Chapter 3 Flashcards
neurophysiology
study of the electrical and chemical signaling communication process
action potential
rapid electrical signal that travels along the axon of a neuron within a neuron
resting cell
- inside of a resting cell is more negatively charged than the extracellular fluid on the outside
- resting membrane potential = -50 to -80 millivolts (mV)
- neuron at rest = balance of electrochemical forces
ions
electrically charged molecules
- anions: negatively charged
- cations: positively charged
lipid bilayer
2 layers of fatty molecules that make up the cell membrane
- where specialized proteins (receptors) float
- build natural boundary against other molecules -> semipermeable via ion channels
- two opposing forces that drive ion movement through cell membrane: diffusion and electrostatic pressure
diffusion
ions flow from areas of high to low concentration (move down concentration gradient)
- cell membranes permit some ions/substances to pass through but not all
electrostatic pressure
ions flow towards oppositely charged areas
- like charges repel each other (e.g. + +)
- opposite charges are attracted to each other (e.g. + -)
ion channels
proteins that span the membrane and allow ions to pass
- open all the time for potassium (K+) ions only
- gated: open and close in response to voltage changes, chemicals, or mechanical action
- neuron shows selective permeability to K+ - can enter and leave freely
ion pump
located at the cell membrane, actively pumps ions to maintain resting potential
- sodium potassium pump: 3 Na+ out for every 2 K+ in
establishment and maintenance of resting potential
- at rest, K+ ions move into negative interior of cell because of electrostatic pressure
- K+ ions build up inside cell and diffuse out through the membrane
- K+ reaches equilibrium (movement out = movement in) aka resting membrane potential -60 mV (range: -50 to -80 mV)
distribution of ions
- K+: more found inside (cell interior)
- Na+, Cl-, Ca+: more outside (extracellular fluid)
- they are exchanged through specialized ion channels in cell membrane; large & negatively charged proteins stay inside neuron
action potentials
brief but large changes in membrane potential of axons
- function: transmit information within a neuron
- originate in axon hillock, propagated along axon
- all-or-none property: neurons fire at full amplitude or not at all
+ does NOT reflect increased stimulus strength
+ stimulus strength increases -> action potential frequency increases
hyperpolarization
increase in membrane potential (interior of membrane becomes even more negative compared to the outside)
depolarization
decrease in membrane potential (interior of cell becomes less negative)
- process through which action potentials are generated
generation of an action potential
- stimulus is sent out from cell body -> sodium channels open and Na+ ions surge in
- inside of cell becomes more positive
- when threshold (-40 mV) is reached -> membrane triggers an action potential and send the electrical signal down the axon
return to resting potential
- action potential = Na+ ions moving into the cell -> at peak, concentration gradient pushing Na+ ions in = positive charge driving them out
- membrane shifts from resting state to active state and back (depolarization)
- voltage-gated Na+ channels open and more Na+ ions enter
- membrane reaches equilibrium (+40 mV) -> inside of cell becomes positive
- voltage-gated K+ channels open -> K+ ions move out -> resting potential restored
refractory period
time during which stimulus given to neuron, no matter how strong, will not lead to an action potential
- Na+ channels are inactivated
- types:
+ absolute refractory period: complete insensitivity to stimuli; about 1-2 millisecond
+ relative refractory phase: reduced sensitivity; only strong stimulation produces an action potential
ion channels-blocking toxins
- tetrodotoxin (TTX - found in pufferfish) and saxitoxin (STX): block voltage-gated Na+ channels -> prevent production of action potentials, leading to paralysis and death
- batrachotoxin (found in poison arrow frogs): forces Na+ channels to stay open -> prevent restoration to resting potential & block neuronal transmission, leading to paralysis
speed of action potentials along axons
- as fast as 150 m/s for some neurons
- factors:
+ size: larger diameter allows depolarization to spread faster through the interior
+ species: vertebrates have much faster conduction thanks to nodes of Ranvier, aka small gaps in the myelin sheath
+ myelin insulation provides resistance to ion flow -> with these nodes, action potentials can jump from node to node instead moving down entire membrane channel by channel like in insects (SALTATORY CONDUCTION)
multiple sclerosis
demyelinating disease
- myelination: process in which glial cells wrap axons with a fatty sheath (myelin) to insulate and speed conduction
- immune system attacks myelin sheath and causes communication problems between brain and rest of the body
- results: nerve deterioration and permanent damage to body
- theories: genetics? virus?
- no cure yet
neurotransmitter
chemical messenger that transmits information between neurons through synapses
postsynaptic potentials
- excitatory postsynaptic potential (EPSP): produces small local depolarization, pushing cell closer to threshold
- inhibitory postsynaptic potential (IPSP): produces small hyperpolarization, pushing cell further away from threshold
+ result from chloride ions (Cl-) entering the cell and making the inside more negative - integration of excitatory and inhibitory inputs = INFORMATION PROCESSING
transmission at a chemical synapse
- action potential travels down the axon to the axon terminal
- voltage-gated calcium channels open and Ca2+ enter
- synaptic vesicles fuse with membrane and release transmitter into the cleft
- transmitters cross the cleft and bind to postsynaptic receptors -> cause an EPSP or IPSP
- transmitter is inactivated (by enzymatic degradation) or removed (by transposter for reuptake and recycling) -> action is brief
- transmitter may activate presynaptic autoreceptors, decreasing release
summation of action potentials from pre-synaptic neurons
- spatial summation: summing of potentials that come from different parts of the cell
+ if overall sum of EPSPs and IPSPs can depolarize cell at axon hillock -> action potential occurs - temporal summation: summing of potentials that arrive at the axon hillock of postsynaptic neuron at different times
+ the closer together in time they arrive, the greater the summation
electrical synapses
gap junctions
allow axon potentials to jump directly to the postsynaptic region without first being transformed into a chemical signal with no time delay
- for behaviors that require fastest possible responses like defensive reflexes
