some stimulus evoke a specific, short-lived
response at the unconscious level. There are
more than two hundred reflexes, which follow
the sequence from stimulus to reflex, along a
neural pathway called reflex arc.
The reflex pathway comprises at least one
afferent neuron (receptor) and one efferent
(effector or excitor) neuron appropriately
arranged in a series. The afferent neuron
receives signal from a sensory organ and
transmits the impulse via a dorsal nerve root
into the CNS (at the level of spinal cord). The
efferent neuron then carries signals from CNS
to the effector. The stimulus and response thus
form a reflex arc as shown below in the knee
jerk reflex.
Sensory Reception And Processing:
Sense organ is the one which responds to
external stimuli by conveying impulses to the
sensory nervous system.
Sense organs are specific for one kind
of stimulus. They transform the received
stimulus into nerve impulses. They are the
biological transductors.Eye:
Eyes are organs of the visual system. They
provide organism’s vision, the ability to
process visual detail, as well as enabling
several photo response functions that are
independent of vision.
Eyes detect light and convert it into electro-
chemical impulses in neurons. In higher
organisms, the eye is a complex optical system
which collects light from the surrounding
environment, regulates its intensity through
a diaphragm, focuses it through an adjustable
assembly of lenses to form an image, converts
this image into a set of electrical signals, and
transmits these signals to the brain through
complex neural pathways that connect the eye
via the optic nerve to the visual cortex and
other areas of the brain.
Eye Structure:
The human eye is a complex optic instrument.
Its main goal is to “transfer” the correct image
to the optic nerve.
Cornea:
It is a transparent coat covering the front part
of the eye. It has no blood vessels, but its
refraction is great. It is part of the eye
optic. Cornea borders sclera which is a non-
transparent eye coat.
Anterior chamber:
It is a space between cornea and iris. It is filled
with intra-ocular fluid.
Iris looks like a circle with an opening in