5.1.3 Neuronal Communication Flashcards
what is the nervous impulse made out of
- made up of a complex network of cells called neurones
- sends information as nerve impulses
what are nerve impulse
- electrical signals
- also called action potentials
- how the nervous system sends information
what are the three types of neurones
sensory
motor
relay
what are sensory neurones
transmit nerve impulses from receptors to the central nervous system CNS
what is the CNS made out of
the brain and spinal chord
what are motor neurones
transmit nerve impulses from the CNS to effectors
what are relay neurones
transmit nerve impulses between sensory neurones and motor neurones
what happens after a stimulus is detected by receptor cells
a nerve impulse is sent along a sensory neurone
what happens when a nerve impulse reaches the end of a neurone
- chemicals called neurotransmitters take information along to the next neurone, which then sends a nerve impulse
what does the CNS do
processes the information, decides what to do, and sends the information along a motor neurone to an effector
what is the process of seeing a stimulus to a response being formed
STIMULUS: seeing someone waving
RECEPTORS: light receptors in your eye detect the wave
→ sensory neurone
CNS: processes the information and decided what to do about it
→ motor neurone
EFFECTORS: muscle cells are stimulated
RESPONSE: muscles contract to make your arm wave
what is a transducer
something that converts one form of energy into another
how do sensory receptors act as transducers
- different stimuli have different forms of energy (e.g. light and chemical)
- your nervous system is only able to send information in the form of nerve impulses (electrical energy)
- so sensory receptors convert the energy of a stimulus into electrical energy
what happens in a nervous system receptor that is not being stimulated
- the receptor is in its resting state
- there’s a difference in charge between the inside and outside of the cell
- this is generated via ion pumps and ion channels
- this results in a voltage (potential difference) across the membrane
what is resting potential
the potential difference when a cell is at rest
what happens to a sensory receptor when a stimulus is detected
- the cell membrane is excited and becomes more permeable
- this allows more ions to move in and out of the cell
- this alters the potential difference (generator potential)
what is the generator potential
the change in potential difference due to a stimulus
what does a bigger stimulus cause in sensory receptors
- bigger stimulus
- excited the membrane more
- causes a bigger movement of ions
- and a bigger change in potential difference
- so a bigger generator potential is produced
what happens if a generator potential is big enough
- it will trigger an action potential (nerve impulse) along a neurone
when will an action potential only be triggered
if the generator potential reaches a certain level called the threshold level
- (if the stimulus is too weak, the generator potential won’t reach the threshold and there will be no action potential)
what are Pacinian corpuscles
mechanoreceptors, so detect mechanical stimuli
- e.g. pressure and vibrations
- found in your skin
- contain the end of a sensory neurone, called a sensory nerve ending
- the sensory nerve ending is wrapped around lots of layers of connective tissue called lamellae
what happens when a Pacinian corpuscle
- e.g. by a tap on the arm
- the lamella are deformed, and press on the sensory nerve ending
- this causes deformation of stretch-mediated sodium channels in the sensory neurones cell membrane
- this means that the sodium ion channels open and sodium ions diffuse into the cell
- this creates generator potential
- if it reaches threshold
- triggers an action potential
what do all neurones contain
- a cell body with a nucleus (plus cytoplasm and all other organelles normally present in cells)
- the cell body has extensions that connect to other neurones, called dendrites and dendrons
- and axons
what do dendrites and dendrons do
carry nerve impulses towards the cell body
what do axons do
carry nerve impulses away from the cell body
what does a sensory neurone look like
- receptor cells
- attached to SHORT DENDRITES
- attached to ONE LONG DENDRON, carrying the nerve impulse all the way to the cell body (receptor cell → cell body)
- after cell body, ONE SHORT AXON is attached to the axon terminal [where the axon branches off into many separate lines as the end of it] at the end carrying nerve impulse (cell body → CNS)
what does a motor neurone look like
- MANY SHORT DENDRITES with the cell body in the middle of them, carrying nerve impulses to the cell body (CNS → cell body)
- ONE LONG AXON attached to the cell body middle bit, ending with the axon terminal attached to effector cells, carrying the nerve impulse (cell body → effector cells)
- UNMYELINATED
what does a relay neurone look like
- MANY SHORT DENDRITES branching off of the cell body in the middle, transmitting nerve impulses (sensory neurone → cell body)
- attached to the cell body middle bit, you have ONE AXON that carries the nerve impulses away (cell body → motor neurone)
- responsible for transmitting action potentials through the CNS
what is always the direction of impulse in a neurone
dendrite → cell body → axon
when is a neurone said to be in a resting state
when its not being stimulated
what is the charge like in a resting neurone
the outside of the membrane is more positively charged than the inside
- more positive ions outside the cell than the inside
- SAID TO BE POLARISED = a difference in charge
what is resting potential
the voltage across a membrane when it is at rest
- about -70mV
what maintains the resting potential in a neurone
the sodium-potassium pumps and potassium ion channels in a neurones membrane
what is an electrochemical gradient
a concentration gradient of ions
how is a resting potential maintained in a neuron
- the sodium potassium pump uses active transport [needs ATP] to move 3 Na+ ions out of the neurone for every 2 K+ ions that move in
- creates a concentration gradient of more Na+ ions outside neurone than inside, so should move back in via diffusion
- HOWEVER, most “gated” sodium ion channels are closed, so membrane is not permeable to sodium ions, so they cannot diffuse back in, creating a sodium ion electrochemical gradient, with more positive Na+ ions outside the cell than inside the cell
- there is also a concentration gradient of K+ ions, as more inside cell than outside
- HOWEVER, potassium ion channels are open, so the facilitated diffusion of K+ ion channels out of the neurone is possible, down the concentration gradient
- means that the outside of the cell is positively charged, compared to the inside
what is the action potential
- when a stimulus is detected by neurones
- causes sodium ion channels to open
- if stimulus is big enough, triggers rapid change in potential difference = action potential
what are all the different words for an action potential
nerve impulse
electrical impulse
what are the steps that takes place in an action potential
stimulus
depolarisation
repolarisation
hyperpolarisation
resting potential
what happens during the stimulus stage of an action potential
- a stimulus excited the neuronal cell membrane
- causes the sodium ion channels to open
- the membrane is able to become more permeable to sodium
- so sodium ions can diffuse into the neurone (down the sodium ion electrochemical gradient
- this makes the inside of the neurone less negative
what happens during depolarisation of an action potential
- if the potential difference reaches threshold (-55mV)
- the voltage-gated sodium ion channels open [only open at certain voltage]
- more sodium ions diffuse into the neurone
- example of POSITIVE FEEDBACK
what happens during repolarisation of an action potential
- when potential difference reaches around +30mV
- the sodium ion channels close
- and the voltage gated potassium ion channels open
- this means the membrane is more permeable to potassium ions, and they diffuse out of the neurone down the potassium ion concentration gradient
- this means the membrane will begin to get back to its resting potential
- = NEGATIVE FEEDBACK
what happens during hyperpolarisation of action potentials
- potassium ions are slow to close
- this means there is a slight “overshoot”
- means too many potassium ions diffuse out of the neurone
- so the potential difference becomes more negative than resting potential
- less than -70mV
what happens during the final stage of an action potential
- the ion channels are reset
- the sodium-potassium pump returns the membrane to its resting potential
- is maintained until excited by another stimulus
why can’t action potentials occur straight after another
- after an action potential, the neurone cell membrane cannot be excited straight away again
- this is because the ion channels are recovering and cannot be made to open
- sodium ion channels are closed during repolarisation and potassium ion channels are closed during hyperpolarisation
- period of recovery called REFRACTORY PERIOD
what would a diagram showing changes in potential difference during an action potential look like
- AT THE START: a straight line at -70mV
- STIMULUS: as soon as stimulus hits, Na+ channels open, so increases slightly until about -55mV
- DEPOLARISATION: from -55mV when the voltage-gated sodium ion channels open, a sharper line going up to +30mV
- REPOLARISATION: from +30mV as soon as the voltage-gated Na+ channels close and K+ channels open, line going all the way down
- HYPERPOLARISATION: the line continues to go down below the resting potential line, until eventually the K+ channels actually fully close
- RESTING POTENTIAL: reached again when the line goes back up to -70mV and stays here
where would the refractory period take place on a diagram showing changes in action potential
from the peak where repolarisation starts all the way to until resting potential is reached after the hyperpolarisation
how do action potentials move along a neurone
as a wave of depolarisation
how does the wave of depolarisation occur in a neurone
- when an action potential happens, some of the sodium ions that enter the neurone diffuse sideways
- this causes the sodium ion channels in the next region of the neurone to open and sodium ions to diffuse into that part
- this causes the wave of depolarisation to travel along the neurone
where does the wave of depolarisation move away from in a neurone
- the wave moves away from the parts of the neurone in the refractory period
- as these parts cannot fire an action potential
how would you show the wave of depolarisation in a diagram
- at first, the stimulus hits the neurone at one end
- causes an action potential to take place here, and Na+ ions enter the membrane here
- rest of the neurone is still in resting potential
- then, the Na+ ions diffuse away from this area as it enters the refractory period, traveling in the wave of depolarisation
- the same happens again, and the Na+ ions diffuse further away, and the refractory period takes place at the previous spot
what happens no matter the size of a stimulus
the action potential will always fire with the same change in voltage (once the threshold is reached)
- a bigger stimulus WILL NOT CAUSE a bigger action potential
what explains the all-or-nothing nature of an action potential
- if threshold is reached, the action potential is always fired with same voltage
- if threshold isn’t reached, the action potential will not fire at all
what does a bigger stimulus cause to happen
a bigger stimulus WILL cause action potentials so fire more frequently
- means that if the brain receives a high frequency of action potentials, it will be able to interpret this as a big stimulus, and respond accordingly
what do myelinated neurones have
a myelin sheath
- looks like two curved lines coming from the axon, with a nucleus inside for a Schwann cell
what does the myelin sheath act as
an electrical insulator
in the peripheral nervous system, what is the myelin sheath made up of
a schwann cell
what are present between each schwann cell
- nodes of Ranvier
- tiny patches of bare membrane
- sodium ion channels are concentrated at these nodes
how does depolarisation happen in a myelinated neurone
- depolarisation only happens at the nodes of Ranvier, where sodium ions can get through the membrane
- the neurones cytoplasm conducts enough electrical charge to depolarise the next node
- so the impulse jumps from node to node
- VERY FAST
what is depolarisation in myelinated neurones called
saltatory conduction
how does an impulse travel along a non-myelinated neurone as opposed to myelinated
- the impulse travels along the whole length of the axon membrane
- slower than saltatory conduction
- though still pretty fast
what is a synapse
a junction between a neurone and another neurone
- or between a neurone and an effector cell, e.g. muscle or gland cell
what are all the different parts of a synapse labelled
- the presynaptic and postsynaptic neurones
- the presynaptic and postsynaptic membranes
- the synaptic knob
- vesicles filled with neurotransmitter
- receptors
- synaptic cleft
- specific enzyme in present on the postsynaptic membrane
what is a synaptic cleft
the tiny gap between cells at a synpase
what is the structure of the presynaptic neurone
- this is the one before the synapse
- has a swelling called the synaptic knob
- this contains synaptic vesicles that are filled which chemicals called neurotransmitters
what happens when an action potential reached the end of a neurone
- it causes neurotransmitters to be released into the synaptic cleft
- they diffuse across to the postsynaptic membrane (the one after the synapse)
- and bind to a specific receptor
what happens when neurotransmitters bind to receptors on the postsynaptic membrane
- they can trigger an action potential if another neurone
- cause muscle contraction in a muscle cell
- cause a hormone to be secreted in a fland cell
what stops the response of a neurotransmitter from continuing
- neurotransmitters are removed from the cleft so that the response doesn’t keep happening
- can be taken back to the presynaptic neurone
- or can be broken down by enzymes, and these products are taken back to the neurone
what are two examples of neurotransmitters
acetylcholine (ACh)
noradrenaline
what are cholinergic synapses
synapses that use acetylcholine
what are the specific structures in a cholinergic synapse called
- acetylcholine binds to CHOLINERGIC receptors
- they’re broken down by an enzyme called ACETYLCHOLINESTERASE (AChE)
what happens when an action potential arrives at the presynaptic neurone [step 1]
1) the action potential arrives at the synaptic knob of the presynaptic neurone
2) this action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open
3) calcium ions diffuse into the synaptic knob (pumped away after via active transport)
what happens when there is a calcium influx in the presynaptic neurone [step 2]
1) the influx of calcium ions in the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane
2) here, they fuse with the membrane
3) and release the neurotransmitter into the synaptic cleft via exocytosis (ACTIVE PROCESS)
what happens once the neurotransmitter has been released from the presynaptic membrane
1) the neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
2) this causes sodium ion channels to open in the postsynaptic neurone
3) this influx of sodium ions causes depolarisation (only if an excitatory synapse)
4) an action potential will be generated on the postsynaptic membrane if the threshold is reached
5) the neurotransmitter is removed from the synaptic cleft so the response doesn’t keep on happening (e.g. acetylcholine is broken down by AChE and diffuses back to the presynaptic membrane)
what are the two types of synapses
excitatory or inhibitory
what is an excitatory synapse
where the neurotransmitters depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached
what is an inhibitory synapse
when the neurotransmitters bind to the receptors on the postsynaptic membrane, they hyperpolarise the membrane
- this makes the potential difference more negative
- PREVENTING an action potential from being fired
what is synaptic divergence
when one neurone connects to many neurones, so that information can be dispersed to many parts of the body
what is synaptic convergence
when many neurones connect to one neurone, so that information can be amplified (made stronger)
what happens with neurotransmitters when a stimulus is only weak
- only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft
- this may not be enough to excite the postsynaptic membrane to the threshold level
- and not enough to stimulate an action potential
→ summation
what is summation
where the effect of neurotransmitters can be combined
what are the two types of summation
spatial and temporal
what types of neurones does spatial summation take place in
convergent
what happens in spatial summation
- when neurones converge, the small amount of neurotransmitter from each neurone can be enough to reach the threshold of the postsynaptic neurone and trigger an action potential
what happens if some of the neurones in a spatial summation release an inhibitory neurotransmitter
then the total effect of all the neurotransmitters might be no action potential
what is spatial summation especially good for
- when stimuli arrive from different sources
- spatial summation allows signals from multiple stimuli to be coordinated into a single response
what is temporal summation
when two or more nerve impulses arrive in a quick succession from the same presynaptic neurone
- this makes an action potential more likely because more neurotransmitter is released from the synaptic cleft
what does summation allow
- means synapses accurately process information
- and can finely tune the response
- BOTH TYPES
how do synapses make sure that impulses are transmitted only one way
- receptors for neurotransmitters are ONLY on the postsynaptic membranes
- so synapses make sure that impulses can only travel in one direction
