Autonomic Nervous System (ANS) Anatomy

Organization of the Autonomic Nervous System (ANS)

• The ANS is also known as the visceral, vegetative, or involuntary nervous system due to its involuntary nature and functions.
• It is composed of efferent neurons that innervate smooth muscle of the viscera, cardiac muscle, vasculature, and the exocrine glands, controlling digestion, cardiac output, blood flow, and glandular secretions.

Anatomy of the ANS

• The ANS carries nerve impulses from the CNS to the effector organs through two types of efferent neurons: the preganglionic neurons and the postganglionic neurons.
• The cell body of the preganglionic neuron is located within the CNS, emerges from the brainstem or spinal cord, and makes a synaptic connection in ganglia.
• The ganglia function as relay stations between the preganglionic neuron and the postganglionic neuron.
• The cell body of the postganglionic neuron originates in the ganglion and terminates on effector organs, such as visceral smooth muscle, cardiac muscle, and the exocrine glands.

Efferent Neurons

• The efferent ANS is divided into the sympathetic and the parasympathetic nervous systems, as well as the enteric nervous system.
• The sympathetic and parasympathetic neurons originate in the CNS and emerge from two different spinal cord regions.

Functions of the Sympathetic Nervous System

• The sympathetic nervous system is continually active to some degree, adjusting in response to stressful situations, such as trauma, fear, hypoglycemia, cold, and exercise.
• The effect of sympathetic output is to increase heart rate and blood pressure, mobilize energy stores of the body, and increase blood flow to skeletal muscles and the heart while diverting flow from the skin and internal organs.
• Sympathetic stimulation results in dilation of the pupils and the bronchioles, affecting GI motility and the function of the bladder and sexual organs.

Fight-or-Flight Response

• The changes experienced by the body during emergencies are referred to as the “fight or flight” response.
• These reactions are triggered both by direct sympathetic activation of the effector organs and by stimulation of the adrenal medulla to release epinephrine and lesser amounts of norepinephrine.

Functions of the Parasympathetic Nervous System

• The parasympathetic division is involved in maintaining homeostasis within the body, required for life, and maintains essential bodily functions, such as digestion and elimination of wastes.
• The parasympathetic division usually acts to oppose or balance the actions of the sympathetic division and generally predominates in “rest-and-digest” situations.

Role of the CNS in the Control of Autonomic Functions

• The ANS requires sensory input from peripheral structures to provide information on the current state of the body.
• Feedback is provided by streams of afferent impulses originating in the viscera and other autonomically innervated structures that travel to integrating centers in the CNS, such as the hypothalamus, medulla oblongata, and spinal cord.

Innervation by the ANS

• Most organs in the body are innervated by both divisions of the ANS.
• Dual innervation allows for antagonism, fine-tuning of homeostatic organ functions, and dynamic regulation.
• Some effector organs, such as the adrenal medulla, kidney, pilomotor muscles, and sweat glands, receive innervation only from the sympathetic system.