Anatomy & Physiology

Anatomy & Physiology Study Guide • Functionally, the PNS can be split into an afferent division, which brings sensory communication from receptors to the CNS, and an efferent division, which carries motor commands to muscles and glands called effectors. • The efferent division of the PNS consists of the somatic nervous system (SNS), which controls skeletal muscle contractions, and the autonomic nervous system (ANS), which controls smooth muscle, cardiac muscle, adipose tissue, and glandular activity. Neurons are nerve cells specialized for intercellular communication: • The perikaryon of a multipolar neuron contains organelles, including neurofilaments, neurotubules, and neurofibrils. The axon hillock connects the initial segment of the axon to the cell body, or soma. The axoplasm contains numerous organelles. • Collaterals may branch from an axon, with telodendria branching from the axon's tip. • A synapse is a site of intercellular communication. A synaptic knob is the most common type of synaptic terminal. Neurotransmitters discharged from the synaptic knob of the presynaptic cell affect the postsynaptic cell, which may be a neuron or another type of cell. • Neurons are structurally classified as anaxonic, bipolar, unipolar, or multipolar. • The three functional divisions of neurons are sensory neurons, motor neurons, and interneurons. • Sensory neurons, which create the afferent division of the PNS, deliver communication received from interoceptors, exteroceptors, and proprioceptors to the CNS. • Motor neurons, which create the efferent division of the PNS, arouse or change the activity of a peripheral tissue, organ, or organ system. • Interneurons (association neurons) are always found in the CNS and may be situated between sensory and motor neurons. They distribute sensory inputs and coordinate motor outputs. CNS and PNS neuroglia support and protect neurons: • Neuron cell bodies in the PNS are clustered into ganglia. • Satellite cells, or amphicytes, surround neuron cell bodies within ganglia. Schwann cells ensheath axons in the PNS. A single Schwann cell may myelinate one segment of an axon or enclose segments of several unmyelinated axons. • In the PNS, functional repair of axons may follow Wallerian degeneration. In the CNS, many factors obscure the repair process and reduce the chances of functional recovery. • The four types of neuroglia, or glial cells, in the CNS are ependymal cells, with functions related to the cerebrospinal fluid (CSF); astrocytes, the biggest and most abundant neuroglia; oligodendrocytes, which are accountable for the myelination of CNS axons; and microglia, or phagocytic cells. The transmembrane potential is the electrical potential of the cell's interior relative to its surroundings: • All normal neural signaling depends on events that orginate at the plasma membrane. Achieve Page 156 of 368 ©2018