(a) Reflex actions performed under the influence of sensory impressions other than those of touch or muscular sense.
(b) Involuntary muscular contractions due to nervousness.
(c) Contractions of the muscles of certain members, caused by the turning of the attention specially to the members.
(d) Involuntary contractions of muscles, accompanying the exertion of other associated and antagonist muscles, and due to the radiation of nerve impulse.
(a) Reflex Actions due to Sensory Impressions other than those of Touch or Muscular Sense
A wide range of movements is included under this heading. Of these it is necessary to mention only a few, such as the sudden start on the hearing of an unexpected noise, the instinctive movement of dodging to escape an approaching missile, and the raising of the arm to ward off an expected blow.
Actions of a somewhat similar character normally occur in which it is not easy to point to the excitation of any sense or senses. These include the instinctive cowering attitude of fear, the play of facial expression caused by sentiment and emotion, etc.
(b) Involuntary Actions due to Nervousness
A condition of marked nervousness generally causes the involuntary contraction of muscles. Who does not recall his earliest attempts at "speaking a piece" in school? The trembling of the lips, the twitching of the arms and hands, and the vain attempts to govern the bodily movements, are an experience painful even in the recollection.
Movements and contractions due to nervousness are entirely purposeless; they even defy the most earnest efforts at inhibition. A marked feature of this type of involuntary action is the contraction of antagonist groups of muscles, productive of muscular stiffness of the members.