CH 4 – The Nervous System Flashcards
Neurons
specialized cells capable of transmitting electrical impulses and then translating those electrical impulses into chemical signals.
The nucleus is in the __ or __, which is also location of endoplasmic reticulum and ribosomes
cell body; soma
Cell has many appendages emanating from the soma called __, which receive incoming messages from cells. Information received from the __ is transmitted through the cell body before it reaches the __, which integrates the incoming signals
dendrites; axon hillock
Axon hillock plays important role in __, or transmission of electrical impulses down the axon. The signals arrive from the dendrites, and the axon hillock sums up these signals. If the result is excitatory enough, it will initiate __
action potentials
__ is a long appendage that terminates near a target structure (muscle, gland, other neuron). __ carry neural signals away from the soma; dendrites carry signals toward the soma.
Axon(s)
Most mammalian nerve fibers are insulated by __, a fatty membrane, to prevent signal loss or crossing over of signal. Sort of like insulation of wires, __ maintains the electrical signal within one neuron. __ also increases the speed of conduction in the axon.
myelin; myelin sheath; Myelin
Myelin is produced by:
oligodendrocytes in the CNS 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 __. These are critical for rapid signal conduction
nodes of Ranvier
At the end of the axon is the __ or __. This structure is enlarged and flattened to maximize transmission of the signal to the next neuron and ensure proper release of __, the chemicals that transmit information between neurons.
nerve terminal; synaptic bouton (knob); neurotransmitters
Immune response against own myelin:
Sometimes the body mounts an immune response against its own myelin, leading to destruction of this insulating substance. Common demyelinating disorder is MS, where myelin of the brain and spinal cord is selectively targeted. This can cause weakness, lack of balance, vision problems, and incontinence.
Neurons are not physically connected to each other. Between neurons there is a small space into which the terminal portion if the axon releases neurotransmitters, which bind to dendrites of the adjacent neuron (postsynaptic neuron). This space known as the __. Together the __, __, and __ are known as a __.
synaptic cleft;
nerve terminal; synaptic cleft; postsynaptic membrane; synapse
Multiple neurons may be bundled together to form a __ in the peripheral nervous system. Nerves may be __, __, or __ in reference to the type of information they carry. The cell bodies of neurons of the same type are clustered together into __.
nerve;
sensory; motor; mixed
ganglia
Pathway of neural signals
Dendrites->Soma->Axon hillock->Axon->nerve terminal->synaptic cleft
Description of neuron structure and function:
Axon: Transmits electrical signals (action potential) from the soma to the synaptic knob
Axon hillock:Integrates excitatory and inhibitory signals from the dendrites and fires an action potential if the excitatory signals are strong enough to reach threshold.
Dendrites: Receive incoming signals and carry them to soma
Myelin sheath: Acts as insulation around the axon and speeds conduction
Soma: Cell body and contains the nucleus, ER, and ribosomes.
Synaptic bouton: lies at the end of the axon and releases neurotransmitters
In the CNS, axons may be bundled together to form __. Unlike nerves, __ only carry one type of information. Cell bodies of neurons in the same tract are grouped into nuclei.
tracts; tracts
nuclei
Other neural cells
Neurons must be supported and myelinated by other cells. These cells are often called glial cells, or neuroglia. Glial cells play both structural and supportive roles
- Astrocytes nourish neurons and form the blood-brain barrier, which controls the transmission of solutes from the bloodstream into 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 are phagocytic cells that ingest or break down waste products and pathogens in the CNS
- Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin around axons
action potentials
all or nothing messages neurons use to relay electrical impulses down the axon to the synaptic bouton. Action potentials cause release of neurotransmitters into the synaptic cleft.
resting membrane potential - explain ion as well
A cells resting membrane potential is the net electric potential difference that exists across the cell membrane, created by movement of charged molecules across that membrane. For neurons, this potential is ~ -70mV, with the inside of the neuron being negative relative to the outside.
2 most important ions in generating and maintaining the resting potential are K+ and Na+. Concentration of K+ is higher inside the cell than outside, making it favorable for K+ to move outside the cell due to K concentration gradient. To facilitate this movement, cell membrane has transmembrane potassium leak channels, which allow the slow leak of potassium outside the cell. As K+ continually leaks out, the inside of the cell loses positive charge leaving behind negative charge and making the outside of the cell positively charged.
However as negative charge builds up in the cell, some potassium ions will be drawn back into the cell due to the attraction between positive K ions and the negative potential building inside the cell. As this potential difference grows, K will also be more strongly drawn back into the cell. At a certain potential, each K cation pushed out the cell due to the concentration gradient will be matched by a K cation pulled back in the due to electric potential. At this point there is no net movement of the ion, cell is in equilibrium with respect to K. The potential difference of this is called equilibrium potential of potassium. This is ~ -90mV. Negative sign due to convention, and because a positive K ion is leaving the cell. Equilibrium potential of potassium is established almost instantly, as only a small amount of K need to exit the cell before resulting electrostatic force equals force of the concentration gradient.
Na concentration gradient is the revere of K. There is a driving force pushing Na into the cell. This movement is facilitated by sodium leak channels. The slow leak of Na into the cell causes a buildup of electric potential. The equilibrium potential of sodium is ~60mV, positive b/c Na is moving into the cell.
In a living system, Na and K are flowing across the cells membrane at the same time. In a certain sense, Na undoes the effect of K movement. The resting potential is thus a tug-of-war: K movement pulls the cell potential towards -90mV while Na movement pulls the cell potential towards +60mV. Neither ion wins, a balance is reached ~ -70mV for average nerve cell. This balance, the net effect of Na and K equilibrium potentials, is the resting membrane potential. The resulting potential is significantly closer to K equilibrium b/c the cell is more permeable to K.
Given the continual ion leaking at the membrane, Na+/K+ ATPase continually pumps Na and K back against their gradients to maintain the resting potential and the ions respective gradients. In a person’s body more ATP is spend by the Na+/K+ ATPase than for any other single purpose.
Neurons can receive both excitatory and inhibitory input.
Excitatory input causes depolarization (raising the membrane potential from its resting potential) and makes the neuron more likely to fire an action potential. If the axon hillock receives enough excitatory input to be depolarized to the threshold value (between -55 to -40), an action potential will be triggered.
Inhibitory input causes hyperpolarization (lowering membrane potential from its resting potential), and thus makes the neuron less likely to fire an action potential.
Not every stimulus generates a response. A postsynaptic neuron may receive information from several different pre-synaptic neurons, both excitatory and inhibitory. The additive effects of multiple signals is known as summation.
2 types of summation:
temporal and spatial.
Temporal summation is the integration of multiple signals close to each other in time. Several small excitatory signals firing at nearly the same moment could bring a post-synaptic cell to its threshold.
Spatial summation is the integration of multiple signals close to each other in space. Many inhibitory signals firing directly on the soma will cause more profound hyperpolarization of the axon hillock then the depolarization caused by a few excitatory signals firing on the dendrites of a neuron.
If the cell is brought to threshold:
voltage-gated Na channels open in the membrane. These ion channels open in response to the change in potential of the membrane (or depolarization of the membrane) and permit passage of Na ions. There is a strong electrochemical gradient that promotes the migration of Na into the cell. From an electrical standpoint, interior of the cell more negative than exterior , which favors the movement of + charged Na cations into the cell. From a chemical standpoint, higher concentration of Na outside the cell than inside, which favors the movement of Na into the cell. As Na passes through these ion channels, the membrane potential becomes more positive (cell rapidly depolarizes). Na channels can be opened or inactivated by changes to membrane potential. When Vm approaches +35mV, Na channels are inactivated and will have to be brought back near the resting potential to be deinactivated.
The + potential inside the cell triggers Na channels to inactivate and triggers voltage-gated K channels to open. Once Na has depolarized the cell, there is an electrochemical gradient favoring the efflux of K from the neuron. As K+ cations are driven from the cell, negative membrane potential will be restored called repolarization. The efflux of K+ cause on overshoot of the resting membrane potential, hyperpolarizing the neuron. This makes the neuron refractory to further action potentials
Na channels exists in 3 states:
Closed: before the cell reaches the threshold, and after inactivation has been reversed
Open: from threshold to ~ +35mV
Inactive: ~ 35mV to resting potential
2 types of refractory periods:
During absolute refractory period, no amount of stimulation can cause another action potential to occur. During relative refractory period, there must be a greater than normal stimulation to cause an action potential b/c the membrane is starting from a potential that is more negative than its resting value.
Na+/K+ ATPase acts to restore both resting potential and Na & K gradients that have been partially dissipated by the action potential. Action potential relies on these electrochemical gradients
impulse propagation
For a signal to be conveyed to another neuron, the action potential must travel down the axon and initiate neurotransmitter release. As Na rushes into one segment of the axon, it will cause depolarization in the surrounding regions of the axon. This depolarization will bring subsequent segments of the axon to threshold, opening the Na channels in those segments. Each of these segments then continues through the rest of the action potential in a wave-like fashion until the action potential reaches the nerve terminal. After the action potential has fired in one segment of the axon, that segment becomes momentarily refractory. Thus, information only flows in one direction.
