Question

What is meant by threshold stimulus in muscle contraction? Name any two stimuli other than nerve threshold stimulus.

Answer 1

 Hint: Muscles are made up of contractile fibres called myofibrils which form the basic units of muscle cells. The muscles contract when they receive the stimulus from nerve fibres. However, any stimulus does not result in muscle contraction.

Complete Step by Step Answer:
For muscle contraction, a specific magnitude of stimulus is required. The minimum strength required for the stimulus to initiate the response of muscle contraction is known as the threshold stimulus. No response is seen if the magnitude of the stimulus is below this threshold value.
The other types of stimuli, other than the nervous stimuli, include mechanical stimuli like pressure, electrical stimuli as in the case of electric shock, and some chemicals that induce chemical stimuli
The sliding filament theory explains the contraction of muscles when the threshold stimulus is achieved. According to this theory, the thick myosin filaments slide over the thin actin filaments during muscle contractions, while both the groups of filaments remain relatively constant in length.
A sarcomere is the functional contractile unit of striated muscles. They consist of the following parts:
‘A’ and ‘I’ bands that are arranged alternately throughout the length of the myofibrils. The actin-containing light bands are called as I-bands or Isotropic bands, whereas the dark bands are called ‘A’ or Anisotropic bands and contain myosin.
In the centre, each ‘I’ band has an elastic fibre called the ‘Z’ line which bisects it. The portion of the myofibrils between two successive ‘Z’ lines is called a sarcomere and is considered as the functional unit of contraction.

Note:
- This threshold value or the strength of the stimulus to initiate muscle contraction varies from one muscle fibre to another and also varies for different stimuli.
- However, according to the ‘All or none’ theory, the magnitude of contraction does not depend on the magnitude of the stimulus as long as it crosses the threshold value. The muscle fibres contract as a single unit as soon as the threshold value is achieved and is not a gradual process.
- This means that a lower value of the stimulus will not produce any response and a supramaximal stimulus or magnitude of stimulus more than the threshold value will still produce the same magnitude of contraction as in threshold value.