Transcript 9-18-04 Nervous System Peripheral No1

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
The Synapse
Fig. 8.13
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
Receptors
2 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