The Reflex Arc and Autonomic Nervous System

The autonomic nervous system (ANS) regulates internal organs through reflex pathways.
Reflexes in the ANS are structurally similar to somatic reflex arcs but differ in effectors.
Somatic reflexes control skeletal muscle.
Autonomic reflexes target smooth muscle, cardiac muscle, and glands.

Somatic reflex arc: One motor neuron connects the central nervous system (CNS) to skeletal muscle.
The somatic motor neuron is myelinated.
The central fiber projects directly to the target effector.
Target effector: skeletal muscle.
Axon ends at synapse (synaptic terminal).

Autonomic (visceral) reflex arc: Involves two neurons—preganglionic and postganglionic.
Preganglionic neuron: From the CNS to the ganglion.
Postganglionic neuron: From the ganglion to the target effector.

Both somatic and autonomic reflexes use similar sensory (afferent) input pathways.
Sensory neurons have cell bodies in dorsal root ganglia or cranial nerve ganglia.
Afferent signals initiate reflexes in the CNS or at peripheral ganglia.

Visceral refers to internal organs, especially those that function automatically (like the heart, lungs, stomach, and glands).
Visceral efferent pathways: Motor pathways that control involuntary functions of smooth muscle, cardiac muscle, and glands. This contrasts with somatic efferent pathways, which control skeletal muscles voluntarily.
Visceral sensory input: Sensory information coming from internal organs (e.g., stretch in the stomach, chemical changes in the blood, pain from organ inflammation). These sensations are often poorly localized or even subconscious.
Some visceral reflexes rely on unconscious sensory input, like blood pressure or organ stretch.
Baroreceptors in the aorta and carotid sinuses detect vessel stretch.
These inputs travel via vagus (X) and glossopharyngeal (IX) nerves to the brainstem.

Referred pain occurs when visceral pain is perceived in somatic regions.
Often due to shared spinal cord entry points between visceral and somatic sensory fibers.
Example: Kehr’s sign: Left shoulder pain from a ruptured spleen.
Cause: Blood irritates diaphragm, which shares spinal segments (C3–C5) with the shoulder region via the phrenic nerve. The brain interprets the diaphragm signal as coming from the shoulder.

Long reflexes: Involve the CNS—sensory input enters the spinal cord or brain; motor output exits via autonomic pathways.
Short reflexes: Bypass the CNS—sensory neurons synapse directly on ganglionic neurons in autonomic ganglia or local plexuses.

Dual innervation: Most organs receive both sympathetic and parasympathetic input.
These systems compete to maintain homeostasis (e.g., heart rate, digestion, pupil size).
Balance is achieved through differing neurotransmitters and receptor types.

Light hits the retina, and the signal travels through the optic nerve to the brain.
A parasympathetic signal leaves the brainstem via the oculomotor nerve.
It synapses in the ciliary ganglion, then goes to the circular muscles in the iris.
Acetylcholine (ACh) is released, causing muscles to contract, and the pupil constricts to limit light entering the eye.

A sympathetic signal exits the upper thoracic spinal cord, travels to the superior cervical ganglion, then to the iris.
Norepinephrine is released onto the radial muscles of the iris.
Muscles contract, and the pupil dilates to allow more light in.
The parasympathetic nervous system constricts the pupil (via circular muscles).
The sympathetic nervous system dilates the pupil (via radial muscles).
Dual innervation means that both divisions of the autonomic nervous system (sympathetic and parasympathetic) innervate the same organ—in this case, the iris—but produce opposing effects.

Organ systems are balanced between the input from the sympathetic and parasympathetic divisions.
When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state.
For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system.

Hypothalamus = master regulator of autonomic and endocrine functions.
Sends output via:
- Medial forebrain bundle
- Dorsal longitudinal fasciculus
These tracts link the hypothalamus to:
- Edinger–Westphal nucleus (parasympathetic)
- Thoracic spinal cord (sympathetic)

Forebrain input (amygdala, olfactory cortex, septal nuclei) modulates autonomic tone.
The amygdala influences the hypothalamus based on emotional state.
Example: fear → sympathetic "fight-or-flight" activation.
Also triggers an endocrine stress response (e.g., adrenaline).

Medulla: controls cardiovascular functions (heart rate, vessel tone).
- Sympathetic: $↑$ heart rate (cardiac accelerator nerves), vasoconstriction (vasomotor nerves).
- Solitary nucleus: receives sensory input from glossopharyngeal & vagus nerves, adjusts heart and vessels.
- Dorsal motor nucleus of vagus: slows heart rate, constricts airways, stimulates digestion.
- Nucleus ambiguus: involved in swallowing, speech, and heart parasympathetic tone.

The sympathetic system activates during exercise:
- $↑$ heart rate & respiration
- $↓$ digestion
- $↑$ sweat
- The body temporarily leaves homeostasis to meet demands.
The parasympathetic system maintains resting heart rate (~70 bpm).
Without it, heart rate would default to ~100 bpm.