3.1 Reflexes of the spinal cord. Determination of the reflex time and analysis of the reflex arc

The nervous system performs a complex role in the body. Its function ensures the interrelation of the organism with the surrounding environment. The functional unit of nervous activity is the reflex, by means of which the physiological functions of the organism are regulated. Reflex refers to the reaction of the body to stimulation, which is carried out through the central nervous system. The path through which excitation passes from the onset of stimulation to the response is called the reflex arc (figure). In a complete organism, all parts of the central nervous system participate in the implementation of the reflex reaction, the structural unit of which is a neuron with its processes.

The purpose of the lesson: get acquainted with the reflex arc and clarify the role of separate links of the reflex arc. Make sure that the reflex response occurs when the receptors are irritated, and the time of its realization depends on the strength of the irritant.

The following is necessary for work: frogs, preparatory set, tripod with clamp, water, 0.5% sulfuric acid solution, electrostimulator, filter paper.

 

Work 1 Spinal reflexes of the frog

Spinal reflexes include reflexes, the centers of which are located in the spinal cord. Depending on the location of the receptors they are divided into exteroceptive, proprioceptive and interoceptive reflexes. Spinal reflexes of the frog are studied on the spinal frog (with the removed brain) (Figure 2.9). The experiment is carried out 5 minutes after the removal of the brain, since decapitation is accompanied by depression of the spinal cord functions (spinal shock).

Work progress: The frog is wrapped in a cloth and the upper jaw is cut with scissors. The spinal frog is suspended from the lower jaw to the tripod. After 5 minutes, the frog calms down and experiments start. A filter paper soaked with 0.5% sulfuric acid solution is applied to the skin of the frog on different parts of the body: on the surface of the left, then right thigh, on the symmetrical parts of the forelimbs. After the stimulation, there is always a protective reflex with a contraction of the muscles of that limb, each surface of which has been irritated (Figure 3.1).

Each unconditioned reflex has its specialized meaning. The leg is rinsed with water and checked for other stimuli: electrical and mechanical. The frog's leg is irritated by induction current and squeezed by forceps.

Work 2 Determining the reflex time

The time of the reflex is the time from the moment of application of stimulation to the appearance of a response. The central time of the reflex is the time of excitation in the central nervous system. The greater it is, the more complex the reflex is, (the more intermediate neurons participate in its implementation, the more synaptic switching occurs). The time of the reflex depends on the strength of the stimulus.

Work progress: The spinal frog is suspended on a tripod. The hind leg is immersed up to the knee joint in a 0.1% solution of sulfuric acid (Figure 3.2).

The time from the moment of stimulation of the leg to the beginning of the reflex reaction is marked. The leg is washed, and after 3-5 minutes, as in the first case, the reflex time is determined when the leg is immersed in 0.5% solution of sulfuric acid. The data is compared and the dependence of the time of reflex on the strength of the stimulus is concluded.

 

Work 3 Reflection arc analysis

For the implementation of the reflex, the integrity of all links of the reflex arc is necessary (Figure 3.3). Turning off any of its links leads to the disappearance of the reflex.

Work progress: The experiment is carried out in a certain sequence. In the beginning, the shin of the leg is irritated by immersing it in a glass with 0.5% sulfuric acid solution and a reflex bending of the leg is obtained. After that, the leg is rinsed with water. Then, the skin is separated from the shin and legs, and the exposed leg is immersed in a glass with sulfuric acid of the same concentration as in the first case. There is no response, since the muscle does not have receptors that react to the acid. The skin of the thigh of the other hind leg of the same frog is cut, the muscles are pulled apart and the sciatic nerve is found and dissected.

A thread is put under the nerve, causing reflex flexion by stimulating the skin of the leg with a 0.5% solution of sulfuric acid. Then, a cotton wool soaked with novocaine is put under the nerve. After a while, the flexion reflex in the leg disappears as a result of a violation of the stimulation in the nerve using novocaine, which confirms the role of the conducting neural pathways in the reflex arc. After making sure that there are exteroceptive reflexes in the frog, the spinal cord is destroyed. In such frogs with the integrity of the receptors and conductive pathways, the reflex can not be induced. This determines the value of the central nervous system in reflex reactions.

 

Control questions

 

1. The importance of the central nervous system in the processes of control and communication in the living body?
2. The concept of reflex. Classification of reflexes. The main components of the reflex arc?
3. Reflex theory, its development?
4. Time of the reflex, factors affecting the time of the reflex. The receptive field of the reflex?
5. Biological significance of the reflex?

 

3.2 Some properties of nerve centers

The nervous center is the aggregate of neurons in the central nervous system necessary for the implementation of a certain reflex during the regulation of a particular function. Nerve centers have a number of characteristic properties: irradiation of excitation, transformation, one-sided excitation, inhibition, aftereffect, sequential and spatial summation, relief, protraction, domination, torpidity, tonicity, fatigue.

The purpose of the lesson: establish certain properties of nerve centers; irradiation of excitation, sequential and spatial summation, correlation

The following is necessary for work: frogs, preparatory set, a set of tools, cloth, plaque for fixing a frog. A tripod with a clamp, a 0.5% solution of sulfuric acid, an electrostimulator, a filter paper.

 

Work 1 Properties of nerve centers

Work progress: а) Sequential summation. The experiment is conducted on a thalamic frog suspended from a tripod by its lower jaw. Two small needles connected to electrodes of electrostimulator for single stimulations are stuck into the leg in the region of the membrane. By means of single strokes, a threshold is found for direct stimulation of the nerves and muscles, during which only the tip of the leg shudders, but there is no reflex. 5-6 consecutively alternating stimuli are used with the same current strength, and then the flexion reflex is observed. This is because weak excitations, caused by subthreshold stimuli, are accumulate in the nerve centers

1. b) Spatial summation. The experiment is conducted on the same frog. Additionally to the first electrodes, the second pair of electrodes of the electrostimulator is punctured into the shin of the leg. First, irritations are applied through one pair of electrodes and the minimum number of strokes is set so that the reflex is caused. After 2-3 minutes, the same rhythm of irritation is used, but now the current alternates (by hand) between the two pais of electrodes. The latent period of reflexes becomes shorter.
2. c) Transformation of the rhythm of excitation. The frog's leg is irritated with strokes (80 times per minute). However, the reflex response is performed not in the form of individual shudders in this rhythm, but in the form of a long tetanic contraction, i.e. with a change in the rhythm of excitation.
3. d) Aftereffect. The indicator of excitation of a long center is the bending of the leg. As follows from experience, it lasts for a while after the cessation of irritation. Consequently, nerve centers have the property of a long-term effect.
4. e) Irradiation of excitement. The tip of the frog's hind leg is squeezed weakly with forceps; this causes slight contraction of the muscle of the hindlimb. When the strength of the mechanical stimulus increases, a stronger bending of the leg is observed (due to the irradiation of the excitation), a larger number of muscles of the same limb is involved in the contractile process. With a further increase in the strength of the stimulus, all the four limbs of the frog contractdue to excitation irradiation. Irradiation of excitation can also be caused by the use of a strong chemical stimulus. A spinal frog is suspended on a tripod, and a filter paper soaked with a 0.5% solution of sulfuric acid is placed to the right side of the frog. The right leg is raised in order to reset the stimulus. The leg is held in that position, and after a while, the left leg bends. This is because the excitation, coming to the central nervous system, irradiates to the motor neurons of the leg of the opposite side.
5. f) Dominant excitation. To obtain a grasping reflex, a male frog is chosen. The spinal frog is fixed in the tripod by the lower jaw. A finger is put against the breast of the frog. The frog grasps it with front limbs. The irritation of the hind limb is stimulated using forceps; as a result the reflex of the grasp increases.

Control questions

 

1. What is a nerve center?
2. List the main properties of nerve centers?
3. What are the different types of summation?
4. Synapse as a functionally organized contact between neurons and effector cells. Classification of synapses?
5. Stages and mechanisms of synaptic transmission. Synthesis of the mediator. Secretion of the mediator. Mediator interactions with postsynaptic membrane receptors?
6. Electrical theory of excitation transfer. Synapse properties?
7. Ways of regulation of synaptic transmission. Synaptic transmission in the brain?

 

3.3 The phenomena of inhibition in the central nervous system

Function of the nervous system is accompanied by one of the main processes - excitement. But along with this process, there is inhibition in the nerve centers, which by its nature is an active process. Inhibition is a nerve process that weakens existing activity, or prevents it from occurring.

The purpose of the lesson: show the presence of an inhibition process in the central nervous system and observe that when the two receptive fields are stimulated simultaneously, there is an inhibition process in the central nervous system.

The following is necessary for work: frog, cloth, preparatory set, tripod with clamp, cotton wool, Ringer's solution, 0.5% sulfuric acid solution, salt crystals, filter paper, scalpel.

                                        

Work 1 Studies of central inhibition - the “Sechenov inhibition” experiment

The phenomena of central inhibition were first observed by I.M. Sechenov in 1862. He found that when the frog's optic thalamus is irritated, the inhibition of spinal cord reflexes is observed.

Work progress: The frog is wrapped in a cloth so that the head remains free. The skin is removed from the head; the bones are dissected from the skull and removed. Bleeding is stopped with a tampon. Carefully, without damaging the brain substance, the opic thalamus is exposed. A cross section of the brain  is carefully made in front of the opic thalamus (superior colliculus of the midbrain) on the posterior edge of the cerebral hemispheres using an acute scalpel and the large hemispheres are removed (Figure 3.3).

The frog is suspended from a tripod by the lower jaw. After waiting 2-3 minutes, the reflex time is determined, immersing the hind leg in a 0.5% solution of sulfuric acid. After the reflex the leg is washed with water. After drying the surface of the incision with a tampon, a crystal of table salt is placed on the optic thalamus and every 2 minutes it is checked for a reflex. Note that the time is noticeably lengthened. As soon as the inhibition process appears, the salt crystal is removed; the optic thalamus area is washed with Ringer's solution and the reflex time, which usually returnes to the original value, is determined again. Then the spinal cord is cut and the speed of the reflex is determined.

 

Work 2. Mutual inhibition of spinal reflexes

The inhibition of spinal reflexes can also occur during simultaneous strong stimulation of the receptive areas of two reflexes

Work progress: A spinal frog is prepared and suspended from a tripod. After 2-3 minutes the experiment starts. The hind leg of the frog is immersed in a 0.5% solution of sulfuric acid and the flexion reflex is observed. With the help of a stopwatch or a metronome the time of the reflex is determined. Then, while immersing the leg in the sulfuric acid, the other leg is simultaneously squeezed with forceps - the bending reflex either does not occur or the reflex time is very prolonged.

 

Work 3. Goltz's inhibition experiment

Work progress: A male frog is used for the experiment; the forebrain is removed, cutting off the head behind the eyes. The frog should be taken with two fingers from the side surface of the back. This irritation causes a croak reflex. The front limbs are clamped with forceps - the croak reflex is inhibited.

 

Control questions

 

1. Nerve centers and their properties?
2. The discovery of the phenomenon of central inhibition by I.M. Sechenov (1862)?

• Types of inhibition in the central nervous system?
• The role of excitation and inhibition in the coordination of the reflex activity of the CNS?

3. Physiology of the medulla oblongata, its structure and functions?
4. Physiology of the diencephalon and its functions?

 

3.4 Pozotonic reflexes. The role of the cerebellum in the regulation of motion

Animals that are at rest are characterized by a certain characteristic pose. The posture in animals is formed by special pozotonic reflexes (reflexes of the body orientation). In the presence of pozotonic reflexes, an important role belongs to the receptor-sensing receptor of 3 groups:

1) Proprioceptors of muscles and tendons, which are excited by the tension created by the weight and activity of the muscles.

2) Exteroceptors of the skin, which, when perceiving various irritations (mechanical, chemical, temperature), are excited asymmetrically in an abnormal position and thereby makes the body to react by restoring the normal position of the body in space.

3) Specialized receptors of the inner ear (otolith device of the threshold, receptive device of semicircular canals).

All positonic reflexes are divided into the following three groups:

1) static reflexes, which are characterized by a certain distribution of the tonus in different groups and form the usual resting position of the animal. They can also be called tonic.

2) adjusting reflexes, which provide the restoration of the usual posture in animals, when due to some circumstances the normal posture was broken.

3) statokinetic reflexes that arise only in the situation of moving the body in space with the acceleration.

The cerebellum, which is associated with all the departments of the central neural system, participates in coordination of complex movements of the animal.

The purpose of the lesson: study pozotonic reflexes, observe the distonia of skeletal muscles and a disorder of coordination of movements during damage or removal of a cerebellum.

The following is necessary for work: frogs, rabbit, preparatory sets, cotton tampons, a bowl with water.

 

Work 2. Observation of frog movement after unilateral removal of the cerebellum

Work progress: The brain is exposed and the cerebellum is found; in frogs' cerebellum is poorly developed and is represented by a narrow strip near the anterior edge of the medulla oblongata. Using a sharp scalpel, the cerebellum is cut along the middle line and the half is cut out. After 5 minutes after the operation, the behavior of the frog is observed (Figure 3.4). In this case, the frog sits, tilting its head and bending the corpus toward the damage. During the jump, it can turn in the air. Then the frog is put in the water and its inability to swim is observed.

Control questions

 

1. The concept of muscle tonus and its components?
2. The mechanism of regulation of the muscle-antagonists tonus at the spinal cord level?

3. Physiology of the midbrain?
4. The role of the red nucleus and Deiters' nucleus in these reflexes?

5.  Physiology of the cerebellum and its functions?