Neurons and Synapses Flashcards
Central nervous system (cns)
Brain and spinal cord
Peripheral nervous system (pns)
Peripheral nerves outside the brain and spinal cord (cranial nerves: emerge directly from the brain stem - 12 pairs; spinal nerves: emerge directly from the spinal cord - 31 pairs)
Stimulus response pathway
Stimulus - transmit - response (for the transmission of electrochemical signals)
Change the neurons membrane potential
Stimulus (external or internal)
Detected by a sensory receptor in the peripheral nervous system (skin, eyes, ears, etc)
Transmit
Sensory neurons (nerves in PNS) transmit the signal (via electrical impulses) to the central nervous system (to relay neurons)
Response
“Selected” by CNS (conscious or unconscious) and a signal is sent (via relay neurons in the CNS to motor neurons in the PNS) to effectors (muscles or glands) to generate a response
What is a response
Change in an organism due to detection of a stimulus
Neurons (function)
To transmit electrical impulses
Basic structure of a neuron
Dendrite, cell body (soma), axon
Dendrite (what is it, function)
Short branched fibers (branch off the cell body)
Function: receive chemical signals (from sensory or other neurons) and transform them into electrical signals which are sent to cell body (into neuron)
Cell body (soma; what is it, function)
Nucleus and cytoplasm + organelles
Function: metabolism and summation of input cells
Axons (what is it, function)
Carry signals away from cell body to the end of the axon (axon terminal)
Axon terminal
Where neurotransmitters are released for communication with other neurons or effectors
Schwann cells
Supporting cells that form the myelin sheath
Myelin sheath
Insulating layer on the axon: increases the speed of the saltatory conduction
Saltatory conduction
Signal along the axon
Sensory neurons (peripheral)
Transmit impulses from body to CNS
Motor neurons (peripheral)
Transmit impulses from CNS to body (effectors)
Relay neurons (central)
Transmit impulses within CNS
Nerve
Bundle of neurons grouped together into a single structure (can be sensory, motor, or mixed)
Role of neurons
Pump sodium and potassium ions across their membranes to generate a resting potential
Membrane potential
The difference in electrical charge across the plasma membrane
Resting potential
The difference in electrical charge across the plasma membrane when s neuron is at rest (not sending an impulse) = -70mV
Inside of the cell at rest
More negative
Cations: lots of K+ and few Na+
Anions: proteins, sulfate, phosphate (collectively A-) and few chloride (Cl-)
Outside of cell at rest
More positive
Cations: lots of Na+ and few K+
Anions: chloride (Cl-)
Resting membrane potential steps
1) sodium-potassium pumps in the membrane of the axon maintain the resting potential
2) using active transport (atp) sodium potassium pumps pump 3 Na+ ions out of the axon while pumping 2 K+ ions into the axon
Result of resting potential membrane steps
Outside of neuron is more positive compared to the inside
Outside of axon and inside of neuron are polarized (negative resting potential of -70mV)
Role of cytoplasm inside axon in resting membrane potential
Contains negatively charged ions that are permanently fixed there (A-) help maintain -70mV
Nerve impulse
Action potentials propagated along the axons of neurons
Action potential is generated and sent down the axon ONLY WHEN:
A stimulus caused a voltage gated ion (Na+) channel to open in the axon membrane
Threshold voltage is reached
An action potential is…
ALL OR NOTHING
Action potential consists of…
Depolarization and repolarization of the neuron
Depolarization
Voltage-gated Na+ channels open when threshold potential of -55mV is reached
Na+ rushes into axon (more Na+ outside the cell)
Causing more Na+ channels to open
Domino effect down the axon
Membrane potential becomes more positive
Repolarization
K+ channels open (Na+ channels close)
K+ rushes out of the axon
Domino effect down the axon
Membrane potential becomes more negative (hyperpolarized)
Resting potential restored
Sodium potassium pumps: 3 Na+ out for every 2 K+ in
(Refractory period: reset stage - action potential can’t be fired until this is done; the resting potential and Na+/K+ ion concentration gradients need to be restored
In myelinated neurons, actions potentials travel…
Faster down the axon bc ion channels are only positioned between myelinated portions (saltatory conduction)
Also require less atp to return to resting potential
Synapse
Junction between two neurons (axon to dendrite)
Transmitting cell- presynaptic neuron
Receiving cell- post synaptic neuron
Synaptic cleft
Gap that separated the neurons
Neurotransmitters
Chemically Transmit messages across the synaptic cleft
Either excitatory or inhibitory
Excitatory neurotransmitter
Open ion channels (Na+ into post-synaptic cell) Cause depolarization (action potentials) Ex: acetylcholine
Inhibitory neurotransmitter
Open ion channels (Cl- into post synaptic cells)
Prevent depolarization (action potentials)
Hyperpolarize post-synaptic cell
Example GABA
Acetylcholine (Ach)
Neurotransmitter
Made by combining choline and an acetyl group
Ach is usually released by presynaptic neurons at…
Neuromuscular junctions in order to trigger muscle contractions by binding to receptors (cholinergic/nicotinic) in the membrane of postsynaptic muscle fibers to allow Na+ ions to diffuse into postsynaptic muscle fiber cells
Acetylcholinesterase (enzyme)
Released by presynaptic cell or found in membrane of postsynaptic cell
Continually breaks down Ach into choline and an acetyl group
Overstimulation of muscle fibers by Ach can lead to…
Fatal convulsions and paralysis. Break down in necessary
Reabsorption
Choline is taken back into the presynaptic cell to be used to make Ach again
Pesticides: blocking synaptic transmission at cholinergic synapses is done by…
Irreversibly Binding neonicotinoid pesticides to acetylcholine receptors (in postsynaptic muscle fiber cell membranes)
It’s an effective pesticide bc…
Ach receptors in insects are different than in mammals, so the neonicotinoid binds stronger to the receptor
When the neonicotinoid binds to Ach receptors…
It blocks normal Ach binding (block synaptic transmission)
AchE cant break down neonicotinoids, so it’s effect is permanent (paralysis/no muscle contraction/death)
Environmental considerations of neonicotinoids
Reduces bees and bird populations
