The Nervous System - Structure
Nerves and Ganglia: The PNS consists of all nervous tissue outside the CNS, primarily organized into nerves (bundles of axons) and ganglia (clusters of neuron cell bodies). It acts as the communication relay between the CNS and the rest of the body.
Cranial and Spinal Nerves: The PNS includes 12 pairs of cranial nerves that emerge directly from the brain and 31 pairs of spinal nerves that emerge from the spinal cord. These nerves branch out to reach every part of the body, including skin, muscles, and internal organs.
Functional Pathways: The PNS is divided into the sensory (afferent) division, which carries impulses from receptors to the CNS, and the motor (efferent) division, which carries impulses from the CNS to effectors like muscles and glands.
Somatic Nervous System (SNS): This division is responsible for voluntary control of body movements via skeletal muscles. It also mediates conscious sensation, such as touch, pain, and temperature, by transmitting sensory information to the CNS.
Autonomic Nervous System (ANS): The ANS regulates involuntary physiological processes, including heart rate, blood pressure, respiration, and digestion. It operates largely without conscious effort to maintain internal homeostasis.
Sympathetic vs. Parasympathetic: The ANS is further divided into the sympathetic division, which prepares the body for 'fight or flight' during stress, and the parasympathetic division, which promotes 'rest and digest' activities during calm periods.
Neuron Structure: The neuron is the functional unit of the nervous system, consisting of a cell body (soma), dendrites that receive signals, and a single axon that transmits signals away from the cell body. This specialized shape allows for rapid, long-distance communication.
Myelin Sheath: Many axons are covered by a fatty layer called the myelin sheath, which is produced by glial cells. This sheath acts as an electrical insulator, significantly increasing the speed at which nerve impulses travel along the axon.
The Synapse: This is the microscopic gap between the axon terminal of one neuron and the dendrite or cell body of another. Communication across this gap occurs via chemical messengers called neurotransmitters, allowing signals to pass from cell to cell.
| Feature | Central Nervous System (CNS) | Peripheral Nervous System (PNS) |
|---|---|---|
| Components | Brain and Spinal Cord | Nerves and Ganglia |
| Primary Role | Integration and Command Center | Communication Relay |
| Protection | Skull, Vertebrae, Meninges | Connective tissue sheaths (Epinerium) |
| Regeneration | Very limited capacity | Higher capacity for axonal repair |
Afferent vs. Efferent: Afferent neurons are 'incoming' (sensory), moving toward the CNS, whereas efferent neurons are 'outgoing' (motor), moving toward the periphery. Remembering 'A' for Arrive and 'E' for Exit is a helpful mnemonic for students.
Somatic vs. Autonomic: The somatic system is generally under conscious control and targets skeletal muscles, while the autonomic system is involuntary and targets smooth muscle, cardiac muscle, and glands.
Identify the Division: When presented with a specific nerve or function, first determine if it belongs to the CNS or PNS. If it is outside the brain or spinal cord, it is always PNS.
Check the Direction: Pay close attention to whether a signal is moving toward the CNS (sensory) or away from it (motor). Misidentifying the direction of flow is a common source of error in multiple-choice questions.
Structural vs. Functional: Distinguish between anatomical parts (like the spinal cord) and functional roles (like the sympathetic response). Exams often ask which structural part performs a specific functional role.
Glial Cell Context: While neurons get the most attention, remember that glial cells provide the structural framework and insulation (myelin) necessary for the system to function. Knowing which cells produce myelin in the CNS vs. PNS is a frequent high-level exam topic.