Describe the mechanism and pathway for accomodation of the human eye?
Please describe the pathway and mechanism, using terms like stimulus,receptor,sensory nerve , relay nerve , brain, motor nerve and effector organ.
Visual accommodation refers to a reflex action of the eye to focus on nearby objects after looking at distant objects, and vice versa. It has to do only with focus of the image, not color or quality.
The stimulus for accommodation is the recognition by the occipital cortex of a nearbyor distant image type.
The pathway for accommodation is as follows:
Information from light is received by the rods and cones of the retina (receptors), and then sent via the optic nerve (sensory nerve) through the optic chiasm to the optic radiations in the occipital lobe of the brain where it is interpreted as vision. Here the image type (nearby image) constitutes the stimulus for accommodation. Nearby images are recognized because they are large and formed of light rays coming in at a wide angle. This perception of nearby image occurs in the peristriate area of the brain where accommodation signals are sent via the Edinger-Westphal nucleus along the III Cranial nerve (relay nerve) to the ciliary muscle. The ciliary muscle contracts and changes the lens shape to focus the nearby image. At the same time impulses from sympathetic nerves cause papillary constriction, another form of accommodation.
The effector organs for accommodation are the ciliary body and lens, and the pupil.
The receptors in the eye, rods and cones, recieve impulses based on interpreting what was seen. Those impulses travel along post ganglia, sensory neuron, to the synapse to the preganglia, relay neuron, using neurotransmittters like acetylcholine and epinephrine as well as others. The receptor sites bind with the appropriate transmiter and send the message to the brain which interpret the visioin. The transmisions also flow from the brain to the eye to control pupil size in response to light timuli, contracting cone and rod 'openings' in the retina. The eye is the effector organ at the terminal end of that reaction and the particular post ganglia which controls those muscles is the motor neuron.