REFLEX ACTION AND REFLEX ARC :
- Sudden spontaneous, involuntary reaction to a stimulus without involvement of brain is called reflex action.
- Some examples of such actions are –
- Sudden withdrawal of the body part which comes in contact with objects that are extremely hot, cold, pointed.
- Reflex arc: sensory organ → sensory neuron → spinal cord → motor neuron → effector organ.
SENSORY RECEPTION AND PROCESSING :
Eye :
- Eye is the sensory organ of vision.
- Our paired eyes are located in sockets of the skull called orbit.
- Eye consists of three layer –
- Sclera: tunica fibrosa.
- External layer composed of dense connective tissue.
- It is the only complete layer of the eye.
- The anterior portion of this layer is transparent and called cornea.
- Choroid: tunica vascularis.
- It is the middle layer of the eye.
- It is well vascularized and looks bluish color.
- Posterior two third parts is thin.
- Anterior part is thick and form ciliary body.
- The ciliary body itself continues forward to form a pigmented and opaque structure called iris (the visible coloured portion of the eye).
- Iris contains a central aperture called pupil.
- The diameter of pupil is regulated by the muscle of iris.
- Retina or tunica nervosa.
- It is the innermost layer of the eye.
- It consists of three layer of cells – from inside to outside
- Ganglion cells
- Bipolar cells
- Photoreceptor cells.
- There are two types of photoreceptor cells namely rods and cones.
- Cones contain photopigment called iodopsin.
- Cones responsible for daylight (photopic) vision and color vision.
- Rods contain photopigment called rhodopsin or visual purple, which contain a derivative of Vitamin-A.
- Rods responsible for twilight (scotopic) vision.
- The optic nerves leave the eye and the retinal blood vessel enters it at a point where rods and cones are absent hence called blind spot.
- At the posterior pole of the eye lateral to blind spot there is a yellowish pigmented spot called macula lutea.
- Macula lutea with highly concentrated cones, where the vision is sharpest ( high resolution vision)
- In the centre of macula lutea there is a central pit called fovea centralis, a tightly packed array of specialized photosensor-receptor cells. It prevents the entry of high intensity light by closing the eye by reflex action.
- The lens composed of crystalline protein, is suspended behind the pupil by a suspensory ligament attached to the ciliary body.
- The lens and suspensory ligament divide the cavity of the eye ball into two chambers.
- Chamber in front of lens called aqueous chamber filled with aqueous humor.
- Chamber behind the lens is called vitreous chamber filled with transparent gel called vitreous humor.
Mechanism of vision :
- The light rays in visible spectrum focused on the retina through the cornea and lens generate potentials (impulses) in rods and cones.
- Photosensitive pigments composed of opsin (a protein) and retinal (an aldehyde of vitamin-A).
- Light induces dissociation of the retinal from opsin resulting changes in structure of opsin.
- This causes change in membrane permeability. As a result, potential differences are generated in the photoreceptor cells.
- This produces a signal that generates action potential in the ganglion cells through bipolar cells.
- These action potentials transmitted by optic nerves to the visual cortex area of brain where the neural impulses are analyzed and the image formed on the retina is recognized.
THE EAR :
- The ear performs two sensory function, hearing and maintenance of body balance.
- Anatomically, the ear can be divided into three major section –
- Outer ear or external ear.
- Middle ear.
- Internal ear or inner ear
External ear :
- Outer ear consists of the pinna and external auditory meatus (canal).
- Pinna collects the vibration in the air which produces sound.
- Auditory meatus extends upto the tympanic membrane (the ear drum).
- Tympanic membrane is made of connective tissue covered with skin.
