Efferent Division of the Peripheral Nervous System

Overview of Efferent Neurons

The efferent division of the peripheral nervous system is responsible for transmitting commands from the central nervous system (CNS) to muscles and glands throughout the body. It is divided into two main subdivisions: the somatic motor division and the autonomic division.

• Somatic motor neurons: Control skeletal muscles and are mostly voluntary in action.

• Autonomic neurons: Control smooth muscle, cardiac muscle, many glands, and some adipose tissue; their actions are mostly involuntary.

The Autonomic Division

Subdivisions of the Autonomic Division

The autonomic division is further divided into two anatomically and functionally distinct branches: the sympathetic and parasympathetic branches. These branches are best distinguished by their relative activity under different physiological conditions.

• Sympathetic branch: Often referred to as the "fight-or-flight" system, it prepares the body for stressful or energetic activity.

• Parasympathetic branch: Known as the "rest-and-digest" system, it promotes maintenance activities and conserves energy.

• Anatomical distinction: The two branches can be distinguished by their anatomical origins and the location of their ganglia.

Homeostasis and Autonomic Balance

Homeostasis is maintained by a dynamic balance between the sympathetic and parasympathetic branches. The dominance of one branch over the other depends on the body's current needs.

• Rest-and-digest: Parasympathetic activity dominates during restful states.

• Fight-or-flight: Sympathetic activity dominates during stress or danger.

Autonomic Reflexes and Homeostasis

Role of Autonomic Reflexes

Autonomic reflexes are crucial for maintaining homeostasis and work in conjunction with endocrine and behavioral state systems.

• Sensory input: Comes from somatosensory and visceral receptors.

• Hypothalamic sensors: The hypothalamus contains neurons that act as sensors for internal conditions.

• Homeostatic control centers: Located in the hypothalamus, pons, and medulla.

• Motor output: Leads to autonomic, endocrine, and behavioral responses.

• Behavioral integration: Behavioral responses may be integrated in brain centers involved in motivated behaviors and movement control.

• Emotional influence: Sensory information integrated in the cortex and limbic system can create emotions that influence autonomic output.

• Spinal reflexes: Some autonomic reflexes are mediated at the spinal cord level and do not require brain integration.

Example: Autonomic Response to Stress

During a stressful event, such as encountering a threat, the sympathetic branch increases heart rate and energy mobilization, while the parasympathetic branch is suppressed.

Additional info: The autonomic division is essential for involuntary regulation of vital functions such as heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.