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