Nervous system Flashcards
Neurons receive impulses via the …
Dendrites
How do neurons alter their branching ?
By strengthening or weakening synapses and changing connections between neurones
What is the cell body of a neutron called ?
Perikaryon
How are neutrons adapted for the precise control if action potential generation ?
No neuronal surface is exposed to the ECM
Neurons are fully covered by glial cells or synapses
This allows tight regulation of the the ionic composition of the thin layer of liquid surrounding neurons
What are glial cells ?
Neurons cannot survive without them
They provide nutrition and signals and help control ionic composition at cell membrane
What is the “you never walk alone” principle?
Neurons are completely covered by contacts of other cells from the outermost dendritic end to the synapses
Explain saltatory conduction
Small stretches of the axon (or dendrite in sensory neurones ) are electrically isolated by myelin sheaths
Depolarisation only occurs in non-isolated areas
What cells comprise the myelin sheath in the central nervous system ?
Oligodendrocytes
Is it possible to distinguish between axon or dendrites in electron micrograph?
No
What are the principal glial cells of the PNS?
Schwann cells
Features of myelin sheath
Schwann cells are very tightly wound around to form a tight layer
They prevent the free movement of ions in the glycocalix(the macromolecules attached to the cell membrane) of the extracellular space/intracellular space
Features of Nodes of Ranvier
Not fully uncovered
Covered by thin outgrowths of Schwann cells
What is another name for sensory neurons?
Pseudo-unipolar neurone
What encases the CNS?
Minimally permeable , multi-layered tough sheet of connective tissues
This is called the meninges
Cerebrospinal fluid is found inside the meninges
What covers the PNS nerounes?
How does their environment compare to the typical interstitial fluid between cells
Schwann cells and satellite cells
surrounded by specialised connective tissue cells
Their environment is less controlled than in the CNS more similar to interstitial fluid
Explain the term gray matter
Areas with clusters of neurons
More cell bodies present so higher blood supply and mito so appear more gray
Explain the term white matter
Areas with predominantly nerve connections (axons/dendrites)
White due to high lipid proportions from myelin sheaths
Explain the term white matter
Areas with predominantly nerve connections (axons/dendrites)
White due to high lipid proportions from myelin sheaths
Function of neurones
Integrators of large number of inputs to generate specific outputs in the form of action potentials
Function of nociceptor
Detect negative properties e.g excess heat
Draw a basic diagram of a sensory neurone
Explain its adaptation for maximal speed of transduction
Dendrite and axon are an uninterrupted strand
This is achieved by moving the perikaryon to the side
Perikaryon depolarised ‘on the fly’
What are efferent neurones?
Where are the perikarya and axons found ?
Cause a response in the effector
Perikarya in CNS
Axons in PNS
What are afferent neurones?
Where are the perikarya axons and dendrites found?
Relay information from a receptor
Cell bodies/dendrites in PNS
Most of axons in CNS
What does ganglion mean (ganglia pl)
a structure containing nerve cell bodies, and often forming a swelling on a nerve fibre.
found on the outside of the CNS
Where are the perikarya of sensory neurones found ?
In the dorsal root ganglion found in the thickening of the canal between vertebrae (called the intervertebral foramen)
What is a periphery nerve?
Bundle of sensory and motor neurone associated with a particular muscle
they are found running in the grooves called septa between muscles
Label this cross section of a peripheral nerve
Each bundle of axons/dendrites is called a fascicle
some nerves have several fascicles others have only one
the tough layer of connective tissue encasing the nerve is called the epineurium
What is the epineurium ?
Dense collagenous red connective tissue that encases peripheral nerves
description and function of the perineurium
description:
- Sheath with flat cells of epithelial character
- they are brownish
- have tight junctions
- collagen fibres between the cells
function :
- they bundles axons/dendrites into fascicles and form a seal
- protects the axons/dendrites against large changes in ion composition
What is the endoneurium?
Loose connective tissue between axons/dendrites within a fascicles
Helps support Schwann cells and maintain the ionic environment
Label the epineurium , perineurium and endoneurium in the photos
Explain how Schwann cells are involved in non-myelinated neurons
Several axons surround the cytoplasm of 1 Schwann cell
no nodes of ranvier
How to identify myelinated and non-myelinated axons in a micrograph
Myelinated axons have clear dark rings , white interior
non-myelinated axons appear as dark spots
How can you tell the difference between blood vessels and nerve fibre bundles in a histology section ?
Small nerves appear wavy and may have elongated nuclei ( due to the Schwann cells)
blood vessels have an empty interior or have red blood cells without nuclei ; cells do not have elongated nuclei ; arteries have thick walls
What are satellite cells?
The support cells in the PNS ganglia
they have flat nuclei
Fill in the blanks :
Nerves consists of bundles of axons and dendrites, held together and supported by 1_________, bundled into fascicles by 2________ which are then packed together by 3__________
use the words perineum , epineurium and endoneurium
1 endoneurium
2 perineurium
3 epineurium
Label this synapse
What is the Nernst potential ?
Another term for the diffusion potential
Define diffusion potential
Voltage when effect of electrostatic forces = effect of conc gradients
no net force
no net ion flow
Membrane potential stays the same
voltage required to maintain a conc grad for a particular ion
How can cells have Vm>Ek at rest?
At resting potential , Vm < ENa = net movement of Na+ Into cell
Vm>Ek so K+ are not at eqm , net movement of K+ Out of cell
no net movement of electrical charge so steady Vm achieved
Membranes have some Na+ permeability
Role of Na+/K+ ATPase pump
To replenish K+ lost from cell and remove Na+ accumulated within cell , thereby maintaining resting potential by keeping [K+] and [Na+] gradients constant
Resting potential
Different in different cells
always negative
inside of cell is negative
outside of cell is positive
established by the Na+/K+ pump
What three factors influence ion movement?
Effect of concentration gradients - movement from high to low conc
electrostatic force - movement towards opposite charge
conc grads and electrostatic forces act against each other
Membrane permeability
Effect of external [K+] on membrane potential of skeletal muscle
Linear relationship
other terms for depolarisation ?
Upstroke/rising phase
Other terms for repolarisation
Downstroke / falling phase
Formation of an AP
Depolarisation to threshold
Na+ influx
Na+V (voltage-gated Na+ channels) must open must faster than Kv
Which ion is responsible for depolarisation in AP in:
nerouns?
Na+
this allows rapid propagation
Which ion is responsible for depolarisation in skeletal muscle ?
Na+ for rapid propagation
Which ion is responsible for depolarisation in cardiac muscle
Na+ initially for rapid propagation and synchronisation
then Ca2+ finally to allow contraction
Which ion is responsible for depolarisation in smooth muscle
Ca2+ for propagation synchrony and contraction
Which ion is responsible for depolarisation in endocrine cells
Ca2+ for secretion
Benefits of myelination
Saltatory conduction allows :
- Reduced leakage across membrane between nodes
- currents travel further before being dissipated
- voltage gated channels concentrated at nodes of ranvier
- depolarisation only has to occur at nodes not across entire axon length
Describe how the resting potential is established
- Na+/K+ pump uses energy from ATP hydrolysis to actively transport 3 Na+ out and 2 K+ in to the neurone
- some permeability to K+ remains (and very little Na+ permeability)
- this generates an electrochemical gradient
- higher Na+ conc outside
- higher K+ conc inside
- the inside of the membrane is more negative than the outside
- the membrane is said to be polarised
- this creates a potential difference called the resting potential
describe the action potential
- if the stimulus is large enough, more Na+ channels proteins open = more Na+ diffuse into the neurone = greater GP
- membrane depolarised to threshold value
- influx of Na+ = AP initiated
- after, resting potential restored by Na+/K+ pump
Explain what happens at the cell membrane when an action potential is initiated at a certain point
- a stimulus causes a GP that is large enough to esceed threshold
- neighbouring voltage-gated Na+ channels open
- Na+ diffuse into neurone
- this causes more Na+ channels to open so the process repeats, resuliting in an influx of Na+ in
- therefore, the membrane becomes fully depolarised (the inside of the neurone is now more positive than the outside) to +40 mV. this is an AP and will be propagated down the neurone
- at +40 mV, Na+ channels close and K+ channels open
- K+ diffuse out of the neurone
- the inside of the neurone becomes more and more negative compared to the outside. this is called repolarisation
- too many K+ diffuse out causing the p.d. to be more negative than resting potential. the membrane is now hyperpolarised
- the Na+/K+ pump restores resting potential by pumping 3 Na+ out for every 2 K+ brought in via facilitated diffusion
Describe the refractory period
- includes the period after an AP has occurred where another AP cannot be initiated at that same point in the membrane. 2 phases:
- absolute refractory period = between depolarisation and repolarisation; impossible to initiate another AP as Na+ channels inactivated
- relative refractory period = when the membrane is hyperpolarised ; another AP can be initiated but it must be stronger than the initial AP (as more Na+ ions need to diffuse in to reach threshold value as p.d. is more negative than resting potential)
What is the importance of the refractory period? [2]
- ensures AP sent as separate impulses - more than 1 AP cannot be initiated at the same point in the membrane at the same time ( this is important because if APs overlap, the brain can’t distinguish the stimuli)
- AP only travel in the forward direction as they can only move along the part of the membrane that has a resting potential (if they go backward they will reach an area in the refractory period)
what is the disadvantage of the refractory period?
limits the frequency of APs
Describe how an action potential is propagated in unmyelinated neurones
- at the point in the membrane where an AP has occured, Na+ channels open so Na+ diffuse into neurone
- there is a high Na+ conc inside the neurone at the point where the Na+ channels are open
- there is a low Na+ conc inside the neurone at the neighbouring section of the membrane at resting potential
- therefore Na+ diffuse sideways across the membrane down a conc gradient creating a local current
- the p.d. across the neighbouring section of the membrane increases
- threshold reached = voltage-gated Na+ channels open = Na + diffuse into neurone = membrane fully depolarises
- this process repeats itself, causing the AP to spread down the axon by local currents
describe the propagation of APs in myelinated neurones
- APs can only occur at nodes of Ranvier
- therefore the Na+ ion have do “jump” from node to node, elongating the local currents. this is called saltatory conduction
- much faster than depolarising the entire neurone membrane bit by bit
- requires less ATP as less active transport occurs as less repolarisation occurs
explain why a myelinated axon conducts impulses faster than a non-myelinated axon
- in myelinated axons, APs occur at nodes only
- the nerve impulse moves down the axon via saltatory conduction (jumps from node to node) so it does not travel along the whole length of axon
describe the adaptations of the presynaptic knob
- many mitochondrion to provide ATP for:
- active transport
- resynthesis of neurotransmitter
- transport of neurotransmitter via vesicles
- many SER for neurotransmitter synthesis (which are made from lipids)
- vesicles carry the neurotransmitter to the membrane
- they are already formed ready for an AP to signal them to release the neurotransmitter = faster impulse transmission
- voltage-gated Ca2+ in membrane to allow Ca2+ to enter when a particular voltage is achieved
describe the adaptations of the post-synaptic neurone membrane
- specialised Na+ channels (not the same as voltage-gated Na+ channels)
- made of 5 polypeptides - 2 of which have receptor sites complementary to the neurotransmitter
- when neurotransmitter binds to receptor sites, the Na+ channels open
what does the “all or nothing response” mean?
- all APs have the same magnitude
- if threshold is met, an AP is generated
- if theshold is not met, an AP is not generated
Describe the transmission of an AP across a synapse between two neurones (fast excitatory synapses)
- an AP arrives at presynaptic membrane
- AP causes voltage-gated Ca2+ channels to open
- Ca2+ diffuses into presynaptic knob, causing vesicles containing ACh to move and fuse with presyntaptic membrane
- ACh released into synaptic cleft via exocytosis
- ACh diffuses across synaptic cleft and binds to receptor sites of Na+ channels in postsynaptic membrane
- Na+ channels open so Na+ diffuse into postsynaptic neurone (other cation channels may open/close but mainly Na+)
- GP created across postsynaptic membrane
- GP is great enough= threshold potential reached = new AP initiated in postsynaptic neurone
Describe the structure and adaptations of the neuromuscular junctions
- neuromuscular junctions are specialised cholinergic synapses which connect motor neurones to muscle fibre membranes
- therefore only ACh is used
- sarcolemma (muscle fibre membrane) is highly folded
- provides higher SA for many Na+ channels
- again many mitochondria and SER in motor neurone knob
- each motor neurone has multiple motor end plates so connects with many muscle fibres
- all muscle fibres contract at the same time after an AP = strong muscle contraction
what are the differences between a cholinergic synapse and a neuromuscular junction?
describe how the neurones recover after an AP is initiated
- enzymes found in the synaptic cleft break down the neurotransmitter
- acetyl cholinesterase AChE hydrolyses ACh
- ACh→ acetic acid + choline
- these products diffuse across synaptic cleft back to presynaptic knob where they are recombined to from the neurotransmitter again
- this requires ATP
- this causes the neurotransmitters to no longer be complementary to receptor sites so the neurotransmitter is removed from Na+ channels in postsynaptic membrane
- Na+ channels therefore close
- Na+/K+ pumps work to restore resting potential
describe the features of synapses
- they allow APs to be transmitted in one direction only from the presynaptic neurone to the postsynaptic neurone because:
- neurotransmitter receptor sites only found in post-synaptic membrane
- only presynaptic knob contains vesicles with neurotransmitter
- convergence = many presynaptic neurones (both excitatory and inhibitory) synaps onto a single postsynaptic neurone.
- this filters out low level stimuli
- allows impulses from various neurones to cause the same response
- divergence = a single presynaptic neurone can join onto several postsynaptic neurones
- this is useful when a single stimuli requires responses from several parts of the body
what is meant by spatial summation?
when simultaneous APs from several presynaptic neurone cause an AP to be initiated in the postsynaptic neurone
Describe how inhibitory synapses prevent an AP from being formed in the next neurone
- an AP arrives at the presynaptic knob
- Ca2+ channels open = Ca2+ diffuse into presynaptic knob causing vesicles containing GABA to move and fuse with membrane
- GABA released into synaptic cleft via exocytosis
- GABA binds to receptor sites of Cl- channels on postsynaptic membrane
- Cl- channels open= Cl- diffuse into postsynaptic neurone
- Potential difference across postynaptic membrane becomes more negative so is said to be hyperpolarised
- stronger stimuli needed to reach threshold (in order to open more Na+ channels so more Na+ can enter)
where are synapses found? [2]
- at axons terminals (in presynaptic neurone)
- at dendrites (in postsynaptic neurone)
Types of neurotransmitters
Amino acids e.g. GABA
catecholamines - adrenaline/noradrenaline
peptides
others e.g ACh
Neurotransmitter used in fast excitatory synapses in the CNS
The amino acid Glutamate
What is the term used to describes the depolarisation of the postsynaptic neurone
Excitatory post-synaptic potential
multiples EPSPs required for one AP
Inhibitory neurotransmitter used in the spinal chord
Amino acid Glycine
Inhibitory neurotransmitter used in the CNS
GABA
Ways to interact with synaptic transmission
- Enzyme action
- receptor desensitisation
- reuptake of NT by post synaptic neurone
what is the difference between the meninges and blood-brain barrier?
the blood brain barrier describes the adaptations of the endothelium within brain capillaries. Whereas the meninges provides protection against physical trauma, the blood-brain barrier provides protection against pathogens and toxins from blood
