2. Excitement is carried out at neuromuscular joints.
3. The conduction of skeletal muscle cell action potential.
4. Excitation-contraction coupling of skeletal muscle cells.
5. Skeletal muscle myofilament sliding theory.
I. Excitement and excitability
1. Organisms have the ability to respond to stimuli, which is called excitability. Excitability is the most important physiological characteristic of neuromuscular. Excitability is the most important physiological characteristic of neuromuscular.
2. Nerve, muscle and gland cells have the highest excitability and are customarily called excitable cells.
3. After tissue cells are stimulated, they will produce a potential change, which can spread to both sides of the cell membrane. This potential is called action potential.
4. Excitability also refers to the ability of tissue cells to generate action potential when stimulated. Excitement is synonymous with generating action potential itself or action potential.
Second, the stimulating conditions that cause excitement.
Stimulation is the motivation to arouse the excitement of the organization. Experiments show that any stimulus must reach a certain stimulus intensity, last for a certain time, and have a certain intensity-time change rate, in order to arouse tissue excitement.
1. Threshold intensity and threshold stimulation.
Generally, the critical stimulation intensity that causes tissue excitement at a certain stimulation time and intensity-time change rate is called threshold intensity or threshold. The stimulus with this critical intensity is called threshold stimulus, the stimulus with intensity less than threshold is subthreshold stimulus, and the stimulus with intensity greater than threshold is suprathreshold stimulus.
2. Strength-time curve
Describing the relationship between stimulus intensity and the time required to excite tissue in rectangular coordinate system, we can get a curve, which is called intensity-time curve. The lowest or most basic threshold strength is called basic strength.
Third, the evaluation index of excitability
1. threshold intensity is the simplest index to evaluate tissue excitability. The relationship between excitability and threshold intensity is mutual.
2. chronaxie is the shortest action time required to stimulate the tissue with twice the basic intensity, just to arouse the excitement of the tissue. The relationship between excitability and duration is mutual.
Fourthly, the excitability changes in the process of recovery after excitement
Organizational excitability goes through four stages.
1. Absolute refractory period: a very short absolute refractory period occurs immediately after excitement, lasting about 0.3ms, and the excitement drops from the original level to zero, and no matter how strong the test stimulus is, it can not cause the second excitement;
2. Relative refractory period: then a relative refractory period lasting for 3ms appears, which shows that the excitability gradually rises, but it is still lower than the original level, and it needs stimulation above the normal threshold to arouse the excitement;
3. Abnormal period: the second is abnormal period, about 12ms, the excitability is higher than the original level, and the stimulation below the normal threshold can also cause the second excitement;
4. Sub-normal period: then there is a period of 70ms, and finally the excitability returns to the original level.
Five, the bioelectric phenomenon of neuromuscular cells
1. Rest potential and action potential.
(1) resting potential is the potential difference between inside and outside the cell membrane at rest, which is called resting potential. The resting potentials of mammalian nerves and muscles are -70~-90mV.
(2) When the action potential is given to the nerve axon for effective stimulation, the potential difference inside and outside the membrane rapidly decreases or even disappears, and then becomes negative outside the positive membrane. This rapid and short-lived fluctuation of membrane potential is called action potential.
2. Mechanism of resting potential and action potential.
At present, there is sufficient evidence about the mechanism of membrane potential, and Hodgkin ion theory is accepted by most scholars. According to this theory, bioelectricity depends on the inhomogeneity of ion distribution on both sides of the cell, the permeability of the membrane to ions and its changes under different conditions, and the direct cause of membrane potential is the movement of ions across the membrane.
When explaining the formation of resting potential and action potential, the ion channels of the membrane are mentioned. Modern physiological research shows that the so-called ion channel of the membrane is actually a specific protein embedded in the plasma bilayer of the cell membrane.
3. Action potential conduction
Between the excited part and the adjacent non-excited part, the potential difference will generate local current, which will make the adjacent non-excited part generate action potential and push it to the two sides in turn for conduction.
4. local excitement
If the intensity of the stimulus is less than the threshold, it can not cause the transmissible action potential, but it is limited to the local range of the stimulus, so it is called local response or local excitement.
Sixth, the transmission of neuromuscular junction excitement.
Nerve and muscle are completely different tissues, and there is no axonal connection between them. Why is excitement transmitted from nerve to muscle? A large number of studies have confirmed that this excitement is transmitted through neuromuscular joint devices.
1. Structure of neuromuscular joint
Neuromuscular joint refers to the device that motor nerve endings transmit information near skeletal muscle.
The structure of neuromuscular junction includes three parts:
(1) The anterior membrane of the connector is the thickened part of the axonal membrane of God. Axle pulp contains a large number of vesicles containing acetylcholine with a diameter of about 50 nm.
(2) Posterior articular membrane, that is, the part of muscle cell membrane corresponding to nerve axon membrane, is also called motor endplate. There are acetylcholine receptors on the motor endplate, which can specifically bind acetylcholine.
(3) Joint space refers to the space between nerve endings and muscle cell membranes.
2. Transmission mechanism of neuromuscular junction excitation.
When motor neurons are excited, nerve impulses propagate along nerve fibers to axonal endings and stimulate the anterior membrane of connectors. The depolarization of the anterior membrane of the connector opens the Ca 2+ channel on the membrane, and part of Ca 2+ in the extracellular fluid enters the anterior membrane of the connector, triggering the vesicles in the axoneme to approach the anterior membrane of the connector, and releasing acetylcholine into the gap of the connector after fusion with the anterior membrane. When acetylcholine diffuses to the posterior membrane of the linker, it binds to the specific receptor on the posterior membrane of the linker, causing the permeability of the posterior membrane to Na+ and K+ plasma to change, and the posterior membrane of the linker depolarizes to form endplate potential, which is then caused by local current. In addition, acetylcholine in the gap is hydrolyzed by cholinesterase on the endplate membrane and inactivated, maintaining the normal transmission function of neuromuscular junction.
2. Mechanism characteristics of excitation transmission in neuromuscular junction;
(1) chemical transfer (2) excitability transfer is 1 to 1 (3) unidirectional transfer (4) time delay (5) high sensitivity.
Master knowledge points
Ultrastructure, transmission mechanism and characteristics of neuromuscular junction of resting potential and action potential in absolute refractory period and relative refractory period
The principle of muscle contraction in the second quarter
Teaching essentials
First of all, the microstructure of muscle fibers
Each muscle fiber is wrapped by a membrane, which is called the sarcolemma. There is sarcoplasm in the sarcolemma, and it is also full of myofibrils arranged in parallel and complex myotube system. Fibrillation and muscle system are the heaviest structures to realize muscle contraction.
1. myofibril
The area between two adjacent Z lines of myofibrils is sarcomere, which is the most basic unit of muscle contraction and relaxation. The sarcomere consists of thin and parallel thick myofilaments and thin filament.
2. Muscle tube system
The myotube system refers to the membrane-like tubular structure wrapped around each myofibril. Realize the storage, release and reunion of Ca 2+ during muscle activity.
Second, the mechanism of muscle contraction
Molecular composition of 1. myofilament
Coarse myofilaments are mainly composed of myosin (also known as myosin) molecules. The spherical head of the molecule forms a so-called transverse bridge. The transverse bridge has two important functional characteristics: first, it has a site that can bind to adenosine triphosphate (ATP) and has AIP enzyme activity, but this enzyme is activated only when the transverse bridge is connected to the cell filament; Secondly, under certain conditions, the transverse bridge can have a reversible structure with the corresponding position of thin filament, and it will swing obliquely, pulling thin filament to slide to the middle of the thick muscle filament.
The filament consists of at least three protein molecules. An actin accounts for 60% of the protein of filament, which constitutes the main body of filament. Actin is directly related to myofilament sliding, and it has a reversible binding site with myosin, both of which are called contractile proteins. The other two are called tropomyosin and troponin, which play a regulatory role in myofilament sliding, so they are called regulatory proteins.
2. Muscle contraction process
Starting from the excitement of muscle cells, the process of muscle contraction should include three interrelated links: cell excitement triggers muscle contraction, that is, excitement-contraction coupling; The movement of the transverse bridge causes the myofilament to slide; And relaxation of contraction muscles.
(1) excitation-contraction coupling
Excitation-contraction coupling includes at least three steps: the action potential is transmitted to the depths of muscle cells through the transverse tube system; Triple pipeline structure transmits information; Release and accumulation of Ca 2+ by longitudinal pipeline system.
(2) The movement of the transverse bridge causes the myofilament to slide.
It is generally believed that the basic process of muscle contraction is that when the concentration of Ca 2+ in sarcoplasm increases, troponin with affinity for Ca 2+ on filaments binds enough Ca 2+, which causes the change of its molecular configuration. This change is transmitted to tropomyosin molecules, which changes the configuration of tropomyosin molecules. Results The double helix of tropomyosin molecule slipped from the groove edge of actin double helix structure to the groove bottom, the inhibitory factor was released, and the sites on actin that could bind to the transverse bridge were exposed. The transverse bridge combines with actin to form actin, which activates the activity of ATPase on the transverse bridge. With the participation of Mg ~ (2+), ATP on the transverse bridge decomposes and releases energy, and the transverse bridge gains energy, which tilts and swings towards the center of the thick myofilament, pulling the filament to slide towards the center of the thick myofilament. When the angle of the transverse bridge changes, the ATP binding site on the transverse bridge is exposed, new ATP is bound to the transverse bridge, and the transverse bridge dissociates from actin and returns to its original vertical position. Then the transverse bridge begins to combine with the next actin site, and the above process is repeated to further pull the filament to slide toward the center of the thick myofilament.
(3) Relaxation of contractile muscle: When the stimulation is terminated, the combination of Ca 2+ and troponin is eliminated, troponin returns to its original configuration, and then tropomyosin also returns to its original configuration, and the joint of actin and transverse bridge is covered up again, and the myofilament returns to its original position due to its own elasticity, so the contractile muscle relaxes.
Three, single contraction and tonic contraction
1. Single muscle twitch
After short-term stimulation, the whole muscle or a single muscle fiber first produces action potential, and then mechanical contraction occurs, which is called single muscle contraction. Muscle contraction is divided into three periods, namely incubation period, contraction period and relaxation period. The relaxation time is much longer than the contraction time, and the single contraction curve is asymmetric.
2. Tetanus
In the experiment, if a series of stimuli are given to muscles, a series of single contractions will occur as long as the interval between each stimulus is not shorter than the time required for a single contraction. If the frequency of stimulation is increased so that the interval between each stimulation is shorter than the duration of a single contraction, muscle contraction will merge, that is, the muscle cannot relax completely, which is called tonic contraction. There are two kinds of tension contraction. When the stimulation frequency increases, the second contraction occurs when the muscle is not fully relaxed, and partial fusion of muscle contraction occurs, which is called incomplete tetanus. On the other hand, if we continue to increase the frequency of stimulation, the muscles will begin to contract again at the end of the last contraction, and the muscle contraction reaction will be completely integrated, which is called complete tetanus. When the human body is engaged in various sports, its muscle contraction belongs to complete tension contraction, and the duration of tension contraction is controlled by impulses from nerves.
Master knowledge points
The sliding mechanism of muscle contraction is coupled with the excitement and contraction of muscle segment tension and contraction.
The form and mechanical characteristics of muscle contraction in the third quarter
Teaching essentials
First, the form of muscle contraction
The forms of muscle contraction are divided into three categories: shortened contraction, elongated contraction and isometric contraction.
1. Shorten contraction
Shortening contraction means that when the tension produced by muscle contraction is greater than the external resistance, the muscle shortens and pulls the bone lever to move in the opposite direction. When shortening the contraction, the starting point and the end point of the muscle are close, which is also called centripetal contraction. For example, when doing elbow flexion, leg running, swing arm spike and other sports, shortening contraction can be divided into unequal length contraction and constant speed contraction.
Constant velocity contraction is realized by special constant velocity load equipment. The tension produced by muscles in the whole joint range is always equal to the load, and muscles can contract at a uniform speed or have the same strength. During the maximum isokinetic contraction, the tension produced by muscles in the whole joint range is 100% of their ability.
Elongation and contraction
When the tension produced by muscle contraction is less than the external force, the muscle actively contracts but is elongated, which is called elongated contraction. Elongation and contraction play a role in braking, slowing down and overcoming gravity in human movement.
In the process of running, the hips and knees are bent by supporting the legs, so that the gluteus maximus and quadriceps femoris are stretched in advance, which creates conditions for the stretching of hips and knees when pushing back.
When the tension produced by muscle contraction is equal to the external force, the muscle actively contracts, but the length remains the same. This form of contraction is called isometric contraction. Isometric contraction is the basis of muscle static work, and human movement plays an important role in fixing, supporting and maintaining a certain posture of the body.
Second, the mechanical characteristics of muscle contraction
1. The effect of afterload on muscle contraction-the relationship between tension and speed
The load or resistance encountered when muscles begin to contract is called afterload. In a certain range, the tension produced by muscle contraction is roughly inversely proportional to the speed; When the afterload increases to a certain value, the tension can reach the maximum, but the contraction speed is zero, and the muscle can only contract in equal length. When the afterload is zero, the tension is zero theoretically and the muscle contraction speed reaches the maximum.
2. The effect of preload on muscle contraction-the relationship between length and tension.
Pre-load refers to the load applied to the muscle before contraction, which makes the muscle in a certain elongated state before contraction. Experiments show that the initial length of muscle contraction increases gradually, and the tension generated during muscle contraction also increases gradually. When the initial length continues to increase to a certain value, the tension can reach the maximum; If you continue to increase the length of muscle contraction, the tension will decrease and the contraction effect will be weakened. Generally, the initial length that causes the maximum muscle contraction tension is called the appropriate initial length.
The so-called appropriate initial length of muscle is generally considered that the appropriate initial length of human muscle is slightly longer than the "resting length" of muscle in the body.
Master knowledge points
Shortening shrinkage, isokinetic shrinkage, prolonged shrinkage, isometric shrinkage and appropriate initial length.
Effect of muscle connective tissue on muscle contraction
Teaching essentials
First, the composition of muscle connective tissue
The connective tissue of muscle includes tendons at both ends of muscle, endocardium, tunica fasciculata and epicardium inside muscle. Collagen is the most important component of connective tissue and exists in the form of collagen fibers.
Second, the influence of exercise on muscle connective tissue
1. Long-term exercise can improve the tensile strength and breaking strength of tendons.
According to the calculation, the tensile stress of tendon during exercise is about 350 ~ 420 kg cm-1. Experiments show that exercise training can improve the tensile stress of tendon, especially the bonding ability and strength between tendon and bone, so that tendon can bear greater tension.
2. Exercise can make the connective tissue in muscle hypertrophy.
Muscle overload training causes muscle hypertrophy, and the connective tissue in muscle also increases accordingly.