The Stimulus

A stimulus is an external force or event which when applied to an excitable tissue produces excitation of ac characteristic type. Examples of various types of stimuli are given below.

1- Electrical: These can be used to produce action potentials in neurons.

2- Hormonal: Acetylcholine acts on the post-junctional membrane of the neuro-muscular junction producing depolarization of the muscle cell.

3- Thermal: Stimulation of thermal receptors in the skin by cold or warmth.

4- Mechanical: Stimulation of the outer lamellae of pacinian corpuscles and of the auditory hair cells by mechanical force.

5- Electromagnetic radiation: Stimulation of photoreceptors (rods and cones) I the retina by light.

6- Chemical: Stimulation of the gustatory (taste) receptors of the tongue by salt and H+ and of olfactory receptors by odoriferous substances.

7- Osmotic: Stimulation of osmoreceptors in the hypothalamus by changes in the osmolarity of the blood plasma.

A stimulus which is too weak to produce a response is called sub-threshold or subliminal stimulus. On gradually raising the strength of the subliminal stimulus, a stage reaches when the stimulus becomes effective; this minimum strength of the stimulus that can produce excitation is called the threshold stimulus. Stimuli having strengths higher than threshold are termed as supra-threshold stimuli.