BIOL 472 - Mammalian Physiology - Topic III - Velocity of Nerve Signal Conduction, Physiology of Synapses

Penn State - Fall 2026 - Dr. James Strauss

Resistance of membrane (Rm), capacitance of membrane (Cm), resistance of interior (Ri)

What three qualities influence nerve conduction velocities?

Inversely proportional

What is the relationship between capacitance and resistance?

ER is directly proportional to sqrt((1/(2pirl))(1/pir²))) (2pirl)

Effective Resistance Equation

Saltatory Conduction

The process of a nerve impulse jumping from one Node of Ranvier to the next

Capacitance is decreased two-fold and resistance of the membrane is increased two-fold. Resistance of the interior is unaffected

How does one myelination affect conduction velocity?

Electrical Synapse

Close (2-3 nm), gap junction, electrical current crosses without chemicals

Chemical Synapse

Wide (30-50 nm), begins with arrival of action potential and accounts for the delay in signal transduction

Orthodromic

Flow from cell body to axon (99.9%)

Antidromic

Flow from axon to cell body (0.1%)

Excitatory Post-Synaptic Potential (EPSP)

Moves cells closer to threshold, and makes them easier to depolarize

When sodium (Na+) conductance is increased

When are EPSPs faster?

When potassium (K+) conductance is decreased

When are EPSPs slower?

Inhibitory Post-Synaptic Potential (IPSP)

Hyperpolarizes cell, making it harder to depolarize

When Chloride (Cl-) conductance is increased

When are IPSPs faster?

When Potassium (K+) conductance is increased

When are IPSPs slower?

Spatial Summation

2 nerve impulses at the same time

Temporal Summation

2 nerve impulses in rapid succession

Presynaptic Facilitation

Stimulation of Calcium (Ca2+) channel opening, more neurotransmitter released

Presynaptic Inhibition

Inhibition of Calcium (Ca2+) channel opening, reduces amount of neurotransmitter released

Choline + Acetyl CoA

How is acetylcholine synthesized?

Parasympathetic stimulation and neurotransmitter for neuromuscular junction

What is the general action of acetylcholine?

Muscarinic and Nicotinic

Two types of acetylcholine receptors

Smooth, cardiac muscle, glands, and brain

Where are muscarinic receptors located?

Muscarine

What toxin stimulates muscarinic receptors, overstimulating the parasympathetic nervous system?

Atropine

What drug inhibits muscarinic receptors, inhibiting the parasympathetic nervous system?

Neurons, skeletal muscle, and brain

Where are nicotinic receptors located?

Nicotine

What substance stimulates nicotinic receptors

Curare and Bungarotoxin

What toxins block nicotinic receptors, resulting in muscle relaxation?

Acetylcholinesterase

What substance degrades acetylcholine?

Binds to acetylcholinesterase to prevent signal from stopping

What is the mechanism of action of Sarin, VX nerve gas?

Atropine and pralidoxime. Atropine blocks ACh stimulation and pralidoxime regenerates bound acetylcholinesterase

What do the autoinjectors that fight against nerve gas have in them / what do those drugs do?

Tyrosine → Dopa → Dopamine → Norepinephrine → Epinephrine

Pathway of biogenic amine synthesis

Monoamine Oxidase (MAO) and Catechol-O-Methyltransferase (COMT)

What are the two methods of norepinephrine deactivation?

Adrenergic Fibers

Nerve tracts using epinephrine / norepinephrine

Sympathetic stimulation

General action of epinephrine and norepinephrine

Alpha Receptors

Which receptor has a greater affinity for norepinephrine?

Beta Receptors

Which receptor has a greater affinity for epinephrine?

a1 Receptors

Stimulate contraction of smooth muscle, especially in vessels and sympathetic nerve tracts, increase IP3

a2 Receptors

Stimulate contraction of smooth muscle in vessels, but inhibit smooth muscle contraction in GI tract / pancreas, decrease cAMP

B1 Receptors

Stimulates heart rate, contractility, also kidney, fat lypolysis, increase cAMP (helpful for fight / flight)

B2 Receptors

Inhibits smooth muscle, increase cAMP, especially in lung, heart, skeletal muscle blood vessels (found in regions that need more oxygen during fight / flight)

B3 Receptors

Fat exclusively, lipolysis, increase cAMP

ACh at nicotinic receptors

Where are neurotransmitters distributed in the somatic nervous system?

Preganglionic fibers are short and postganglionic fibers are long

In sympathetic pathways, how are preganglionic and postganglionic fibers organized?

Preganglionic fibers are long and postganglionic fibers are short

In parasympathetic pathways, how are preganglionic and postganglionic fibers organized?

Cocaine

What drug inhibits reuptake of dopamine, resulting in a greater concentration in the synapse?

Parkinson’s Disease

Disease caused by a lack of dopamine in the substantia nigra, resulting in shaky tremors and impaired movement initiation. Can be treated using L-Dopa therapy

Tryptophan

What amino acid is serotonin derived from?

Psilocybin and LSD

What drugs directly stimulate serotonin?

Valium

What drug enhances the action of GABA?

Activates GABA, activates adenosine receptors, blocks glutamate, slows down second messenger pathways

Four modes of action of alcohol

Learning, but also neuronal damage during stroke

What pathways (double-edged sword) is NO responsible for?