Tag: coordination - the linking system

The Sympathetic Nervous System manifests 'fight or flight' responses. It decreases the tear secretion and saliva secretion but increases sweat secretion. The parasympathetic nervous system increases tear secretion and saliva secretion.

The electrical potential difference between outside and inside of a nerve fibre before excitation (polarized nerve fibre) is known as resting potential whose value is $-70$ mV.

The second type of water-current system found also in calcareous sponges, is at first characterized by choanocytes that surround fingerlike projections of the sponge wall. So the correct option is ' sycon'.

Nerve net found in many cnidarians is a part of more advanced nervous systems. Cytoplasmic processes join the nerve cells (neurons) of nerve nets. In cnidarians, the neurons are joined to epithelial receptors and to contractile cells. In vertebrates, nerve nets may be found around blood vessels and the alimentary tract. The nerve net is the simplest form of a nervous system found in multicellular organisms. Unlike, central nervous systems where neurons are typically grouped together, neurons found in nerve nets are found spread apart. This nervous system allows cnidarians to respond to physical contact. They can detect food and other chemicals in a rudimentary way. While the nerve net allows the organism to respond to its environment, it does not serve as a means by which the organism can detect the source of the stimulus. For this reason, simple animals with nerve nets, such as Hydra, will typically produce the same motor output in response to contact with a stimulus regardless of the point of contact. 

Heliotropism is a growth movement or bending of stem in the direction of the light. It is also called as solar tracking. Photoperiodism is the phenomenon in which various physiological reactions like flowering response to the relative length of dark or light periods. Photonasty is nastic movement of plant towards sunlight. Hydrotropism is a growth movement of plant part in response to water.