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Medical Pathophysiology > Week 5 - Lecture 1 - Alterations in neuronal impulse conduction > Flashcards

Week 5 - Lecture 1 - Alterations in neuronal impulse conduction Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the two principle type of cells of nervous tissue

A

neuroglia and neurones

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2
Q

What are neuroglia cells

A

small cells that surround and wrap delicate neurons

in the central nervous system there’s astrocytes, microglial, ependymal and oliodendrocytes

in the PNS there is satellite and Schwann cells

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3
Q

what are neurons

A

excitable cells that transmit electrical signals

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4
Q

5 neuron characteristics

A
  1. extreme longevity
    - 100 years or more
  2. amitotic ( do not divide)
  3. high metabolic rate
    - require continuous supply of oxygen and glucose
  4. large, highly specialised cells - conduct impulses
  5. All have cell body and processes
    - processes are dendrites and an axon
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5
Q

Functional classification of neurones

A

grouped by direction in which nerve impulse travels relative to CNS

  1. sensory - afferent
  2. motor - efferent
  3. interneurones (association)
    - lie between motor and sensory neurones
    - 99% of body’s neurones
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6
Q

dendrites

A

dendrite are the receptive (input) region of a neurone, they receive incoming signals toward cell body as graded potentials (changes in membrane potential)

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7
Q

Axons

A

Axons are the conducting region of a neurone

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8
Q

What is the myelin sheath

A

The myelin sheath is a whitish, protein lipoid substance
- it is segmented sheath around most long, or large diameter axons

myelinated fibres

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9
Q

What are the functions of myelin

A

protects and electrically insulates axon
increase speed of nerve impulse transmission
nonmyelinated fibres conduct impulses slowly

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10
Q

Plasma membranes of myelinating cells have less protein, why

A

because there are no channels or carriers, good electrical insulators and interlocking proteins bind adjacent myelin membranes

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11
Q

what are and why are there myelin sheath gaps

A

gaps between adjacent Schwann cells

called nodes of ranvier

regions rich in sodium channels to promote nerve impulse over long distances

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12
Q

Membrane potentials

A

neurons communicate with other neurons in the body through action potential (nerve impulse/electrical signal)

  • nerve impulse is always in the form of an action potential regardless of stimulus (eg. pain, light, heat)
  • Action potential is a result off charged ions
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13
Q

What are the two main types of ion channels

A

leakage (non gated) and gated

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14
Q

Leakage non gated channel

A

always open

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15
Q

Gated channels

A

part of protein changes shape to open/close channel

3 channels

  1. chemically gated (ligand - gated) channel
  2. voltage gated channel
  3. mechanically gated channel
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16
Q

chemically gated ( ligand - gated) channel

A

open with binding of a specific neurotransmitter

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17
Q

voltage gated channels

A

open and close in response to changes in membrane potential

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18
Q

mechanically gated channels

A

open and close in response to physical deformation of receptors, as in sensory receptors

19
Q

The resting membrane potential

A

the bulk solution inside and outside of the cell are electrically neutral

potential difference exists in a very defined area only across the membrane of resting cells

  • approx - 70mV In neurons
    (cytoplasmic side of membrane negatively changed relative to outside)

membrane termed polarised

This is generated by the differences in ionic makeup of ICF and ECF
differential permeability of the plasma membrane
- ECF has higher concentration of sodium NA+ than ICF, balanced chiefly by chloride ions (Cl-)
-ICF has higher concentration of K+ than ECF, balanced by negatively charged proteins

K+ plays most important role in membrane potential

20
Q

Differences in plasma membrane permeability

A

slightly permeable to Na+ (through leakage channels)
- sodiums diffuses into cell down concentration gradient

25 times more permeable to K+ than sodium (more leakage channels)
- potassium diffuses out of cell down concentration gradient

more potassium diffuses out of the cell than sodium diffuses into the cell

  • cell more negative inside
  • establishes resting membrane potential

sodium potassium pump stabilises resting membrane potential

21
Q

How do membrane potentials change

A

membrane potential changes when

  • concentrations of ions across membrane change
  • membrane permeability to ions changes
22
Q

Changes in membrane potential

A

terms describing membrane potential relative to resting membrane potential

Depolarisation

  • decreased in membrane potential (towards zero and above)
  • inside of membrane becomes less negative than resting membrane potential
  • increase probability of producing a nerve impulse

Hyperpolarisation

  • an increase in membrane potential (away from zero)
  • inside of cell more negative than resting membrane potential)
  • reduces probability of producing a nerve impulse
23
Q

threshold

A

not all depolarisation events produce an action potential

for an axon to ‘fire’ depolarisation must reach threshold
- that voltage at which the action potential is triggered

at threshold

  • membrane has been depolarised by 15 to 20 mV
  • Na+ permeability increases
  • Na+ influx exceeds K+ efflux

an action potential either happens completely, or it does not happen at all

24
Q

Propagation of an action potential

A

propagation allow an action potential to serve as a signalling device

Na+ influx causes local currents

  • local currents depolarisation of adjacent membrane areas in direction away from action potential origin (toward axon’s terminal)
  • local currents trigger an action potential there
  • this causes the action potential to propagate away from the origin

since Na+ channels closer to an action potential origin are inactivated no new action potential is generated there

once initiated an AP is self propagating

25
Q

The synapse

A

nervous system works because information flows from neurone to neurone

neurons functionally connected by synapses

  • junctions that mediate information transfer
    1. from one neurone to another neurone
    2. or from one neurone to an effector cell

synapses are electrical or chemical in nature

26
Q

axodendritic

A

between axon terminals of one neurone and dendrites of others

27
Q

axiomatic

A

between axon terminals of one neurone and soma of others

28
Q

Electrical synapses

A

less common than chemical synapses

neurones electrically coupled (joined by gap junctions that connect cytoplasm of adjacent neurones and can allow the passage of charged ions)

  • communication very rapid
  • may be unidirectional or bidirectional

More abundant in embryonic nervous tissue

29
Q

chemical synapses

A

specialised for release and reception of chemical neurotransmitters
typically composed of two parts
1. axon terminal of presynaptic neurone
- contains synaptic vesicles filled

two parts separated by synaptic cleft : fluid filled space

electrical impulse changed to chemical across synapse, then back into electrical

transmission across synaptic cleft

  • chemical event (slower than an electrical one)
  • depends on
    • release of neurotransmitter
    • diffusion of neurotransmitter
    • receptor binding of neurotransmitter
  • ensure unidirectional communication between neurones
30
Q

information transfer across chemical synapses 9 stages

A
  1. action potential arrives at axon terminal of presynaptic neurone
  2. causes voltage-gated Ca2+ channels to open, Ca2+ floods into the cell
  3. intracellular proteins binds Ca2+ and promote fusion of synaptic vesicles with axon membrane
  4. exocytosis of neurotransmitter into synaptic cleft occurs
  5. neurotransmitter diffuses across synapse
  6. binds to receptors on post synaptic neurone
  7. ion channels are opened
  8. causes an excitatory or inhibitory event
  9. neurotransmitter effects terminated
31
Q

termination of neurotransmitter effects

A

within a few milliseconds neurotransmitter effect terminated in of three ways

  1. reuptake
    - by astrocytes or axon terminals
  2. degradation
    - by enzymes
  3. diffusion
    - away from synaptic cleft
32
Q

post-synaptic potentials

A

neurotransmitter receptors cause changes in membrane potential that vary in strength with

  • amount of neurotransmitter released and
  • time neurotransmitter stays in the area

types of post synaptic potentials

  • EPSP - excitatory postsynaptic potentials
  • IPSP - inhibitory postsynaptic potentials
33
Q

Neurotransmitters

A

50 or more neurotransmitters have been identified
most neurones make two or more neurotransmitters
- neurons can evert several influences
classified by chemical structure and by function

34
Q

Acetylcholine

A

first identified : best understood

released at

  • neuromuscular junctions
  • some ANS neurons
  • some CNS neurones
    • synthesised from acetic acid and choline by enzyme choline acetyltransferase
    • Degraded by enzyme acetylcholinesterase (AChE)
35
Q

Biogenic amines

A
  1. catecholines
    - dopamine, norepinephrine (NE) and epinephrine
    - excitatory/inhibitory
  2. indolamines
    - serotonin (inhibitory) and histamine (excitatory/inhibitory)

Broadly distributed in brain

  • play role in emotional behaviour and biological clock
  • imbalanes associated with mental illness
36
Q

Amino acids

A

glutamine : excitatory

  • stroke neurotransmitter
  • excess release : cell stimulated to death

aspartate : excitatory
glycine : inhibitory
- spinal cord main inhibitory

GABA - gamma - aminobutyric acid : inhibitory
= main inhibitory neurotransmitter of the brain

37
Q

classification of neurotransmitters : function

A

great diversity of functions

can classify by

effects: excitatory versus inhibitory
actions: direct versus indirect

function of the neurotransmitter is determined by the receptor to which it bunds

effects - excitatory versus inhibitory

  • neurotransmitter effects can be excitatory (depolarising) and or inhibitory (hyper polarising)
    • GABA and glycine usually inhibitory
    • glutamine usually excitatory
    • acetylcholine and NE bind to at least two receptor types with opposite effects

ACh excitatory at neuromuscular junction in skeletal muscles

ACh inhibitory in cardiac muscles

38
Q

Direct vs indirect actions

A

direct action

  • neurotransmitter bind to and opens ion channels
  • promotes rapid responses by altering membrane potential
  • examples : Each and amino acids

indirect action

  • neurotransmitter acts through intracellular second messengers, usually G protein pathways
  • broader, longer lasting effects similar to hormones
  • example : biogenic amines, neuropeptides and dissolved gases
39
Q

neurotransmitter receptors

A
  1. channel linked receptors : mediate fast synaptic transmission
    - ligand gated ion channels
    - action is immediate and brief
  2. G protein linked receptor : mediate slow synaptic responses
    - responses are indirect, complex, slow, and often prolonged
40
Q

neurotransmitters vs neuromodulators

A

nitric oxide, adenosine often referred to as neuromodulators

41
Q

neuromodulators

A

chemical messengers released by the neurone

does not can EPSP or IPSP

affects the strength of synaptic transmission (may inhibit, potentiate, or prolong the effects of neurotransmitters)

42
Q

Processes of neuronal injury

A

mature neurones do not divide
- new neurones do not replace damaged neurone that lost function

loss of neurones leads to impaired neurologic transmission

impaired neurologic transmission leads to neurologic disorders

  • cell damage
  • ageing

manifestations reflect

  • site of injury
  • functions that are controlled by the dysfunctional neuronal signals
43
Q

processes of neuronal injury

A

result of injury at the cellular level

chromatolysis

  • for of apoptosis
  • swelling of a neurone and dissolution of chromophil substance in neurones
  • in certain pathological conditions or following an injury to the cell or axon

atrophy
- decrease in cell size

neuronophagia
- phagocytosis and inflammatory responses causes by a dead neurone damaging neighbouring cells

intraneuronal inclusions
- distinctive structures formed in the nucleus or cytoplasm

Medical Pathophysiology (30 decks)

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