How does the cerebral cortex overwrite the reflex?
The structure of the reflex arc. Reflex ring. Nervous system physiology
Each of us has said the phrase “I have a reflex” at least once in our lives, but few people understand what it says. Almost all of our lives are based on reflexes. In childhood, they help us survive into adulthood - to work effectively and stay healthy. Obeying reflexes we breathe, walk, eat and more.
Reflex - is the body's response to a stimulus carried out by the nervous system. They indicate the beginning or end of any activity: the movement of muscles, endocrine glands, changes in vascular tone. This enables you to quickly adapt to a changing environment. The importance of reflexes in human life is so great that even their partial elimination (removal during surgery, trauma, stroke, epilepsy), lead to permanent disability.
The study of the central and peripheral nervous systems involved in IP Pavlov and IM sections. They left a lot of information for future generations of doctors. So far we have not shared psychiatry and neurology, but after their appearance neurologist began to practice alone, gathering experience and analyzing it.
Types of reflexes
Worldwide reflexes are divided into conditional and unconditional. The first in human life and in the process arises are connected, for the most part, with what he did. Some of the skills acquired over time will disappear and a new one will take their place as appropriate in the circumstances. These include biking, dancing, playing musical instruments, crafting, driving a car, and much more. These reflexes are sometimes referred to as "dynamic stereotypes".
Unconscious reflexes, as set the same in all people, and we have had them since birth. They exist throughout life as they support our existence. People don't know that they have to breathe, reduce the heart muscle, hold your body in a certain posture in space, blink, sneeze, etc. This happens automatically because nature has taken care of us.
Classification of reflexes
There are several types of reflexes that reflect their function or indicate a level of perception. One can name a few of them.
Biological meaning was isolated reflexes:
- Reflexes determining body position (pozotonicheskie);
- Moving reflexes.
Depending on the location of the receptors that perceive the stimulus can be identified:
- exteroreceptors that are on the skin and mucous membranes;
- interoreceptors, located in the internal organs and blood vessels;
- proprioceptors perceiving irritation to muscles, joints and tendons.
Knowing the three classifications can be submitted to describe each reflex: it is acquired or innate, what function it performs and how to call it.
Flat reflex arc
For important neurologists to know the level to which a reflex closes. It helps to better identify the affected area and predict health problems. Distinguish spinal reflexes, motor neurons that are located in the spinal cord. They are responsible for body mechanics, muscle contraction, the work of the pelvic organs. Rising to a higher level - in the medulla, found bulbar centers that regulate the salivary glands, some facial muscles, breathing and heart function. Damage to this department is almost always fatal.
The midbrain closed mesencephalic reflexes. Basically it is the reflex arc of the cranial nerves. A distinction is also made in diencephalic reflexes of terminal neurons that are located in the diencephalon. And cortical reflexes that are controlled by the cerebral cortex. Usually she acquired skills.
Note that the structure of the reflex arc involved in the higher coordinating centers of the nervous system always includes the lower levels. That is the way corticospinal pass through intermediate, middle, elongated and in the spinal cord.
The physiology of the nervous system is arranged so that each reflex duplicates several arcs. This keeps the body functioning, even with injuries and illnesses.
The reflex arc
Reflex arc - a nerve impulse transmission path from the sensor body (receptor) for execution. Nervous reflex arc is made up of neurons and their processes that form a circuit. This concept was introduced into M.Hollom medicine in the mid-nineteenth century, but over time it is transformed into a "reflex ring." It was decided that this term would better reflect the processes occurring in the nervous system.
Physiology distinguishes monosynaptic, as well as two- and trehneyronnye arches, sometimes found polisinapticheskie reflexes, that is, including more than three neurons. The simplest arc consists of two neurons: the perceiver and the motor. The pulse enters the nerve site through the long process of the neuron, which in turn transmits it to the muscle. These reflexes are usually unconditional.
Departments reflex arc
The structure of the reflex arc consists of five departments.
The first - a receptor that receives the information. It can be located both on the body surface (skin, mucous membrane), and in its depth (retina, tendons, muscles). Morphologically, the receptor looks like a long process of neurons or a cluster of cells.
Second segment - sensitive nerve fibers that transmit excitation on an arc of a circle. The bodies of these neurons are located outside the central nervous system (CNS), in spinal locations. Their function is comparable to the arrow on the tracks. That is, the neurons distribute information that comes to them to different levels of the central nervous system.
Third segment - space switch sensitive to power fibers. For most, it's reflexes in the spinal cord, but some of the complex arcs pass right through the brain, such as protective, tentative, nutritional reflexes.
Fourth section presents motor supplies fiber that carries nerve impulses from the spinal cord or motor neurons to effector cells.
Last, the fifth division - a body, the activity reflex. Usually a muscle or gland, like the pupil, heart, sex or salivary gland.
The physiological properties of the nerve centers
The physiology of the nervous system is variable at different levels. The department formed later, the more difficult its work and hormonal regulation will be. There are six properties that are common to all nerve centers, regardless of topography:
Conducting excitation only from the receptor to the effector neurons. Physiologically, this is because the synapses (connecting points of neurons) only work in one direction and cannot change it.
Delay in nerve excitation is also linked to the large number of neurons in the arch and, consequently, the synapses. In order to synthesize a neurotransmitter (chemical stimulus), let yourself in the synaptic cleft and spend so exciting, it takes more time than if the impulses only propagate along the nerve fibers.
Summing up excitements. This happens when the stimulus is weak, but rather constantly and rhythmically repetitive. In this case, the mediator accumulates in the synaptic membrane until it is not a significant amount, and then transmits an impulse. The simplest example of this phenomenon - the act of sneezing.
The transformation of the excitation frequency. The structure of the reflex arc, as well as functions of the nervous system, so that even in the slow rhythm of the stimulus it hits part of the dynamic - 50-200 times per second. Therefore, muscles in the human body are reduced in a more tetanic manner, that is, discontinuously.
Reflex aftermath. The neurons of the reflex arc are in an excited state for some time after the stimulus has ceased. For this reason there are two theories. The first states that the nerve cells to transmit the excitation of a second longer than the stimulus and to lengthen it, creating reflex. The second is based on the reflex ring that is closed between two intermediate neurons. They transmit excitation as long as one of them cannot generate the pulse or until no external brake warning signal can be generated.
Drowning nerve centers occurs with prolonged stimulation of the receptors. This is first manifested in decline, and then completely lacking sensitivity.
Autonomic reflex arc
Of nervous system type that implements the excitation and carries out nerve impulse, isolated, somatic and autonomic nerve arc. A distinctive feature is that the reflex on the skeletal muscle is not interrupted, and the autonomic ganglia are required to switch. All nerve centers can be divided into three groups:
- Vertebral (spinal) ganglia become the sympathetic nervous system. They are on either side of the spine that form pillars.
- Prespinal nodes are at a distance and from the spine, and are removed by the authorities. These include the ciliary ganglion, cervical sympathetic nodes, the solar plexus, and mesenteric nodes.
- Intraorganic nodes are not difficult to guess, are located in the internal organs: heart muscle, bronchial, intestinal tube, the endocrine glands.
These differences between the somatic and autonomic systems are deeply rooted in phylogeny and are linked to the speed of propagation of the reflexes and their necessity.
Realization of reflex
On the outside of the reflex arc there is receptor stimulation, which causes excitation and the appearance of a nerve impulse. The basis of this procedure is the change in the concentration of calcium and sodium ions, which are on both sides of the cell membrane. Changing the number of anions and cations leads to a shift in the electrical potential and the occurrence of a discharge.
From receptor stimulation occurs centripetal movement of the afferent connection reflex arc - cerebrospinal node. Sprout it comes to the sensitive nuclei of the spinal cord, and then switches to the motor neurons. This is the central connection of the reflex. Processes motor nuclei of the spinal cord are located together with other roots and sent to the appropriate actuators. The thicker muscle fibers end up in motor plaque.
The baud rate impulse depends on the type of nerve fiber and can be from 0.5 to 100 meters per second. Excitation is not transferred to neighboring nerves due to the presence of shells separating them from one another.
Meaning of reflex inhibition
Since the nerve fiber is able to keep stirring for a long time, braking is an important adaptive mechanism of the organism. Thanks to him, the nerve cells do not experience constant fatigue and overstimulation. Contact afferentation, being realized and braking, is involved in the formation of the conditioned reflexes and eliminates the need for the central nervous system to analyze the secondary tasks. This provides coordination reflexes, for example movements.
Feedback afferentation also prevents nerve impulses from spreading to other nervous system structures while maintaining their performance.
Coordination of the work of the nervous system
In a healthy person, the organs all work harmoniously and in concert. They are subject to a uniform system of coordination. The structure of the reflex arc - this is a special case that confirms a single rule. As with any other system, in humans, there are a number of principles or laws by which it operates:
- Convergence (impulses from different sections can come to a section of the CNS);
- Irradiation (prolonged and strong irritation causes excitation of neighboring sides);
- Reciprocity (inhibiting some other reflexes);
- a common end segment (based on the discrepancy between number of afferent neurons and efferent ones);
- Feedback (self-regulation system, based on the number of pulses received and generated);
- Dominant (the presence of the center of gravity of the suggestion that overlaps the other).
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