Fundamentals of the Nervous System - Chapter 11 (Part 2)
- Nerve impulses pass from neuron to neuron at synapses
- Prevents nerve impulses from directly passing from one neuron to the next
- Ensures unidirectional communication between neurons
- Types of Synapses
- Axodendritic - synapses between axon of one neuron and dendrite of another
- Axosomatic - synapses between the axon of one neuron and soma of another
- Axoaxonic (axon to axon)
- Electrical synapses are less common than chemical synapses
- Correspond to gap junctions found in other cell types
- Are important in the CNS in:
- Arousal from sleep
- Mental attention
- Emotions and memory
- Ion and water homeostasis
- Chemical Synapses
- Specialized for the release and reception of neurotransmitters
- Typically composed of two parts:
- Axonal terminal of presynaptic neuron, which contains synaptic vesicles
- Receptor region on the dendrite(s) or soma of the postsynaptic neuron
- As the nerve impulse enters the ______ ____ of the axon terminal it opens voltage gated channels in the _________________ membrane that allows _____to enter.
- The Ca++
promotes synaptic _____________ containing a neurotransmitter to fuse with the axon membrane and release the ___________________ into the ___________ cleft.
- The ________________ crosses the synapse and joins with the chemically gated protein _______________ in the __________________ neuron
membrane which allows _____ to enter the cell causing the resting membrane to be __________________.
- Excitatory synapses - Excitatory postsynaptic potentials(EPSPs)
- graded may or may not reach _______ __________
- depolarizes membrane of postsynaptic neuron
- Na+ gates are opened. Na+ ions flow _______the cell. Why?
- Internal cell membrane become _______ negative?
- This is called ________________.
- Will a current be likely to flow in the cell?
- Inhibitory synapses - Inhibitory-postsynaptic potentials (IPSPs)
- hyperpolarizes membrane of postsynaptic neuron
- b. action potential of postsynaptic neuron becomes less likely
- Cl - or K+ channels are opened
- Does the Cl - ions go into the cell or out? K+
into or out?
- Does the internal cell become more or less negative?
- This is called _______________________________.
- Will current be likely to flow in the cell?
ON TO THE AXON HILLOCK (trigger zone)
- This depolarization causes a current to flow. The charge flows along an electrical concentration __________ to the more - areas of the cell membrane toward the______________ of the
- This is a ___________ (local) potential meaning if it is not strong enough it will dissipate before it reaches the axon hillock. Graded
potentials occur at the synapse in ________ and ___ ______.
- Threshold potentials
- Subthreshold potentials
- Summation of EPSPs an IPSPs at the axon hillock.
- More than one axon is stimulating a single neuron. (spatial summation)
- 2 or 3 subthreshold EPSP's can add up
to one _______________ potential.
- A threshold EPSP and threshold IPSP stimulates a neuron simultaneously. Would an action potential result?
- A neuron sending repeated stimuli (temporal summation) at subthreshold to another neuron in rapid succession, may sum up to an
ACTION POTENTIAL GENERATION
- If the synaptic (local
graded) potentials add up to _____________ they will reach the _____ ______. The nerve impluse than continues down the axon as an _________
- Only the _____________ can generate an action potential.
- The action potential is generated
along the axon membrane by opening voltage gated ________channels and Na+ diffuses in and the membrane is _____________
- Soon after voltage
sensitive ________ channels open and K+ diffuses out, and the membrane is _________________,
- This depolarization and repolarization occurs all the way down the axon
with out diminishing in strength which is characterized as _______ ____ ___________
- unmyelinated axons conductions are slower because
- In myelinated axons the action potential jumps from ________ to _______ this type of conduction is called _____________ and is much faster then in
Meanwhile back at the synapse.......REPOLARIZATION
- Another nerve impulse may not be received by a postsynaptic neuron, until it goes back to the ________________ state.
- Absolute refractory period
- Relative refractory period
- Neurotransmitter bound to a postsynaptic neuron producing a continuous postsynaptic effect must be removed from its receptor
- Blocks reception of additional "messages"
- Removal of neurotransmitters occurs when they:
- Are degraded by enzymes
- Are reabsorbed by astrocytes or the presynaptic terminals
- Diffuse from the synaptic cleft
- Example - __________________ swiftly removes ACh from the receptors thus cutting off the influx of Na+ and eflux of ____.
The ______ pump
is activated to reestablish the ionic conditions.
The membrane than returns to its __________ __________ potential of ________ mV.
Thus the membrane is _____________ to polarized.
- How drugs or poisons control/or destroy:
- Stop the chemical reactions that create neurotransmitters.
-->effects serotonin and midbrain dopamine systems resulting in the progressive degeneration of nerve terminals.
- Interfer with vesicle storage or release
- opiates -->increase release of dopamine
- Interfers with deactivation of
the neurotransmitter. - MAOI's inhibit the
action of the enzyme Monoamine oxidase (reduces levels of dopamine and norepinephrine)
- Bind to receptors in place of neurotransmitters - mimics
(agonists) & blockers (antagonists)
- LSD ---replaces serotonin ---> hallucinates
- caffeine --->replaces adenosine --> facilitates
- beta blockers
- Preventneurotransmitters from returning to their sending neuron (reuptake) -
- Paxil to control panic disorders
- Interfer with second messengers, the chemical and electrical changes that take place in a receiving neuron.
- Inhibiting Postsynaptic Potentials (IPSP)
- Neurotransmitters that inhibit -Serotonin, GABA,
endorphins, (ACh, epinephrine -effect depends on receptor)
- Opiates (morphine) - inhibit the release of substance P
- Valium - enhances binding effect of GABA (agonist)
- Fluoxetine ""Prozac" blocks reuptake of serotonin
- Excitatory postsynaptic potentials (EPSP)
- Neurotransmitters that excite -substance P, glutamate, dopamine, Serotonin, ACh , epinephrine
- LSD - competes with serotonin for receptor (antagonist)
- Amphetamines -enhance the release and decrease reuptake of dopamine.(agonist)
- caffeine - lowers the threshold at synapse (facilitation)
- nicotine - increase release of dopamine
- sarin inhibits acetylcholinesterase
- Effect of alcohol
- Increased turnover of norepinephrin and dopamine
- slow acetylcholine systems
- increased production of endorphins
- increased transmission in GABA systems
- Strychnine Poisoning
- In spinal cord, Renshaw cells normally release an inhibitory neurotransmitter (glycine) onto motor neurons preventing excessive muscle contraction
- Strychnine binds to and blocks glycine receptors in the spinal cord
- Massive tetanic contractions of all skeletal muscles are produced
- when the diaphragm contracts & remains contracted, breathing can not occur
- Effect of ions on nerve impulse transmission
- Decrease of K+ in extracellular fluids causes hyperpolarization of RMP
- caused by starvation, alkalosis, kidney disease
- symptoms - muscle weakness, sluggish reflexes
- Increase or decrease threshold?
- Stimulus needs to be stronger or weaker?
- Increase of K+in extracellular fluids causes depolarization of RMP
- nervousness, convulsions, paralysis
- Increase or decrease threshold?
- Stimulus needs to be stronger or weaker?
- Total depolarization would result in which of the symptoms?
- Local anesthetics and certain neurotoxins
- Prevent opening of voltage-gated Na+ channels
- Nerve impulses cannot pass the anesthetized region
- Novocaine and lidocaine
- Puffer Fish poison
Helpful Activities to do after you finish reading this chapter