Lesson 9 Excitement processes investigation in cns. Inhibition processes investigation in cns.

1. The topic studied actuality.

Synapse is nervous system morphological unit while nervous center is a functional one.

Pharmacological agents allow influence on central inhibition process. Topic studied is important in clinical practice: it is possible well-singlemindedly and separately influence on corresponding nervous centers which is connected with these chemicals structure because they can coincide nervous centers corresponding mediators. Excitement developmental regularities knowledge, excitement afteraction give opportunity to understand memory, study process, striated and non-striated muscular fibers tone as well as the one of blood vessels walls and so on.

2.Study aims:

To know: information conductance mechanisms in central synapses, neuromediators and neuromodulators role; excitement development and summation mechanisms and their role in CNS integrative function; excitement conductance peculiarities through central synapses; different inhibition types developmental mechanisms and these processes role in CNS integrative function.

To be able to: draw schematically excitement conductance mechanisms through central synapse as well mechanisms of EPSP temporary and spatial summation; to draw schematically pre- and postsynaptic inhibiting as well as recurrent inhibiting neuronal mechanisms.

3. Pre-auditory self-work materials.

3.1.Basic knowledge, skills, experiences, necessary for study the topic:

Subject	To know	To be able to
Medical biophysics	Nervous impulse occurrence biophysical mechanisms explanation.	Manage the skills of work on electrical and medical devices
Histology	Neurons structure and functions; synapses and nervous centers structure	Draw chemical synapses structure
Biochemistry	Nervous tissue metabolism distinguishing features
Neurology	Nervous centers features and their possible disorders, representation about inhibition types, their significance and possible disorders.

3.2.Topic content.

NERVOUS CENTERS PHYSIOLOGY

Organism reflectory activity is defined by general features of nervous centres in more extent. In narrow context, nervous centre is synapse as itself and in wider aspect it is neurons complex located at different floors of CNS (this term is not anatomical, but physiological one). So, nervous center is multi-leveled structure. Main function of any nervous center is definite reflectory acts performance or managing one of organism functions.

Principle of dynamic functions localization - functional nervous centre may be localized into different anatomical (morphological) structure. For instance, respiratory center:

• medulla oblongata – working part;

• diaphragmal nerves center (C2-C4 spine segments);

• hypothalamus – respiration changing during emotional reactions (for instance, breathing acceleration at lovely person touching, kissing et al.);

• cortex – respiration adaptation to separate conditions (in singers, in highlands, in sea depth and so on).

Nervous centres hierarchy – separate sides of one organism function are managed by nervous centres localized at different levels of nervous system.

Nervous centres common features are the following:

1. 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.