Nervous System 1 - Neurons, Impulse generations, & Transmission Flashcards
Lec 5, 6, 7 & 8
What are neurons?
neurons are excitable cells, meaning they are responsive to stimuli. When they’re stimulated an electrical impulse may be generated and propagated along the axon ie a nerve impulse.
What are electrochemical gradients?
when cells have a diff btwn the conc of ions and mols in the ICF and ECF, these diffs result in electrochemical grads that can be used for signalling by some cells.
What are electrochemical gradients (that give cells their electrical properties) due to?
1) ion conc diffs across membrane (gradients). 2) permeability of cell membrane to ions.
What are some NB ions?
Na+, Cl-, K+, Ca++, and large negatively charged organic molecules which are non-diffusable proteins.
What is the conc diff of Na and K across memb ?
[Na+] high in ECF and low in ICF. [K+] is low in ECF and high in ICF. this is due to the maintained activity of the Na/K ATPase pump in cell memb.
What is the conc diff of Ca++ across memb?
[Ca++] is higher in the ECF and lower in ICF due to various transporters in cell memb and ER membs.
What is the conc diff of Cl- across memb?
[Cl-] is higher in ECF and lower in ICF bc its repelled by org- in the cell.
What is the conc diff of org- across memb?
org- stay inside the cell, bc they cant pass the bilayer.
What is the permeability of the cell memb determined by?
ion channels - ions diffuse through them down conc grad
What are the types of ion channels?
non-gated channels (leak channels), and gated channels.
What is a non-gated ion channel?
channels that are always open. have more K channels then Na channels on the cell memb, therefore the memb is more permeable to K at rest. these channels are NB in establishing the RMP.
What is a gated channel and its types?
not involved in rest bc they open in response to a stimuli. 1. voltage gates - respond to changes in memb voltage. 2. chemical gates - respond to chemical like hormones and nt binding. 3. thermal gates - respond to temp changes 4. mechanical gate - respond to mechanical deformation.
What is a membrane potential?
the difference in electrical charges btwn the inside and outside of a cell.
What is a membrane potential created by?
the mvmt of ions (charged particles) across the memb.
What is resting membrane potential?
the charge difference/potential diff across the cell memb in a resting cell. so the diff in charge btwn inside and outside of cell. -70mV. inside the cell is more negative.
What are the factors establishing the RMP?
Na/K ATPase pump, org- inside cell, more non-gated K channels than non-gated Na channels,
How does the pump effect the establishment of the RMP?
uses atp, 3 Na out and 2 K in, ions pumped against conc grad. this maintains the conc grad of Na and K. also contributes a little to the rmp bc it pumps more pos ions out than in.
How does org- effect the establishment of the RMP?
large negatively charged organic ions cant cross the membrane which means theyre stuck inside the cell, making it more negative.
How does the permeability of the membrane establish the RMP?
MOST NB FACTOR. memb is more permeable to K at rest bc there are more non-gated K channels than Na channels, this means that a) more K ions are able to move down their conc grad to the outside of the cell, making the inside more negative. b) as inside of cell becomes more neg, opposite charges attract and less K diffuses out. c) Na diffusion into cell increases bc attracted to oppo charge. d) until -70mV is reached, the amount of K+ moving out of cell is greater then the Na+ moving into cell. (inside still becoming more neg) e) once -70mV is reached the amount of pos charges moving in and out is the same. (same amount of K and Na moving - bc of the electrical gradient holding back K and accelerating Na). f) now no net mvmt of ions is 0
What are electrically excitable cells?
only mm and nerve cells. capable of producing departures form RMP in response to a stim. (changes in internal or external enviro).
What happens when a neuron is stimulated?
a) gated ion channels open b) mp changes, producing a graded potential, and then if this meets the threshold… c) action potential triggered.
What is a graded potential?
when a stimulus causes a small change in the mp (so the diffs in charge across the memb) on the dendrite or cell body by opening gated channels which changes the memb permeability.
What are the possible results of GPs?
- more pos than rmp; depolarization ie -70mV to -65mV (closer to zero). 2. more neg than rmp; hyperpolarization ie -70mV to -75mV.
What are the characteristics of graded potentials?
a) ions move passively bc oppo charges attract, so they go to adjacent areas of the membrane that have the oppo charge, so current flow, which causes depol or hyperpol. b) gps are short distance signals that die away quickly c) magnitude and distance travelled varies directly with the strength of the stimulus (larger stim, larger distance traveled) d) gps can summate so gps can add together to create a larger gp.
What happens after a graded potential?
Ap, or repolarization where return to rmp.
When does a GP cause an AP?
if it causes depol, and if it is large enough. steps; 1. critical stimulus 2. gp reaching threshold -55mV 3. ap
What is an AP?
a nerve impulse/signal. a large change in membrane potential (diff in charge across memb) that propagates along an axon with no change in intensity. initiates at trigger zone ie axon hillock of multipolar and bipolar neurons, and past the dendrites of unipolar neurons.
What are the events of an action potential?
a) rmp and start depol b) depol c) repol d) hyperpol e) rmp
What is step A of the ap?
GP; memb pot at axon hillock reaches -55mV bc of the depol from the gp.
What is step B of the ap?
depol; voltage-gated Na channels respond to change in voltage/mp and open. now increase in Na permeability and more Na diffuses into cell making it more positive, leading to more channels opening and more na coming in (pos fb). depol reaches =30mV.
What is step C of the ap?
repol; voltage-gated K channels open (slow response) and cell starts to become more neg bc K diffuses out. Na channels close and become inactivated and Na mvmt returns to resting lvls.
What is step D of the ap?
hyperpol; charge drops below -70, K channels slow to close. Na channels reactivated so can respond to stim again.
What is step E of the ap?
back to rmp; all voltage gated channels are closed, now its just reg diffusion and pump working that get mp back to normal quickly.
What are the refractory periods called and their order?
Absolute refractory period and Relative refractory period.
What is the absolute refractory period and its purpose?
spans from depol to the end of repol. purpose is to prevent AP summation, so no AP can be generated regardless of stimulus size. this happens bc all the Na channels are open during depol (so there’s no more to open), and bc all Na channels become inactivated during repol (they cant open until the MP reaches the RMP and they become activated again). Summary; ARP during depol no AP bc all Na channels are already open, during repol no AP bc Na channels are inactivated and cant be activated/opened until reach RMP.
What is the relative refractory period?
during hyperpol. an AP can be generated by a greater than norm stim, bc MP is more neg so need a more intense stim to get to threshold. this can happen bc Na channels are closed but become reactivated when voltage passes RMP into hyperpol (so theyre still closed but can be opened), K channels are still open making cell more neg.
What is the all-or-nothing principle of APs?
All; if thresh is reached AP is produced - same every time. None; below thresh - no AP.
How does propagation of an AP work?
goes across entire length of axon so it can communicate signal. in depol of an AP the pos ions move toward the more neg area of the adjacent memb, which then depolarizes (more pos) that part of the memb (bc Na voltage gated channels have opened with the change in voltage from pos ion flow) to reach thresh and an AP occurs.
Which way does AP propagation happen and why?
happens in one direction bc the preceding memb is in the refractory period and no AP can be produced.
What does the rate of propagation depend on?
fiber (axon) diameter; the larger the diameter, the faster the propagation bc theres less resistance in ion flow. myelination; a. unmyelinated b. myelinated.
When does continuous conduction happen and what is its speed?
when the fibres are unmyelinated, so APs have to happen all along the fibre. so propagation is therefore slower.
When does saltatory conduction happen and what is its speed?
when the fibres are myelinates, so APs occur at the nodes of ranvier where the ion channels are present. so its leaping over parts of the memb making it faster.
What is fibre type A?
large diameter, myelinated, propagates APs at ~130 m/s, most sensory neurons and motor neurons in skel mms. fastest type.
What is fibre type C?
small diameter, unmyelinated, propagate APs at ~0.5 m/s, in autonomic NS and some pain fibres. slowest type.
Compare the location of GPs and APs?
GP; in cell body or dendrites. AP; at axon hillock.
Compare the strength of the MP btwn GPs and APs?
GP; variable. AP; all or nothing (has to pass thresh).
Do both GPs and APs summate?
GP; yes (stim can add to current GP) AP: no (refractory period inhibits this)
Compare repol in GPs and APs?
GP; current dies away and settles back to RMP AP; K channels open and make cell more neg, Na channels become inactive and close making the cell more neg
How are the types of gates diff in GPs and APS?
GP; chemical gates, mechanical gates, thermal gates AP; voltage gates
Compare the distance traveled in GPs vs APs?
GP; shorter distance (1-2mm) AP; propagates along length of axon so long distances
Is there a refractory period in both GPs and APs?
GP; no AP; yes
What are synapses?
chains of neurons connected by junctions (so where neurons connect and communicate with eachother)
What is synaptic transmission at neuronal junction and its steps?
presynaptic neuron to postsynaptic neuron transmission (so signal communication). Steps: 1. AP arrives at axon terminal (synaptic end bulb). 2. Ca voltage gated channels open (bc of AP changing the voltage) and Ca enters end bulb (Ca higher outside cell so going down conc grad). 3. rise in Ca triggers exocytosis of nt containing vesicles (so they bring nt outside of cell) 4. nt diffuses across synaptic cleft and binds to spec receptors on the postsynaptic memb. the receptors are chemically gated ion channels that open in response to nt binding. 5. gated ion channels open and ions move in or out of postsynaptic memb, which creates a GP -> postsynaptic potential (PSP).
What are the two types of postsynaptic potentials?
excitatory PSPs which is depol. (can lead to AP) happens due to Na or Ca channels opening or K channels closing (making inside of cell more pos). nt usu acetylcholine or glutamate. inhibitory PSPs which is hyperpol. (inhibits AP) happens due to K or Cl channels opening (making cell more neg). nt usu glycine or GABA.
Where do PSPs occur?
on the cell body or dendrite (bc they’re GPs).
Can PSPs summate?
yes, many neurons can synapse onto one, so if theres many EPSPs, leads to summation and a large area of the memb is depolarized, spreading to axon hillock and if the EPSPs reach the thresh theres an AP. However, some are IPSPs so the sum of all IPSPs and EPSPs determines of the AP will occur ar the axon hillock. so there might be inhibitory PSPs that make the memb more neg whic hmoves in oppo direction of thresh.
What is the neuromuscular junction?
it is the jxn btwn the axon terminal of a neuron and an individual mm fibre.
What are the steps to the synaptic transmission at the NMJ?
similar to that of neuronal jxn with some modifications;
a. nt released is always ACh so always excitatory. b. Na chemical gates on mm motor end plate (sarcolemma of mm fibre) open and causes a GP/end plate potential on sarcolemma. c. EPP triggers AP on sarcolemma, lots of ACh is released therefore always get an AP from an EPP (bc lost of channels are opened and EPPs summate across memb).
