17.18: Peripheral Nervous System: Ganglia and Nerves
The Peripheral Nervous System (PNS) is a crucial component of the body's neural network, extending beyond the central nervous system (CNS) to bridge the gap between the CNS and the external environment. It encompasses nerves, ganglia, and sensory receptors.
Nerves
The nerve is a bundle of axons that serves as the communication highway in the PNS. Each nerve is ensheathed in a protective layer of connective tissue called the epineurium. This outermost layer safeguards the nerve and supports the blood vessels supplying it. Within the nerve, axons are organized into smaller bundles called fascicles, each surrounded by the perineurium. This arrangement facilitates the efficient transmission of electrical signals along the axons. Encasing each axon and its Schwann cells is a delicate sheath called the endoneurium. This layered structure ensures a precise and protected transmission of nerve impulses across the PNS.
The PNS is functionally divided into two principal categories — the cranial and spinal nerves. The 12 pairs of cranial nerves have their origins in the brain. They are instrumental in various functions ranging from sensory input — such as vision, smell, and taste — to motor control, which includes facial expressions, eye movement, and speech.
On the other hand, the 31 pairs of spinal nerves emerge from the spinal cord, branching out to innervate the entire body. These nerves transmit sensory information from the limbs and trunk to the CNS and carry motor commands from the CNS to the muscles. Each spinal nerve is a mixed nerve, containing afferent (sensory) and efferent (motor) fibers, allowing for the bidirectional information flow between the CNS and the body.
Ganglia
Ganglia are clusters of neuron cell bodies located outside the CNS that are integral to the PNS's operation. They are categorized based on their location and the specific functions they perform.
The dorsal root ganglia, situated along the spinal cord, house the cell bodies of sensory neurons. They relay sensory information from peripheral receptors to the spinal cord, facilitating the sense of touch, pain, temperature, and proprioception.
The autonomic ganglia are associated with the motor division of the PNS. They play a key role in regulating involuntary functions, such as digestion, respiration, and heart rate. These ganglia are subdivided into sympathetic and parasympathetic ganglia, reflecting their roles in the body's fight-or-flight response and rest-and-digest activities, respectively. Sympathetic ganglia are positioned near the spinal cord, ready to mobilize the body's resources under stress. In contrast, parasympathetic ganglia are located close to visceral organs, overseeing the conservation and restoration of the body's energy.
