Download Chapter 3 Sherringtons Reflex Work - Reflex arc (the circuit between

Chapter 3
Sherringtons Reflex Work
- Reflex arc (the circuit between a sensory neuron and a muscle response)
- Stimulated puppy paws to determine what would cause a reflex to occur. He found 2
types of summation…
1. Temporal summation (repeated light stimuli within a brief time frame causes response)
2. Spatial Summation (several stimuli inputs at different locations can cause a response)
- From this, he inferred that there are excitatory postsynaptic potentials (EPSP), which
are small depolarizations below threshold level that can compound eachother to exceed
the threshold and therefore incite an action potential.
- Conversely, there are inhibitory postsynaptic potentials (IPSP), which are purposely
induced hyperpolarizations of the membrane that serve to prevent action potentials
Note: Neurons have a spontaneous firing rate which creates action potentials in the
absence of stimulation.
Neurotransmitters
- Loewi worked on frog hearts and showed that there is a chemical messenger that
communicates to heart tissue. Neurotransmitters!
- Neurotransmitter (A chemical messenger that relays a signal from one neuron to another
at the synapse)
- Neurotransmitters are stored on the presynaptic terminal in a vesicle
- Neurotransmitters are made in the cell body and transported to the vesicle
- Neurotransmitters are released through the physical process of exocytosis
- Neurotransmitters bind to receptors on neighboring dendrites to pass messages along
Types of Neurotransmitters
- Amino Acid (made from a single amino acid. Ex. glutamine, glycine, tryptophan etc…)
- Peptide (made from chains of amino acids. Ex. Endorphins)
- Acetylcholine (similar to an amino acid except that the amino group is changed to a
N(CH3)3 )
- Monoamines (non-acidic transmitters that have an NH2 group. Ex. Indolamines like
serotonin and Catecholamines like epinephrine, norepinephrine and dopamine)
- Purines (made from the nucleic acid base adenosine)
- Gasses (Nitric Oxide)
Note: Hormones are basically neurotransmitters on a large scale. Hormones act globally
while neurotransmitters act very locally.
Effects of Neurotransmitters
- Ionotropic Effects (the neurotransmitter crosses the synapse and binds to a receptor on
the postsynaptic terminal, i.e. dendrite, which causes a gated channel to open which
allows ions to flow through. The signal is then propagated electrically. Response occurs
within 10ms and lasts up to 30ms)
- Metabotropic Effects (the neurotransmitter crosses the synapse and binds to a G-linked
receptor on the postsynaptic terminal, i.e. dendrite, which causes an activation of a
second chemical messenger inside of the cell. The signal is then propagated chemically.
Response occurs within 30ms and lasts up to hours)
Inactivation and Reuptake of Neurotransmitters
- Enzymatic breakdown (Ex. Acetylcholinesterase destroys activity of acetylcholine
through chemically modifying it. The inactivated acetylcholine can be sent back to the
presynaptic neuron where it can be reactivated by another enzyme and used again)
- Reuptake (some neurotransmitters like serotonin and the catecholamines detach from
the target receptor and can be absorbed back into the presynaptic neurons vesicles)
How Drugs Effect Synapses
- Antagonist (blocking the neurotransmitter from reaching its target receptor. This can be
done by binding the neurotransmitter, by binding the receptor, or by preventing release of
the neurotransmitter)
- Agonist (mimicking or increasing the effects of a neurotransmitter. This can be done by
directly activating the receptor, stimulating the release or neurotransmitter from the
vesicle, or by blocking reuptake or breakdown of the neurotransmitter)
- Affinity (how well a drug binds a receptor)
- Efficacy (how well a drug activates a receptor)
Note: A good antagonist can have high affinity and low efficacy. A good agonist can
have both high affinity and high efficacy.