How Nerves Work Flashcards
What are the two main subdivisions of the nervous system?
Central Nervous System
Peripheral Nervous System
What makes up the central nervous system?
Brain
Spinal cord
What makes up the peripheral nervous system?
The peripheral nervous system includes the nerves and ganglia outside the brain and spinal cord.
What are the branches of the peripheral nervous system?
Autonomic Nervous System (involuntary)
Somatic Nervous System (voluntary)
What makes up the autonomic nervous system?
Sympathetic nervous system - fight or flight
Parasympathetic nervous system - relax
Enteric nervous system
What is the cerebrum?
The largest part of the brain, located superiorly and anteriorly in relation to the brainstem.
What is the cerebral cortex?
Brain’s outermost layer of nerve cell tissue.
Wrinkled appearance from its many folds and grooves.
AKA grey matter.
What is the cerebellum?
A part of the brain responsible for balance and coordination
What are the bumps on the brain called?
Gyri (gyrus in singular form)
What are the grooves on the brain called?
Sulci (sulcus in singular form)
What is the purpose of the bumps and grooves of the brain?
Increase surface area of the cerebrum and pack in a lot more neurons
What are the lobes of the cerebrum called?
Frontal lobe
Temporal lobe
Parietal lobe
Occipital lobe
What are the meninges of the nervous system?
Three layers of membranes known as meninges protect the brain and spinal cord.
What are the three layers of the meninges?
The delicate inner layer is the pia mater.
The middle layer is the arachnoid, a web-like structure filled with fluid that cushions the brain.
The tough outer layer is called the dura mater.
What is the diencephalon divided up into?
Epithalamus, thalamus, subthalamus, and hypothalamus.
What are the parts of the brain stem?
Midbrain
Pons
Medulla oblongata
What is the role of the frontal lobe?
Personality
Mood
Social conduct
Language (dominant hemisphere side only)
What is the role of the parietal lobe?
Processing the sense of touch, limb position and spatial awareness
What can damage to the parietal lobe result in?
Damage to the parietal lobe typically involves loss of abilities in the parts of their body opposite the lesion.
What is the temporal lobe responsible for?
Processing auditory stimuli
What is the role of the occipital lobe?
Visual processing
Where can the midbrain be found?
Top of the brain stem
What is the midbrain involved in?
Auditory and visual processing, eye movement
What is the pons involved in?
It handles unconscious processes and jobs, such as your sleep-wake cycle and breathing.
What is the role of the medulla oblongata?
Its location means it’s where your brain and spinal cord connect, making it a channel for nerve signals to and from your body.
What is the role of the thalamus?
Body’s information relay station.
All information from your body’s senses (except smell) must be processed through thalamus before being sent to brain’s cerebral cortex for interpretation.
What is the role of the hypothalamus?
Acts as body’s control coordinating center.
Its main function is to keep your body in homeostasis.
How many nerve pairs can be found in the spinal cord?
31
Cervical - 8
Thoracic - 12
Lumbar - 5
Sacral - 5
Coccygeal - 1
What part of the body do cervical nerves supply?
Arms, shoulders and neck
What part of the body do thoracic nerves supply?
Chest and abdomen
What part of the body do lumbar nerves supply?
Hips and legs
What part of the body do sacral nerves supply?
Genitalia and gastrointestinal tract
What is white matter made up from?
Axons of nerve fibres
What is grey matter made up of?
Neuronal cell bodies
What horns are in the grey matter?
Dorsal (posterior)
Ventral (anterior)
What can be found in the dorsal root ganglion?
Cell bodies of sensory nerves
Where can cell bodies of motor fibres be found?
In the ventral horn of the grey matter
Describe the sensory reflex arc
- Pain stimuli detected by nociceptors in the skin after stepping on a lego brick
- Single is activated and sent along the fibre through the dorsal root and then synapse in the dorsal horn
- Interneurons would relay the signal to the motor fibre
- Motor fibre sends single along the ventral root towards muscle and tells leg ot contract, moving foot off of lego brick
What are the five parts of a neuron?
Dendrites
Cell body (Soma)
Initial segment (axon hillock)
Axon
Axon (presynaptic) terminals
What is the role of the dendrites?
Receives information/communications from other cells
What is the role of the soma?
Contains the nucleus and maintain the cell and keep neuron functioning efficiently
What is the role of the axon hillock?
The axon hillock serves as a junction between the cell body and an axon.
Triggers action potential.
What is the role of the axon?
Transmit information to different neurons, muscles and glands.
Aka sends the action potential
What is the role of the axon terminal?
Release neurotransmitter
What are the types of neurons?
Afferent neurons
Interneurons
Efferent neurons
What is the role of afferent neurons?
It is a sensory neuron and its role is to detect stimuli like pain or fine touch.
It sends a signal to the CNS and these come into the spinal cord via the dorsal root ganglion and will meet some interneurons.
What is the role of efferent neurons?
It is a motor neuron and it is responsible for carrying signals from the brain to the peripheral nervous system in order to initiate an action.
What is the role of interneurons?
They connect and transferring signals between sensory and motor neurons.
What type of shape do afferent neurons have?
Bipolar
Pseudounipolar
What type of shape do interneurons have?
Multipolar
Anaxonic
What type of shape do efferent neurons have?
Multipolar
What are the four types of glia?
Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
What is the role of the astrocyte?
Maintain external environment for the neurons
Surround the blood vessels and form blood-brain barrier
What is the role of the oligodendrocytes?
Form myelin sheaths in the CNS
What forms myelin sheath in the PNS?
Schwann cells
What is the role of the microglia?
Macrophages of the CNS, fight infection
What is the role of ependymal cells?
Produce cerebrospinal fluid
What three potentials enable the neurons to send electrical signals?
Action P
Graded P
Resting membrane P
What is the role of the action potential?
Once the cell reaches a certain threshold, an action potential will fire, sending the electrical signal down the axon.
What is the role of graded potentials?
Graded potentials are responsible for the initial membrane depolarization to reach threshold.
They decide when an action potential should be fired.
What is the role of the resting membrane potential?
The electrical potential inside the cell relative to the adjacent extracellular space.
Keeps neurons ready to respond to action potentials.
Do most cells have a resting membrane potential?
Yes
The voltage inside of the cell is negative compared to the outside.
What is the typical value of the resting membrane potential?
It typically sits between -50 and -75mV.
How is a resting membrane potential created in a cell?
Through the addition of a leaky potassium (K+) channel
How does a leaky potassium (K+) channel create a restingmembrane potential?
Some potassium leaks down the concentration gradient
This builds up an electrogradient gradient and the cell becomes more negative in charge
Equilibirum is reached when the electrical gradient is equal and opposite to the concentration gradient
Resting membrane potential created
What happens in the sodium potassium pump?
It moves two potassium ions into the cell where potassium levels are high, and pumps three sodium ions out of the cell and into the extracellular fluid.
What determins the resting membrane potential size?
The size of initial concentration gradient.
Small concentration gradient = small resting membrane potential
Large concentration gradient = large resting membrane potential as more K+ is needed to leak out to reach eqillibirum
What is the equilibirum potential?
The membrane potential at which the electrical gradient is exactly equal and opposite to the concentration gradient - noticed by Nernst
What do the following mean in the Nernst equation?
R
T
Z
F
R = gas constant
T = absolute temp
Z = valence of the ion
F = faraday constant
Does the Na+/K+ pump generate the resting membrane potential?
Yes and no
Exchanges 3 Na+ for 2 K+
Electrogenic (makes the inside of the cell slightly negative) but only 5mV
Na+/K+ needed to set up the ion gradients
Without the leaky K+ channels, only a small membrane potential would be generated
What is the threshold a membrane potential has to reach for an action potential to be fired?
-55mV
What are examples of graded potentials?
Generator potentials - at sensory receptors
Postsynaptic potentials - at synapses
End plate potentials - at neuromuscular junctions
Pacemaker potentials - in pacemaker tissues
What does it mean for graded potentials to be “graded”?
The size of the stimulus has a direct impact on the size of the response
Small stimulus -> small number of mechanoreceptors open causing small depolarisation
Large stimulus -> large number of mechanoreceptors open causing large depolarisation
What does it mean when saying graded potentials are decremental?
As graded potentials travel along the cell membrane they become smaller because they decay.
This is caused by positive ions leaking out of the cell membrane.
Therefore, graded potentials are only useful in short distances and are called local potentials.
What does it mean when saying graded potentials can be depolarising or hyperpolarising?
Neurotransmitters open channels that depolarise or polarise the cell
Graded potentials at synapses can therefore excite or inhibit a cell
What is depolarisation?
A change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside.
What is repolarisation?
During normal resting state, membrane potential is a negative value but the negative membrane potential changes to positive following depolarization of an action potential.
Negative to positive value is temporary.
Membrane potential returns to the resting membrane potential (which is negative value).
The process of returning to a negative membrane potential is repolarization.
What does it mean for a graded potential to excite a cell?
Bring the cell towards threshold and making it more likely to fire an action potential.
What does it mean for a graded potential to inhibit a cell?
Bringing the cell away from threshold, making it less likely to fire on action potential and is therefore inhibiting the cell.
What does it mean when saying graded potentials can summate?
Temporal summation occurs when graded potentials within the postsynaptic cell occur so rapidly that they build on each other before the previous ones fade.
How do neurons generate graded potentials?
They manipulate different ion channels
What channels need to be opened to depolarise the cell?
Sodium channels
Calcium channels
What channels need to be opened to hyperpolarise the cell?
Chloride channels
Potassium channels
What channels need to be opened to depolarise the cell after hyperpolarising it?
Leaky potassium channels
What channels need to be opened to depolarise the cell after hyperpolarising it?
Leaky potassium channels
What are the ways a neuron can be hyperpolarised?
When the neurotransmitter binds receptor of a postsynaptic cell membrane, that channel opens and it will allow chloride ions to flow into the cell.
Chloride flows into the cell and it very quickly depolarises it and move the cell away from threshold.
This is a fast IPSP.
Define IPSP
An inhibitory postsynaptic potential is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.
What is the second way a neuron can be hyperpolarised?
A G protein is used and potassium channels open.
When channels open, potassium flows out of the cell down its concentration gradient hyperpolarising the cell.
Slow IPSP.
What is a EPSP?
A postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential.
What are the two ways you can depolarise a postsynaptic potential?
FAST EPSP
An excitatory neurotransmitter binds to an ionotropic receptor and it opens its integral ion channel.
Ions must have a charge of +1
Non-specific monovalent cation channel.
This will let some potassium flow out of the cell down its concentration gradient.
When this channel opens, lots of sodium is going to flow into the cell down its large concentration gradient and its electrical gradient.
That’s going to very quickly depolarise the cell and make it more likely to fire and action potential.
We call this a fast EPSP.
What are the two ways you can depolarise a postsynaptic potential?
SLOW EPSP
The second way that a cell can be depolarised is again through a metabotropic receptor.
When that excitatory neurotransmitter binds to this metabotropic receptor,
Its G protein will act as a doorman to find and close those leaky potassium channels.
With those being closed, potassium is not going to continually leak out the cell, but it still getting pumped into the cell by the Na+/K+ pump.
This is a slow EPSP
What properties of graded potentials that are relevant to the function of synaptic integration?
Graded
Decremental
Depolarisiing/hyperpolarising
Can summate
What is synaptic integration?
A complex process which describes how neurons integrate the receiving inputs from thousands of presynaptic neurons before the generation of a nerve impulse (action potential)
How can the threshold of -55mV be reached when excitatory inputs are not able to reach threshold alone?
Think of there being A and B inputs. A is smaller than B.
Temporal summation - B is fired twice in rapid succession and that’s enough to bring the cell up to threshold for firing an action potential.
Spatial summation - we first of all fire a, we’ve got this first fast EPSP then b is stimulated and it gets added on top and that brings the cell up to threshold.
Spatial summation is really a mix of both temporal and spatial summation.
What happens to synaptic integration when neurotransmitter is released?
An inhibitory neurotransmitter is released, it diffuses across the synaptic cleft and it binds to different ionotropic receptors.
These ones have an integral chloride channel. When that opens, chloride enters the cell and it causes a rapid hyperpolarisation.
That moves the cell away from threshold, makes it less likely to fire on action potential and therefore that generates a fast IPSP.
What happens to synaptic integration when IPSPs and EPSPs are added together?
Pushes the cell to threshold and fires an action potential or keeps cells away from threshold
What are the properties of action potentials?
Have a threshold
All or nothing response
Self propagating
Have a refractory period
Travel slowly
Only encoded stimulus intensity in firing frequency, not amplitude
What are action potentials mediated by?
Voltage gated channels as opposed to the ligand gated channels that generate postsynaptic potentials
What is the action potential generated by?
Voltage gated Na+ channels mediated the depolarising phase
Voltage gated K+ channels mediating the repolarising and hyperpolarising phase
What does myelination cause?
Increased membrane resistance (less current leaks out membrane)
Decreased membrane capacitance (less current wasted charging up the membrane)
What happens to the action potential in regards to myelination?
Action potentials spreads passively from node to node and still reaches threshold - this is known as saltatory conduct
What are clinical consequences of demyelination?
Multiple Sclerosis in the CNS
Guillain Barre Syndrome in the PNS
Both diseases attack myelin sheath
What are physiological consequences of demyelination?
Decreased membrane resistance (more current leaks out of the membrane)
Increased membrane capacitance (more current wasted)
Conduction
What are the nerve fibre types?
(general)
Small and large unmyelinated and myelinated axon
Different classifications - each conduct different velocities
What are nerve fibre types?
(classifications)
A Alpha (Aα)
A Beta (Aβ)
A Gamma (Aγ)
A Delta (A𝛿)
C
Discuss Aα nerve fibre
Largest myelinated
Velocity: 70-120 m/sec
Function: proprioception, motor neurons
Discuss Aβ nerve fibre
Large myelinated
Velocity: 30-70 m/sec
Function: touch, pressure
Discuss Aγ nerve fibre
Small myelinated
Velocity: 15-30 m/sec
Function: Motor neurons of muscle spindles
Discuss A𝛿 nerve fibre
Smallest myelinated
Velocity: 12-30 m/sec
Function: Touch, cold, “fast” pain
Discuss C nerve fibres
Unmyelinated
Velocity: 0.5-2 m/sec
Function: Warmth, “slow” pain
What is the relationship between conduction velocities and fibre types?
The more myelination on a nerve fibre, the faster the conduction velocity per second
What is the difference between action potentials and compound action potentials?
AP:
- Intracellular recording
- Microelectrode through membrane
- Relative to the outside the cell
CAP:
- Extracellular recording
- Electrodes outside the axons
- Relative to earth
- Each action potential very small but add up to large waves
What can speed up action potential conduction?
Large diameter axons and by myelination
How can large axons help speed up the action potential?
Electric currents flows more easily down a large axon, allowing the Na+ channels to be more spaced out along a membrane, conduction velocity is going to be faster because the opening of those channels is what takes the time.
What is the neuromuscular junciton?
The synapse between motor neuron and sensory muscle
What is the first step in triggering muscle contraction?
To evoke an action potential in the skeletal muscle membrane (sarcolemma)
What are the three parts that make up the neuromuscular junction?
Presynaptic terminal filled with vesicles containing acetylcholine (Ach)
Synaptic cleft
Posysynaptic endplate of the skeletal muscle fibre - there are folds in the end plate
What are the steps in depolarisation of the motor neuron to depolarisation of the skeletal muscle, an action potential being evoked leading to contraction of the muscle?
- Action potential in motor neuron
- Opens voltage-gated Ca2+ channels in presynaptic cleft
- Fusion of vesicles (Ca2+ dependent exocytosis)
- Ach diffuses across synaptic cleft
- ACh binds to ACh (nicotinic) receptors
- Opens ligand-gated Na+/K+ channels
- Evokes end plate potential (graded potential)
- (Always) depolarises membrane to threshold
- Opens voltage gated Na+ channels
- Evokes action potentials
- Muscle contracts
- Acetylcholine cleared up by acetylcholinesterase
What are the key characteristics of the neuromuscular junction?
There are two channels involved - ligand-gated Na+/K+ channels evoke the end plate potential
Very large graded potential always big enough to reach threshold
No synaptic integration
Post-junctional folds increase number of voltage-gated Na+ channels close to where it is evoked
What is the structure of the CNS synapse similar to?
The structure of the neuromuscular junction
Same series of events but the CNS synapse has more complexities
What is an important messenger in the CNS synapse?
Calcium is a really important intracellular messenger, and it affects the activity of a number of proteins involved in vesicle trafficking.
How many transmitters act at the neuromuscular junction and what is it called and what does it bind to?
One.
Acetylcholine.
Binds to cholinergic nicotinic and ionotropic receptors.
What are the classes of transmitters used in the CNS synapse?
Amines
Amino acids
Peptides
Purines
Gases
What are amines derived of and what are the most common ones used as neurotransmitters in the CNS synapse?
Derived from a single amino acid
Adrenaline
Noradrenaline
Dopamine
Serotonin (5HT)
Histamine
What class of neurotransmitters are the most commonly used as neurotransmitters in the CNS synapse and what are examples of them?
Amino acids
Glutamate
GABA
Glycine
What are examples of purines and gases used as neurotransmitters in the CNS synapse?
Purines
ATP
Adenosine
Gases
Nitric oxide
What is the one postsynaptic potential in the neuromuscular junction?
Very large end plate potential
Excitatory and always leads to a muscle cell reaching threshold to fire an action potential, leading to muscle contraction.
What are the range of postsynaptic potentials used in the CNS?
Fast EPSPs (ionotropic)
Slow EPSPs (metabotropic)
Fast IPSPs (ionotropic)
Slow IPSPs (metabotropic)
Generally small (1mV)
Enables complex synaptic integration
What is the mV of the end plate potential in the neuromuscular junction?
40mV
Describe the synapses of the CNS
Axo-dendritic = excitatory
Axo-somatic = inhibitory
Axo-axonic = typically inhibitory, so they would decrease how much neurotransmitter is released,
but they can also be excitatory and increase neurotransmitter release.
What is divergence in synaptic connectivity?
One motor neuron can synapse onto multiple muscle fibres and this can range from as few as three up to several thousand depending on the muscle.
What is convergence in synaptic connectivity?
Allows a neuron to receive input from many neurons in a network. Lateral inhibition.
Not in neuromuscular junction.
Describe feedback inhibition.
Found in the CNS synapse but not in the neuromuscular junction.
When an action potential is fired, collateral (branch) activates an inhibitory interneuron
Inhibitory neurotransmitter is released and the neuron hyperpolarises, preventing repeat firing
What are the three pathways in the CNS?
Monosynaptic reflex
Polysynaptic reflex
Inhibitory reflex pathways
What pathway do most of our reflexes follow?
Polysynaptic
Discuss the monosynaptic reflex
Made up of two neurons.
A sensory neuron which will detect its stimulus of interest out in the periphery, fire an action potential and then it synapses directly onto the motor neuron and it would then tell it to fire and ultimately lead to muscle contraction.
Discuss the polysynaptic reflex
Polysynaptic reflex channels are directed particularly toward flexor (withdrawal) responses through one or more interneurons to produce coordinated patterns of muscle activity to remove a portion of the body from a potentially damaging or offending stimulus.
Discuss the inhibitory reflex
If interneurons for inhibitory interneuron that releases an inhibitory neurotransmitter like GABA are taken out, that would then lead to this motor neuron being inhibited and not causing muscle contraction.
What is synaptic plasticity?
Changes in strength of synapses
Can be activity-dependent
How can you develop synaptic plasticity?
CNS injury like stroke or spinal cord injury
What are the different types of synaptic plasticity?
Long-term potentiation
Long-term depression
To summarise, why are CNS synapses more complex than the neuromuscular junction?
Range of neurotransmitters
Range of postsynaptic potentials
Arrangement of synapses
Arrangement of wiring
