Autonomic Nervous System

Adrenal Gland

  • The adrenal gland will be reviewed at the end of the section.

Neurotransmitters

  • Different neurotransmitters are released in different places.

  • Key to understanding the system's coordination is understanding neurotransmitters and receptors.

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

  • Motor neuron extends to skeletal muscle.

    • Releases acetylcholine.

    • This is a cholinergic neuron.

    • Cholinergic: A neuron that releases acetylcholine.

Somatic Nervous System

Autonomic Nervous System

Sympathetic Division (Fight or Flight)

  • Two-neuron chain.

  • Sends information to smooth muscle and heart (cardiac muscle).

  • Preganglionic neuron is a cholinergic neuron, releasing acetylcholine at the synapse.

Parasympathetic Division

  • Preganglionic neuron is also a cholinergic neuron, which releases acetylcholine onto its postganglionic neuron.

  • The postganglionic neuron of the sympathetic division of the autonomic nervous system is the only place in the peripheral nervous system where a neurotransmitter other than acetylcholine is found.

    • This is an adrenergic neuron.

    • Releases epinephrine and norepinephrine.

  • In the parasympathetic division, the postganglionic neuron releases acetylcholine (cholinergic).

Acetylcholine

  • Most common neurotransmitter in the peripheral nervous system.

  • Neurotransmitters land on receptors (chemically gated ion channels) on the surface of cells.

    • Called cholinergic receptors.

    • Found on cell bodies and neuromuscular junctions.

    • Cause an end-plate potential, leading to skeletal muscle contraction.

  • Acetylcholine is released at the synapse between preganglionic and postganglionic neurons and lands on cholinergic receptors on the postganglionic neuron.

Adrenergic Receptors

  • Smooth muscle and heart are innervated by both sympathetic and parasympathetic divisions.

    • These cells have receptors for both neurotransmitters (acetylcholine and epinephrine).

  • If smooth muscle from the digestive tract is exposed to acetylcholine, it contracts.

  • If the same muscle is exposed to epinephrine/norepinephrine, it relaxes.

  • Heart: Acetylcholine slows contraction; epinephrine/norepinephrine speeds it up.

  • Skeletal muscle only responds to acetylcholine; epinephrine/norepinephrine has no effect.

Bilingual Analogy

  • Smooth and heart muscle are "bilingual," understanding both acetylcholine and norepinephrine.

  • Skeletal muscle only understands acetylcholine (like speaking only Spanish).

Gas and Brake

  • Skeletal muscle: Only has a "gas pedal" (acetylcholine).

  • Autonomic nervous system: Has both "gas" (sympathetic) and "brake" (parasympathetic).

  • Allows for coordinated control.

  • Autonomic nervous system enables automatic responses without conscious thought.

    • Sympathetic division activates some organs while inhibiting others.

    • Parasympathetic division reverses the effects when the emergency is over.

  • Two programs: parasympathetic is usually running until the sympathetic program is needed.

Cholinergic Receptors

  • When you hear the word cholinergic, you should think acetylcholine.

Two Types of Cholinergic Receptors:

  • Nicotinic

    • Nicotine is a poison that binds to these receptors.

  • Muscarinic

    • Muscarine is a mushroom poison that binds to these receptors.

Naming Conventions

  • Receptors are named after substances found to bind to them, not necessarily the substance they were designed for.

  • Plants evolve chemicals to disrupt animal physiology as a defense mechanism.

Examples of naming conventions

  • Cannabinoid receptors

  • Opioid receptors

Locations and Effects of Cholinergic Receptors

  • Nicotinic receptors:

    • Found on all skeletal muscle.

    • Found on all postganglionic neurons.

    • Found on the adrenal gland.

    • Always stimulatory.

  • Muscarinic receptors:

    • Found on target organs of the parasympathetic division (smooth and cardiac muscle).

    • Can be inhibitory or stimulatory.

Conceptual Understanding

  • The complicated system exists for automatic control.

  • The rule: acetylcholine on a nicotinic receptor.

  • The exception: target organs, which can use muscarinic receptors and adrenergic receptors.

  • The change in the activity depends on modifying the receptors on the organs themselves.

  • Control comes from changing the setup on the target organs, which involves acetylcholine on muscarinic receptors and epinephrine/norepinephrine on adrenergic receptors.

Muscarinic Receptors

  • The effect can be inhibitory or stimulatory, which allows control over different organs.

    • Inhibitory on heart muscle.

    • Stimulatory on digestive tract muscle.

  • Usually stimulatory, except for cardiac muscle.

Adrenergic Receptors

  • Epinephrine and norepinephrine receptors.

Two Kinds of Adrenergic Receptors:

  • Alpha receptors

    • Alpha 1

    • Always stimulatory when affected by epinephrine/norepinephrine.

  • Beta receptors

    • Beta 2

    • Always inhibitory when affected by epinephrine/norepinephrine.

Review of the System

Somatic Nervous System

  • One neuron from the spinal cord to the muscle.

  • Cholinergic neuron (releases acetylcholine).

  • Acetylcholine lands on cholinergic receptors on the skeletal muscle (stimulatory effect).

Autonomic Nervous System

  • Two neurons.

  • Preganglionic neuron releases acetylcholine (cholinergic neuron).

  • Acetylcholine lands on cholinergic receptors on the postganglionic neuron (nicotinic receptors).

  • Postganglionic neuron releases neurotransmitter on the effector organ.

Parasympathetic Division

  • Preganglionic neuron releases acetylcholine.

  • Acetylcholine lands on nicotinic receptors on the postganglionic neuron.

Sympathetic Division

  • Exception: norepinephrine/epinephrine is released from sympathetic postganglionic neurons.

  • Lands on alpha one or beta two receptors on effector organs.

Location of Receptors

  • Nicotinic receptors: skeletal muscle, postganglionic neurons, adrenal gland.

  • Muscarinic receptors: effector organs.

Dual Innervation

  • Antagonistic effects: one system stimulates, the other inhibits.

  • Programs: parasympathetic runs unless there is an emergency, then sympathetic increases and parasympathetic decreases.

  • Both systems are always active; it is a matter of adjusting the balance.

Tone

  • Parasympathetic and sympathetic tone refers to the slight, constant signaling each system provides.

Exception: Blood Vessel Smooth Muscle

  • Only responds to sympathetic nervous system signaling.

  • Only has adrenergic receptors.

  • Works like skeletal muscle (all gas, no brake).

  • More epinephrine/norepinephrine constricts the blood vessel; less dilates it.

Effects of Sympathetic Signaling

  • Metabolic rate increases.

  • Blood vessels to the skin dilate for cooling.

  • Sweat glands become more active.

  • Blood pressure increases.

Adrenal Gland

  • Releases epinephrine/norepinephrine into the blood.

  • Circulates throughout the body.

  • Effects last longer due to slow degradation of epinephrine/norepinephrine.

Comparison of Parasympathetic and Sympathetic Divisions

Parasympathetic

  • Acetylcholine is destroyed quickly by acetylcholinesterase.

  • Effects are localized with long preganglionic neurons and short postganglionic neurons.

Sympathetic

  • Epinephrine/norepinephrine takes longer to degrade.

  • Has a more widespread effect due to the adrenal gland releasing hormones into the blood.

  • Effects last longer.

Cholinergic vs Adrenergic

  • Nicotinic: always stimulatory.

  • Muscarinic: stimulatory or inhibitory.

  • Alpha one: stimulatory.

  • Beta two: inhibitory.

Controls

Somatic Nervous System

  • Neurons originate in the frontal lobe of the cerebral cortex.

Autonomic Nervous System

  • Neurons originate in the hypothalamus of the diencephalon.

  • Hypothalamus controls both the autonomic nervous system and the endocrine system.

  • Hypothalamus receives information about blood pressure, heart rate, digestion, and metabolic activity.

High Blood Pressure (Hypertension)

  • Blood vessels are narrower than they should be.

  • Can be caused by plaque (atherosclerosis).

  • Causes the heart to work harder, leading to weakening over time.

Treatment

  • Beta blockers decrease sympathetic signaling, allowing blood vessels to dilate.

  • Beta blockers bind to beta one receptors on the heart. Although they may not perfectly block this signaling, they are effective at reducing the force of contraction in the heart, and reducing blood pressure.

Low Blood Pressure

  • Body has many ways to maintain it.

  • Not enough oxygen or glucose to the brain.

  • Body is more concerned with short-term survival.

Raynaud Syndrome

  • Overstimulation of sympathetic activity causes blood vessels to constrict suddenly.

  • Usually affects fingers.

  • Can be caused by cold or emotional stress.

Aging

  • Autonomic nervous system becomes less efficient.