ANS structure neurotransmitters receptors pt.1

How is the nervous system subdivided?

CNS (brain and spinal cord) and PNS; the PNS has a sensory division (somatic and visceral) and a motor division (somatic NS and autonomic NS); the ANS splits into sympathetic and parasympathetic

What are the two efferent divisions of the PNS?

Somatic nervous system and autonomic nervous system (ANS)

What does the ANS control?

Smooth muscle; cardiac muscle; glands; and adipose tissue

What is the ANS control type?

Subconscious and involuntary; makes fast adjustments to support body activities

How does the ANS differ from the SNS?

Effectors; efferent pathways; and target organ responses

Effector organs of the somatic NS

Skeletal muscle

Effector organs of the ANS

Cardiac muscle; smooth muscle; and glands

Somatic NS efferent pathway

One neuron from the CNS to the effector

ANS efferent pathway

Two-neuron chain (preganglionic then postganglionic)

Somatic NS control

Voluntary

ANS control

Involuntary

Somatic NS myelination

Heavily myelinated axons (fast conduction)

ANS myelination

Preganglionic axon lightly myelinated; postganglionic axon unmyelinated (slower conduction)

Somatic NS neurotransmitter

ACh

ANS neurotransmitters

Preganglionic: ACh; postganglionic: ACh or NE

Somatic NS target organ response

Stimulatory (excitatory)

ANS target organ response

Stimulatory or inhibitory depending on the receptor

Effect of denervation in the somatic NS

Paralysis and atrophy of skeletal muscle

Effect of denervation in the ANS

Denervation hypersensitivity (target organs become supersensitive)

Two-neuron ANS pathway

CNS then preganglionic neuron then ganglion then postganglionic neuron then effector

Preganglionic neuron

Runs from the CNS to the ganglion

Postganglionic neuron

Runs from the ganglion to the target organ (effector)

Heart transplant effects (denervated heart)

Elevated resting heart rate; blunted exercise response; loss of heart rate variability; altered response to ANS drugs

Sympathetic division role

Fight or flight; mobilizes the body during extreme situations

Parasympathetic division role

Rest and digest; housekeeping activities that conserve body energy

The four E's of the sympathetic system

Exercise; excitement; emergency; and embarrassment

The three D's of the parasympathetic system

Digestion; defecation; and diuresis

Autonomic tone

Homeostasis is a dynamic balance between the two divisions; parasympathetic dominates at rest and sympathetic dominates in fight or flight

Parasympathetic tone effects

Slows the heart and sets normal activity of the digestive and urinary systems

How do the sympathetic and parasympathetic systems interact?

Dynamic antagonism (e.g. heart rate) and complementary actions (e.g. sexual function)

Antagonistic control of heart rate

Sympathetic acts on beta-1 to increase heart rate; parasympathetic acts on M2 to decrease heart rate

Origin of the sympathetic system

Thoracolumbar (spinal cord T1-L2)

Origin of the parasympathetic system

Craniosacral (brainstem cranial nerves and sacral cord S2-S4)

Sympathetic preganglionic fiber length

Short

Sympathetic postganglionic fiber length

Long

Parasympathetic preganglionic fiber length

Long

Parasympathetic postganglionic fiber length

Short

Sympathetic ganglia location

Near the spinal cord (paravertebral or prevertebral)

Parasympathetic ganglia location

Usually near or within the target organ (some outside)

Paravertebral ganglia

Sympathetic ganglia alongside the spinal column (sympathetic chain)

Prevertebral (collateral) ganglia

Sympathetic ganglia in front of the spinal column

Name the collateral ganglia

Celiac; superior mesenteric; and inferior mesenteric ganglia

Splanchnic nerves

Sympathetic neurons exiting below the diaphragm (T5-L3) that bypass the chain and synapse in collateral ganglia

What do collateral ganglia postganglionic neurons innervate?

Digestive; urinary; and reproductive systems

Which fibers pass directly to the adrenal medulla?

Thoracic splanchnic nerve fibers

Divergence

One preganglionic neuron synapses on several postganglionic neurons at different levels

Convergence

Several preganglionic neurons synapse on one postganglionic neuron

Why is divergence important?

Lets the sympathetic division act as a single unit (mass activation); stay tonically active; and produce a strong fight-or-flight response

Why are sympathetic responses widespread?

Extensive branching and divergence; responses are more widespread than the localized parasympathetic responses

Structural similarities of sympathetic and parasympathetic systems

Both originate in the CNS; have preganglionic and postganglionic fibers; and mostly innervate the same organs

Which neurotransmitter do ALL autonomic preganglionic neurons release?

Acetylcholine (ACh)

Which neurotransmitter do parasympathetic postganglionic neurons release?

ACh (the only parasympathetic neurotransmitter)

Which neurotransmitter do most sympathetic postganglionic neurons release?

Norepinephrine (NE)

Sympathetic postganglionic neurons that release ACh

Those innervating sweat glands

Sympathetic neurons that release dopamine

Some neurons (e.g. to the kidney); dopamine is a precursor of NE

Is epinephrine released by peripheral neurons?

No; epinephrine is released only by the adrenal gland (adrenal medulla)

Where is ACh released?

All autonomic preganglionic neurons; all parasympathetic postganglionic neurons; somatic motor neurons at the NMJ; and sympathetic neurons to sweat glands

How is ACh synthesized?

Choline + acetyl-CoA yields ACh in the cytoplasm via choline acetyltransferase (ChAT); synthesis is fast

Enzyme that synthesizes ACh

Choline acetyltransferase (ChAT)

What is stored with ACh in vesicles?

ACh and cotransmitters such as ATP

How is ACh action terminated?

Rapid hydrolysis by acetylcholinesterase (AChE)

What happens if acetylcholinesterase is inhibited?

ACh builds up in the synapse and its effects are prolonged (enhanced cholinergic activity)

Two types of cholinergic receptors

Nicotinic (Nn and Nm) and muscarinic (M1-M5)

Why are the receptors named nicotinic and muscarinic?

Named after drugs that bind them and mimic ACh effects (nicotine and muscarine)

Nicotinic receptor locations

Neuromuscular junction; all autonomic postganglionic cell bodies (ganglia); adrenal medulla; some CNS pathways

Nicotinic receptor signaling

Ligand-gated ion channel; fast; short-acting; always stimulatory

Nn vs Nm

Nn = autonomic ganglia (neuronal); Nm = skeletal muscle (neuromuscular junction)

Nicotinic mnemonic

Neuron + Neuromuscular junction

Muscarinic receptor locations

All parasympathetic effector cells and sweat glands

Muscarinic receptor signaling

G-protein-coupled receptor; slower and longer lasting; can be excitatory or inhibitory depending on subtype and tissue

How many muscarinic subtypes are there?

Five (M1; M2; M3; M4; M5)

How does M2 slow the heart?

ACh activates M2 and opens K+ channels; hyperpolarization slows spontaneous depolarization and lowers heart rate

M3 and M5 effects

Depolarization and contraction of smooth muscle and secretion of glands

Which receptor is found on the cell bodies of all autonomic postganglionic neurons?

Nicotinic receptor

Which receptors are found on effector organs?

Muscarinic and adrenergic (alpha and beta) receptors

Catecholamines

Dopamine; norepinephrine; and epinephrine

Catecholamine synthesis pathway

Tyrosine to dopamine to norepinephrine to epinephrine

Where can catecholamine synthesis stop?

At dopamine in some neurons; usually at NE; and sometimes continues to epinephrine

Which division uses adrenergic neurotransmitters?

Only the sympathetic nervous system

Two families of adrenergic receptors

Alpha (alpha-1 and alpha-2) and beta (beta-1; beta-2; beta-3)

Adrenergic receptor signaling

All are G-protein-coupled receptors

Alpha-1 G-protein and pathway

Gq to increased PLC to IP3 and DAG to increased Ca2+

Alpha-2 G-protein and pathway

Gi to decreased adenylyl cyclase to decreased cAMP

Beta-1 G-protein

Gs (increased adenylyl cyclase and cAMP)

Beta-2 G-protein

Gs (some Gi)

Beta-3 G-protein

Gs

Alpha vs beta sensitivity

Alpha receptors are more sensitive to norepinephrine; beta receptors are more sensitive to blood epinephrine

Alpha-1 locations and effects

Most blood vessels (vasoconstriction); radial muscle of iris (pupil dilation); GI and urinary sphincters (constriction)

Alpha-1 memory aid

Alpha-1 = contraction

Alpha-2 locations and effects

Presynaptic autoreceptors (inhibit NE release); CNS; pancreas (inhibits insulin secretion); GI tract (inhibits secretions)

Beta-1 locations and effects

Heart (increased contraction and heart rate); kidney (increased renin release)

Beta-1 memory aid

Beta-1 = 1 heart

Beta-2 locations and effects

Skeletal muscle vessels (vasodilation); lung smooth muscle (bronchodilation); uterus and GI smooth muscle (relaxation)

Beta-2 memory aid

Beta-2 = relax smooth muscle

Beta-3 locations and effects

Adipose tissue (triglyceride breakdown/lipolysis); bladder detrusor (relaxation and prevention of urination)

What is an autoreceptor?

A presynaptic receptor that responds to the neuron's own neurotransmitter

What does the alpha-2 autoreceptor do?

NE binds alpha-2 on the presynaptic terminal and inhibits further NE release (negative feedback)

What happens if you block the alpha-2 autoreceptor?

Negative feedback is lost so more NE is released

How can epinephrine constrict some vessels and dilate others?

It binds different receptor subtypes: alpha-1 causes vasoconstriction and beta-2 causes vasodilation

Which receptor causes vasoconstriction?

Alpha-1