Nerve centers. Synaptic and junctional transmission. Central inhibition презентация

specialized junction that transfers nerve impulse information between neurons

neuro-synapses or just synapse
To an effector cell
Neuromuscular synapse if muscle involved
Neuroglandular synapse if gland involve
Presynaptic neuron – conducts impulses toward the synapse
Postsynaptic neuron – transmits impulses away from the synapse
Two major types:
Electrical synapses
Chemical synapses

Synapses

neuron and the dendrite of another
Axo-somatic – synapses between the axon of one neuron and the soma of another
Other types of synapses include:
Axo-axonic (axon to axon)
Dendro-dendritic (dendrite to dendrite)
Dendro-somatic (dendrites to soma)

transmission in the CNS of human being are chemical synapses.
First neuron secretes a chemical substance called neurotransmitter at the synapse to act on receptor on the next neuron to excite it, inhibit or modify its sensitivity.

nerve impulse information from a presynaptic membrane to a postsynaptic membrane using neurotransmitters.

Axo-dendritic synapse Axo-somatic synanpse Axo-axonic synapse

vesicles in the presynaptic neuron

3) Released from the presynaptic neuron under
physiological conditions

4) Rabidly removed from the synaptic cleft by uptake or degradation

5) Presence of receptor on the post-synaptic neuron.

6) Binding to the receptor elicits a biological response

R.E.B, 4MedStudents.com, 2003

the experimental situation described above characteristically do not lead to the formation of a propagated action potential in the postsynaptic neuron. Instead, the stimulation produces either a transient, partial depolarization or a transient hyperpolarization.

to enter than K+ to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated.

If K+ or chlorine ion (Cl−) gates open (allowing K+ to exit or Cl− to enter), the membrane becomes more depolarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.

presynaptic neuron opens volage-gated Ca++ channels.
2. Ca++ influx into presynaptic term.
3. Ca++ acts as intracellular messenger
stimulating synaptic vesicles to fuse with
membrane and release NT via exocytosis.
4. Ca++ removed from synaptic knob by
mitochondria or calcium-pumps.
5. NT diffuses across synaptic cleft and
binds to receptor on postsynaptic membran
6. Receptor changes shape of ion channel
opening it and changing membrane potential
7. NT is quickly destroyed by enzymes or
taken back up by astrocytes or presynaptic
membrane.
Note: For each nerve impulse reaching the presynaptic terminal, about 300 vesicles are emptied into the cleft. Each vesicle contains about 3000 molecules.

released
The amount of time the neurotransmitter is bound to receptors
The two types of postsynaptic potentials are:
EPSP – excitatory postsynaptic potentials
IPSP – inhibitory postsynaptic potentials

Postsynaptic Potentials

to become more permeable to potassium and chloride ions
Leaves the charge on the inner surface more negative (flow of K+ out of the cytosol makes the interior more negative relative to the exterior of the membrane
Reduces the postsynaptic neuron’s ability to produce an action potential

Inhibitory Synapses

current to flow between adjacent cells. Connexons: protein tubes in cell membrane.
Rare in CNS or PNS
Found in cardiac muscle and many types of smooth muscle. Action potential of one cell causes action potential in next cell, almost as if the tissue were one cell.
Important where contractile activity among a group of cells important.

point of contact)

The narrow gap between the two neurons at the synapse is the synaptic cleft; the cleft is filled with extracellular fluid and spans an area of approximately 20 nm

A neuron that conducts impulses toward a synapse is called a pre-synaptic neuron

A neuron that conducts impulses away from a synapse is called a post-synaptic neuron

on its dendrites and cell body

synaptic knob and depolarises
the pre-synaptic membrane

Events at the synapse

Voltage-gated calcium ion
channels open in the
pre-synaptic membrane

Calcium ions diffuse
into the synaptic knob

synaptic knob and depolarises
the pre-synaptic membrane

Voltage-gated calcium
channels open in the
pre-synaptic membrane

Calcium ions diffuse
into the synaptic knob

Events at the synapse

The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane

membrane

Calcium ions diffuse
into the synaptic knob

The uptake of calcium ions triggers the fusion of the synaptic vesicles with the pre-synaptic membrane

Events at the synapse

Neurotransmitter diffuses across the cleft and binds to specific protein receptors
embedded in the post-synaptic membrane

specific protein receptors
embedded in the post-synaptic membrane

Binding of neurotransmitter opens Na+ gates in the membrane and there is an influx of Na+ into the post-synaptic neuron

An excitatory post-synaptic potential (EPSP) builds up across the membrane and if this reaches threshold, an action potential is triggered in the post-synaptic neuron

which is located at the surface of the post-synaptic membrane

Following activation of the post-synaptic membrane, neurotransmitter is removed from the synaptic cleft to enable further stimulation to occur

The neurotransmitter, noradrenaline, is actively transported back into the axon terminals

stored and released only on the pre-synaptic side of the synaptic cleft

This arrangement allows for the transmission of impulses between neurons in one direction only

Receptors for neurotransmitter are only located on the post-synaptic membrane

of the post-synaptic membrane

During hyperpolarisation, the post-synaptic membrane potential becomes more negative than its resting potential and results from either the efflux of positive charge or the influx of negative charge

The nature of the neurotransmitter determines the response of the post-synaptic membrane

the other neurons. Another neuron may excite several neurons. But if from both neurons which is divergented excitement will be simultaneously the total quantity of excited neurons will be decrease.

of axons of inhibition neurons form synaptic connection with neighboring excitive cells in these cells develop lateral inhibition

synapses on a dendrite.
Two or more presynaptic inputs are active at the same time
A space (spatial) dependent process.
Occurs in a Convergent Synapse

if they occur in rapid succession, within 1-15 msec of one another.
The same presynaptic fiber fires AP in quick succession
A Time (Temporal) dependent process
Occurs in a Divergent Synapse