Cell Communication, Nervous System, and Neurons Flashcards
Accommodation
Threshold stimulus is reached too slowly to trigger action potential
Summation
Nervous system screens and aggregates signals
Spatial:
Multiple dendrites receive a signal at the same time
Temporal:
Adds up effects of signals received by single dendrite in quick succession
What chemical and electrical gradient forces are the ions in a neuron at resting potential experiencing?
Resting potential is -60mV
Na+: chemical concentration into the cell and electrical gradient into the cell
K+: chemical concentration out of the cell and electrical gradient into the cell
Organic anions: chemical concentration out of the cell and electrical gradient out of the cell
Cl-: chemical concentration into the cell and electrical gradient out of the cell
Ca2+: chemical concentration into the cell and electrical gradient into the cell
Electrical synapse
Rare, but occur when coordinated action is required from a group of cells (I.e. the cardiac muscle, or smooth visceral muscle)
Much quicker signal transmission, and signal propagation is bidirectional
Which signals are short range and fleeting?
Somatic nervous system with neurotransmitters effecting very specific muscles
Describe the transduction of a signal along your neurons
Input from environment reaches sensory receptors, physics information stimulates the sensory neurons and is translated to electrical signal that travels length of neuron. Chemical signal carries signal from one neuron to the next and then converts back to electrical signal
What factors contribute to the nervous system’s perception about what stimuli is the most important?
Past experience, preconceived notions, evolutionary instinct
Depolarization
Membrane potential reverses polarity so it is positive on inside and negative on outside due to sodium rushing inward
Voltage gated Na+ Channels
Proteins change configuration when voltage across membrane reaches threshold, allowing Na+ to flow into cell
Positive feedback as more Na+ flows into cell, Na+ diffuses down axon and kicks off more depolarization
What methods might a G-protein use as a second messenger system?
G-protein attached to receptor protein on inside of postsynaptic membrane
Alpha subunit breaks free when receptor stimulated and:
- May activate separate specific ion channels
- May activate another second messenger (cGMP)
- May activate intracellular enzymes
- May activate gene transcription
Where does action potential originate in neuron
At axon hillock of signal strength is large enough
How does neurotransmitter stop stimulating the post-synaptic cell?
Neurotransmitter may be destroyed by enzyme in matrix of synaptic cleft
May be directly absorbed by presynaptic cell via active transport
May diffuse out of synaptic cleft
How is a neuron a concentration cell?
Neurons use an insulating membrane (cellular membrane) to separate two conducting solutions; the intracellular and extracellular fluids.
Uneven distribution of large amounts of Na+ outside of cell, and much higher amounts of K+ inside of the cell is responsible for a resting potential difference across the membrane (-70mV). Outside of cell- positive charge buildup
Chemical communication systems in the body
- nervous system: propagates neurotransmitters in short intercellular gaps (nervous system)
- Local mediators: released for cells in general proximity (paracrine system)
- Hormones: sustained long range signal molecules which move through the blood (endocrine system)
Fatigue
Presynaptic cell unable to transmit action potential due to lack of neurotransmitters. Happens when cell fires too often
How does post-synaptic cell receive action potential
Neurotransmitter diffuses across synaptic cleft and binds as a ligand to receptor protein, which causes increase in membrane permeability to ions, which then diffuse into the cell
Can function through a second messenger system, or direct change to protein permeability
Prolonged changes prefer a second messenger system (G-proteins commonly initiate)
What causes the resting potential to be so negative inside a neuron?
K+\Na+ ATPase pumps K+ into cell and Na+ out of cell. Since membrane is very permeable to K+, it diffuses out of the cell, and Na+ cannot diffuse inward. Therefore, as K+ reaches closer to equilibrium in terms of concentration gradient, it brings the resting potential down by decreasing the number of positive ions in the cell. This yields a resting potential of -60mV
What are the rates of diffusion of K+ and Na+ through leakage channels across the membrane?
Membrane is highly permeable to K+, but very impermeable to Na+, so diffusion of K+ out of cell is much faster
Can a synapse have both inhibitory and excitatory stimuli?
A given synapse only inhibits or excites
Neurotransmitters can be excitatory or inhibitory depending on the type of receptor in the post synaptic membrane
Discuss specificity of neurons to neurotransmitters
A single neuron usually secretes only one type of neurotransmitter, so it only has to produce one kind
A neuron may respond to multiple types of neurotransmitters if it’s dendrites have corresponding receptors
Over 50 neurotransmitters have been discovered, many for different parts of nervous system
Frequency of firing
Number of times a signal gets received for a specific unit of time
What signals are long range and sustained?
Endocrine hormone signals that travel through the blood stream
Neurotransmitter
Chemicals that are often derived from amino acids are released into the synapse by a presynaptic neuron and attach to receptors on post-synaptic cell
Small vesicles filled with neurotransmitters rest just inside presynaptic membrane
What are the structures in the Nervous System?
Brain, spinal cord, nerves, neural support cells, and sensory organs (eyes and ears)
Central Nervous System (CNS): brain and spinal cord
Peripheral Nervous System (PNS): nerves that signal voluntary movement and nerves that signal involuntary bodily functions
Ca2+ voltage gated channels in presynaptic neuron
Channels activated when action potential arrives at synapse, allowing Ca2+ to flow into cell
Rush of Ca2+ causes neurotransmitters to be released into synaptic cleft through exocytosis
Cell body
Contains organelles of neuron
Cytosol highly conductive and conducts signals immediately to axon hillock
Small dendrites transfer signals quickly to cel body
What are ways a signal can become high intensity?
High frequency of firing
Large number of receptors that respond
Specific type of receptors that respond
High intensity signals trigger action potentials
Relative refractory period
Only abnormally large stimulus will create action potential due to membrane hyperpolarization

*basic structure of a neuron
Neurons are comprised of many dendrites (branches off of cell body), single cell body, one axon with many branches
Stimuli to stimulate action potential
Total effects of all inputs exceeds threshold stimulus
Stimuli can be excitatory (depolarizing) or inhibitory (hyperpolarizing)
Voltage gated K+ channels
Less sensitive to voltage changes than Na+, so take longer to open. As they open, most Na+ channels are closing and becoming inactivated. They allow K+ to rush out of cell
Cause repolarization -> hyperpolarization
What are the benefits of having one long axon instead of multiple shorter ones?
Minimize error and maximize efficiency in signal transduction
Shorter axons with more synapses could slow down signal and introduce more possible sources of error
What are two ways to modify an axon to make an action potential transduct faster?
Increase the diameter of the axon to allow freer flow of Na+ down the axon
Increase the amount of myelin at ion on the axon to decrease capacitance and therefore decrease the amount of Na+ that needs to rush into the cell to depolarize the cell membrane
Chemical synapse
Space between two neurons that is crossed by neurotransmitters
Unidirectional and slower than electrical synapses
Example: motor end plate, or connection between neuron and a muscle
Neuron
Smallest functional unit of the nervous system. Highly specialized cell capable of transmitting a signal from one cell to another through combination of electrical and chemical processes
Join to form higher level divisions of NS
Cannot divide
Uses mostly glucose and oxygen from blood, and depends on aerobic respiration
What processes does the nervous system carry out to maintain homeostasis?
Sensing the environment, selecting and processing most significant information, communicating it to body, and coordinating a response
What is their Nernst equation and how does it become negative?
EK = - RT/nF ln(K+ intracellular/ K+ extracellular)
Because K+ is greater intracellularly, the fraction is greater than 1, and the natural log becomes positive, then with the negative it becomes negative.
This means that for the concentration difference of K+ to equal out, there must be a negative voltage across the membrane, because positive actions are rushing out of the cell
Resting potential
Electrical potential or voltage across the neuronal membrane at rest
Axon
Long strand that carries a tip potential to other neurons
Contains lots of ion channels to sustain signal.
Carries signal to synapse, which passes signal to another cell
What protein pumps potassium and sodium against concentration gradients to maintain gradient?
Na+/K+ ATPase: pumps three Na+ out for every two K+ it pumps in
What ions are important to creating the resting potential of a neuron and what are their concentrations inside and outside of the cell?
Na+: greater concentration outside of the cell
K+: greater concentration inside of the cell
Cl-: greater concentration outside of the cell
Organic anions (proteins): greater concentration inside of the cell
Ca2+: greater concentration outside of the cell
Resting membrane potential around -60mV
Absolute refractory period
Time in which no stimuli can stimulate action potential
Can occur due to Na+ channels being inactivated or due to the cell still being depolarized
Excitatory postsynaptic potential and Inhibitory postsynaptic potential
Aka EPSP or IPSP
Firing of one or more synapses that creates a change in neuron cell potential that is either depolarizing or hyperpolarizing, respectively
Typically, around 40-80 synapses must fire simultaneously in same neuron for EPSP to create action potential
Action potential
Mechanism by which a signal travels down the length of a neuron
Triggered by a depolarization above a threshold, all or none
Caused by opening of Na+ voltage gated channels at threshold which causes Na+ to rush into cell and move potential towards the equilibrium potential of 50mV for Na+.
Voltage gated K+ channels as Na+ channels begin to deactivate
Voltage gated Na+ channels inactivated and K+ channels from arise membrane
Voltage gated K+ channels close and membrane equilibrates to resting potential
