Neural Physiology: The Autonomic Nervous System and Its Divisions

Neural Physiology: Introduction to the Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system responsible for regulating involuntary physiological functions. It controls smooth muscle, cardiac muscle, and glands, and is not under voluntary control.

• Peripheral Nervous System (PNS): Neurons outside the brain and spinal cord.

• Central Nervous System (CNS): Brain and spinal cord.

• Somatic Division: Controls skeletal muscles (voluntary).

• Autonomic Division: Controls glands, smooth muscle, and heart (involuntary).

Branches of the Autonomic Nervous System

The ANS is divided into three main branches, each with distinct roles in maintaining homeostasis:

• Sympathetic Nervous System (SNS): Prepares the body for 'fight-or-flight' responses.

• Parasympathetic Nervous System (PNS): Promotes 'rest-and-digest' activities.

• Enteric Nervous System: Regulates gastrointestinal function.

Homeostasis and Antagonistic Control

Balance between sympathetic and parasympathetic activity is essential for homeostasis. These systems often exert opposing effects on the same organs, allowing fine control of physiological functions.

• Sympathetic Activity: Increases heart rate, blood pressure, and energy mobilization.

• Parasympathetic Activity: Decreases heart rate, promotes digestion and energy storage.

• Antagonistic Control: Two different signals (SNS and PNS) act on the same target using different receptors and have opposite effects.

Autonomic Reflexes

Autonomic reflexes are critical for maintaining homeostasis. They are integrated in the hypothalamus, pons, and medulla, and regulate functions such as blood pressure, heart rate, temperature, water balance, and plasma osmolarity.

• Example: Baroreceptor reflex controls blood pressure via feedback loops involving afferent neurons, the brain, and efferent neurons to the heart.

Autonomic Reflex Pathway

• Stimulus (e.g., change in blood pressure)

• Sensor (baroreceptors in arteries)

• Afferent neurons transmit input signal to the brain

• Integrating center (brain)

• Efferent neurons send output signal to the heart

• Target (heart)

• Response (adjustment in heart rate and blood pressure)

Neural Physiology: The Sympathetic and Parasympathetic Nervous Systems

Comparing Sympathetic and Parasympathetic Divisions

The SNS and PNS differ in their anatomical origins, ganglia locations, neurotransmitters, and physiological functions.

• CNS Origin:

  • SNS: Thoracic and lumbar spinal cord

  • PNS: Brainstem and sacral spinal cord

• Location of Autonomic Ganglia:

  • SNS: Ganglia close to the spinal cord

  • PNS: Ganglia close to or within the target organ

• Neurons in Pathway:

  • Autonomic pathways: Two efferent neurons (preganglionic and postganglionic)

  • Somatic pathways: One efferent neuron

Neurotransmitters and Receptors

• Preganglionic Neurons (SNS & PNS): Both use acetylcholine (ACh) acting on nicotinic receptors (receptor-channels) for fast post-synaptic responses.

• Postganglionic Neurons:

  • SNS: Use norepinephrine (NE) acting on adrenergic receptors (G-protein coupled receptors, GPCRs) for slower responses.

  • PNS: Use acetylcholine (ACh) acting on muscarinic receptors (GPCRs) for slower responses.

• Somatic Pathways: Always use ACh and nicotinic receptors for fast responses; targets are under voluntary control.

Table: Key Differences Between SNS and PNS

Feature	SNS	PNS
CNS Origin	Thoracic & lumbar spine	Brainstem & sacral spine
Ganglia Location	Close to spinal cord	Close to target organ
Preganglionic Neuron Length	Short	Long
Postganglionic Neuron Length	Long	Short
Preganglionic NT & Receptor	ACh, Nicotinic	ACh, Nicotinic
Postganglionic NT & Receptor	NE, Adrenergic (GPCR)	ACh, Muscarinic (GPCR)
Function	Fight-or-flight	Rest-and-digest

Synapses in the Autonomic Nervous System

Postganglionic synapses in the ANS differ from typical neuronal synapses. Instead of classic axon terminals, autonomic postganglionic neurons form varicosities—swellings along the axon that store and release neurotransmitters, allowing a single neuron to affect a large area of target tissue.

• Varicosities: Enable widespread and coordinated responses in smooth muscle and glandular tissue.

• Cellular Events: Similar to neuronal synapses, involving neurotransmitter release, receptor activation, and cellular response.

The Adrenal Medulla: A Modified Sympathetic Ganglion

The adrenal medulla acts as a specialized sympathetic ganglion. Preganglionic sympathetic neurons synapse with chromaffin cells in the adrenal medulla, which release epinephrine into the bloodstream.

• Epinephrine: Functions as a norepinephrine agonist, binding to adrenergic receptors on target cells and triggering 'fight-or-flight' responses.

• Role: Rapidly mobilizes energy and prepares the body for acute stress.

Key Terms and Definitions

• Autonomic Nervous System (ANS): Regulates involuntary functions via sympathetic, parasympathetic, and enteric branches.

• Sympathetic Nervous System (SNS): Prepares the body for action.

• Parasympathetic Nervous System (PNS): Promotes maintenance and conservation of energy.

• Varicosity: Swelling along an axon that releases neurotransmitter over a broad area.

• Adrenergic Receptor: GPCR that binds norepinephrine or epinephrine.

• Muscarinic Receptor: GPCR that binds acetylcholine.

• Nicotinic Receptor: Ion channel receptor for acetylcholine.

Equations and Pathways

• Autonomic Reflex Arc: $\text{Stimulus} \rightarrow \text{Sensor} \rightarrow \text{Afferent Neuron} \rightarrow \text{Integrating Center} \rightarrow \text{Efferent Neuron} \rightarrow \text{Target} \rightarrow \text{Response}$

Example: Regulation of Heart Rate

• Sympathetic Stimulation: Increases heart rate via norepinephrine acting on adrenergic receptors.

• Parasympathetic Stimulation: Decreases heart rate via acetylcholine acting on muscarinic receptors.

• Antagonistic Control: The final heart rate is determined by the balance of sympathetic and parasympathetic signals.

Additional info: The enteric nervous system, while part of the ANS, is primarily involved in gastrointestinal regulation and can function independently of the CNS.