Which of the following is not characteristic of neurons? A They conduct impulses. B They have extreme longevity. C They are mitotic. D They have an exceptionally high metabolic rate. A axon B dendrite C neurolemma D Schwann cell. Specific motor and sensory functions are localized in specific areas called domains whereas memory and language have overlapping domains. The corpora quadrigemina superior colliculi are visual reflex centers, whereas the inferior colliculi are auditory reflex centers.
Cell bodies of the somatic motor neurons of the spinal nerves are located in the ventral horn of the spinal cord. Cerebrospinal fluid circulates within the ventricles of the brain and in the subarachnoid space outside the brain. The sole of the foot is stimulated with a dull instrument.
Match the following: 26 Controls the outputs of the cortex and regulates motor activity. Diff: 1 Page Ref: ; Fig. Reciprocal inhibition means that while one sensory nerve is stimulated, another sensory neuron for synergistic muscles in the same area is inhibited and cannot respond. The stimulus energy must be converted into the energy of a graded potential called a transduction potential. Sign in. Consequently, the image focal point would be beyond the retina if the eye's lens system were not adjusted.
During accommodation, the lens curvature increases, increasing the refractive power of the eye and focusing the image on the retina. If a viewed object is brought closer to the eye, the light rays from the object diverge at a greater angle relative to the eye Figure Consequently, the nearer the object of view, the greater the angle of incidence of light rays on the cornea, and the greater the refractive power required to focus the light rays on the retina.
The cornea has a fixed refractive power i. However, altering the tension of the zonules on the elastic lens capsule can alter the lens shape. The change in the refractive properties of the eye is called the accommodation or "near point" process.
In the normal eye under resting distant vision conditions, the ciliary muscles are relaxed and the zonules are under tension Figure In this case, the lens is flattened, which reduces the refractive power of the lens to focus on distant objects.
When an object is closer to the eye i. The ciliary muscle contracts, pulling the ciliary processes toward the lens remember the muscle acts as a sphincter. This action releases tension on the zonules and the lens capsule. The reduced tension allows the lens to become more spherical i. The increase in lens curvature increases the lens refractive power to focus on near objects. Consequently, as an object is moved closer to the viewer, his eyes accommodate to increase the lens curvature, which increases the refractive power of his eye Figure The lens is flattened by the tension on the zonules and the lens capsule.
However, in the accommodation process, the ciliary muscles contract and, acting like a sphincter muscle, decrease the tension on the zonules and lens capsule.
The lens becomes more spherical with its anterior surface shifting more anteriorly into the anterior chamber. Presbyopia : In presbyopia, there is normal distance vision, but lens accommodation is reduced with age. With age, the lens loses its elasticity and becomes a relatively solid mass. During accommodation, the lens is unable to assume a more spherical shape and is unable to increase its refractive power for near vision Figure As a result, when an object is less than 30 ft.
For the presbyopic eye a corrective lens that converges the light rays i. A convex lens i. These lenses refract the light rays so they strike the surface of the cornea at a smaller angle.
However, because the corrective lens increases the refractive power, the presbyope with convex lenses will have problems with distance vision.
Consequently, the corrective lenses are often half lenses i. Hyperopia : In hyperopia Figure When viewing distant objects, the image is focused at a point beyond the retina. The hyperopic lens system is too weak and the image is focused beyond the retina. The young hyperope can compensate by using lens accommodation, i. We call the hyperope "far-sighted" hypermetropic because the power of accommodation used for distance vision cannot be used for near vision.
As the hyperope ages and becomes presbyopic, the power of accommodation is diminished. Consequently, the middle aged hyperope may have a limited range near and far of vision. To correct this effect of aging, the refractive power of the eye is increased with convex lenses Figure Myopia : In myopia Figure When viewing distant objects, the image is focused at a point in front of retina.
The refractive power of the eye's lens system is too strong and the image is focused in front of the retina. The uncorrected myopic eye is "near-sighted" because it can focus unaided on near objects.
That is, the young myope will see distant objects as blurred, poorly defined images but can see nearby small objects clearly remember nearby objects emit divergent light rays. For distance vision, the refractive power of the myopic eye lens system is corrected with concave lenses that diverge the light rays entering the eye Figure Note that as the power of accommodation diminishes with age, near vision is also affected in the presbyopic-myopic eye.
The mature myope may require bifocals, the upper half of the lens diverging light rays for distance vision and the lower half with no or low converging power for near vision. Astigmatism : An astigmatism results when the cornea surface does not resemble the surface of a sphere e.
In an eye with astigmatism, the image of distant and near objects cannot be focused on the retina Figure Astigmatism is corrected with a cylindrical lens having a curvature that corrects for the corneal astigmatism.
The cylindrical lens directs light waves through the astigmatic cornea to focus a single, clear image on the retina. You will now learn about the retinal neurons and the laminar structure of the retina, and the ways in which the light-sensitive receptors of the eye convert the image projected onto the retina into neural responses. The light sensitive retina forms the innermost layer of the eye Figure The retina is the innermost coat of the eye and consists of the retinal pigment epithelium and neural retina.
The retina covers the choroid and extends anteriorly to just behind the ciliary body. The retina consists of neurons and supporting cells. The retina is derived from the neural tube and is, therefore, part of central nervous system. It consists of two parts, the retinal pigment epithelium, which separates the middle, choroid coat of the eyeball from the other innermost component and the neural retina Figure The neural retina contains five types of neurons Figure The retina is a laminated structure consisting of alternating layers of cell bodies and cell processes Figure The neural retina consists of at least five different types of neurons: the photoreceptors rods and cones , horizontal cell, bipolar cell, amacrine cell and ganglion cell.
The receptor cells synapse with bipolar and horizontal cells in the outer plexiform layer. The bipolar cells, in turn, synapse with amacrine and ganglion cells in the inner plexiform layer The axons of the retinal ganglion cells exit the eye to form the optic nerve. The innermost layers are located nearest the vitreous chamber, whereas the outermost layers are located adjacent to the retinal pigment epithelium and choroid. The most important layers, progressing from the outer to inner layers, are:.
Notice that light passing through the cornea, lens and vitreous must pass through most of the retinal layers before reaching the light-sensitive portion of the photoreceptor; the outer segment in the receptor layer. Notice also that in the region of the fovea where the image of the central visual field center is focused, the retina consists of fewer layers Figure The area around the fovea, the surrounding macula, is thicker because it contains the cell bodies and processes of retinal neurons receiving information from the receptors in the fovea.
The optic disc is formed by the retinal ganglion cell axons that are exiting the retina. It is located nasal to the fovea Figure This region of the retina is devoid of receptor cells and composed predominantly by the optic nerve layer. Consequently, it is the structural basis for the 'blind spot" in the visual field.
The fovea and macula are colored as they appear when stained for Nissl substance, which is most abundant in the neuron cell body. The human has two types of photoreceptors : the rods and cones Figure They are distinguished structurally by the shapes of their outer segments.
The photopigments of the rods and cones also differ. The rod outer segment disks contain the photopigment rhodopsin, which absorbs a wide bandwidth of light. The cones differ in the color of light their photopigments absorbs: one type of photopigment absorbs red light, another green light, and a third blue light.
As each cone receptor contains only one of the three types of cone photopigment, there are three types of cones; red, green or blue. Each cone responds best to a specific color of light, whereas the rods respond best to white light 2.
The rod and cone photopigments also differ in illumination sensitivity; rhodopsin breaks down at lower light levels than that required to breakdown cone photopigments. Consequently, the rods are more sensitive - at least at low levels of illumination.
The human visual system has two subsystems that operate at different light energy levels. The scotopic, dark-adapted system operates at low levels of illumination, whereas the photopic, light-adapted system operates at high levels of illumination. The photoreceptors are neurons that have a dendritic component the outer segment and an axonal component that forms synaptic terminals. Notice that the location of the optic disc relative to the fovea corresponds to the location of the blind spot relative to the visual field center.
Biochemical processes in the photoreceptors participate in dark and light adaptation. Notice when you enter a darkened room after spending time in daylight, it takes many minutes before you are able to see objects in the dim light. This slow increase in light sensitivity is called the dark-adaptation process and is related to the rate of regeneration of photopigments and to the intracellular concentration of calcium 3.
A contrasting, but faster, process occurs in high levels of illumination. When you are fully dark-adapted, exposure to bright light is at first blinding massive photopigment breakdown and stimulation of photoreceptors and is followed rapidly by a return of sight. This phenomenon, light adaptation, allows the cone response to dominate over rod responses at high illumination.
The photoreceptors exhibit a fairly high basal release of glutamate. When light strikes the photoreceptor cell, it initiates a biochemical process in the cell that reduces the release of glutamate from its axon terminal. The glutamate , in turn, affects the activity of the bipolar and horizontal cells, which synapse with the photoreceptor. The bipolar cells, in turn, synapse with amacrine and retinal ganglion cells. It is the axons of the retinal ganglion cells that exit the eye as the optic nerve and terminate in the brain.
Notice that the direct pathway for the transmission of visual information from the eye to the brain includes only the receptor cell, bipolar cell and ganglion cell. The horizontal cells modulate the synaptic activity of receptor cells and, thereby, indirectly affect the transmission of visual information by bipolar cells. Similarly the amacrine cells modulate the synaptic activity of the retinal bipolar and ganglion cells, thereby affecting the transmission of visual information by the ganglion cells.
All multipolar neurons have two characteristics: -- more than two processes emanate from the cell body, and -- the cell body receives synaptic input just like the dendrites.
Neurons Three major categories of neurons are recognized: Bipolar neurons are relatively rare. The arrow points to an axon of a Purkinje neuron.
At middle of the slide, cell bodies of three Purkinje cells neurons are visible as black, rounded, profiles.
Cell processes, termed dendrites , extend superficially from the cell body into the molecular layer. A small axon arrow emerges from each cell body and runs through the granue cell layer. Note: Axons and dendrites of neurons are normally visible only in special stained sections such as this Golgi preparation. Schematic cartoon of a ventral horn Somatic Efferent SE multipolar neuron. SE neurons have their cell bodies in the CNS; their axons join peripheral nerves and innervate skeletal muscle.
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