A21 Nerve Impulses Questions/def Flashcards Preview

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Flashcards in A21 Nerve Impulses Questions/def Deck (16):

What is the role of the myelinated sheath

Insulates the axon and prevents ion movement between axon and surrounding tissue

Protective layer around axon
Speeds up nerve conduction


What is the myelinated sheath made of

Many layers of cell membrane of Schwann cells which wrap themselves around the axon


Difference between sensory neurone and motor neurone

Sensory neurone conducts impulse from a receptor to CNS

Motor neurone conducts impulse from CNS to effector


What are nodes of randier

Small gaps between neighbouring Schwann cells. Axon comes in direct contact with intercellular fluid at modes of ranvier


What happens during an action potential

The cell surface membrane becomes permeable to sodium ions

Positive ions outside membrane diffuse into neurone down concentration gradient


What does polarised mean

If a potential difference is maintained across he neurone with the inside being negative and the outside being positive


What does depolarisation mean

The decrease and then temporary reversal of the potential difference so that the inside of the membrane becomes positive in respect to the outside

Membrane is actually still polarised but the polarity is reversed


Impulse propagation

Depolarisation of one part of the membrane sets up a local circuit with the areas either side of it

Local circuits occur as positive ions are attracted by neighbouring negative regions

The region behind the depolarised zone becomes repolarised.

Positive ions from the depolarised zone pass along the inside of the membrane towards the polarised zone immediately in front

These processes occur continuously at creates a wave of depolarisation that moves rapidly along the neurone


What are local circuits

Movements of ions along the neurone.

The flow of ions is caused by an increase in concentration at one point which causes diffusion away from the region of higher concentration


Myelinated neurone impulse propagation

Local circuits cannot be Set up in parts of the neurone that are covered in myelinated sheath

Action potentials jump from one node of ranvier to the next - increases speed at which impulse is propagated along the axon - called saltatory conduction


Definition of threshold stimulus

Level of stimulus a neurone requires before an action potential is produced


Factors affecting speed of impulse

Myelin sheath - acts as an electrical insulator in myelinated neurones. Prevents depolarisation in that part of neurone.
Sheath is disturbed along neurones between adjacent Schwann cells - nodes of ranvier, depolarisation takes place here
Local circuits are formed here, action potentials jump from one node to another known as saltatory conduction

Diameter of axon - thicker the axon, faster the impulse. 'Less leakage' of ions in neurones with large diameter.

Temperature - temperate affects rate of diffusion of ions involved in neurone action potentials. Affects sped that neurones conduct impulses


Advantages of synapses

Ensures unidirectionality - neurotransmitter only made in pre synaptic membrane, receptors only in post synaptic membrane

Prevent over stimulation of effectors - too many impulses passing alone same neurone in short period of time will exhaust supply of neurotransmitter more quickly than it can be built up


Points to remember in transmission at synapses

Calcium ions diffuse due to impulse

Causes synaptic vesicles to make their way towards pre synaptic membrane

Neurotransmitter acetylcholine diffuses by exocytosis across cleft

Attaches to specific and complimentary receptor sites on post synaptic membrane

Causes depolarisation to occur as calcium ions diffuse in

Generates EPSP

if it reaches threshold action potential generated and impulse propagated through axon

Enzyme acetylcholinesterase attached to post synaptic membrane breaks down acetylcholine into constitute parts acetate and choline

Choline taken up by pre synaptic cell and combined with acetyl co enzyme A to reform acetylcholine

Mitochondria present to provide ATP to drive this process


Spatial and temporal summation at synapses

Spatial - number of different pre synaptic neurones can release together enough neurotransmitter to produce an EPSP above threshold to produce an action potential whereas one on its own may not

Temporal summation - single pre synaptic neurone releases neurotransmitter several times over a short time frame. Each pulse of neurotransmitter contributes to depolarisation of post synaptic membrane although any one action potential on its own may not be enough


Substances affecting action potentials

Nicotine - stimulates the release of acetylcholine and other neurotransmitters making action potentials more likely

Curare - blocks receptors at neuromuscular junctions preventing transmission - loss of muscle function

Opioids - block calcium channels in pre synaptic neurone. Less transmitter substance is released and action potentials are less likely. Can reduce pain relief by blocking pain impulses coming from pain receptors