PS1090 - Understanding Neuroscience Flashcards
What happens when neurons connect to other neurons?
- Can stimulate other neurons (excitatory synapses/ neurons)
- Can inhibit other neurons (inhibitory synapses/neurons)
- Can receive input from many neurons
- Can send output to many neurons
What are dendrites?
Dendrites are protrusions where neurons collect information from other neurons, it integrates that information and makes a ‘decision’ about whether to pass information through the axon, it conducts signals through the axon.
What happens at axon terminals?
At the axon terminals, the neuron connects to other neurons, it can stimulate other neurons and either excite them and make them likely to fire a signal or inhibit them from sending a signal. Each neuron can receive information from many other neurons and also send output to other neurons.
What are glia or glial cells?
Glia or glial cells are non-neuronal cells in the central nervous system – brain & spinal cord
How can the shape of neurons indicate function?
if they have a very widespread dendritic tree, it could mean that they receive input from lots of neurons. If they have a widespread axon terminal ending, it could mean it connects with many other neurons.
What do neurons do?
They receive, process and transmit information.
What’s a simple reflex arc?
A receptor (neuron that detects the stretch of a muscle) connects to a motor neuron (neuron that controls muscles in the spinal cord) and that neuron then causes the muscle to contract.
How do neurons connect to make networks?
In the brain, there are complex patterns of convergence and divergence. There are neurons in one area passing on information to other areas. They spread out their information to multiple neurons (divergence) Each neuron will receive input from multiple neurons (convergence).
What does the function of a neural circuit depend on?
- How the neurons are connected
- How strong are the connections (synapses)? They can be very strong and pass signals easily or be weak and only pass a signal if there’s a lot of stimulation.
- Whether the connections (synapses) are excitatory (EPSP) or inhibitory (IPSP)
How can the function of a neural circuit change?
The function of a neural circuit can change through changes in synaptic strength – for example, memory!
What are the functions/characteristics of glial cells?
Supportive function
Protective function
Metabolic function - help clean up metabolites
Unlike neurons they divide throughout life
What are glial cells?
Glia, also called glial cells or neuroglia, are non-neuronal cells in the central nervous system and the peripheral nervous system that do not produce electrical impulses.
What are the most abundant type of cell in the central nervous system?
Glial cells are the most abundant cell types in the central nervous system.
What are the 4 types of glia?
• Astrocytes: star shaped (most common)
• Oligodendrocytes: few extensions, have a very specific function (only found in the CNS)
• Schwann cells (PNS – peripheral NS: found outside the brain)
Very similar to Oligodendrocytes but they’re outside the central nervous system.
• Microglia: smaller than the rest
How do astrocytes provide a scaffold by connecting neurons to blood vessels?
- They attach to blood and neurons
- Provide physical support
- Help control blood flow
How do astrocytes help to control blood flow?
How is this the basis of neuroimaging techniques?
They react to active neurones by controlling blood flow. They cause the capillaries (that they are attached to) to dilate when neurons around them are active. Bringing oxygen and nutrients to the neurones.
When neurons are active, blood flow increases and that’s caused by the astrocytes – this is the basis of neuroimaging techniques.
Apart from being a physical support and controlling blood flow, what are the other functions of astrocytes?
- Separate synapses (astrocytes maintain the gap between synapses which is important for neuronal communication.)
- Clean up debris that is generated by neuronal signalling (when one neuron communicates with another, it does so by excreting transmitter substances – small molecules that have to be cleaned up after each firing session)
Briefly bullet point the 5 functions of astrocytes.
- Attach to blood and neurons
- Provide physical support
- Help control blood flow
- Separate synapses
- Clean up debris that is generated by neuronal signalling
Where are oligodendrocytes found?
In the central nervous system
What are oligodendrocytes called in the peripheral nervous system (PNS)?
Schwann cells
Briefly bullet point the function of oligodendrocytes.
- Support axons
* Insulate axons and speed up information transmission
How do oligodendrocytes speed up information transmission.
They have a round shape and extend these sheath-like growths around the axon and surround each axon with a layer of fatty substance known as myelin which insulates them.
They provide electrical insulation which underlies the ability of neurons to send signals from one place to another really fast.
Explain what happens to the immune system of Multiple Sclerosis patients.
• It’s a demyelinating disease
• Induces numerous scars (multiple scleroses) in the brain
• Immune system attacks the myelin produced by oligodendrocytes
• Inflammation of CNS nerves
• Affects insulating layer of axons
• As the insulation becomes degraded, neurons can’t communicate as fast with one another. This causes problems for M.S. patients. • • First, they get problems with movement and motor control and then starts affecting cognition.
This is a very debilitating disease but fortunately the treatment for it is getting better.
What are microglia?
• Aren’t really glia • Precursors of blood cells • Part of the brain’s immune system • Related to macrophages Eat up debris and hostile bits
Why doesn’t the normal immune system work in the brain?
There’s a blood brain barrier, so there’s a very tight control of what comes into the brain. The normal immune system doesn’t work in the brain as the cells and large molecules that mediate immune functions can’t penetrate the blood brain barrier. The microglia work inside the brain and do a similar role.
What is included in the CNS?
- Brain
* Spinal cord
What is included in the PNS?
All nerves and neurons that reside outside, or extend beyond the CNS
What is a nerve?
A nerve is an enclosed bundle of axons.
What are the structural subdivisions of the PNS?
- Cranial nerves - nerves that emanates from the brain directly
- Spinal nerves – nerves that emanate from the spinal cord
What are cranial nerves?
The cranial nerves are a group of 12 nerves (bundles of axons) controlling muscles in the neck and head. Ten of the twelve cranial nerves originate in the brainstem while the remaining two originate in the cerebral cortex (the olfactory and optic nerves I and II respectively). They are unique because they have very specific functions.
What are the functional subdivisions of the PNS?
- Somatic nervous system
- Spinal nerves:
- Sympathetic nervous system
- Parasympathetic nervous system
What does each spinal nerve have?
Ventral (toward front) root
• Contains efferent fibres = projecting away from the CNS and towards the stomach.
Dorsal (toward back) root
• Contains (toward back) root = projecting towards the CNS
What is the somatic nervous system?
• Part of the PNS that controls voluntary body movements and conducts sensory information
• The elements of the PNS that aren’t part of the autonomic nervous system
This is what allows us to make any movement and to also receive physical sensations from the body. Pain, touch heat etc. Every nerve that controls movement is part of the somatic nervous system.
What does ‘soma’ and ‘somatic nervous system’?
Soma means body so somatic nervous system means the bodily nervous system.
What is the autonomic nervous system?
- Was thought to be independent
- Part of the PNS that controls homeostasis
- In charge of circulation, breathing, digestion, sexual function
- Usually not subject to voluntary control
- Made up of the sympathetic nervous system and parasympathetic nervous system
What does the autonomic nervous system regulate?
It regulates body functions, what we call homeostasis, maintaining the body in a functional state. You can’t control it with a lot of training. This is what controls our breathing, heartbeat, digestion, sexual function etc.
What is the autonomic nervous system made up of?
The autonomic nervous system is divided into two sections – sympathetic and parasympathetic.
What is the sympathetic nervous system?
The sympathetic division is anatomically distinct it’s driven by a chain of swellings (ganglia) that sit outside the spinal cord and it connects to lots of different organs in the body such as the heart, liver, kidneys, digestive system, genitals etc.
When it’s activated it stops you from salivating, dilates pupils, stops digestion, constricts blood vessels in the skin and it releases adrenaline or epinephrine. It puts the body in a mode of alertness.
What is the parasympathetic nervous system?
The parasympathetic system does the exact opposite. It constricts the pupil, makes the heart beat slower, increases digestion, dilates blood vessels. This could kick in after a big meal. This may explain why after a big meal you don’t feel like going for a run.
What is the brain and spinal cord protected by?
Cushioned by fluid, protected by bone
Brain: skull
Spinal cord: vertebra
What membranes cover the inside of the skull and spinal column?
Pia mater
Arachnoid mater
Dura mater
Describe each layer of membrane and their function.
The membranes have a protective and nutritious function. The pia mater is a soft matter, outside this there is the arachnoid mater, which is spider-like as it consists of lots of blood vessels which give a spiderweb like appearance. Then you have the dura mater which is a hard casing which serves a protective function. These cover the inside of the skull and the inside of the spinal column.
What is cerebrospinal fluid and its function.
It’s a watery cushion that allows the CNS to float.
Functions:
Protection - If you make any sudden movements, e.g. hitting your head, the hard impact isn’t immediately translated to your brain. The cerebrospinal fluid cushions and protects it.
Nutrition - It picks up excess neurotransmitters and metabolites from the brain and recirculates them.
What are ventricles? How many are there inside the CNS?
The ventricles of the brain are a communicating network of cavities filled with cerebrospinal fluid (CSF). There are 4 ventricles, the 2 lateral ventricles are the large ones.
What happens to the ventricles in schizophrenia patients?
In some individuals, these ventricles get larger for instance in patients with schizophrenia.
What is grey matter and what parts of the spine and brain are made of grey matter.
Grey matter contains the cell bodies of neurons
It includes??? look this up
• Cortex (rind, bark)
• Nuclei (Kernels – if embedded in white matter)
• Inner part of spinal cord
What is white matter and what parts of the spine and brain are made of grey matter.
White matter: axons of neurons It includes??? look this up • Fibre tracts • Corpus callosum • Outer part of the spinal cord
Why are axons white?
They are surrounded by a fatty insulating sheath: myelin
What does lateral mean?
To the side
What does medial mean?
To the middle
What does Ipsi- mean e.g. ipsilateral.
Ipsi- means the same.
E.g. ipsilateral means belonging to or occurring on the same side of the body.
What does Contra- mean e.g. contralateral.
Contra- means opposite.
E.g. contralateral means belonging to or occurring on the same side of the body.
What does ipsilateral mean?
On the same side
What does contralateral mean?
On the opposite side
What does bilateral mean?
On both sides
What does superior mean?
To the top
What does inferior mean?
To the bottom
What does anterior mean?
To the front
What does posterior mean?
To the back
What words do orientations change for?
The orientations change from spine to brain for these words: Dorsal Ventral Rostral Caudal
What does dorsal mean for the the spine and brain?
Towards the backbone for the spine.
Or towards the top of your head for the brain.
What does ventral mean for the the spine and brain?
Towards the stomach for the spine.
Or down towards your feet for the brain.
What does rostral mean for the the spine and brain?
Rostral is towards the top of your head for the brain.
Or towards the snout for the brain.
What does caudal mean for the the spine and brain?
Caudal is towards the back of your head for the brain.
Or down towards your toes for the spine.
What is a coronal slice?
Split in the middle vertically and facing the front.
What is a sagittal slice?
Split in the middle vertically and to the side.
Apart from coronal and sagittal slice, what other ways can the brain be sliced?
Horizontally
How can the central nervous system be divided?
- Spinal cord and the brain
- Brainstem (=hindbrain + mesencephalon – cerebellum)
- Diencephalon (=thalamus + hypothalamus)
- Forebrain (=isocortex + basal ganglia + limbic system)
+ cerebellum (as a separate part)
What are the three sections can the brain be divided into?
The hindbrain (back of the brain), the mesencephalon (midbrain) and the forebrain (front of brain).
What components of the hindbrain (back of the brain) are there?
The hindbrain can be subdivided into the different components. The myelencephalon is the lowest part of the brain, which is really an extension of the spinal cord (also known as the medulla). The metencephalon are two structures next to the medulla – the cerebrallum and the poms.
What components of the forebrain (front of the brain) are there?
The forebrain consists of two sets of structures: the diencephalon and telencephalon. The diencephalon consists of a structure called the thalamus, which is important for connecting the brain to the rest of the world and the hypothalamus which helps regulate hormonal control over the body. The telencephalon consists of three sections; the limbic system, basal ganglia and isocortex (biggest part of the brain)
What does the spinal cord do?
- Connects brain and body
* Houses local reflex pathways e.g. patellar reflex
What does the brainstem do?
- Controls vital body functions
- Breathing
- Heartbeat
- Artery dilation
- Salivation
- Vomiting
What does the midbrain contain?
The midbrain contains important sensory and motor centres
What does the cerebellum do?
• Interfaces with the pons • Receives sensory and motor information. • Massive fibre bundles connect it to the brainstem • Coordinates movement: Balance Motor planning Motor learning Eye movement control
Apart from being critically involved in most of our actions, why is there also increasing evidence that the cerebellum is involved in controlling conditions.
It seems to check on any movement and tells the brain whether that movement was carried out the way the brain wanted. E.g. If you’re going to walk across the room your brain sets up a plan for that, a programme for controlling your muscles and a copy of that command (called an efferent copy) is sent to the cerebellum. The cerebellum then compares the intended movement with the sensory information to see if we moved in the way that was intended. If we didn’t, it will adjust the pathways to correct the error. When a child is learning to walk, the cerebellum is incredibly active, learning all the time. Possibly the same thing could happen for cognitive processes.
What does the diencephalon mean and what does it consist of?
It means ‘between-brain’
It consists of the thalamus and hypothalamus?
What is the thalamus?
The thalamus are two egg shape structures above the midbrain – act as a gateway between the brain and outside world.
Complex cluster of nuclei (nuclei are groups of nerve cells that perform some function)
• Motor nuclei
• Sensory nuclei
Connected to almost any area of cortex
what are nuclei?
Nuclei are groups of nerve cells that perform some function.
What does the thalamus do?
It’s the most important relay station for outputs from inputs to the cortex.
Involved in regulating sleep and wakefulness.
The electrical activity driven by the thalamus is how we can use electrodes to measure the states of sleep, e.g. REM.
How is the brain involved in regulating sleep and wakefulness?
The thalamus can block signals if it wants to, this happens during sleep, which is why you don’t get disturbed by sights and sounds. Similarly, the thalamus blocks nerve impulses going from the brain to the muscle, so when you’re dreaming of running but can’t move your body, the thalamus is blocking your muscles.
What are the two little knobs on the thalamus?
These two little knobs are really important, there’s the lateral geniculate nucleus and the medial geniculate nucleus. The lateral geniculate nucleus is where all your visual information passes, without this you would be blind. The medial geniculate nucleus is where all your auditory information passes on its way to the brain, without this you would be deaf.
What is the hypothalamus and what does it mean?
The hypothalamus means ‘below the thalamus’
It’s a cluster of complex nuclei.
What does the hypothalamus do?
• Regulates homeostasis, metabolic processes, autonomic activities
Body temperature
Hunger
Thirst
Circadian cycles (sleep cycle)
Reproductive behaviour
• Links nervous and endocrine (hormone) systems via pituitary gland
What is the basal ganglia compromised of?
- Caudate nucleus
- Putamen
- Globus pallidus
- Substantia nigra (midbrain)
What does the basal ganglia do?
Basal ganglia are subcortical nuclei important for movement control.
What happens to the substantia nigra in certain diseases like Parkinson’s?
Substantia nigra is important for movement control, in certain diseases like Parkinson’s, cells in this part of the brain break down – they are called dopaminergic cells and they excrete dopamine. When you have this deficit, you have difficulty initiating movements and you can correct this by administering dopamine as a drug.
What is the limbic system involved in?
Involved in emotion (recognition & production), motivation and emotional memory. Hippocampus is essential for memory formation.
What makes up the limbic system?
Cingulate cortex Thalamus Mammillary bodies Hippocampus Olfactory bulb Fornix
What is the function of the hippocampus and what happens to patients with damage to their hippocampus?
The hippocampus is essential for forming new memories.
Patients with damage to their hippocampus typically form anterograde amnesia which means they have the inability to create new memories. They have perfect recollection of events in the past, but they can’t learn new things.
Emotional memories and feelings can still be created as this is mediated by the amygdala. You can associate things with feeling sensations.
In patients with Alzheimer’s, the hippocampus is the part of the brain that tends to degenerate first which is the reason why Alzheimer’s patients have the inability to form new memories.
You can form motor memories and learn new skills as this isn’t mediated by the hippocampus. E.g. you could learn how to play tennis and how to hit the ball but you’d never learn the rules.
What is the amyglada and what is its function?
A small nucleus, critical for emotional information or memory.
What is the function of the cingulate cortex?
It responds to emotional conflicts and motivation
What is the cerebrum?
The cerebrum is the largest part of the human brain.
It consists of the two cerebral hemispheres connected by a large white matter fibre bundle – the corpus callosum.
The outer layer of the cerebrum is grey matter – the cerebral cortex.
Underneath the cerebral cortex are massive nerve fibre bundles – cortical white matter.
What is the corpus callosum and its function?
The corpus callosum is a set of fibres that connect the left and right hemispheres.
They’re really important in transmitting information between the two hemispheres.
Each hemisphere is able to generate perceptions on its own. They are very similar, the left hemisphere is verbal, it can produce language and the right hemisphere can understand language.
Why may the corpus callosum be cut?
You can cut through the corpus callosum without any obvious harm. This was sometimes done for patients with intractable epilepsy to stop epileptic seizures from spreading from one hemisphere to the other.
What does the cortex mean?
It’s latin for bark, rind.
What is the structure of the cerebral cortex?
- The outer surface of the forebrain
- 2-4mm thick
- The grey matter surrounding the white matter
- The type of cortex in the cerebrum is called isocortex
What are the anatomical divisions of the cerebral cortex?
- Each hemisphere of the cerebral cortex forms a single deeply folded surface
- Folds to allow larger surface area
- Patterns of folds unique to each person
- Gyrus (pl. gyri) = ridge
- Sulcus (pl.sulci) = groove
- Fissure = deep sulcus
What are the functional divisions of the cerebral cortex?
• The surface of the cerebral cortex is organised into cortical areas.
• There may be 100-150 such areas in the human brain.
• Different areas perform different functions, e.g.
V1 (primary visual cortex – visual processing)
M1 (motor cortex – controls voluntary movements).
• Each area is defined by having a unique combination of 3 [or 4] specific criteria
- Physiology (function
- Architecture (anatomy)
- Connectivity (connections)
- [Topography (maps)]
How many lobes are there?
There are four lobes – relatively arbitrary divisions.
However, cortical areas within each lobe tend to perform similar (or related) functions.
Lobes are names after the overlying skull bone.
What are the four lobes called?
- Frontal lobe
- Parietal lobe
- Temporal lobe
- Occipital lobe
What is the occipital lobe?
- Exclusively concerned with visual processing
- Separated from parietal lobe by parieto-occipital sulcus
- Calcarine sulcus divides the primary visual cortex into the upper and lower parts
What is the parietal lobe?
Separated from the frontal lobe by central sulcus (fissure).
Important for: Somatosensory perception Intersensory integration Spatial vision Spatial attention
What happens if the parietal lobe is lesioned?
- Visual neglect
- Gerstmann’s syndrome (dysphagia, dyscalculia, finger agnosia, left-right confusion)
- Balint’s syndrome (optic ataxia, optic apraxia, simultaneous agnosia)
What is the temporal lobe?
Separated from frontal lobe by Sylvian fissure (lateral sulcus).
• Superior temporal
Primary auditory cortex A1
• Inferior temporal gyrus
High-level visual processing:
Object recognition
Face recognition
• Medial temporal lobe
Memory
• Amyglada
Emotion, fear
What is the frontal lobe?
Separated from the parietal lobe by central sulcus
Separated from temporal lobe by lateral sulcus
The brain’s largest lobe that makes us human and makes us grown up.
Functions:
Movement (primary motor cortex M1)
Impulse control, judgement, language production, memory, problem solving, sexual behaviour, social behaviour (Broca’s area)
Involved in planning, coordinating, controlling and executing behaviour
What are the different types of neurons?
- Multipolar: many dendrites, one axon
- Bipolar: one extended dendrite at one end, one axon at the other end (e.g. the retina has lots of bipolar cells)
- Unipolar: one branch leaves the cell body, axon spreads in two directions. The dendrite and axon hillock emanate from one part of the axon and the axon terminal emanates from the other side of the axon.
What are the positive ions that are crucial for neural signalling and what is their role?
- Sodium (Na+): generating action potentials (nerve impulses)
- Potassium (K+): maintaining resting potential
- Calcium (Ca2+): synaptic transmission (communication between one neuron and the next)
What are the negative ions that are crucial for neural signalling and what is their role?
- Chloride ions (Cl-): suppressing action potentials (opposite effect of sodium ion)
- Proteins (An-): maintaining resting potential
Where are the positive ions (sodium, potassium and calcium) found?
• Sodium (Na+): mainly outside neurons (extracellular)
Found in extracellular fluid, so they are largely outside cells e.g. if you cut yourself the blood tastes salty due to the plasma being full of sodium ions.
• Potassium (K+): mainly inside neurons (intracellular)
When you cut yourself and feel pain, part of that is driven by potassium ions being released out of the cells and spilling into the tissues which triggers pain receptors.
• Calcium (Ca2+): (almost) exclusively extracellular
In a much smaller concentration but almost entirely extracellular.
Where are negative ions (chloride and proteins) found?
• Chloride ions (Cl-): mostly extracellular
These ions are outside like the sodium, it’s like regular table salt being outside the cells.
• Proteins (An-): mostly intracellular
They are everywhere but most of them are inside the cells.
Explain the different levels of permeability in membranes.
- If a membrane allows all particles to diffuse through it is “fully permeable”
- If a membrane only allows some particles to diffuse through, but not others, it is semipermeable or selectively permeable
- Semipermeability can arise if a membrane contains pores that are too small for large particles (such as proteins) but large enough for small particles (such as K+ ions) to cross it.
What are ion channels?
Neuronal cell membranes contain ‘pores’ made up of large proteins that allow certain ions to pass through the membrane.
Ion channels are selective: they only allow one type of ion through
How permeable are neuronal cell membranes?
Because of ion channels, neuronal cell membranes are semi-permeable: they only allow some types of ions to diffuse through.
What is the membrane potential the result of and what can it be measured by?
1) differences in ionic concentrations between the inside and outside of the neuron and
2) ion channels in the neuronal cell membrane that only allow certain ions to pass in and out of the neuron
The membrane potential can be measured using tiny electrodes
What is meant by electrical potential?
- All charged particles (such as ions) are surrounded by an electrical field
- The strength of this electrical field is the electrical potential
- Usually one is more interested in the electric potential difference or voltage
- Voltage is measured in volts (V)
- For example, the voltage of a battery is the electric potential difference between the plus (+) and minus (-) poles
What is a resting membrane potential?
Resting potential is the point where you have a balance between the electrostatic force which is pulling potassium ions into the cell due to the cell being negative and the concentration gradient (diffusion) that is forcing potassium ions out of the cell. The equilibrium potential is the same as the resting potential: it’s when there are equal numbers of ions moving in and out and the forces are the same.
Why do neurons have a resting potential (and why is it negative)?
- The cell membrane is semi-permeable
- When neurons are at ‘rest’, only potassium ions (K+) can freely cross the cell membrane (because only K+ channels are open)
- Because the concentration of K+ ions is higher inside the cell than outside, some K+ ions leave the cell by diffusion
- Negative protein ions (An-) cannot cross the membrane and remain inside the cell – leaving more negative than positive ions inside
- This causes a negative charge to build up inside the neuron
- The resting potential is about -60 to -70 millivolts (mV)
What determines the resting potential?
Determined by 2 opposing forces:
1. Concentration gradient which drives the (K+) out of the cell & makes the inside more negative.
- Electrostatic force (electrostatic gradient)
makes opposite charges attract, K+ is drawn back into the negative inside the cell
- When these forces are balanced, equal numbers of K+ ions leave and enter the cell
- The membrane potential is then at the equilibrium potential
- This is the resting potential – what happens when you have the same number of (K+) ions going out of the cell as are coming back into the cell
But why is there more (K+) inside the cell in the first place?
- Because of the sodium-potassium pump (Na+/K+ pump)
- An ion channel that pumps Na+ out of the cell and K+ into the cell
- Because this is against the concentration gradients of Na+ and K+ it requires energy
How does the Na+/K+ pump work?
- The Na+/K+ pump uses energy from ATP to pump Na+ out of the cell and K+ into the cell
- Pumps 3 Na+ out for every 2 K+ in
- Most of the brain’s energy consumption is used to fuel this pump
What is ATP?
ATP is the molecule that drives all our metabolic processes. It binds to the potassium pump and in the process causes the pump to move sodium ions out of the cell.
What is polarisation?
Polarization means there is an electrical potential difference and a voltage across the cell membrane – if it didn’t have any voltage it wouldn’t be polarized.
What is hyperpolarization?
Hyperpolarization is when the membrane potential gets more negative.
What is depolarisation?
Depolarisation is when the membrane potential gets less negative.
What is an action potential?
• If depolarization exceeds a threshold, an action potential (AP) occurs:
o Sudden and brief (0.5-2ms)
o Momentarily reverses membrane potential
o Repolarizes quickly and overshoots
o Magnitude is fixed: all-or-nothing response
What is the sequence of changes in the ion channels in the membrane that drive the action potential?
- Start off with the potassium channels open and the sodium channels closed – that’s the resting potential.
- The sodium channels are voltage gated which means they open when the voltage becomes more positive.
- As the voltage increases the sodium channels start to open so the sodium can start to move into the cell.
- Then they start opening very quickly and letting in lots of sodium making the potential of the inside of the cell more positive.
- It stops at the point where the sodium channels become inactive and start to close again so sodium cannot get in.
- Finally, another potassium channel which is voltage gated opens up when positive so the potassium can diffuse back out of the cell because there is more potassium inside the cell. It couldn’t leave because the inside of the cell was negative, and the electrostatic force pulled it back. But when the cell membrane potential is positive there is nothing holding the potassium back, so they leave the cell very quickly taking the positive charge with them. Making the cell membrane negative and then we are back to where we started with all the channels closed except for the potassium channel which is always open.
What are APs initiated by?
APs are initiated by the voltage-gated Na+ channel:
How are APs initiated?
- APs are initiated by the voltage-gated Na+ channel
- The ion channels are closed at resting potential
- Begin to open when membrane is depolarized to ~ -40 - -55 mV
- This allows Na+ ions to diffuse from outside the cell (high concentration) to the inside (low concentration) pulling in lots of positive charge into the cell
- This further depolarizes the cell, causing more channels to open
- Causes rapid increase in membrane potential from -70 mV to +40mV
What happens at the peak of the AP?
The potential rises very quickly and at the peak they close, and another potassium channel opens, and the potassium leaves the cell as the electrostatic force keeping the potassium inside is weaker and the potential drops down. Then it comes back to the resting potential of -70.
How do voltage-gated channels work?
- They are open or closed depending on the membrane potential of the cell.
- Contain a voltage sensor “paddle” – a protein structure that changes shape depending on membrane potential
- The change in shape causes the channel to open or close
- Different types of voltage-gated channels open or close at different membrane potentials
What happens at the end of the AP during the return to resting potential?
• When membrane potential reaches ~ +40 mV, voltage-gated Na+ channels close and become “inactivated”
• No more Na+ ions can enter the cell
At the same time, voltage-gated K+ channels open (these are in addition to the regular potassium channels that always allow potassium to go in or out)
• K+ ions now diffuse from inside the cell (high concentration) to the outside (low concentration), reducing the positive charge inside
• This causes the membrane potential to become negative again
• The membrane potential briefly becomes hyperpolarized – more negative than the resting potential – before returning to normal.
What is the refractory period? Explain the toilet analogy.
- The period immediately after an AP during which another AP cannot be elicited
- The refractory period is due to the inactivation of the voltage-gated Na+ channel
Like flushing a toilet, when the toilet is flushed there is a period of time that it can’t be flushed again as the system is emptied and needs to be refilled. This is like the refractory period. In the same way action potentials have a refractory period where during this time it can’t happen again.
Explain the propagation of action potentials.
- If an AP depolarizes the neighbouring membrane region beyond threshold, it sets off an AP in the neighbouring region
- This AP in turn depolarizes the membrane in its neighbouring region, which depolarizes the next region – and so on (a chain reaction)
- The result is a wave of depolarization that spreads along the axon.
Why can’t a nerve impulse move in different directions?
The nerve impulse can’t move backwards due to the previous region being in the refractory period.
What is saltatory conduction?
The way impulses leap from node to node.
What is the role of saltatory conduction in AP propagation?
Saltatory means to leap, the action potential jumps from one part of the axon to the other. It does this through structures called nodes of Ranvier. In between the insulated sections (where the myeline provides electrical insulation) are open parts of the axon which are the nodes of Ranvier. Because the myelin electrically insulates the axon any changes in electrical potential doesn’t have to go through every segment of the membrane and it can jump directly from one node to the next, very quickly. This gives a massive increase in the speed of conduction.
What are ranvier nodes?
In between the insulated sections (where the myeline provides electrical insulation) are open parts of the axon which are the nodes of Ranvier.
Why do you feel both a fast and slow dull pain when stepping on a nail?
If you step on a nail you will feel an immediate pain and then a few seconds later a dull pain. This is because there are two types of nerve fibres that conduct pain information. A fast one that allows you to respond quickly and a slow one reminding you that you’re injured and to take a break. The fast one is mediated through myelinated fibres (axons that have this myelin sheet around them) and the slow pain response is mediated through these slow fibres that don’t have myelin on them and take several seconds to arrive. In the brain almost all fibres are myelinated.
How do local anaesthetics (such as lidocaine – used for tooth extraction and small surgical procedures )work?
- Most local anaesthetics block voltage gated Na+ channels
- Action potentials cannot be transmitted
- Signals from pain receptors cannot reach the brain – no sensation
How do neurons communicate?
- Action potentials (APs) travel down the axon until they reach the axon terminals
- At the axon terminals, signals are transmitted from one neuron to another through structures called synapses
