PROPERTIES OF SYNAPTIC TRANSMISSION 1 FORWARD CONDUCTION Impulses

PROPERTIES OF SYNAPTIC TRANSMISSION 1 -FORWARD CONDUCTION: Impulses are conducted only from presynaptic to postsynaptic neurons. 2 -SYNAPTIC DELAY: This is the time required for transmission of an impulse across a synapse. This time is needed for release of the neurotransmitter, its diffusion across the synaptic cleft, its combination with the postsynaptic receptors, Na+ influx to produce EPSP and summation of the EPSPs to the threshold value.

2 -SYNAPTIC DELAY n n n The minimal synaptic delay is about 0. 5 ms , and from it a certain reflex can be known whether it is monosynaptic or polysynaptic. Also the number of synapses in a polysynaptic reflex can be calculated as follows: a-The total reflex time is determined. b-The time needed for impulse conduction in the afferent and efferent neurons is calculated.

PROPERTIES OF SYNAPTIC TRANSMISSION n n n 2 -SYNAPTIC DELAY c-The difference between a&b represents the time of transmission of the impulses in the NS across the synapses this time is known as the central delay. If the central delay is about 0. 5 ms, this means that the reflex is monosynaptic, while if it is more than 1 ms it is means that the reflex is polysynaptic. (central delaysynaptic delay=number of synapses)

PROPERTIES OF SYNAPTIC TRANSMISSION n n n 3 -FATIGUE: Repetitive stimulation of presynaptic fiberes at a rapid rate , leads at first to maximal discharge from postsynaptic neurons then , with time, such disharge gradually decreases. This is known as fatigue of synaptic transmission, and it is due to exhaustion of the neurotransmitter stores in the presynaptic terminal knobs

4 -FACTORS AFFECTING SYNAPTIC TRANSMISSION n a-Effect of p. H : Alkalosis increases the excitability of neurons and synaptic transmission. A rise of p. H of arterial blood to 7. 8 leads to convulsions due to increased excitability of the cerebral neurons. In patients suffering epilepsy, slight alkalosis(hyperventilation) may precipitate an epileptic fit.

4 -FACTORS AFFECTING SYNAPTIC TRANSMISSION n n n Acidosis depresses neuronal activities. A fall of the arterial blood p. H to about 7(uremia or severe diabetes) often causes coma. b-Effect of hypoxia: Hypoxia decreases neuronal excitability and synaptic transmission.

4 -FACTORS AFFECTING SYNAPTIC TRANSMISSION n n c-Effect of drugs: 1 -Drugs increasing neuronal excitability &synaptic transmission: -Theophylline, and caffeine decrease threshold for excitation of the neurons. -Strychnine: This substance blocks the action of the inhibitory neurotransmitter glycine, they discharge repetieively leading to sever convulsions.

Effect of drugs n -LSD(lysergic acid diethylamide): This is a hallucinogenic agent which is serotonin agonist and produces its effects by activating certain serotonin receptors in the brain.

c-Effect of drugs 2 -Drugs decreasing neuronal excitability and synaptic transmission: -Antianxiety drugs: This include tranquilizers(diazepam’valium’)&hypnotics(drugs that induce sleep ‘barbiturates’)these drugs facilitate the effect of GABA. -Alcohol: This potentiates the effect of GABA. -Anaesthetic drugs: These are often volatile lipid soluble gases (ether) which dissolve in neuronal membranes making them less responsive to excitatory agents. n

THE REFLEXES n A reflex is the involuntary response of an organ to a stimulus. It is the physiological (functional)unit in neuronal activities , and together with hormones they regulate various body functions aiming at homeostasis.

THE REFLEXES n n CLASSIFICATION OF REFLEXES I-Local axon reflexes: These are responsible for functions in the GIT (e. g. secretion of gastrin hormone and intestinal juice, and production of peristaltic movements).

CLASSIFICATION OF REFLEXES n II-Conditioned reflexes: These are acquired reflexes that develop as aresult of training and previous experience, and they need an intact cerebral cortex. These reflexes play an important role in the secretion of various digestive juices&in the regulation of other functions e. g. the heart & respiratory rates can be changed as a result of conditioned reflexes.

CLASSIFICATION OF REFLEXES n n III-Unconditioned reflexes: These are inborn(inherent) reflexes that need neither training nor an intact C. C. Thet are classified into the following groups: A-Hypothlamic reflexes: These regulate the body temperature, the body water and the general metabolism through the hypothalamic control of endocrine glands.

III-Unconditioned reflexes n n B-Medbrain reflexes: These include the various postural reflexes that regulate the body equilibrium and some visual reflexes(e. g. the papillary light reflex. C-Medullary reflexes: These include various reflexes that regulate the H. R &A. B. P , the R. R&many digestive functions.

III-Unconditioned reflexes n n D-Spinal reflexes: These are the reflexes mediated by the spinal cord. They are simplest reflexes in the CNS and include superficial, deep&visceral reflexes. 1 -Superficial reflexes: These are initiated by stimulation of the skin receptors, and they include: -Planter reflex-withdrawal reflex-abdominal reflexes-corneal reflex-crossed extensor reflex 2 -Deep reflexes: These are initiated by stimulation of receptors in the muscles and tendons, and they include: Stretch reflex.

III-Unconditioned reflexes n n 3 -Visceral (autonomic) reflexes: Micturation reflexes-Defecation reflex-Erection reflex The center of the above 3 reflexes is the 2, 3&4 th sacral segments(sacral parasympathetic)-reflexes V. C or VD that occurs as a result of exposure to cold or heat.

ACCOMMODATION FOR NEAR VISION n 1 -An object at infinity (i. e. more than 6 m from the eye)emits parallel rays which are refracted by the cornea and lens to form a clear focused image on the retina without any effort of accommodation. under such condition the focus will be 24 mm behind the anterior surface of the cornea(on the retina)

ACCOMMODATION FOR NEAR VISION n n 2 -An object at a distance of 6 m infront of the eye will have its focus still on the retina. Again under such condition the eye needs no accommodation. 3 -An object at distance less than 6 m infront of the eye has its focus behind the retina. In this case accommodation for near vision must take plase to have a clear focused image on the retina.

ACCOMMODATION FOR NEAR VISION n So accommodation is the ability of the crystalline lens to increase its curvature and hence its refractive power. The refractive power of the crystalline lens of the eye can be voluntarily increased from 15 diopters to approximately 29 diopters in young children with a total accommodation of 14 diopters.

ACCOMMODATION FOR NEAR VISION n To do this, the shape of lens is changed from that of a moderately convex lens to that of a very convex lens.