Physiology of the Nervous System Flashcards
voltage
- the measure of potential energy generated by separated electrical charges
- measured in volts or millivolts (1mV = 0.001 V)
- always measured between 2 points
- called the potential difference or the potential
current
- the flow of electrical charge from one point to another
- can be used to do work
resistance
- the hindrance to charge flow provided by substances through which the current must pass
What 2 factors does the amount of charge that moves between 2 points depend on?
voltage and resistance
chemically gated channels
- also called ligand-gated channels
- open when the appropriate chemical (neurotransmitter) binds
voltage-gated channels
- open and close in response to changes in membrane potential
mechanically gated channels
- open in response to physical deformation of the receptor (sensory receptors for touch and pressure)
electrochemical gradient
determine the direction an ion moves (into or out of the cell)
What are the 2 components of the electrochemical gradient?
- concentration gradient
- electrical gradient
concentration gradient
ions move along chemical concentration gradients from an area of their higher concentration to an area of lower concentration
electrical gradient
ions move towards an area of opposite electrical charge
Which ion plays the most important role in generating the membrane potential?
potassium (K+)
At resting membrane potential, what causes the negative interior of the cell?
Due to a much greater ability for K+ to diffuse out of the cell than for Na+ to diffuse into the cell
sodium-potassium pump
- ejects 3 Na+ from the cell
- transports 2 K+ back into the cell
- stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium
What can produce a change in membrane potential?
- anything that alters ion concentrations on the 2 sides of the membrane
- anything that changes membrane permeability to any ion
graded potentials
- usually incoming signals operating over short distances that have variable (graded) strength
- short-lived, localized changes in membrane potential, usually in dendrites of the cell body
- can be either depolarizations or hyperpolarizations
- these changes can cause current flows that decrease in magnitude with distance
- their magnitude varies directly with stimulus strength
- triggered by some change in the neuron’s environment that opens gated ion channels
action potential
- long-distance signals of axons that always have the same strength
- buried reversal of membrane potential with a total amplitude of ~ 100 mV
depolarization
decrease in membrane potential
hyperpolarization
increase in membrane potential
receptor potential / generator potential
- produced when a sensory receptor is excited by its stimulus
postsynaptic potential
- produced when the stimulus is a neurotransmitter released by another neuron
- neurotransmitter released into the synapse
Why can graded potentials only act as signals over very short distances?
because the current dissipates quickly and decays with increasing distance from the site of initial depolarization
What is the principal way neurons send signals over long distances?
by generating and propagating action potentials
Which cells can generate action potentials?
cells with excitable membranes (neurons and muscle cells)
step 1 of generating an action potential
resting state: all voltage-gated Na+ and K+ channels are closed
step 2 of generating an action potential
depolarization: voltage-gated Na+ channels open
step 3 of generating an action potential
repolarization: Na+ channels are inactivating, and voltage-gated K+ channels open
step 4 of generating an action potential
hyper-polarization: some K+ channels remain open and Na+ channels reset
How is stimulus intensity coded?
it is coded for by the number of impulses per second (by the frequency of action potentials) rather than by increases in the strength (amplitude) of the individual action potentials
absolute refractory period
- begins with the opening of the Na+ channels and ends when the Na+ channels begin to reset to their original resting state
- ensures that each AP is a separate all-or-none event
- enforces one-way transmission of the AP
- because the area where the AP originated has just generated an AP, the Na+ channels in that area are inactivated and no new AP is generated there
relative refractory period
- follows the absolute refractory period
- most Na+ channels have returned to their resting state
- some K+ channels are still open
- repolarization is occurring
- axon’s threshold for AP generation is elevated, so a stimulus that would normally generate an AP is no longer sufficient
- only an exceptionally strong stimulus can reopen the Na+ channels that have already returned to their resting state and generate another AP
Where are nerve fibres that transmit impulses most rapidly (100 m/s or more) found?
found in neural pathways where speed is essential (such as those that mediate postural reflexes)
What do the axons that conduct impulses more slowly serve?
typically serve internal organs (gut, glands, blood vessels) where slower responses are not a handicap
What 2 factors does the rate of impulse propagation depend on?
- axon diameter
- degree of myelination
axon diamter
- the larger the axon’s diameter, the faster it conducts impulses
- larger axons offer less resistance to the flow of local currents
degree of myelination
- the presence of a myelin sheath dramatically increases the speed of propagation
- conduction velocity increases with the degree of myelination
What are the 2 ways that action potentials can be propagated?
- continuous conduction
- saltatory conduction
continuous conduction
- action potential propagation in nonmyelinated axons occurs by continuous conduction
- voltage-gated channels in the membrane are immediately adjacent to each other
- relatively slow
saltatory conduction
- when an AP is generated in a myelinated fibre
- local depolarizing current does not dissipate through the adjacent membrane regions, which are non-excitable
- the current is maintained and moves rapidly to the next myelin sheath gap (distance of ~ 1mm) where it triggers another AP
- APs are only triggered at the gaps
- about 30 times faster than continuous conduction
group A fibres
- mostly somatic sensory and motor fibres
- serve the skin, skeletal muscles, and joints
- have the largest diameter
- thick myelin sheaths
- conduction impulses at speeds up to 150 m/s
group B fibres
- lightly myelinated
- intermediate diameter
- transmit impulses at ~15 m/s
group C fibres
- smallest diameter
- non-myelinated
- incapable of saltatory conduction
- conduct impulses at a leisurely pace (~1m/s)
What do B and C fibres have in common?
B and C fibre groups include:
- autonomic nervous system motor fibres serving the visceral organs
- visceral sensory fibres
- smaller somatic sensory fibres that transmit sensory impulses from the skin (such as pain and small touch fibres)
