MODULE 2

reflexes

reflex arc

measurements

modify; stronger; weaker

• Reflexes are slower than conduction along an axon • Several weak stimuli presented at nearby places or times produce a stronger reflex than one stimulus alone does • When one set of muscles becomes excited, a different set becomes relaxed

15 m/s

that some ome process must be slowing conduction through the reflex, and that it was inferred that the delay occurs where one neuron communicates with another

repeated stimuli within a brief time have a cumulative effect

pre: neuron that delivers transmission post: one that receives transmission

• Although the subthreshold excitation in the postsynaptic neuron decays over time, it can combine with a second excitation that follows it quickly • With a rapid succession of stimuli, each add its effect to what remained from the previous ones, until the combination exceeds the threshold of the postsynaptic neuron, producing an action potential

graded potential

excitatory; inhibitory

excitatory postsynaptic potential (EPSP)

• If the delay between EPSPs was short enough, the second EPSP added to what was left of the first one, producing temporal summation • A quick sequence of EPSPs combines to exceed the threshold and produce an action potential

summation over space

• Synaptic input from separate locations combine their effects on a neuron • Instead of pinching one point twice, he pinched two points at once • Although neither pinch alone produced a reflex, together they did • Stimulating two points activated separate sensory neurons whose axons converged onto one neuron in the spinal cord • Excitation from either sensory axon excited that spinal neuron, but not enough to reach the threshold and produced an action potential

• EPSPs from several axons summate their effects on a postsynaptic cell • However, if a neuron receives many incoming axons with synchronized input, spatial summation excites the neuron enough to activate it

yes, a neuron might receive input from several axons (spatial part) in close succession (temporal part)

A series of axons active in one order can have a different result from the same axons in different order

• At these synapses, input from the axon hyperpolarizes the postsynaptic cell • Increases the negative charge within the cell, moving it farther from the threshold and decreasing the probability of an action potential

• This temporary hyperpolarization of a membrane • Occurs when synaptic input selectively opens the gates for potassium ions to leave the cell (carrying a positive charge with them) or for chloride ions to enter the cell (carrying a negative charge)

• The EPSPs on a neuron compete with the IPSPs • The balance between the two increases or decreases the neuron’s frequency of action potentials • Some synapses produce fast, brief effects, and others produce slow, long-lasting effects

• The neuron synthesizes chemicals that serve as neurotransmitters. It synthesizes the smaller neurotransmitters in the axon terminals and synthesizes neuropeptides in the cell body • Action potentials travel down the axon. At the presynaptic terminal, an action potential enables calcium to enter the cell. • Calcium releases neurotransmitters from the terminals and into the synaptic cleft: the space between the pre- and postsynaptic terminal • The released molecules diffuse across the narrow cleft, attach to receptors, and alter the activity of the post synaptic neuron. Mechanisms vary for altering that activity • The neurotransmitter molecules separate from their receptors • The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling or they may diffuse away Some postsynaptic cells reverse messages to control the further release of neurotransmitter by presynaptic cells

chemical substances involved in the transmission of neural impulses from one neuron to another