Drugs of the Peripheral Nervous System

The Nervous System Review

Organization of the Nervous System

• Central nervous system (CNS)– Brain and spinal cord

• Peripheral nervous system (PNS)– Neurons outside the CNS

– Sensory division• Afferent fibers transmit impulses from receptors to CNS

– Motor division• Efferent fibers transmit impulses from CNS to effector organs

Relationship between motor and sensory fibers of the PNS and the CNS

Autonomic Nervous System

• Sympathetic– Fight or flight, stress– Excitatory effects elicited by norepinephrine

activating beta receptors– Inhibitory effects elicited by activation of alpha

receptors

• Parasympathetic– Rest and digest– Digestive system activated, heart rate inhibited,

blood vessels dilated– Vagus nerve primarily responsible for

activating parasympathetic responses

Fig. 8.39

Autonomic Neurotransmission

• Two neurons– Presynaptic and postsynaptic

• Presynaptic from the brain to autonomic ganglia– Acetylcholine released from prenaptic neuron across

synapse to activate postsynaptic neuron

• Postsynaptic neurons release norepinephrine (sympathetic) or acetylcholine (parasympathic) at the effector organ

Fig. 8.13The Synapse

Neurotransmitters

--packaged in synaptic vesicles.

Nerve endings of the ANS secrete:• Acetylcholine (ACh)--Cholinergic neuron

– Parasympathetic effector

• Norepinephrine (NE)--Adrenergic neuron– Sympathetic effector

• Neurotransmitters diffuse across the synaptic cleft and bind to receptor on the post-synaptic membrane

• This can cause membrane channels (Na+, K+, or Cl-) to open or close depending on the neurotransmitter

• If stimulatory, Na+ channels will open

• If inhibitory, K+ or Cl- channels will open– Cell becomes more negative, hyperpolarized

Catecholamines

• Norepinephrine and epinephrine– Norepinephrine primarily neurotransmitter– Epinephrine primarily hormone

• Primarily concerned with sympathetic transmission

• Released at synaptic cleft and bind to alpha or beta receptors

Acetylcholine

• Responsible for all parasympathetic neurotransmission– Binds to mucarinic receptors at the end organ

Ganglionic transmission

– Transmits both parasympathetic and sympathetic preganglionic signals to nicotinic receptors (Nn)

– All ganglionic transmission is cholinergic (acetylcholine)

• Drugs that block ganglionic transmission block either parasympathetic or sympathetic depending on which is active

• This is a paradox many have a problem grasping

Fig. 8.39

Neuromuscular transmission

• Somatic motor neurons release acetylcholine at neuromuscular junction– Acetycholine diffuses across synapse and binds

to muscular nicotinic receptors (Nm) causing sodium influx

The Neuromuscular Junction

Central Neurotransmission

• Acetylcholine the primary neurotransmitter in the brain

• Works by interaction with muscarinic receptors

Neurotransmitter degradation

• Neurotransmitters which fail to bind to a post-synaptic receptor are – degraded by enzymes (acetylcholinersterase)– Taken up into the presynapse and recycled– Diffuse out of the synapse

Receptors2 types of cholinergic receptors:• Nicotinic

– Preganglionic sympathetic and parasympathetic (Nn)– Also neuromuscular somatic motor (Nm)

• Muscarinic– parasympathetic

2 types of adrenergic receptors:• Alpha

– Generally inhibitory

• Beta– Generally excitatory

Neurotransmission Enhancers

• Receptor agonists– Compound which bind to the receptor and activate

• Can be cholinergic or adrenergic

• Agents which induce neurotransmitter release– Stimulate release of neurotransmitter in absence of

signal (or reduced signal)

• Inhibitors of neurotransmitter degradation– Inhibit acetylcholinersterase

Neurotransmission Inhibitors

• Presynaptic nerve blockerss

• Receptor antagonists– Bind to receptor and prevent activation

• Ganglion blockers