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Electrifying Connections: Exploring Membrane Potential, Synapses, and Neurotransmitters, NEUROSCIENCES physiology , 2nd year class notes$5.49
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Electrifying Connections: Exploring Membrane Potential, Synapses, and Neurotransmitters, NEUROSCIENCES physiology , 2nd year class notes
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Course
PHY02NS
Institution
Dow University Of Health Sciences,DUHS
Dive into the electrifying world of neuronal communication with this illuminating guide. Membrane potential serves as the electrical foundation of neural activity, shaping the dynamics of synaptic transmission and neurotransmitter release. Explore the intricacies of synapses, where the delicate dan...
MEMBRANE POTENTIAL
It is the difference in concentration of ions on the two sides of the cell
membrane.
or in other words it is the separation of charges across the membrane.
TYPES OF MEMBRANE POTENTIAL
RESTING MEMBRANE POTENTIAL (RMP)
It is the difference in concentration of ions on the two sides of the cell membrane, in non-
excitable cells and in excitable cells when they are not being stimulated.The average
voltage ranges from -70 mV to 90 mV, most commonly used is -70 mV
,FACTORS THAT CONTRIBUTE TO THE PRODUCTION OF RMP
a. Concentration difference of ions across the membrane
b. Permeability of Na+(less permeable,+61mV) and K+(more permeable,-94mV) during the
resting state.
c. Na-K pump that pumps 3 Na+ ions out of the cell and 2 K+ions inside the cell and
maintains RMP.
NERNST EQUATION
It is used to calculate equilibrium potential(the potential which neutralizes the diffusion
potential). EMF is Nernst potential in mV on the inside of the membrane. It will be –ve for +ve
ions; and +ve for –ve ions
There are two forces acting on an ion at the cell membrane.
1, chemical force=The chemical force is the concentration difference of the ion between the
inside and outside of cell membrane,its unit is millimolar.
2, electrical force= The electrical potential differences between the inner and outer side of the
cell membrane,its unit is millivolt.
At electrochemical equilibrium, the chemical and electrical driving forces that act on an ion
are equal and opposite.
Driving force = is the sum of chemical and electrical forces.
Nernst potential helps us express the chemical force in units of millivolts.
GRADED POTENTIAL
Graded Potential is local current (local change in RMP),due to subthreshold stimulus that
spreads for only a few micrometers before it dies out and have decremental
propagation.If the stimuli sum up & reach a threshold level then Action Potential is fired
GRADED POTENTIAL PRODUCTION
Stimulus causes ligand or mechanically gated ion channel to open or close → local change is
produced which makes cell either less –ve or more –ve; magnitude depends upon intensity of
stimulus (i.e. response is graded).
ELECTROTONIC POTENTIALS
ACTION POTENTIAL
The change in RMP due to threshold potential and have non decremental propagation.
NEURONAL ACTION POTENTIALS
4,PHASES:
A,RESTING PHASE:
Membrane is much more permeable to K+ than to Na+.
Greater diffusion of K out than Na in, therefore inside is negative and Outside is positive.
Both Na & K voltage gated channels are CLOSED.
B,DEPOLARISATION PHASE
,Mechanical/chemical/vibratory/other stimulus opens some Na+ channels such that Na+ flows
into the cell. Therefore membrane potential becomes less negative (ie. It depolarises). If the MP
reaches approx. -55mV (threshold), the voltage gated Na+ channels open.Na+ influx increases
dramatically – until MP reaches approx. +30mV where the voltage-gated Na+ channels close.
C,REPOLARIZATION PHASE
Approx. +30mV K+ voltage gated channels open. (perm. of K increases & Na decreases). Large
outflow of K+ causes membrane potential to become more negative (repolarises) and returns to
- 70mV.
D,HYPERPOLARISATION (UNDERSHOOT) PHASE
K+ channels remain open past -70mV and MP becomes more negative than at rest. o K+
channels close and Na/K ATPase returns the MP to normal (-70mV).
CHARACTERISTICS OF ACTION POTENTIALS
Action potentials have three basic characteristics:
♦ Stereotypical size and shape. Each normal action potential for a given cell type looks
identical, depolarizes to the same potential, and repolarizes back to the same resting potential.
♦ Propagation. An action potential at one site causes depolarization at adjacent sites, bringing
those adjacent sites to threshold. Propagation of AMP is non decremental.
♦ All-or-none response. An action potential either occurs or does not occur.
,REFRACTORY PERIOD
Refractory period is a period during which another normal action potential cannot be elicited in
an excitable cell by normal stimulus.
TYPES
1,ABSOLUTE REFRACTORY PERIOD
The absolute refractory period overlaps with almost the entire duration of the action
potential. During this period, no matter how great the stimulus, another action potential
cannot be elicited. The basis for the absolute refractory period is closure of the
inactivation gates of the Na+ channel in response to depolarization.
2,RELATIVE REFRACTORY PERIOD
The relative refractory period begins at the end of the absolute refractory period and overlaps
primarily with the period of the hyperpolarizing afterpotential. During this period, an action
potential can be elicited, but only if a greater than usual depolarizing (inward) current is
applied. The basis for the relative refractory period is the higher K+ conductance than is
present at rest. Because the membrane potential is closer to the K+ equilibrium potential,
more inward current is needed to bring the membrane to threshold for the next action
potential to be initiated.
SPEED OF IMPULSE
Speed of impulse depends upon:
1. Axon Diameter, Larger = Quicker
2. Presence of Myelin (white matter), Impulse jumps from exposed axon-region to the next
instead of having to open & close ion channels across the axon’s entire length(saltatory
conduction).
Speed of propagation ∝ diameter ∝ internodal distance.
, Compound AMP which is AMP of nerve trunk can be graded due differences in
speed of conduction, and threshold stimulus of different nerve fibres.
SYNAPSES
A synapse is a junction between two excitable cells(neuron, glands and muscles)
across which electrical impulses pass.
STRUCTURE OF A SYNAPSE.
a. Axon terminal of pre synaptic neuron contains a large number of mitochondria &
membrane-bound vesicles containing NT which release its content by exocytosis.
b. A gap called synaptic cleft separates pre & post synaptic membrane.
c. Post synaptic membrane has receptor proteins to which NT bind; Action Potential is
transferred from one excitable cell to another.
, TYPES OF SYNAPSES
A. DEPENDING UPON THE SITE:
i. Axo dendritic(common)
ii. Axo somatic
iii. Axo axonic(inhibitory)
B. DEPENDING UPON THE NATURE:
i. Electrical
ii. Chemical
i. ELECTRICAL SYNAPSES
They are actual communications b/w cells. Channels extend from cytoplasm of one
neuron to other; flow of ions through gap junctions. Electrical synapses are
bidirectional, and faster.Commonly occurs in invertebrates; in humans a few places
in CNS. In electrical synapses, the rapid spread of activity from one cell to another
ensures that a excitable tissue performing an identical function act together.
LOCATION
Gap junctions between excitable cells form electrical synapses
a. Gap junctions between neurons → form electrical synapse(hippocampus and
thalamic reticular nuclei).
b. In the heart – intercalated disk → form electrical synapse
c. Smooth muscle → form electrical synapse
d. In the bone – between osteocytes
e. Skin
f. Retina
ii. CHEMICAL SYNAPSES
It is the most common type of synapse. A neurotransmitter passes across the
narrow space b/w cells & become attached to a protein receptor on post synaptic
membrane of excitable cell.“One way” conduction, allowing signals to pass in one
direction only.Synaptic cleft is 20nm.
PHASES OF NEUROTRANSMISSION:
1. Action potential reaches axon terminal, opens voltage-gated Ca+ channels.
➢ Influx of Ca+ into axon terminal causes vesicles of neurotransmitter to migrate to
the axon terminal.
➢ Neurotransmitters are released by exocytosis from the sending (pre-synaptic)
neuron.
, ➢ Neurotransmitter (acetylcholine/nor adrenaline/dopamine/glutamate/gaba/etc)
diffuses across synaptic cleft between 2 neurons.
2. Neurotransmitters bind to ligand-gated ion channels, causing change in MP of
post-synaptic neuron (dendrite) → creating graded potentials.
➢ -Short-lived, localised changes in membrane potential.
➢ -Current flow decreases in magnitude with distance.
➢ -The stronger the stimulus, the greater the GP (and further distance)
○ If GP depolarises membrane, it is excitatory
○ If GP hyperpolarises membrane, it is inhibitory.
➢ - The sum of the GP may cause MP to reach threshold, triggering an action
potential on the next neuron.
3. Neurotransmitter Inactivation stops continued stimulation of post-synaptic neuron.
FATE OF NEUROTRANSMITTER
a. Diffusion into nearby ECF where astrocytes absorb them
b. Degradation in synaptic cleft e.g. Acetylcholine (ACh)
i. ACh inactivated by enzyme acetylcholinesterase located in synaptic cleft &
post synaptic membrane breaks down it into acetic acid & choline, thus making it
ineffective within fraction of a second; Synaptic knob reabsorbs choline and uses it to
synthesize more ACh
ii. Serotonin is also acted upon by Catechol O Methyl Transferase(COMT) in
synaptic cleft & then restored to vesicles; reutilized in the synthesis of neuro
transmitters
c. Reuptake of amino acids and monoamines by endocytosis & breaks them down by
an enzyme called Mono Amine Oxidase(MAO) so transported back into vesicles;
some anti-depressant drugs work by inhibiting MAO
TYPES OF POST-SYNAPTIC RECEPTORS:
Ionotropic: (Ligand-Gated Ion Channels)
Mechanism: Binding of Neurotransmitter→ Opening of Ion Channel
→Excitation/Inhibition of Cell.
● Excitatory: Na+ /Ca+ Channel opening→ Na+/Ca+ Influx → Depolarisation
of Membrane →→ Excitatory Post Synaptic Poƚenƚial͟ (EPSP) which is due to
excitatory neurotransmitters( ACh, NE, glutamate)
● Inhibitory: →Inhibitory Post Synaptic Potential (IPSP) due to inhibitory
neurotransmitters(GABA, glycine, dopamine, serotonin)
○ Cl Channel – opening → Cl Influx → Hyperpolarisation of Membrane
○ K+ Channel – opening →K+ Efflux → Hyperpolarisation of Membrane
Metabotropic: (G-Protein Linked Receptors)
SUMMATION
combination of graded potential In the post synaptic neuron to integrate multiple
signals.
TYPES
TEMPORAL SUMMATION
More than 1 GPs arrive in close succession at pre-synaptic membrane; before the
1st local depolarization returns to resting value, 2nd is produced. Less effective
SPATIAL SUMMATION
More than 1 local depolarization produced simultaneously, summate at axon hillock
to produce AP.More effective
In a neuron all EPSPs and IPSPs are combined; the product of summation is
determined by which influence is greater; if EPSPs are greater than IPSPs AP will fire; if
IPSPs are greater then neuron will be inhibited.
●
NEURONAL CIRCUITS
, CONVERGENCE
When many pre synaptic neuron converge on a single post synaptic neuron.Good for
incoming sensory information.
DIVERGENCE
When axon of a pre synaptic neuron divides into many branches that diverge to end on
many post synaptic neurons.Helpful in large scale muscle contraction.
REVERBERATION
Chains of neuron containing collaterals synapses with previous neurons in
chain.Important in feedback mechanism.
SYNAPTIC DELAY
Time required by the impulse to transfer from pre to post synaptic membrane(0.3 to
5.0ms).It ensures one way transmission of NTs.
FACILITATION
The increase in strength of response of post synaptic neuron due to repeated
stimulation followed by brief rest.
SYNAPTIC FATIGUE
Due to repeated stimulation at a rapid rate, the decrease in strength of response of post
synaptic neuron
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