Health and Disease Week 7 Flashcards
What type of cells are neurones?
postmitotic
define postmitotic
cells that are fully differentiated and unable to divide
define soma
the cell body of a neurone, an expanded area of cytoplasm where nucleus and organelles are found
Which organelle do neurones NOT have?
centrioles -> important in cell division, so don’t need them
What is the structure and function of the myelin sheath?
made up of Schwann cells and they insulate the axon
What are the 4 shape classes of neurones?
multipolar, bipolar, unpolar, anaxonic
What are the 3 classes of neurones?
afferent, efferent, interneurone
define afferent neurone
transmits signal from receptors to CNS
define efferent neurone
transmits signal from CNS to effectors
define interneurone
transmits signal from afferent to efferent neurones
What is a resting membrane potential (RMP)?
an electrical charge across the plasma membrane, with the interior of the cells negative compared to exterior
define chemical gradient
gradient based on concentration
define electrical gradient
gradient based on charge
What are the 2 types of movement across the plasma membrane involving proteins?
facilitated diffusion and active transporters (pumps)
define facilitated diffusion
when ions diffuse down their concentration gradient and the channels are selectively permeable to different ions
define active transport
when pumps move ions against their concentration gradient and create a concentration gradient across the membrane
How do Na+ and K+ pumps maintain excitability?
active transporters exchange 2K+ in for every 3NA+ out
What is the resting membrane potential in most neurones?
-70mV
Which equation is used to calculate the equilibrium potential for an ion?
the Nernst equation
give the Nernst equation
E ion = RT / zF x log [ion]outside / [ion]inside
Which other equation is used to calculate the resting membrane potential?
Goldman equation
define action potential
a brief change in the voltage across a membrane due to the flow of ions into and out of the neurone
What does the ‘all or nothing law’ describe?
if a stimulus is strong enough, an action potential occurs, which is always a full response i.e. can’t be strong or weak
What are the 5 phases of action potentials?
- hypopolarisation
- depolarisation
- overshoot
- repolarisation
- hyperpolarisation/undershoot
define hypOpolarisation
the initial increase of the membrane potential to the value of threshold potential
define depolarisation
the potential moving from the RMP to less negative values
define overshoot
the peak of the action potential being reached at about +40mV
define repolarisation
the potential moving back to the RMP (-70mV)
define hyperpolarisation/undershoot
the potential moves away from the RMP in a MORE negative direction, but the RMP is eventually restored due to K leak channel and sodium-potassium pump
action potential stage 1
membrane is at resting potential and VGNC and VGKCs remain closed
action potential stage 2
a stimulus causes depolarisation the threshold potential and VGNCs open
action potential stage 3
Na+ ions flow in and the membrane rapidly depolarises and more VG sodium ion channels open but VGKCs remain closed
action potential stage 4
VG sodium ion channels are inactivated and Na+ entry slows
action potential stage 5
VGKCs open and K+ begin to flow out, beginning repolarisation
action potential stage 6
VGNC channels fully close and VGKC remain open to give delayed hyperpolarisation -> eventually resting potential is restored
What are the properties of VGNCs?
- open rapidly with depolarisation at around -55mV
- moving away from -55mV causes inactivation
- inactivated channels are blocked during continued depolarisation
- inactivated channels move to a closed state during repolarisation
What are the properties of VGKCs?
- K+ channels slowly open during depolarisation at about +30mV
- they close slowly on repolarisation
What are the 2 classes of refractory period?
absolute refractory period, relative refractory period
What happens during the absolute refractory period?
action potentials cannot be generated as VGNCs are inactivated and can’t be activated again until the membrane is repolarised and resting state is restored
When does the refractory period occur?
from the start of an action potential to the point that voltage first returns to RMP
What happens during the relative refractory period?
the membrane potential is hyperpolarised by VGKCs and action potentials CAN be generated if the stimulus is strong enough to overcome hyperpolarisation and reach threshold
What is action potential propagation?
the process by which an action potential travels across a neurone
How does action potential propagation work (local currents)?
- stimulation at point A exceeds threshold
- local currents spread along axon and exceed threshold
- this opens VGNC and generate AP at point B
- VGNC inactivation occurs and open VGKC prevent back-propagation
What 2 factors does the velocity of action potential propagation depend on?
- axon diameter
- myelination
How does axon diameter affect action potential propagation velocity?
the larger the diameter, the faster the propagation as there is more space for ions to travel
define saltatory conduction
the propagation of action potentials along myelinated axons from one node of Ranvier to the next, increasing velocity
What causes Guillain-Barre syndrome?
the destruction of Schwann cells in the peripheral nervous system
What causes multiple sclerosis (MS)?
a loss of oligodendrocytes in the brain and spinal column
What are the 2 types of graded potential?
excitatory and inhibitory
What do inhibitory graded potentials do?
take the membrane further away from the threshold potential
What are the 4 types of excitatory graded potentials?
- temporal summation
- spatial summation
- no summation
- spatial summation of EPSPs and IPSPs
define temporal summation
2 excitatory stimuli close in time causes EPSPs to add together
define spatial summation
2 simultaneous stimuli at different locations cause EPSPs to add together
define no summation
2 stimuli separated in time cause EPSPs that do NOT add together
define spatial summation of EPSPs and IPSPs
changes in membrane potential that cancel each other out
define synapse
the point at which an electrical signal moves from one nerve cell to another
What are the 2 types of synapse?
electrical and chemical
What is the structure of an electrical synapse?
the pre and post-synaptic neurones are directly connected by gap junctions
What is the structure of an chemical synapse?
the pre and post-synaptic neurones are physically separated by the synaptic cleft and chemical transmitter from pre-synaptic neurone binds to receptors on post-synaptic neurone
What are gap junctions?
channels formed by connexons in both cell membranes and allow direct passage of ions and small molecules through the channels
What are the features of chemical synapses?
- synaptic vesicles filled with neurotransmitter on the inside of the presynaptic neurone
- voltage-gated calcium ion channels in presynaptic neurone
- neurotransmitter receptors on postsynaptic neurone
What are the 3 types of synapse in the CNS?
axo-dendritic, axo-somatic, axo-axonal
define axo-dendritic synapse
an axon synapsing onto a dendrite
define axo-somatic synapse
an axon synapsing onto a cell body
define axo-axonal synapse
an axon synapsing onto another axon or axon terminal
define neurotransmitter
a chemical messenger that is released from a pre-synaptic neurone
What are the sequence of events involved at a chemical synapse?
- neurotransmitter is synthesised and stored in vesicles
- AP arrives at presynaptic terminal
- AP depolarises the terminal and VGCC opem
- Ca2+ enter and trigger exocytosis
- transmitter diffuses down synaptic cleft and binds to receptors
- ion flow causes postsynaptic response
- transmitter is removed by enzyme breakdown or reuptake
- the vesicle is retrieved from the terminal membrane
What are the 4 key criteria for a substance being a neurotransmitter?
- substance must be present within the presynaptic neurone
- substance must be released in response to presynaptic depolarisation by an AP
- specific receptors for the substance must be present on the postsynaptic cell
- there must be a mechanism for removal and/or breakdown
What are the 2 types of neurotransmitter receptor?
ionitropic receptor, metabotropic receptor
What is an ionotropic receptor?
a ligand (transmitter) gated ion channel that opens to allow passage of ions through membrane in response to binding
What is a metabotropic receptor?
a g protein-coupled receptors -> binding activates g-protein which activates second messenger which can directly affect ion channels on membrane
What is a neuromuscular junction?
the synapse between a motor neurone and skeletal muscle
What is a neuromuscular junction also known as?
a motor endplate
Which neurotransmitter and receptor are used at neuromuscular junctions?
acetylcholine and nicotinic receptors
What type of receptor in the nicotinic acetylcholine receptor?
ionotropic or neurotransmitter-gated ion channel
How is acetylcholine synthesised?
in the presynaptic terminals by the enzyme choline acetyltransferase
How is acetylcholine broken down?
by acetylcholinesterase into acetate and choline
What is the main excitatory transmitter in the CNS?
glutamate
What type of receptor does glutamate activate?
both ionotropic and metabotropic receptors
What is the main inhibitory transmitter in the CNS?
GABA (G-aminobutyric acid)
What type of receptor does GABA work on?
a small family of ionotropic and metabotropic receptors
What type of potential does glutamate generate?
an excitatory postsynaptic potential (EPSP)
What type of potential does GABA generate?
an inhibitory postsynaptic potential (IPSP)
What do IPSPs do?
make a neurone less likely to generate an action potential
Which 2 main parts is the nervous system divided into?
the central and peripheral nervous systems
What is the central nervous system made up of?
the brain and spinal cord
define periphery
the area that is away from the centre of the body
define afferent neurone
a neurone that transmits an action potential from the periphery to the CNS
define efferent neurone
a neurone that transmits an action potential from the CNS to the periphery
define vertebrae
a stack of bones that surround the spinal cord
What are the 4 sections of spinal nerves that receive inputs and send outputs?
cervical, thoracic, lumbar, sacral
How do sensory inputs enter the CNS?
through the dorsal root into the spinal cord
What is the dorsal root ganglion?
where the cell bodies of axons are just outside the vertebrae in the spinal cord
What type of neurone are most sensory neurones?
unipolar -> have a cell body and one long axon
What is the simplest example of a connection between sensory neurone input and output back to the periphery?
the monosynaptic reflex
define monosynaptic
there is ONE synapse between the presynaptic and postsynaptic neurone
What type of neurone are most motor neurones?
multipolar
define reflex
when input is received by the periphery and the body provides output back to the periphery without any conscious thought
What do proprioceptors do?
sense position and movement
How does the knee jerk reflex work?
- tap the tendon attached to the quadriceps
- the tendon gets pulled, which stretches the quadriceps
- this is sensed by the muscle spindle fibres (proprioceptors)
- generates action potential
- action potential goes through dorsal root to spinal cord
- neurotransmitter is released in spinal cord
- this causes an action potential in a motor neurone, which causes the quadriceps to contract
What is the pathway of a reflex arc?
- receptor gives input to integrative centre
- integrative centre gives output to effector
What is USUALLY the integrative centre?
the spinal cord
How do sensory neurones vary?
by thickness and myelination
What does the extent of myelination determine?
the speed of an action potential
define ascending pathway
neuronal input travelling from the spinal cord to the brain
give an example of an ascending pathway in proprioception
- action potential comes into dorsal root
- goes up the spinal cord to a synapse at the brain stem (medulla)
- action potential crosses to the other side of the body
- it reaches another synapse in the middle of the brain called the thalamus
- eventually reaches the somatosensory cortex
define descending pathway
information going from the brain to the spinal cord
Where does all input from the periphery to the CNS BELOW the neck go to?
the spinal cord
Where does all input from the periphery to the CNS go to if ABOVE the neck?
the cranial nerves
define glial cell
a cell that is specialised to help neurones function but are NOT neurones
What are 2 examples of glial cells?
oligodendrocytes and astrocytes
What do oligodendrocytes do?
produce myelin -> there are different glial cells that do this in the peripheral nervous system called Schwann cells
What does astrocytes do?
act as a buffer to equalise concentrations of k+ ions -> there may be high concentrations of K+ ions if axons are generating many action potentials, so astrocytes ‘suck them up’ and move them to other parts of the brain
define the meninges
the 3 layers of membranes that cover and protect the brain and spinal cord
What is the outer membrane called?
dura mater
What is the inner membrane called?
the pia mater
What is the space between the inner and outer membranes called?
the sub arachnoid space
What is the sub arachnoid space filled with?
cerebrospinal fluid
What is the function of the choroid plexus?
it is a gland in the middle of the brain that produces cerebrospinal fluid
What does the cerebrospinal fluid contain?
salt and sugar -> high concentrations of Na+ and low K+ -> glucose and oxygen
What is the blood-brain barrier?
a semipermeable membrane separating the blood from the cerebrospinal fluid and brain
What is the structure of the blood-brain barrier?
tight junctions between the endothelial cells and blood vessel wall (tight junction is a seal between two cells that prevents leakage of content)
What does the blood-brain barrier do?
prevents substances leaking out of the blood vessels into the brain, including blood borne viruses and bacteria
Which cells acts as immune cells in the brain?
microglia
