral cortex associated Name perceptions Light Eyes Visual system Optic (II) Visual cortex Visual perception Sight (vision) Vestibulocochlear Auditory Sound Ears Auditory system Auditory cortex Hearing (audition) (VIII) perception Olfactory perception, Chemical substance Nose Olfactory system Olfactory (I) Olfactory cortex Gustatory Smell (olfaction) perception (taste or flavor) Facial Gustatory Chemical substance Mouth Gustatory system (VII), Glossophary Gustatory cortex perception (taste Taste (gustation) ngeal (IX) or flavor) Trigeminal (V), Tactile Somatosensory Glossopharyngeal Somatosensory perception (mech Position, motion, temperature Skin Touch (tactition) system (IX) + Spinal cortex anoreception, the nerves rmoception) Synapse Synapse Synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from one neuron to another. At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell. In many synapses, the presynaptic part is located on an axon, and the postsynaptic part is located on a dendrite or soma. Synapses (at least chemical synapses) are stabilized in position by synaptic adhesion molecules (SAMs) projecting from both the pre-and post-synaptic neuron and sticking together where they overlap. What is the structure of a synapse? The synapse consists of three main parts:.1 The pre-synapse: this is the axon terminal of the neuron that is sending information. It releases neurotransmitters into the synaptic cleft to communicate with the next neuron or cell..2 The synaptic cleft: this is a tiny gap (about 20-30 nanometers wide) between the pre- synapse and the post-synapse. It is filled with a fluid called the interstitium, which helps facilitate the transmission of neurotransmitters..3 The post-synapse: this is the membrane of the neuron or cell that is receiving the information. It contains protein channels called receptors, which bind to the neurotransmitters released by the pre-synapse. What is the structure of a synapse? The post-synapse can be another neuron, a gland, an organ, or a muscle. The terms pre- and post- refer to the positions of the neuron relative to the synaptic cleft. The pre- synapse is the neuron before the synaptic cleft, while the post-synapse is the neuron after the synaptic cleft. What are the two main types of synapses? 1- Electrical synapse: Electrical synapses are a type of synapse that directly connects other cells through a protein channel called a gap junction, connexon, or pore. These protein channels are made of connexin proteins and allow charged ions and messenger proteins to pass through uninhibited. One of the main advantages of electrical synapses is that they allow for fast and efficient communication between cells. Because the transmission of information is direct and uninhibited, electrical synapses are able to transmit signals much faster than chemical synapses, which rely on the diffusion of neurotransmitters across the synaptic cleft. they are present in the central nervous system, retina, and olfactory bulbs of humans, where they play an important role in synchronizing and coordinating the activity of neurons. Additionally, because the transmission of information in electrical synapses can flow in both directions, they can also allow for bi-directional communication between cells. What are the two main types of synapses? 2- Chemical synapses: Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. At a chemical synapse, one neuron releases neurotransmitter molecules into a small space (the synaptic cleft) that is adjacent to another neuron. The neurotransmitters are contained within small sacs called synaptic vesicles and are released into the synaptic cleft by exocytosis. These molecules then bind to neurotransmitter receptors on the postsynaptic cell. Finally, the neurotransmitters are cleared from the synapse through one of several potential mechanisms including enzymatic degradation or re-uptake by specific transporters either on the presynaptic cell or on some other neuroglia to terminate the action of the neurotransmitter.  CHEMICAL SYNAPSE ELECTRICAL SYNAPSE Chemical synapse is a cell-to-cell connection via which neurotransmitters The electrical synapse is a cell association between two nerve cells where ions transfer nerve impulses in one way. are used to transmit nerve impulses rapidly. Nerve signals are transmitted as electric signals via gap junctions or low- Neurotransmitters transport nerve impulses as a chemical signal. impedance bridges. Found in higher vertebrates. Found in lower vertebrates and invertebrates. Transmission of signals occurs in one-way. Transmission of signals occurs two-way. Large in size (10-20 nm). Smaller in size (0.2 nm). Synaptic knobs are densely packed with synaptic vesicles and mitochondria. Synaptic knobs do not have any synaptic vesicles, only a few mitochondria. The postsynaptic membrane contains chemoreceptors. The postsynaptic membrane is devoid of chemoreceptors. Information is transmitted slowly. Information is transmitted at a high rate. More vulnerable to fatigue. Less vulnerable to fatigue. Sensitive to hypoxia and pH. Insensitive to hypoxia and pH. Identified in the retina, olfactory bulb, cerebral cortex, lateral vestibular Found in most of the neuron junctions. nucleus, and hippocampus. Different types of synaptic connections: 1. Axodendritic: This is the most common type of synapse in the human body. It occurs when the axon of one neuron connects to the dendrites of another neuron. 2. Axosomatic: This type of synapse occurs when the axon of one neuron connects to the cell membrane or body (soma) of another neuron. 3. Axo-axonic: This type of synapse occurs when the axon of one neuron connects to the axon of another neuron. These synapses are usually inhibitory. 4. Dendro-dendritic: This type of synapse occurs between the dendrites of two different neurons. 5. Neuromuscular: This type of synapse occurs when the axon of a neuron connects to a muscle. These synapses are highly specialized and convert electrical impulses in the motor neuron into the electrical activity that causes muscle contractions. All neuromuscular junctions use acetylcholine as a neurotransmitter. It's also worth noting that neurons can connect to all parts of the body, not just other neurons. For example, neurons can send axons into the interstitial spaces or to blood vessels.