Autonomic Nervous System & Motor Control: Key Concepts for Neuroscience

Somatic Nervous System (SNS)

Voluntary, conscious control of skeletal muscles.

Autonomic Nervous System (ANS)

Involuntary, unconscious regulation of smooth muscle, cardiac muscle, and glands.

Effectors

Both systems can stimulate or inhibit its effectors.

Motor Fibers

Both have motor fibers but differ in effectors, efferent pathways and ganglia, and the target organ responses to different neurotransmitters.

ACh at the neuromuscular junction

All somatic motor neurons release ACh at the neuromuscular junction.

Effect on skeletal muscle

Always excitatory (induces contraction).

Postganglionic sympathetic fibers

Typically release norepinephrine (NE).

Postganglionic parasympathetic fibers

Release acetylcholine (ACh).

Target Organ Effects

The effect (excitatory or inhibitory) depends on the specific receptors on the target tissue.

Adrenergic receptors

Respond to NE; effect depends on subtype (α or β).

Cholinergic receptors

Respond to ACh; nicotinic or muscarinic subtypes, each with different responses.

Rest-and-Digest System

Supports relaxation, energy conservation, and normal body maintenance.

Key responses of the Parasympathetic Division

↓ Heart rate, constricts pupils, stimulates digestion, promotes nutrient absorption, stimulates urination and defecation, conserves energy.

Fight-or-Flight System

Prepares the body for stress, emergencies, and increased activity.

Key responses of the Sympathetic Division

↑ Heart rate and blood pressure, dilates bronchioles, dilates pupils, mobilizes energy, diverts blood flow to skeletal muscles, inhibits digestion and urinary functions.

Homeostasis

These divisions usually have opposite effects on the same organs to maintain homeostasis.

Parasympathetic Division (Craniosacral)

Parasympathetic fibers originate in the brain (cranial nerves III, VII, IX, X) and the sacral spinal cord (S2-S4).

Origin of Parasympathetic fibers

Parasympathetic fibers arise from the brainstem and sacral spinal cord (S2-S4).

Fiber Lengths

Long preganglionic fibers and short postganglionic fibers.

Cranial Portion

Preganglionic fibers travel with cranial nerves III, VII, IX, and X, controlling functions like pupil constriction, tear/salivary secretions, and major thoracic/abdominal organ regulation.

Sacral Portion

Supplies the pelvic organs and the distal half of the large intestine, helping manage urination, defecation, and reproductive organ function.

Sympathetic Division (Thoracolumbar)

Sympathetic fibers originate in the thoracic and lumbar regions of the spinal cord (T1-L2).

Origin of Sympathetic Fibers

Sympathetic fibers arise from the thoracic and lumbar spinal cord segments T1-L2.

Innervation Scope of Sympathetic Division

More complex than the parasympathetic division because it innervates more organs and all superficial structures of the body.

Unique Targets of Sympathetic Division

The sympathetic division exclusively supplies sweat glands (eccrine & apocrine), arrector pili muscles (hair-raising muscles), smooth muscle in the walls of all arteries and veins (deep and superficial), and the kidneys.

Effectors Supplied by Sympathetic Division

It innervates smooth muscle, cardiac muscle, and glands in internal body cavities—but also all superficial smooth muscle and glands.

Segment-Specific Output T1-T4

Preganglionic fibers serving the head.

Segment-Specific Output T1-T6

Preganglionic fibers serving thoracic organs.

Segment-Specific Output T5-L2

Preganglionic fibers serving the abdominal organs.

Segment-Specific Output T10-L2

Preganglionic fibers serving pelvic organs.

Parasympathetic Division Fiber Lengths

Long preganglionic fibers and short postganglionic fibers.

Sympathetic Division Fiber Lengths

Short preganglionic fibers and long postganglionic fibers.

Location of Parasympathetic Ganglia

Ganglia are located in or very close to the visceral effector organs (intramural ganglia).

Location of Sympathetic Ganglia

Ganglia lie close to the spinal cord, either in the sympathetic chain (paravertebral ganglia) or prevertebral ganglia near the abdominal aorta.

Visceral Reflex Arc

Has two consecutive motor neurons (preganglionic and postganglionic).

Somatic Reflex Arc

Has one motor neuron connecting the CNS directly to the skeletal muscle.

Afferent Pathway of Visceral Reflex Arc

Afferent fibers are visceral sensory neurons, detecting internal conditions (stretch, chemicals, etc.).

Afferent Pathway of Somatic Reflex Arc

Afferent fibers are somatic sensory neurons, detecting external stimuli (pain, temperature, touch).

Effectors of Visceral Reflex Arc

Effectors are smooth muscle, cardiac muscle, or glands.

Effectors of Somatic Reflex Arc

Effectors are skeletal muscles.

Acetylcholine (ACh)

Released by all preganglionic neurons in both sympathetic and parasympathetic divisions, released by parasympathetic postganglionic neurons, and released by sympathetic postganglionic neurons to sweat glands (an exception).

ACh

Acts on cholinergic receptors (nicotinic and muscarinic).

Effects of ACh

Can be excitatory or inhibitory, depending on the receptor type.

Norepinephrine (NE)

Released by most sympathetic postganglionic neurons.

Effects of NE

Can also be excitatory or inhibitory, depending on which adrenergic receptor subtype is present.

Epinephrine (EPI)

Acts as a hormone rather than a neurotransmitter.

Source of EPI

Secreted by adrenal medulla into the bloodstream.

Release mechanism of EPI

Sympathetic preganglionic fibers release ACh → stimulates nicotinic receptors on adrenal medulla → epinephrine secretion.

Receptors activated by EPI

α1, α2, β1, β2, β3.

Effects of EPI

Widespread sympathetic 'fight-or-flight' responses: ↑ Heart rate & contractility (β1), Bronchodilation (β2), Glycogen breakdown ↑ glucose (β2, β3), Vasoconstriction (α1) & vasodilation in skeletal muscles (β2).

Somatic motor neurons

Always release ACh, and the effect on skeletal muscle is always excitatory.

Autonomic neurons

Can release ACh or NE, and the effect on target organs varies depending on receptor type.

Autonomic Tone

Both divisions are partially active at all times, even at rest.

Purpose of Autonomic Tone

Allows precise, continuous regulation of organ function (e.g., blood vessel constriction by sympathetic tone).

Dynamic Antagonism

The two divisions often have opposing effects (e.g., sympathetic ↑ heart rate, parasympathetic ↓ heart rate).

Predominance of divisions

One division usually predominates depending on the situation: Parasympathetic: rest-and-digest, Sympathetic: fight-or-flight.

Cooperative Effects

In some cases, both divisions work together to achieve a single, coordinated outcome.

Example of Cooperative Effects

Male reproductive organs — parasympathetic → erection, sympathetic → ejaculation.

Key Concept of ANS

The ANS maintains homeostasis through tone, antagonism, and cooperation.

Thermoregulatory Responses

Sympathetic nerves dilate skin blood vessels → heat loss; constrict skin blood vessels → conserve heat.

Renin Release from Kidneys

Sympathetic impulses stimulate kidneys to release renin, triggering formation of hormones that increase blood pressure.

Metabolic Effects of Sympathetic Division

Increases metabolic rate of body cells, raises blood glucose levels (glycogen breakdown), mobilizes fats for energy (lipolysis).

Key Concept of Sympathetic Division

Helps maintain homeostasis under stress and temperature changes, regulates blood pressure, and supports energy mobilization.

Parasympathetic Division

One preganglionic neuron → one (or few) postganglionic neurons.

Acetylcholinesterase

All fibers release ACh, which is rapidly broken down by acetylcholinesterase.

Effect of Parasympathetic Division

Short-lived, highly localized control over target organs.

Sympathetic Division

Preganglionic axons branch extensively in the sympathetic trunk.

Synapse in Sympathetic Division

Synapse with postganglionic neurons at multiple levels.

Effect of Sympathetic Division

Diffuse, widespread, and highly interconnected responses throughout the body.

Key Concept of Parasympathetic Division

Parasympathetic = precise, localized actions.

Hypothalamus

Master Control Center that integrates emotional, endocrine, and autonomic responses.

Homeostasis Functions of Hypothalamus

Maintains body temperature, hunger, thirst, fluid balance.

Brainstem Functions

Contains cardiovascular, respiratory, digestive, and urinary centers.

Medulla Oblongata

Controls heart rate, blood pressure, and breathing.

Spinal Cord Functions

Mediates simple autonomic reflexes.

Example of Spinal Cord Reflex

Defecation and urination reflexes.

Visceral Sensory Neurons

Monitor internal conditions (stretch, chemicals, pressure).

Hypertension

Chronically elevated blood pressure.

Cause of Hypertension

Often due to overactive sympathetic vasoconstriction, which increases peripheral resistance.

Raynaud's Disease

Episodic spasms of small arteries, usually in fingers and toes.

Cause of Raynaud's Disease

Excessive sympathetic stimulation in response to cold or stress → vasoconstriction.

Autonomic Dysreflexia

Sudden, massive sympathetic reflex in spinal cord injury patients (usually above T6).

Cause of Autonomic Dysreflexia

Uncontrolled sympathetic response to stimuli (e.g., bladder distension, skin irritation).

Beta Blockers

Block β-adrenergic receptors, mainly β1 (heart) and β2 (lungs and blood vessels).

Effects of Beta Blockers

Lower heart rate, reduce blood pressure, decrease tremors.

Beta Blockers in Sports

Reduce physiological tremor and anxiety, improving steadiness and focus.

ANS Connection of Beta Blockers

Interfere with sympathetic nervous system signaling.

Cerebellar cortex

At the highest level of our conscious motor pathways, but not the ultimate planner and coordinator of complex motor behaviors.

Basal nuclei

Plays a role in the ultimate planning and coordination of complex motor behaviors.

Segmental level

Consists of reflexes and spinal cord circuits that control automatic movements as well as central pattern generators.

Projection level

Consists of neurons acting through direct and indirect motor pathways.

Precommand level

Consists of the cerebellum and basal nuclei, and controls the outputs of the cortex and brain stem motor centers.

Inborn (intrinsic) reflex

A rapid, predictable motor response to a stimulus that is unlearned, unpremeditated, and involuntary.

Learned (acquired) reflexes

Results from practice or repetition.

Somatic reflexes

Activate skeletal muscle.

Autonomic (visceral) reflexes

Activate visceral effectors (smooth or cardiac muscle or glands).

Flexor (withdrawal) reflex

Initiated by a painful stimulus, causes automatic withdrawal of the threatened body part.

Crossed extensor reflex

Often accompanies the flexor reflex in weight-bearing limbs and is important for maintaining balance.

Interneurons

Act as a link or bridge between sensory neurons and motor neurons within the central nervous system (CNS).

Function of interneurons

Process and interpret information received from sensory neurons, then decide how the body should respond by sending signals to motor neurons.