Final answer:
The initial segmental division of the brachial plexus is Roots. It precedes the formation of trunks, divisions, and cords in the plexus hierarchy. In the peripheral nervous system, a bundle of axons is referred to as a nerve.
Step-by-step explanation:
The initial segmental division of the brachial plexus is Roots. The brachial plexus is a network of nerves that originate from the spinal cord and are responsible for motor and sensory innervation to the upper limb. It begins with the roots, which are the ventral (anterior) nerve root axons emerging from the anterior or lateral horns of the spinal cord, and then these roots converge to form trunks, divisions, and eventually, cords that give rise to peripheral nerves.
The two major divisions of the peripheral nervous system are the somatic nervous system and the autonomic nervous system (which is further divided into the sympathetic nervous system and the parasympathetic nervous system). The sensory division of the somatic nervous system is responsible for transmitting information from sensory receptors to the central nervous systems, such as pain, temperature, and touch sensations.