Understanding the Peripheral Nervous System
Peripheral Nervous System
Somatic Nervous System
The somatic nervous system consists of:
- Spinal Nerves: These nerves transmit sensory information (touch, pain) from the trunk and extremities to the central nervous system via the spinal cord. They also relay information about the position and state of muscles and joints. Additionally, they carry motor commands from the spinal cord to skeletal muscles.
- Cranial Nerves: These nerves transmit sensory information from the head and neck to the central nervous system and receive motor commands to control the skeletal muscles in those areas. There are 12 pairs of cranial nerves, each with specific functions, including smell (olfactory), vision (optic), eye movement (oculomotor, trochlear, abducens), facial sensation and movement (trigeminal, facial), hearing and balance (vestibulocochlear), taste and throat sensation (glossopharyngeal), and control of internal organs (vagus), as well as neck and tongue movement (accessory, hypoglossal).
Parasympathetic Nervous System
The parasympathetic nervous system is part of the autonomic nervous system. Its nerves originate from the brainstem and sacral spinal cord. Acetylcholine serves as the neurotransmitter for both pre- and postganglionic neurons. Preganglionic fibers branch out through cranial nerves (such as the facial and vagus nerves) and pelvic nerves from the sacral plexus.
Sympathetic Nervous System
The sympathetic nervous system is also part of the autonomic nervous system. It consists of chains of ganglia located on either side of the spine. These ganglia connect to spinal nerves via communicating branches. The sympathetic system operates through reflex arcs, involving afferent and efferent arms.
- Efferent Arm: Originating in the lateral horns of the spinal cord, efferent fibers travel through the anterior root and exit to sympathetic ganglia via white communicating branches. Postganglionic fibers then either rejoin spinal nerves through gray communicating branches, follow blood vessels, or form visceral nerves to reach their target organs.
- Afferent Arm: Visceral sensory fibers enter the sympathetic chain through white communicating branches and join spinal nerves. The cell bodies of these neurons reside in the dorsal root ganglia, and their axons terminate in the posterior horns of the spinal cord. Interneurons connect the dorsal and lateral horns to complete the reflex arc.