One-sided impulse (excitement) conduction.



(BELL-MAGENDIE LAW)

During any reflex activity, the impulses are transmitted in only one direction through the reflex arc as per Bell - Magendie law. The impulses pass from receptors to the center and then from center to effector organ. It is caused by synapse structure: mediator is released only in presynaptic ending.

2. Excitement transduction lack. Excitement transmission retardation depends on synapses and associative neurons quantity.

3. Summation is accumulation (summing) effects of subliminal stimuli; one subliminal stimulus does not give any answer reaction because presynaptic ending releases too few mediator. It has been first described by Russian physiologist Ivan Sechenov in 1863.

There are two main summation types:

a) temporary (consequent) - when one nerve fiber is stimulated repeatedly with subliminal stimuli frequently, these stimuli are summed up to give response in the muscle; as a summation result sufficient mediator quantity is released and answer reflectory reaction occurs; example: sneezing reflex occurs at stimulus prolonged action to nasal mucosa receptors;

b) spatial - when two afferent nerve fibers supplying a muscle are stimulated separately with subliminal stimulus, there is no response; but, if both the nerve fibers are stimulated together with stimulus of same strength, the muscle contracts - this is called spatial summation; with other words, space summation occurs at simultaneous irritation of different sensory nerves conducting excitement in one and the same nervous center; its role is in fatigue liquidating (activity type changing); example: semitendineal muscle contraction reflectory contraction at simultaneous tibular and fibular nerves subliminal irritation because subliminal irritation of one nerves from them does not cause any contraction.

4. Excitement rhythm transformation. Nervous centers are able to transform frequency and rhythm of coming impulses 2 main types:

a) reducing -

– in synapses – 3-5 EPSP (exciting post-synaptic potentials) will give just 1 action potential;

– if impulses come in nervous center with a frequency which is bigger than this nervous center lability than this nervous center will answer with rate corresponding to its abilities id est with more rare impulsation;

b) increasing

– space summation is in the base of it;

– nervous center can answer with a series of impulses to a singular irritation coming outside.

5. Automatism. It is a distinguishing feature of vital nervous centers.

6. Chemothropy is selective high sensitivity to chemicals action. For instance, carbonic acid chemothropy to respiratory center.

7.Reflectory afteraction is expressed in following: answer reflectory reaction is present during some time after stimulus action stoppage. Afteraction duration can be more than stimulus duration in many times. There exists direct dependence: the stronger and more durable irritation acts to the receptor, the more durable is afteraction. This feature reasons are in following: trace depolarization, nervous impulses circulation (ring connection existence between neurons of a given center).

8. Tiredness – they belong to the structures with the biggest tiredness among all nervous system parts. It is determined by nervous center low lability. It is expressed in reflectory answer gradual decreasing and further stoppage in a case of prolonged stimulus action. It is a result of synaptic transmission disorder.

9. Rhythmical activity. All neurons can be divided into 2 groups:

a) “silent”:

– they are non-excited without irritation;

– they answer only to epiliminal stimuli;

– they answer only to stimulating actions;

b) “rhythmically active”:

– they are excited without stimulus;

– they react even to subliminal stimuli;

– they give reaction both to stimulating and to inhibitory actions.

Mechanism: ring connection between neurons.

Role:

– CNS sensitivity increasing to the stimulus;

– CNS functional possibilities expanding;

– CNS flexibility and plasticity providing.

 

INHIBITING IN CNS

Russian physiologist Ivan Sechenov discovered central inhibiting – chemical irritation of thalamus inhibits simple spine non-conditioned reactions. He was the first who underlined that inhibiting is active process. Inhibiting has been considered only as passive process coming after overexcitement before Sechenov.

Inhibiting (inhibition) is a special nervous process expressed externally in answer reaction weakening or complete disappearance. It is characterized by definite intensiveness and duration. Inhibition is an innate process which is improved in course of human onthogenesis.

Primary – it is not connected with excitement process and occurs with inhibitory cells (Ranshow cells) participation.

Secondary – appears without inhibitory neurons. It is nervous system overexcitement result.

Postsynaptic – EPSP formation. It is urgent but particular one.

Presynaptic or protective - it is developed the mostly often in axo-axonal synapses in brain stem and spine. Essence: presynaptic membrane hyperpolarization. Mediator – glycine. It protects from pathological, excessive, unnecessary information. It is non-urgent but more complete comparatively to the previous one.

Pessimal - It is central inhibition type. It appears at irritation high frequency. High rate of answer excitement occurs at first moment. Then stimulated central neuron comes in inhibiting state while working in such a regimen.

Recurrent – it is primary inhibition example. Essence: neuron activity inhibiting caused by recurrent axon collateral of this neuron.

Reciprocal –It belongs to postsynaptic inhibiting. This inhibiting can be belonged to nervous center co-ordination principles (see materials of next lesson). Expiration and inspiration centers are inhibited reciprocally in medulla oblongata, pressor and depressor cardiac-vascular centers. It is rather distinct at spine level at highly co-ordinated acts performance (walking, running, scretching et al.). At spinal segments the excitement of motoneurons causing muscles-flexors contraction is accompanied by reciprocal inhibiting of other motoneurons group leading to extensors relaxation. So, shortly, flexors excitement and parallel extensors inhibiting (and on the contrary) occurs. There are two explanation of spinal reciprocal inhibiting:

– impulses multiplication mechanism is switched on on the way from afferent fiber to muscle extensor motoneurons at muscle flexor motoneurons excitement; as a result – extensors motoneurons are receiving highly-rated impulsation leading to their pessimum state – so, inhibiting;

– inhibitory associative neurons synthesizing inhibitory mediator are switched on on the way to extensors motoneurons.

Lateral – activity neurons or receptors located near to excited neurons or receptors are interrupted. Lateral inhibiting mechanism provides sensory systems discriminative ability. So, it provides sounds rate determining in auditory sensory system; in visual one – increases significantly contrast of percepted image; in tactile one – encourages differentiating 2 points of touching. Lateral inhibiting is connected significantly with recurrent inhibiting mechanisms. It is also postsynaptic inhibiting.

 


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