2/19/99

Physiology II

Nervous System Unit

· Hormonal system all over again

· The neurotransmitters are basically hormones

· Require a transmitter

· Have some kind of affect on the cell

· 2 classes of neurotransmitters

1. small molecular weight—are synthesized in nerve ending, packaged in nerve ending—E, ACTH, glutamate, dopamine—can be recycled—not generally specific—takes a lot of molecules to elicit a response much of it is wasted

2. neuropeptides—synthesized in the soma on ribosomes called Nissel Bodies and packaged in the soma, passed down by axoplasmic transport—more specific ex: ADH—very strong, a little bit goes a long way and if very long lasting

· second messenger hormones that tend to open ionic channels as there response

· in any given area of a cell, have both excitatory and inhibitory receptors

· the excitation of a cell is the sum of the stimulation of the excitatory and inhibitory receptors by the neurotransmitter

· neurotransmitter does not travel, only across the synapse, a couple of Angstroms; unlike hormones that have to travel through the blood to find its receptor

· first time you fire a synapse, there is no receptor on the target cell to receive the neurotransmitter

· each time you fire a synapse, more receptors are produced in response to the neurotransmitter—facilitation

· usually release the same amount of neurotransmitter

· have an accumulated effect to stimulate the receptor cell

· ex: of disorder that does not have receptors—Maisteina Gravis

Releasing transmitters

· output neuron is excited b/c it is releasing transmitter

· inhibited neuron does not release transmitter

What is needed to release transmitter?

1. uptake of calcium into synaptic knob—calcium voltage regulated channels

tells the vesicle where to attach to the membrane

2. energy and mitochondrial activity

· every cell has choline esterase on the cell surface

· E has the ability to circulate somewhat

· Purely electrical synapse disadvantage—almost direct contact as seen w/gap jxns in muscle—huge rxn where everyone reacts together

· Excitatory postsynaptic potential—open sodium channels

· Must have a concentration gradient that is set up by active transport

· 3 Na+ go out and 2 K+ go in for the active transport

· -70mV for the rest potential

· to start an excitation need to overcome the threshold which is different for different types of cells

· get of +20mV is the action potential

· sodium permeablity spike until reach equilibrium and starts to fall off

· potassium doesn’t start to increase until sodium has peaked

· potassium channels stay open longer and are the refractory period

· are chemicals that impede sodium to opens delays the action potential

Summation

· spatial—different neurons all over the cell that starts the action potential

· temporal—one neuron keeps firing (repetitive) until starts the action potential—rare ex: tetany, epilepsy (seizures stop b/c run out of energy)

Synaptic delay—takes time (3 min) from input to getting response

Decremental Conduction

· most synapses are on dendrites

· dendrites are leaky, lose potential as moves down the dendrite and potential doesn’t reach the soma

· axonal hillock—area to stop the signal; very little myelin; not reactive synapses along w/inhibitory receptors

Organization of Input

· neuronal pools—ex: cortex is the largest—feedback areas

· referred pain—

· spinothalamic tract—input to the spine to the thalamus—have pain and temp sensation

· shared neuronal junctions

· discharged zone—maximal contact—whatever source does recipient does

· facilitated (subliminal) zone—some contact but not enough to elicit the same response as output; needs more to make the excitation occur

Sustain a Signal

· feedback loop

· reverbatory circuit—lasts a few seconds, keeps feeding on itself, shuts off b/c run out of energy