Module 2, Week 1 (week 4) Flashcards
What is the nervous system?
main controlling/communicating system in the body
How do cells communicate messages?
electrical and chemical signals
Which is the fastest - electrical or chemical signals?
Electrical
How fast do electrical signals travel?
120m/second or 432km/hour
Examples of types of chemical communication are:
hormones, neurotransmitters
The brain and spinal cord make up the central nervous system. What is the main function of the central nervous system?
integrating centers that control and regulate - i.e homeostasis, movement, body functions
Some cells serve the nervous system. What are they called?
Glial cells
Cells of the nervous system are called?
neurons
Neurons are designed to carry signals over long or short distances?
Long
How long can nerve fibres be?
Over 1m
neurons communicate with:
other neurons and effectors - target cells (muscles cells and glands)
In brain and spinal cord which chemical signals are used?
Neurotransmitters
What is a neuron?
The structural unit of the nervous system
What’s the main role of neurons?
To generate electrical signals that can travel rapidly from cell to cell
Neurons have the capacity for?
Excitability
What is a stimulus in relation to the nervous system?
Change in the environment that is strong enough to lead to an Action Potential
What is Action Potential?
A nerve impulse generated by the cycle of depolarisation, repolarisation, after-hyperpolerisation - changes in charge across the cell membrane - moves along the length of the axon.
Are neurons the same size as each other?
No! Some are short (brain), some are up to 1m or more in length, some go from brain to toe.
Name the three parts of the neuron:
cell body, dendrites, axon
Are responses of the nervous system quick or slow?
Quick
Are responses general or targeted/specific?
Targeted/specific
What is the name of the type of cells that assist the nervous system?
Glial cells
What is the name of specific glial cells that assist neurons?
Schwann cells
How do neurons communicate with other neurons/cells?
Neurotransmitters
What is the primary function of neurons?
To create and move rapid signals from one neuron to another.
What is a stimulus in a neuron?
A change (in charge) in the environment of the cell membrane (instigated by the movement of ions across the membrane) that is strong enough to instigate an action potential.
What is an action potential?
An electrical signal (nerve impulse) that moves along the length of an axon. A wave of charge, that firstly depolarises, then repolarises, and finally hyperpolarises briefly before coming back to a resting state. Then repeat down the axon.
What initiates an action potential/nerve impulse?
The movement of ions (sodium/potassium) across the cell membrane via protein transport channels.
Does the strength of a nerve impulse change between action potentials?
No. The strength of action potentials is always the same.
What is the range of speed a nerve impulse can travel?
0.5-130m per second or 1-290 mile/hour
What is another name for the cel body of a neuron?
Perikayron or soma
List the organelles of the neuron’s cell body:
Nucleus & Cytoplasm
Cytoplasm:
lysosomes
mitrochondria
golgi complex
free ribosomes
clusters of rough endoplasmic reticulum (nissl bodies)
cytoskeleton (neurofibrils: intermediate filament-cell shape; microtubles: move materials around cell body/axon)
Proteins synthesised in Nissl Bodies (clusters of rough ER) are used where?
Inside the cell - for growth/repair
What is Lipofuscin?
Pigment (yellow/brown granules) in the cytoplasm that increases as the neuron ages. Gives a yellowy-brown tinge to the neuron.
What is a nerve fibre?
Any extension that protrudes from the cell body - dendrites and axon.
What are dendrites?
Dendrites receive information (signals) from other neurons.
Dendrites have receptors in the plasma membrane that can bind with neurotransmitters coming from other neurons.
Are short. Look like little trees due to branches.
Cytoplasm has Nissl bodies, mitochondria, other organelles.
What is an axon?
Nerve fibre in which the nerve impulse moves along. Runs from cell body to axon terminals.
List characteristics of an axon:
Long, thin
Wider, tapered section near cell body - Axon Hillock (initial segment = trigger zone)
Organelles include: mitochondria, microtubules, neurofibrils
Cytoplasm in axon called axoplasm
Cell membrane of axon called axolemma
Has side branches called axon collaterals
Axon and axon collaterals end in axon terminals - tapered, fine, thin endings known as axontelodendria
Where does a nerve impulse/action potential begin?
Where the Axon Hillock and initial segment of the axon meet.
The point where the axon hillock and initial segment meet is referred to as the…
Trigger zone
What is a synapse?
The place where two neurons communicate
What are synaptic end bulbs?
Swollen ends of the axon terminals. They can have a row of swollen bumps - varicosities. They store neurotransmitters.
What is a neurotransmitter?
A chemical substance/molecules that either excites or inhibits the postsynaptic neuron/cell (muscle or gland)
How many neurotransmitters will a typical neuron produce?
Two or three
Do neurotransmitters elicit the same response on the postsynaptic neuron/cell?
No, each neurotransmitter affects receiving neurons/cells differently.
How many transport systems does a neuron have?
Two - 1. slow axonal transport 2. fast axonal transport
Describe the slower axonal transport system:
Travel speed 1-5mm/day
Transports axoplasm to axon terminals
Moves in one direction only - cell body to axon terminals
Describe the faster axonal transport system:
Travel speed 200-400mm/day Transport is motorised by proteins Uses microtubles to move along axon Transports chemical substances Two directional - from cell body to axon terminals and back. Forward movement = anterograde - for moving organelle & synaptic vesicles. Backward movement = retrograde - for moving membrane vesicles/cellular bodies for degradation, for recycling or hormones/chemicals/toxins/viruses entering neuron (tetanus toxin, rabies virus, herpes simplex virus etc)
How much does the nervous system weigh?
2kg / 3% body weight
How many neurons in nervous system?
Billions
What are the two main divisions of nervous system?
Central Nervous System & Peripheral Nervous System
The Central Nervous System consists of:
Brain & spinal cord
How many neurons in brain?
85 billion
How do brain and spinal cord connect?
Through foramen magnum (hole in base of skull) & vertebral bones
How many neurons in the spinal cord?
100 billion
Describe the main roles of Central Nervous System:
Receives/analyses incoming sensory information
Decides action necessary in response to sensory information
Manages thoughts, memories, emotions
Gives directives for muscle movement/gland secretion - response to sensory input
Describe the main roles of the Peripheral Nervous System:
Relays information from sensory receptors in body to CNS and back to organs/muscles/glands with response directive.
Describe the structural components of the Peripheral Nervous System:
Nerves, ganglia, enteric plexuses & sensory receptors outside of CNS
What is a nerve?
cord-like bundle of axons - up to hundreds/thousands - plus connective tissue/blood vessels servicing/supporting nerves
How many nerves come out from CNS?
12 pairs of nerves come out from brain
31 pairs of nerves come out from spinal cord
What are ganglia?
Small amount of nervous tissue gathered together - mostly cell bodies. Very similar to CNS nerves, but are outside CNS.
What are enteric plexuses
Networks of nerves in the walls of the gastrointestinal tract - assist with the regulation of digestive system
What are sensory receptors?
Structures involved in monitoring the internal and external environment and changes that occur within the body. They communicate changes to the CNS. Examples of sensory receptors: touch receptors in skin, photo receptors in eyes, olfactory receptors in nose
List the 3 divisions of the Peripheral Nervous System:
Somatic nervous system
Autonomic nervous system
Enteric nervous system
Describe the Somatic Nervous System:
Sensory neurons of the head, body wall, limbs, and those relating to vision, hearing, taste & smell that communicate changes with the CNS.
Motor neurons that deliver instructions from CNS to skeletal muscles - voluntary (as skeletal muscles can be consciously controlled).
Describe the Autonomic Nervous System:
Sensory neurons in internal organs (thoracic/abdonimopelvic areas) that communicate info to CNS.
Motor neurons delivering instructions to smooth muscle, cardiac muscle & glands - involuntary.
Motor neuron part of autonomic nervous system - two branches:
a) sympathetic division (respond to exercise/emergency - flight/fight)
b) parasympathetic (respond to rest/digest)
Do effectors respond the same to sympathetic/parasympathetic nervous systems?
No. Opposite actions.
Describe the Enteric Nervous System:
100 million neurons + in enteric plexuses (network of neurons)
Mostly neurons function without interacting with CNS or autonomic nervous system.
Enteric NS does respond to and communicate with sympathetic & parasympathetic NS.
Sensory neurons observe chemical changes/stretching of GI tract.
Motor neurons govern muscle movement in GI tract (peristalsis), secretions from GI tract (i.e. stomach acid), secretions of hormones from GI tract endocrine cells.
List the hierarchy of nervous systems:
Central Nervous System
Peripheral Nervous System:
Somatic NS
Autonomic NS - Sympathetic/Parasympathetic NSs
Enteric NS
Name five main functions of the nervous system:
talk use senses remember control/regulation of body movement control/regulation of organ function
Name three main functional modes of nervous system:
- Sensory (input)
- Integrative (process)
- Motor (output)
Describe the sensory function of the nervous system:
Sense changes in body through sensory receptors - send input to CNS via neurons/nerves.
Describe the Integrative function of the nervous system:
Process information (input) from sensory receptors. Analyse. Prepare responses.
Describe the motor function of the nervous system:
Effectors (muscle/glands) receive information (output) from the CNS (control centre) through nerves for specific action - muscle contraction/gland secretion)
How do the different divisions of the body’s nervous system work together:
Somatic Nervous System
Communicates messages from sensory neurons/receptors to CNS
CNS communicates to somatic motor neurons
Action of skeletal muscles (voluntary)
Autonmic Nervous System
Communicates messages from sensory neurons/receptors to CNS
CNS communicates to Autonomic motor neurons including sympathetic/parasympathetic NS
Action of smooth muscle, cardiac muscle, glands (involuntary)
Sympathetic/Parasympathetic communicate/deliver messages to Enteric motor neurons
Enteric Nervous System
Enteric sensory neurons/receptors communicate messages (input) to CNS
CNS delivers instructions for action to Enteric motor neurons in enteric plexuses
Enteric motor neurons cause smooth muscle, cardiac muscles, glands and endocrine cells of GI tract to respond (involuntary)
What is resting potential of a cell membrane?
When cell membrane’s negatively charged, -70mV
charge dependent on movement of sodium/potassium ions in/out of cell
Name two factors determining the resting potential of a cell membrane:
Sodium/potassium ion levels in/out cell
Permeability of cell membrane to Na+/K+. Generally plasma membrane more permeable to K+
Describe concentration gradients:
Concentration of ions either in/out of cell. High concentration - lots of ions, low concentration - fewer ions. Ions travel down their concentration gradient - from high to low. Gradients are created by the concentrations of ions in either the intracellular and extracellular fluid.
Does Na+ have higher concentration inside the cell or outside the cell?
Outside - in the extracellular fluid
Does K+ have higher concentration inside the cell or outside the cell?
Inside - in the intracellular fluid
What happens if plasma membranes become more permeable?
More ions move down their concentration gradient, changing the concentration of ions (sodium/potassium specifically) in the intracellular and extracellular fluid.
Does ion migration across the cell membrane affect the cell’s ability to generate an action potential?
Yes, its what causes the instigation of the action potential. K+ moves out of the cell, depolarising the cell. If depolarised enough, bringing the cell membrane to threshold potential, an action potential will begin.
What is sodium’s concentration outside the cell?
145mMol/L
What is sodium’s concentration inside the cell?
15mMol/L
What is potassium’s concentration outside the cell?
15mMol/L
What is potassium’s concentration inside the cell?
150mMol/L
Are there more potassium or sodium leak channels?
Potassium - means more potassium moves out of the cell than sodium moves in
Is the plasma membrane generally more permeable to potassium or sodium?
Potassium
Are cations positively or negatively charged?
Positively
Are anions positively or negatively charged?
Negatively
Which mechanism maintains the cell membrane at resting potential?
The sodium/potassium pump - corrects the shift in the concentration of ions that occurs through leak channels. Actively transports ions to where their highest concentrations should be - in the extracellular fluid for sodium and intracellular fluid for potassium.
In addition to the concentration gradient, is there another gradient type?
Electrical gradient - ions have charge - Na+ and K+ ions have a positive charge. The movement of ions changes the electrical gradient in/out of the cell.
When ions move from positive to negative and visca versa, what do they form?
An electrical charge that changes the charge of the plasma membrane.
What are four types of protein channels?
- Leak channels
- Ligand-gated channels
- Mechanically gated channels
- Voltage-gated channels
List 5 facts about leak channels:
- Opening/closing of gates occurs randomly
- Cell membranes have a higher number of K+ leak channels than Na+ leak channels
- K+ leak channels leak more than Na+
- Cell membranes are more K+ permeable
- Most cells have leak channels
List 4 facts about ligand-gated channels:
- A ligand (chemical) is required to trigger the opening of the protein channel
- Neurotransmitters/hormones are types of ligands, also particular ions
- Ligand contact with protein transporter/receptor opens/closes gate
- Ligand-gated channels found in dendrites of sensory organs/dendrites & cell bodies of interneurons & motor neurons
List 3 facts about mechanically-gated channels:
- Mechanical trigger causes gate to open (vibration, pressure of touch)
- Force makes protein change shape/open
- Found in receptors of ears, internal organs (where stretching occurs), skin (touch)
List 2 facts about voltage-gated channels:
- Changes in electrical voltage/charge in membrane potential triggers opening of gates
- Integral in instigation of action potentials
What is energy potential measured in?
millivolts - 1mV = 0.001V
The more substantial the difference in charge inside/outside the cell the more significant the…
charge/voltage
What’s the name of a piece of equipment that measures charge?
Voltmeter - with microelectrodes
A cell at membrane potential, approx -70mV is referred to as…
polarised
List 3 factors explaining the resting membrane potential:
- Imbalance of ions ICF & ECF: ICF increase Na+ & Cl-(anion), ECF increase K+ phosphate (anion) & molecules
- Anions cannot leavy cell
3.
Polarised means…
charge is more negative inside the cell than outside the cell
When stimulus is strong, what may eventuate with protein channel?
It may stay open for an extended period - more ions will move down their concentration gradient.
What do action potentials enable?
Carrying a signal over large distances
What is the measurement of threshold potential?
-55mV
Where does an action potential start?
at the axon hillock
Why are graded potentials so important?
The instigate depolarisation. If in quick enough succession, or strong enough, they can summate (unite) to generate an action potential.
Once threshold potential is reached, then what happens?
Cell membrane depolarises till approximately +30, at which point a downward turn in charge occurs (repolarising), reducing to below -70mV (hyperpolarising momentarily), setting back at -70mV (resting potential). This process repeats along axon till axon terminals.
Where are voltage-gated channels found?
Axons of neurons
What triggers voltage-gated channels?
Changes in charge from -70mV resting membrane potential
Which protein channels are found in dendrites and cell body?
Ligand-gated channels and mechanically gated channels
What is a graded potential?
A small change in charge from resting membrane potential = slightly depolarised (more negative in cell/positive outside cell)
List two types of graded potentials:
- hyperpolarising (more negative - more polarised)
2. depolarising (less polarised - less negative)
Describe a depolarising graded potential:
- Gate triggered (ligand/mechanical) - opens - ions move across membrane
- small change in charge from resting membrane potential
- K+ and Ca+ move into cell - depolarising it a touch (making it more positive)
Where do graded potentials occur in neurons?
in dendrites and cell body
Do graded potentials differ in intensity?
Yes, some are stronger than others - hence, being graded
What alters the intensity/amplitude of graded potentials?
a) the intensity/strength of the stimulus
b) how many gated-channels are open and the length of time they are open for
Does a graded potential last a long time?
No, they dissipate quickly
Why do graded potentials fade quickly?
Ions leak through leak channels of postsynaptic neuron/cell
How fast can an axon potential move
150m/second
Why do action potentials only occur in the axon?
This is where the voltage-gated channels are - they are essential to action potentials
Which particular ions are voltage-gated channels specific with?
Sodium/potassium
How does the sodium voltage-gated channel differ from the potassium voltage-gated?
It opens when activated, but has an additional mechanism. When its inactivated, it has a ‘lock’ phase, where a ball-type plug blocks the entrance inside the cell. Potassium voltage-gated channels only have one activation mechanism that opens and closes the gate/channel. Potassium voltage-gated channels open and close more slowly than sodium V-G channels.
When membrane is in resting state, how do ions move across the cell membrane?
Diffusion through leak channels & sodium/potassium pumps - active transport
When threshold potential is reached -55mV, what happens next?
Sodium voltage-gated channels open - they are quick responders. Many sodium voltage-gated channels open simultaneously, allowing high concentrations of sodium ions to flood into cell - depolarises cell instantly. Depolarising continues to +30mV. +30mV marks a turning point, and gates start to close and inactivation gate (ball-like plug) blocks channel pore.
What happens once a sodium voltage-gated inactivation gate closes?
This action triggers potassium voltage-gated channel to open. It opens slowly, letting potassium ions out of the cell, polarising the cell. This is known as the repolarising phase.
List the three positions of Sodium (Na+) voltage-gated channels:
- resting = closed
- threshold potential = opening - sodium rushes into cell
- closed/locked = inactivation gate plugs channel
List two positions of Potassium (k+) voltage-gated channels:
- resting = closed
2. open (takes longer to open that Na+ VG channels & also longer to close)
What triggers an action potential?
The graded potentials, summating, with more excitatory than inhibitory effects, depolarising to threshold.
What is action potential propagation?
The continuing of the action potential over and over along the length of the axon. The effect of one action potential depolarises the adjacent section of the plasma membrane, which triggers action potential in the next section of membrane. Repeats until reaches axon terminals.
How long does an action potential last?
Approx 5 seconds per action potential cycle.
List two factors that affect action potential propagation speed:
- myelination - myelinated axons can propagate action potential more rapidly
- diametre of the axon - thicker/wider = more responsive to propagation of action potential
Why does the myelin sheath increase action potential propagation speed?
Myelin sheath conducts the electrical impulse… it essentially jumps the myelinated sections of the axon.
Does the amplitude of the action potentials change?
No - always the same once instigated in strength/amplitude.
When the action potential arrives at the axon terminus, the change in membrane potential triggers which response?
The opening of Ca+ voltage-gated channels - which allow Ca+ to flood into the axon terminal area in the presynaptic cell, moving down its concentration gradient.
Where are Ca+ voltage-gated channels located in neurons?
ONLY in axon terminals.
The role of Ca+ in the axon terminals is?
To guide neurotransmitter carrying vesicles towards the presynaptic membrane, ready for exocytosis.
Which mechanism stops synaptic transmission?
Removal of Ca+ through Ca+ pumps (active transport - channels fueled by ATP) - back into extracellular fluid.
What is an ionotropic acetylcholine receptor?
Receptor and ion channel. Acetylcholine is neurotransmitter that binds with receptor. A cation (positive ion) channel - allows positively charged ions into the postsynaptic cell. If ionotropic receptor is activated/gate opened, more cation substances can flow into cell - primarily sodium (Na+). More positive ions coming into the cell than k+ leaving the cell, has depolarision effect on postsynaptic cell. This has an excitatory response on postsynaptic cell - EPSP.
What does EPSP stand for?
Excitatory Postsynaptic Potential
What is an ionotropic GABA receptor?
Receptor and ion channel. A anion (negative ion) channel - Cl- (chloride). Neurotransmitter GABA - binds with receptor, channel opens, Cl- floods into cell - making cell membrane more negative. Further polarises cell membrane, taking it below resting potential = inhibitory response = takes it further away from threshold. Inibitory response = IPSP
What does IPSP stand for?
Inhibitory postsynaptic potential
What is a metabotrophic receptor?
Receptor that binds with neurotransmitters. Do not act as ion channel. G protein attached to receptor. When neurotransmitter binds to receptor, i.e. acetylcholine, the Gprotein triggers associated ion channel adjacent to metabotrophic receptor or activates another molecule that triggers ion channel to open. Metabotrophic acyetylcholine’s adjacent channel is k+ channel. Gprotein triggers k+ channel to open. K+ travels down concentration gradient out of cell. Cell membrane becomes more negative = inhibitory response. If associated ion channel was sodium, positive ions would flood into cell = excitatory response = depolarising cell membrane.
Do EPSPs & IPSPs have a big or small impact on postsynaptic cell?
Small - need to combine to have a strong enough impact at axon hillock - stimulate an action potential.
Which factors impact the response of the postsynaptic cell?
- type of neurotransmitters released
- the receptors of the postsynaptic cell
- summation of EPSPs & IPSPs - net result
What is a synapse?
Communication between two neurons or one neuron and another cel (often muscle/gland cells)
What does axodendritic mean?
Synapse between axon and dentrites.
What does axosomatic mean?
Synapse between axon and cell body.
What does axoaxonic mean?
Synapse between an axon and another anxon.
What is an electrical synapse?
When the nerve impulse is carried to another cell via tubes and tunnels straight to the cytosol of the other cell. The action potential continues through tubes connecting cells. Called Gap Junctions (100 connexons).
What are the benefits of Gap Junctions/electrical synapses?
Faster
Coordinated action - can connect multiple neurons at ones
Example - heart beat - uses electrical synapses
Which parts of the body employ electrical synapses?
Visceral smooth muscle
Cardiac muscle
Embryo
Brain
What fills synaptic cleft?
Interstitial fluid
Are electrical synapses one-directional or two-directional?
Two-directional
are chemical synapses one-directional or two-directional?
One-directional
Explain why are response is considered excitatory:
Excitatory means the response causes depolarisation, or more positive charge inside the cell, moving towards threshold potential - can trigger an action potential.
Sodium channels open. What happens?
Sodium concentration higher out of cell.
Sodium goes with concentration gradient - flows into cell.
Sodium is positive ion. Makes cell membrane charge more positive = depolarising effect = excitatory = EPSP.
Chloride channels open. What happens?
Chloride contentration higher out of cell.
Chloride enters cell when gates open, going with concentration gradient.
Chloride is negative ion. Further polarises cell, making it more negative - inhibitory response, taking it further away from threshold potential - IPSP.
Potassium channels open. What happens?
Potassium concentration is higher in cell.
Potassium leaves cell, making cell membrane charge more negative - further polarising cell membrane - inhibitory as takes further away from threshold potential - IPSP.
List ways that neurotransmitter is taken out of the synaptic cleft?
- Diffusion in extracellular fluid
- Enzyme breakdown for some neurotransmitters
- Uptake - specialised protein channels (neurotransmitter transporters) actively transport back into presynaptic cell
- Other cells uptake neurotransmitter using active transport (neurolgia)
Graded potentials vary in strength. Does the position on the postsynaptic cell make a difference to the summative effect on the neuron?
Yes. As the graded potentials’ charge dimishes quickly, the closer the point of reception is to the axon hillock, the more chance of reaching threshold potential. Trigger zone is at axon hillock.
Name the two types of summation of postsynaptic potentials:
- Temporal - multiple action potentials in a row = EPSPs in postsynaptic neuron. Effects of EPSPs still active when next arrives - builds momentum/depolarising effect on membrane - travels to axon hillock - together could be enough to reach threshold.
- Spacial - multiple EPSPs happening at the similar time from different presynaptic neurons.
Important to note that these EPSPs, in order to reach threshold, need to occur
Both temporal and spacial occur
What is synaptic transmission?
When neurotransmitters are released into the synaptic cleft to facilitate communication/signalling with another neuron/cell.
Why are there no action potentials in cell body or dendrites?
No voltage-gated channels
What are two methods of signaling neurons?
- In axon - action potential
- In cell body and dendrites: graded potential
Different roles
Why do graded potentials fade easily and quickly?
Cell body and dendrite cell membranes ar leaky - lots of leak channels - lose charge with ions moving with concentration gradient. Graded potentials cannot travel over long distances as a result. Their purpose is only to make it to the axon hillock. Generally, that’s not a long way.
What’s one of the main reasons for graded potentials?
To trigger action potentials.
Where do graded potentials occur?
Mainly in cell body/dendrites of postsynaptic cell.
Which channels are involved in graded potentials?
Ligand-gated and mechanically gated channels in postsynaptic cell.
Whats a word to describe how graded potentials fade/fail to travel over distances?
Decremental - reduce in amplitude
Are all graded potentials the same in amplitude/strength?
No, dependent on strength of stimulus. Range from 1mV-50mV
How strong is the amplitude of an action potential?
approx 100mV
How long does an action potential last?
approx 0.5-2 milliseconds
How long does a graded potential last?
approx milliseconds to minutes (rare)
Do graded potentials have a refractory period?
No. Means multiple graded potentials can occur within brief periods.
Complete. Action potentials are all or
nothing
Summation is:
the culmination of multiple graded potentials that arrive in the postsynaptic cell fairly closely together - the combined effort (net EPSP & IPSPs) - can trigger an action potential.
What is a receptor potential?
Type of graded potential that occurs in sensory nerve endings.
What are the two refractory periods?
- Absolute refractory period
2. Relative refractory period
Does one refractory period follow another?
Yes, relative follows absolute
What happens at -55mV - threshold?
Sodium voltage-gated channels open - influx of Na+ = beginning of depolarisation part of the cycle.
The absolute refractory period is:
The period in which an action potential is occurring. Only one action potential can occur at a specific part of the cell membrane at a time.
Why can an action potential only occur a section of the cell membrane at a time?
Sodium voltage-gated channels can only do one job at a time. For the depolarising stage they need to be open. For the repolarising stage, they need to be closed/locked. If they open too early, the repolarisation part of the process can’t be completed and the action potential fails. Can not be open and closed at same time.
Relative refractory period - when can an action potential fire again?
When the sodium voltage-gated channel unlocks.
Sodium voltage-gated channels close but also have a locking mechanism. This inactivation gate needs to be closed for repolarisation to begin. As repolarisation is nearing the end of its process, the locking mechanism on the Na+ voltage-gated channels unlocks. At this point, if there was a large stimulus, action potential could start again. Na+ channels can get ready quickly.
Why does it take a strong stimulus to instigate a second action potential at the same part of the membrane?
If one action potential isn’t quite finished, it will be repolarising, with the K+ gates being slow to close. This means the voltage of the membrane could be below resting potential. A stimulus would need to be strong to bring the charge to threshold as it would need to depolarise more than usual, and also to trigger enough Na+ voltage-gated channels.
Not being able to accommodate back to back action potentials is significant because….
it means the action potential can only move in one direction
Approximately how many Na+ flood into the cell when voltage-gated channels open?
2000
What happens to the Na+ once action potential has ended?
Active transportation - sodium/potassium pumps transport ions back to where their concentration is to maintain the concentration gradient.
What is the speed of nerve impulses in small neurons?
250 impulses per second - refractory period 4 msec
What is the speed of longer/wider neurons?
1000 impulses per second - refractory period 0.4 msec
What are glial cells?
Specialised cells that assist the nervous system. Involved in myelination of axons
Where are Schwann cells found?
On axons in peripheral nervous system
In axons, what is the name of the gaps in between myelin?
Nodes of Ranvier
What occurs at the Nodes of Ranvier?
Action Potentials
What’s special about Nods of Ranvier?
They have lots of voltage-gated channels
Factors of Continuous Conduction are:
- unmyelinated fibres
- neurons are usually short
- action potential/nerve impulse doesn’t travel far along the axon.
- Action Potentials happen close together. Takes longer for them to travel down the axon
- A lot of APs occur
- a lot of Na+ leaks out of axon
Factors of Saltatory Conduction are:
- occurs in myelinated neurons
- action potentials occur at gaps in between myelinated sections
- conduction of nerve impulse is faster
- action potentials travel faster along axon
- found in parts of the body where speed is necessary
- leaping-type action from node of ranvier to node of ranvier
- current flows in ions in cytosol and extracellular fluid - both sides of myelin sheath
- saves energy - more efficient
- leaks ions less = less ATP required by Na+/K+ pumps to transport ions back to high concentration
Name 3 x factors affection the rate of action intentions along an axon:
- myelination - myelin = greater speed
- diameter of axon - larger surface area = greater speed
- temperature - higher temperature = greater speed
With action potentials, their strength is the same every time. What indicates a stong stimulus is involved then?
The number of action potentials. If a stimulus is strong, the end point of the first and second action potentials may be at or over threshold, so the axon fires again.
Weak stimulus = one AP
Strong stimulus = 2 or more APs
