Introduction to the Nervous System

The Peripheral Nervous System (PNS)

The PNS consists of all the nervous tissue located outside the CNS. It is composed of 12 pairs of cranial nerves (which emerge directly from the brain) and 31 pairs of spinal nerves (which emerge from the spinal cord). The primary role of the PNS is to carry impulses between the CNS and various target areas, such as muscles and organs.

Functional Divisions: Somatic vs. Autonomic

While anatomy divides the system by location, function divides it by what it controls.

The Somatic Nervous System

The somatic nervous system is responsible for the voluntary control of skeletal muscles. It also monitors the external environment through sensory information such as touch, temperature, pain, and proprioception (the sense of body position).

The communication pathway for spinal nerves is highly organized:

• Sensory information enters the spinal cord via the posterior roots.
• Motor impulses exit the spinal cord via the anterior roots. Because they contain both sensory and motor fibers, spinal nerves are often referred to as "mixed nerves".

The Autonomic Nervous System (ANS)

The ANS manages the involuntary control of cardiac muscle, smooth muscle, and glands. It is primarily concerned with monitoring the internal environment (the viscera) to ensure organs function correctly without conscious effort.

The ANS utilizes a two-neuron relay system to reach its targets:

1. A preganglionic neuron that originates in the CNS.
2. A postganglionic neuron that originates in a ganglion (a cluster of nerve cell bodies) and travels to the target organ.

Exam Tip: Remember that the Somatic system is for voluntary movement and external sensation, while the Autonomic system is for involuntary functions and internal monitoring.

Antagonistic Control in the Autonomic System

The Autonomic Nervous System is further divided into two branches that typically work in an antagonistic fashion—meaning they have opposing effects to balance the body's state.

The Sympathetic Division: "Fight or Flight"

The sympathetic division prepares the body for intense physical activity or perceived threats. It is characterized by a thoracolumbar outflow, meaning the nerves emerge from the T1 to L2 segments of the spinal cord.

When activated, the sympathetic system:

• Increases heart rate.
• Dilates airways to improve oxygen intake.
• Diverts blood flow away from the digestive system and toward the skeletal muscles.

Anatomically, the sympathetic division features short preganglionic fibers and long postganglionic fibers.

The Parasympathetic Division: "Rest and Digest"

The parasympathetic division works to conserve energy and maintain "business as usual" functions. It is characterized by a craniosacral outflow, with nerves emerging from the cranial nerves (specifically CN III, VII, IX, and X) and the sacral spinal segments (S2–S4).

When activated, the parasympathetic system:

• Decreases heart rate.
• Promotes digestion and waste elimination.

Anatomically, the parasympathetic division features long preganglionic fibers and short postganglionic fibers, with the ganglia located very near or even within the target organs.

Summary Table: Nervous vs. Endocrine Systems