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Tutorials » The Human Physiology » Sensory systems

Sensory systems
- The Human Physiology

A sensory system is a part of nervous system consisting of sensory receptors that receive stimuli from internal and external environment, neural pathways that conduct this information to brain and parts of brain that processes this information. The information is called sensory information and it may or may not lead to conscious awareness. If it does, it can be called sensation.

Receptors
Specialized endings of afferent neurons or separate cells that affect ends of afferent neurons. They collect information about external and internal environment in various energy forms-and energy that activates a receptor is called a stimulus. Stimulus energy is first transformed into graded or receptor potentials and the process by which a stimulus is transformed into an electrical response is called stimulus transduction.
Each receptor is specific to a certain type of stimulus, which is called its adequate stimulus. Specificity also exists in the range of stimulus energies that the receptor responds to. However, a receptor can be activated by a nonspecific stimulus if its intensity is sufficiently high.

Receptor Potential
Gating of ion channels in specialized receptor membranes allows a change in ion fluxes across the membrane, generating a graded receptor potential. The graded potential initiates an action potential, frequency and NOT magnitude of which is determined by magnitude of the graded potential. Magnitude of the receptor potential is determined by stimulus strength, summation of receptor potentials, and receptor sensitivity. The decrease in sensitivity with a constant stimulus is called adaptation.

Neural Pathways in Sensory Systems
A single afferent neuron with all its receptor endings makes a sensory unit. When stimulated, this is the portion of body that leads to activity in a particular afferent neuron is called the receptive field of that neuron.
Afferent neurons enter the CNS, diverge and synapse upon many interneurons. These afferent neurons are called sensory or ascending pathways and specific ascending pathways if they carry information about a single type of stimulus. The ascending pathways reach the cerebral cortex on the side opposite to where their sensory receptors are located.
Specific ascending pathways that transmit information from somatic receptors and taste buds go to somatosensory cortex (parietal lobe), the ones from eyes go to visual cortex (occipital lobe), and the ones from ears go to auditory cortex (temporal lobe).
Olfaction is NOT represented in cerebral cortex.
Nonspecific ascending pathways consist of polymodal neurons and are activated by sensory units of several types. These pathways are important in alertness and arousal.
Cortical association areas, lying outside primary cortical sensory areas, participate in more complex analysis of incoming information such as comparison, memory, language, motivation, emotion etc.

Primary Sensory Coding

Sensory systems code 4 aspects of a stimulus:
(1)   Stimulus Type (modality). All receptors of a single afferent neuron are sensitive to the same type of stimulus.
(2)   Stimulus Intensity. An increased stimulus results in a larger receptor potential,, leading to a higher frequency of action potential. Stronger stimuli also affect a larger area and recruit a larger number of receptors.
(3)   Stimulus Location.  Coded by site of the stimulated receptor. The precision of location, called acuity, is negatively correlated with the amount of convergence in ascending pathways, size of the receptive field and overlap with adjacent receptive fields. Response is highest at the center of receptive field since receptor density is the highest there. Using lateral inhibition, a process by which information from neurons at the edge of a stimulus is inhibited, acuity can be increased.
(4)   Stimulus Duration. Rapid adapting receptors respond rapidly at the onset of stimulus but slow down or stop firing during the remainder of stimulus (they adapt quickly). They are important in signaling rapid change. Slow adapting receptors maintain their response at or near the initial level of firing through the duration of stimulus and are important in signaling slow changes.

Somatic Sensation
Sensations from skin, muscles, bone are initiated by somatic receptors. Receptors for visceral sensations are similar.

Touch-Pressure
Mechanoreceptors in the skin are of 2 types, rapid and slow adapting ones.

Posture and Movement
Muscle-spindle stretch receptors, occurring in skeletal muscles respond to the absolute magnitude and the rate of muscle stretch. Mechanoreceptors in joints, tendons, ligaments and skin also participate.

Temperature
Thermoreceptors are of two types, one that responds to an increase and the other that responds to a decrease in temperature.

Pain
Nociceptors respond to intense mechanical deformation, excessive heat etc. which cause tissue damage and many chemicals that are released by damaged cells or cells of immune system.
If the initial stimulus of pain leads to an increased sensitivity to subsequent painful stimuli it is called hyperalgesia. If descending pathways inhibit the transmission of pain stimuli, it leads to a suppression of pain and this is called stimulation-produced analgesia.
If both visceral and somatic afferent converge on the same interneuron, excitation of one can lead to excitation of the other, leading to the pain being felt at a site different from the actual injured part. This is called referred pain.
Stimulating non-pain afferent fibers can inhibit neurons in the pain pathway and this therapy is called transcutaneous electric nerve stimulation (TENS). Rubbing on a painful area and acupuncture work for the same reason.

Vision

Optics
Receptors in eye are sensitive to only the visible light of electromagnetic spectrum. Lens and cornea focus impinging light rays into an image at fovea centralis area of retina. Light passing from air into cornea is bent and the curved surface of cornea plays the major role in focusing.
Changes in lens shape make adjustments (accommodation) for distance. Lens shape is controlled by zonular fibers that are in turn controlled by the smooth ciliary muscle. To focus on distant objects, the lens is pulled into a flattened oval shape. For near vision, the pull is removed to make the lens more spherical and provide additional bending for light rays.
Cells of the lens lose their organelles and are therefore transparent. The lens become progressively opaque as newer cells replace older ones, which accumulate in the lens. This is called cataract.
If the lens loses its elasticity (due to age) and cannot assume a spherical shape, it leads to loss of near vision and this is called presbyopia.
If images of far objects focus at a point in front of retina, the eye is nearsighted or myopic and far vision is poor. If images of near objects focus at a point behind retina, the eye is farsighted or hyperopic and near vision is poor. If the lens or cornea is not smooth, it is called astigmatism.
The lens separates an anterior chamber filled with aqueous humor and a posterior chamber filled with vitreous humor. If aqueous humor is formed faster than it is removed, it results in an increased pressure within the eye. This can cause irreversible blindness with the death of optic nerves and it is called glaucoma.
The pigmented, opaque iris that has a central hole, the pupil, controls amount of light entering the eye. The iris has smooth muscles, innervated by autonomic nerves. Stimulation of the sympathetic nerves dilates the pupil to let in mare light when light is poor and stimulation of the parasympathetic nerves constricts the pupil to allow in less light when light is bright.

Photoreceptor Cells

Rods - sensitive and responding to low light and cones - less sensitive and responding to bright light. There are three kinds of cones containing red-, green-, or blue sensitive pigment.
Photoreceptors contain photopigments, which absorb light. There are 4 photopigments, rhodopsin in the rods and one in each of the 3 cone types. Each photopigment contains an integral membrane protein, opsin, which binds a light sensitive chromatophore molecule. The chromatophore - retinal (a derivative of vitamin A) is the same in all the 4 photopigments. The opsin is different in each type of photopigment, absorbing light at different wavelengths of the spectrum.
Light activates retinal, causing it to change shape and triggering a hyperpolarization in the bipolar cells, which synapse with the photoreceptor cells. After its activation, retinal changes back to its resting shape by light-independent mechanisms and the photoreceptor cell is depolarized

Neural Pathways
Photoreceptor cells synapse with neurons called bipolar cells which, in turn, synapse with ganglion cells that produce the first action potentials in the chain. Axons from ganglion cells form the optic nerve, which crosses over to the opposite side of the optic chiasm.

Sound Transmission in the Ear

Outer Ear (Pinna/Auricle) - Directs and amplifies sound waves.

External Auditory Canal –the ear canal leading from the outside to the middle ear cavity
Tympanic membrane (Eardrum) - Vibrates at the frequency of sound waves.
Middle Ear Cavity - Filled with air. Has a movable chain of 3 bones, the malleus, incus and stapes that couple and amplifies the vibrations in the tympanic membrane to the
Oval window- a membrane covered opening separating the middle ear and the
Inner Ear (cochlea)
Scala vestibuli - Filled with fluid
Cochlear duct - Lined by the basilar membrane upon which sits the organ of Corti containing the receptor cells.

Organ of Corti
Receptor cells of the organ of Corti, the hair cells, are mechanoreceptors that have hairlike stereocilia. Vibration of the basilar membrane, with which the hair cells are attached, stimulates the hair cells and the pressure waves are transformed into receptor potentials.

Neural Pathways
Afferent neurons from the hair cells form the cochlear nerve.

Hearing
Entire audible range extends from 20 to 20,000 Hz.

Vestibular System
A series of fluid filled tubes in the inner ear that connect with each other and the cochlear duct containing hair cells that detect changes in motion
2 Parts:

Semicircular Canals
Detect angular acceleration during rotation of the head along the three axes.

Utricle and Saccule
Provide information about linear acceleration and changes in head position relative to gravity.

Vestibular Information
Information from hair cells in the vestibular apparatus is transmitted to the parietal lobe and is integrated with information from other parts of body, leading to sense of posture and movement. Unexpected inputs from the vestibular system leads to vertigo or motion sickness.

Chemical Tastes

Taste
Taste buds found on the tongue respond to 4 basic modalities, sweet, sour, salty and bitter. Each groups has a distinct transductional system. Organized into independent pathways but a single receptor cell may respond to more than one taste category in various degrees.
Smell
Odor is related to chemical structure of a substance. Olfactory receptor cells lie in the olfactory epithelium in upper part of nasal cavity. These cells have several long, non-motile cilia, which contain binding sites for olfactory stimuli. Each cell contains one type of receptor. Axons of olfactory receptor cells of same specificity synapse together. Information is passed into the olfactory cortex in the limbic system.

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