35.4 The Peripheral Nervous System

Peripheral Nervous System

The peripheral nervous system (PNS) connects your brain and spinal cord to the rest of your body, carrying signals back and forth to control everything from your heartbeat to your sense of touch. It splits into two main divisions: the autonomic nervous system (controlling involuntary functions) and the sensory-somatic nervous system (handling voluntary movements and sensations). Together, these divisions let your body respond to its environment and maintain internal balance.

Structure of the Autonomic Nervous System

The autonomic nervous system (ANS) regulates involuntary functions like heart rate, digestion, and respiratory rate. You don't consciously decide to speed up your heart or slow your digestion; the ANS handles that automatically. It has two main divisions that work in opposition to maintain homeostasis.

Sympathetic Nervous System (SNS): This activates the "fight or flight" response during stress or emergencies.

• Increases heart rate, blood pressure, and blood glucose levels to supply energy to muscles
• Dilates pupils (improving vision) and bronchioles (increasing oxygen intake)
• Diverts blood flow away from the digestive system toward skeletal muscles for quick action
• Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord (T1–L2)
• Postganglionic neurons sit in paravertebral ganglia (the chain running along the spinal column) and prevertebral ganglia (located near target organs)

Parasympathetic Nervous System: This promotes "rest and digest" functions during relaxed states.

• Decreases heart rate and blood pressure to conserve energy
• Stimulates digestion by increasing peristalsis and secretion of digestive enzymes
• Constricts pupils and bronchioles
• Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and the sacral region of the spinal cord (S2–S4)
• Postganglionic neurons are located near or within target organs (heart, lungs, digestive tract)

A useful way to remember the difference: sympathetic preganglionic neurons come from the thoracolumbar region, while parasympathetic preganglionic neurons come from the craniosacral region.

The enteric nervous system is sometimes called the "brain of the gut." It's a subdivision of the ANS that independently regulates gastrointestinal functions like motility and enzyme secretion, though it can be modulated by sympathetic and parasympathetic input.

Sensory-Somatic Information Transmission

The sensory-somatic nervous system (SSNS) handles the flow of information between sensory receptors, the CNS, and effector organs (mainly skeletal muscles). It has three key components: sensory receptors that detect stimuli, sensory (afferent) neurons that carry information toward the CNS, and motor (efferent) neurons that carry commands out to effectors.

Sensory Receptors detect stimuli and convert (transduce) them into electrical signals called receptor potentials. The main types include:

• Mechanoreceptors — detect touch and pressure
• Thermoreceptors — detect temperature changes
• Nociceptors — detect pain
• Proprioceptors — detect body position and movement

Sensory Neurons are pseudounipolar, meaning a single process extends from the cell body and splits into two branches. Their cell bodies cluster in the dorsal root ganglia (for spinal nerves) or cranial nerve ganglia. These afferent fibers carry sensory information from receptors to the spinal cord or brainstem, where they synapse with interneurons or, in some reflex arcs, directly with motor neurons.

Motor Neurons are multipolar, with cell bodies located in the ventral horn of the spinal cord (anterior horn cells) or in brainstem motor nuclei. Their efferent fibers carry motor commands from the CNS to skeletal muscles. Upper motor neurons originate in the cerebral cortex or brainstem and synapse with lower motor neurons in the spinal cord or cranial nerve nuclei, which then directly innervate the muscles.

Reflex Arcs represent the simplest sensory-somatic pathway. In a reflex arc, a sensory neuron synapses with a motor neuron in the spinal cord (or brainstem), producing a rapid, unconscious response. The classic example is the knee-jerk (patellar) reflex: tapping the patellar tendon stretches the quadriceps, sensory neurons fire, and motor neurons contract the muscle before your brain even registers what happened.

Cranial vs. Spinal Nerves

Both cranial and spinal nerves connect the CNS to the rest of the body, and both generally contain sensory and motor fibers. They differ in where they originate and what they innervate.

Cranial Nerves (12 pairs) emerge directly from the brainstem and are numbered I through XII.

• Most are mixed nerves carrying both sensory and motor fibers (e.g., the facial nerve VII and the vagus nerve X)
• Two exceptions are purely sensory: the olfactory nerve (I) and the optic nerve (II)
• They innervate structures mainly in the head and neck (eyes, ears, nose, tongue, pharynx, larynx), though the vagus nerve (X) extends into the thorax and abdomen to reach the heart, lungs, and digestive tract

Spinal Nerves (31 pairs) emerge from the spinal cord and are named by vertebral level: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

• All are mixed nerves with both sensory and motor fibers
• They innervate the trunk and limbs
• Spinal nerves combine and redistribute their fibers into nerve plexuses: the cervical and brachial plexuses supply the upper limbs, while the lumbar and sacral plexuses supply the lower limbs

Neurotransmission and Signal Propagation

Neurons communicate at specialized junctions using chemical messengers called neurotransmitters. When an action potential reaches the end of an axon, it triggers the release of neurotransmitter molecules into the synaptic cleft, where they bind to receptors on the next cell.

The myelin sheath, a fatty insulation wrapped around axons by Schwann cells (in the PNS), dramatically increases signal propagation speed. Signals jump between gaps in the myelin called nodes of Ranvier in a process known as saltatory conduction. This is why myelinated neurons transmit signals much faster than unmyelinated ones.

Two specialized junctions are worth knowing:

• The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber. The neurotransmitter here is acetylcholine (ACh), which binds to receptors on the muscle fiber and triggers contraction.
• Neuroeffector junctions are the sites where autonomic neurons release neurotransmitters onto target organs. The sympathetic division primarily uses norepinephrine, while the parasympathetic division uses acetylcholine.