Introduction to the Peripheral Nervous System
The peripheral nervous system (PNS) is one of the two major parts of the body’s nervous system (Figure 2). In conjunction with the central nervous system (CNS), the PNS coordinates action and responses by sending signals from one part of the body to another. The CNS includes the brain, brain stem, and spinal cord, while the PNS includes all other sensory neurons, clusters of neurons called ganglia, and connector neurons that attach to the CNS and other neurons.
Composition of the PNS
Neurons, or nerve cells, are the most basic unit of the PNS ((Figure 1). They carry electrical signals that allow them to communicate with each other. In humans, neurons are composed of various parts: the soma, or cell body which has the nucleus; the axon by which the nerve signal travels; the myelin sheath, which provides conductivity and allows electrical signaling; the dendrites, which receive signals from other neurons; and axon terminals, which nerve cells use to communicate to each other. This communication is achieved by neurotransmitters, which travel across synapses (or the spaces between the axon terminal of one neuron and the dendrites of another neuron) and bind to appropriate receptors.
There are a few different types of neurons in the PNS. Sensory neurons are activated by inputs from outside or inside the body. Motor neurons connect neurons to muscles or other effector organs to create motor output. Interneurons connect neurons to each other, making the process of communication more efficient. Glial cells also support and nourish the communication function of the system. Together, these nerve cells and the interactions between them form the neural circuits that regulate an organism’s body, perception, and behavior.
Divisions of the PNS
The PNS can also be divided into two separate systems: the autonomic nervous system and the somatic nervous system.
Autonomic Nervous System
The autonomic nervous system regulates involuntary and unconscious actions, such as internal organ function, breathing, digestion, and heartbeat. This system consists of two complementary parts: the sympathetic and parasympathetic systems. Both divisions work without conscious effort and have similar nerve pathways, but they generally have opposite effects on target tissues. The sympathetic nervous system activates the ‘fight or flight’ response under sudden or stressful circumstances, for instances by raising the heart rate and dilating the pupils. The parasympathetic nervous system activates a ‘rest and digest’ or ‘feed and breed’ response after these stressful events, which conserves energy and replenishes the system. Together, these two systems regulate homeostasis within the body - one priming the body for action, and the other repairing the body afterward.
The somatic nervous system controls voluntary and otherwise conscious systems, such as those in the skin, bones, joints, and skeletal muscles. It performs this function through the use of afferent fibers that receive information from external stimuli. This data is sent through pathways that connect the skin and skeletal muscles to the CNS for integration and processing. The information is then sent back through the somatic system to neuromuscular junctions—the interfaces between neurons and muscles—for motor output.
The somatic system also provides us with reflexes, which are automatic and do not require input or integration from the brain to perform. These reflexes can be categorized into either monosynaptic or polysynaptic, based on the reflex arc used to perform the function. Monosynaptic reflex arcs have only a single synapse between the sensory neuron that received the information, and the motor neuron that responds. Polysynaptic reflex arcs have at least one interneuron between the sensory neuron and the motor neuron. Through voluntary action and reflexes, the somatic nervous system keeps the body adept and coordinated.