chapter 4: the nervous system Flashcards
1
Q
what are neurons?
A
neurons are specialized cells capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals
2
Q
explain the anatomy of a neuron.
A
- the cell body (also called the soma) contains the nucleus, endoplasmic retiticulum, and the ribosomes
- dendrites are appendages that emanate from the soma; they receive incroming messages from other cells
- incoming info from the dendrites is transmitted through the cell body before it reaches the axon hillock, which integrates incoming signals
- axon hillock plays an important role in action potentials (the transmission of electrical impulses down the axon)
- signals arriving from the dendrites can be excitatory or inhibitory
- the axon is a long appendage that terminates in close proximity to a target structure (a muscle, a gland, or another neuron)
- most mammalian nerve fibers are insulated by myelin (a fatty membrane), to prevent signal loss or crossing of signals
- the myelin sheath maintains the electrical signal within one neuron
- myelin is produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system
- at certain intervals along the axon, there are small breaks in the myelin sheath with exposed areas of axon membrane called nodes of Ranvier, which are critical for rapid signal conduction
- at the end of the axon, the nerve terminal or synaptic bouton (knob); the structure is enlarged and flattened to maximize transmission of the signal to the next neuron and ensure proper release of nuerotransmitters (the chemicals that transmit information between neurons)
3
Q
what are neurons?
A
neurons are specialized cells capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals
4
Q
are neurons physically connected to each other? how does this impact release of neurotransmitter?
A
- no, neurons are not phsyically connected to each other
- betwen the neurons, there is a small space in which the** terminal portion of the axon releases neurotransmitters, which bind to the dendrites of the adjacent neuron (postsynaptic neuron**); this space is known as the synaptic cleft
- the nerve terminal, synaptic cleft, and postsynaptic membrane are known as a synapse
- neurotransmitters released from the axon terminal traverse the synaptic cleft and bind to receptors on the postsynaptic neuron
5
Q
what are bundled neurons refered to in the peripheral nervous system? what are the different types of nerves?
A
- neurons bundled together in the peripheral nervous system are known as nerves
- the different types of nerves are: sensory, motor, and mixed (this refers to the type of info they carry)
- mixed nerves carry both sensory and motor info
- the cell bodies of neurons of the same type are clustered together itno ganglia
6
Q
in the central nervoys system, axons bundled together are referred to as what?
A
- axons may be bundled together to form tracts
- unlike nerves, tracts only carry one type of information
- the cell bodies of neurons in the same tract are grouped into nuclei
7
Q
what are the other cells in the nervous system?
A
glial cells!!! (aka neuroglia)
the different types:
* astrocyte: nourish neurons and form the blood-brain barrier, which controls the transmission of solutes from the bloodstream into the nervous tissue
* ependymal cells: line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber
* microglia: phagocytic cells that ingest and break down waste products and pathogens in the central nerous system
* oligodendrocytes (central nervous system) and Schwann cells (peripheral nervous system) produce myelin around axons
8
Q
describe neurons’ “all or nothing” messages
A
- neurons use “all or nothing” messages called action potentials to relay electrical impulses down the axon to the synaptic bouton
- action potentials ultimately cause the release of neurotransmitters into the synaptic cleft
9
Q
explain how potassium and sodium are important for generating and maintaining a neuron’s resting membrane potential.
A
- there is a higher concentration of potassium inside the cell, and a higher concentration of sodium outside the cell
- potassium’s high concentration inside the cell makes it favorable to move outsid the cell, which it does through potassium leak channels which allow the slow leak of potassium outside the cell
- as potassium leaks out the cell, the cell loses a small amount of positive charge, leaving behind a negative charge w/in the cell and making the outside of the cell slightly positive relative to inside the cell
- as the negative charge builds up, potassium will be drawn back into the cell, where eventually there will be no net movement of potassium reaching equilibrium potential of potassium, which is around -90mV (the sign is negaive bc a cation is leaving the cell)
- sodium’s high concentration outside the cell makes it favorable to move inside the cell via sodium leak chnnels
- with the flow of sodium into the cell, a positive charge will build up outside the cell, drawing sodium back out, where it will eventually reach equilibrium potential of sodium, which is around 60mV (positive bc an cation is moving into the cell)
- in a living system, sodium and potassium are flowing at the same time, so the ions never establish their own equilibrium
- a balance of these two is reached, at -70mV; this is the net effect of sodium and potassium’s equilibrium potentials, and this is known as the resting membrane potential
- the resting membrane potential for the cell is much closer to potassium because the cell is more permeable to potassium
- Na+/K+ ATPase restores sodium and potassium so that the resting potential is maintained; it does so by continually pumping potassium in the cell and sodium out of the cell
10
Q
explain excitatoy and inhibitory input
A
- excitatory: causes depolarization (raising them mebrane potential, Vm, from its resting membrane potential); makes the neuron more likely to fire an action potential
- inhibitory: causes hyperpolarization (lowering the membrane potential from its resting membrane potential; makes neuron less likely to fire an action potential
- if an axon hillock receives enough excitatory input to be depolarized to the threshold value (ranging from -55mV to -40mV), an action potential will be triggered
11
Q
explain the different types of summation.
A
- summation: the additive effect of multiple signals
- temporal summation: multiple signals are integrated during a relatively short period of time; a number of small excitatory signals firing at nearly the same moment could bring a postsynaptic cell to threshold, enabling action potential
- spatial summation: the additive effects are based on the number and location of the incoming signals; a large number of inhibitory signals firing directly on the soma will cause more profound hyperpolariztio of the axon hillock than the depolarization caused by a few excitatory signals firing on the dendrites of a neuron
12
Q
explain the sodium and potassium ion channels, along with membrane potential.
A
- if the cell is brought to threshold, voltage-gated soidum channels open in the membrane
- these ion channels open in response to the change in potential of the membrane (depolarization) and permit the passage of sodium ions
- there is a strong electrochemical gradient that promotes the migration of sodium ions into the cell
- from an electrical standpoint, the interior of the cell is more negative than the exterior of the cell, which favors the movement of positively charged sodium cations into the cell
- from a chemical standpoint, there is a higher concentration of sodium outside the cell than inside the cell, which also favors the movement of sodium into the cell
- as sodium passes through these ion channels, the membrane becomes more positive; the cell rapidly depolarizes (this occurs when the membrane potential is raised from its resting potential)
- sodium channels can also be also be inactivated; this occurs when the Vm approaches +35mV; to be deinactivated the membrane potential needs to be brough back around its resting potential
- positive potential also triggers the voltage-gated potassium channels to open
- once sodium has depolarized the cell, there is an electrochemical gradient favoring the efflux of potassium from the neuron
- as positively charged potassium cations are driven out of the cell, there will be a restoration of the negative membrane potential called repolarization
- the effluc of K+ causes an overshoot of the resting membrane potential, hyperpolarizing the neuron
- this hyperpolarization is important in that it makes the neuron refractory to further action potentials
- the Na+/K+ ATPase acts to restore the resting potential, and the sodium potassium gradients that have been partially dissipated by the action potential
13
Q
what are the three states sodium channels can exist in?
A
- closed: before the cell reaches threshold and after inactivation has been reversed
- open: from threshold to approximately +35mV
- inactive: from approximately +35mV to resting potential
14
Q
what are two types of refractory periods?
A
- absolute refractory period: no amount of simulation can cause another action potential to occur
- relative refractory period: there must be greater than normal simulation to cause an action potential becayse the membrane is starting from a potential that is more negative than its resting value
refractor periods occur when the neuron is hyperpolarized
15
Q
what is impulse propagation?
A
- for a signal to be conveyed to another neuron, the action potentual must travel down the axon and initiate neurotransmitter release; this movement is called impulse propagation
