Final answer:
The term that best defines 'horses tail,' the terminal (caudal) portion of the spinal cord, is 'Cauda equina' (Option 3). It describes the bundle of nerves at the end of the spinal cord. The cauda equina extends from the lower end of the spinal cord to provide nerve supply to the lower body.
Step-by-step explanation:
The terminal (caudal) portion of the spinal cord is best defined by the term Cauda equina. Option 3: Cauda equina is the correct answer because it refers to the bundle of nerves at the end of the spinal cord that resembles a horse's tail. This is where the spinal cord tapers and ends, and these nerves extend downward to innervate the lower parts of the body.
The spinal cord is divided into regions that correspond to the vertebral column levels, and the cauda equina emerges from the lumbar and sacral parts of the spinal cord. In contrast, the other options provided in the question represent different concepts: Hyperkinesis is an increase in muscular activity, Action potential is a neural impulse, and Hyperesthesia is increased sensitivity to stimulation.