When Is an End Organ?

It is the terminal part of nerve fibers and is distributed in various organs and tissues. According to their functions, they are divided into sensory nerve endings and motor nerve endings.

[shén jng mò sho]

for

feel

Feeling (incoming)

Sensory nerve endings

(Encapsulate nerve endig) This kind of sensory nerve endings come in many forms and different sizes, but they all have connective tissue capsules outside the nerve endings. The common ones are:

(1) Tactile corpuscle: Also known as Meissner corpuscle, it is distributed in the dermal papilla of the skin, with the palms of the fingers and the bottom of the toes the most. The body is oval in shape, with a diameter of about 30 to 100 m. It is surrounded by connective tissue to form a capsule, and there are many horizontal flat tactile cells in it. Myelinated nerve fibers lose their myelin sheath at the capsule, penetrate into the capsule, and branches coil. The main function is to feel the sense of touch.

(2) Lamellar corpuscle: Also known as Pacinian corpuscle, this type of corpuscle is widely distributed and is found in deep dermis, subcutaneous tissue, mesentery, and connective tissue of the pancreas. The corpuscles are mostly circular or elliptical in shape, with different diameters of about 0.5 to 3 mm. The capsule of the body is a concentric circular layer of flat connective tissue cells and fibers, and the layers are filled with gel-like substances. The central axis of the body is a homogeneous cylinder. It is said that the inner bulb nerve fibers enter the inner stick after losing their myelin sheath, mainly for feeling pressure, vibration and tension.

(3) muscle spindle: slender spindle-shaped body widely distributed in the whole body skeletal muscle, about 2 to 5 mm long, with connective tissue capsules on the surface, which contains 3 to 10 thinner skeletal muscle fibers , Called intrafusal muscle fiber. There are two types of sensory nerve fibers entering the muscle spindle: one is a thick myelinated nerve fiber that loses myelin before the human muscle spindle, enters the muscle spindle in the middle of the muscle spindle, and branches repeatedly, showing a circular or spiral wrap It is wrapped around the nucleus-containing part of the inner spindle muscle; the other is a thin myelinated nerve fiber, which appears as a flower branch after losing the myelin sheath and is distributed at the two ends of the nerve endings. In addition, there is a thin motor nerve fiber in the muscle spindle, which comes from small neurons (gamma neurons) in the anterior horn of the spinal cord. The end forms a motor end plate and is distributed at both ends of the muscle fiber in the spindle. The muscle shuttle is located between the muscle fiber bundles. When the muscle contracts or stretches, the muscle fibers in the shuttle are stretched, which stimulates the nerve endings, generates nerve impulses, and transmits to the center to produce sensations. Therefore, the muscle shuttle is the body that senses the movement of the muscles and the position of the limbs. Receptor. The tendon spindle in the tendon is similar in structure to the muscle spindle.

Motor end plate neuron tip

(Motor end plate) It comes from the axonal ends of motor neurons in the anterior horn of the spinal cord or the brainstem. It reaches the myometrium of skeletal muscle fibers and loses its myelin sheath. It is then divided into claw-shaped thin branches, and its terminals swell on the surface of skeletal muscle fibers. An oval plate-shaped hump is formed with a diameter of 10 to 80 m. The mitochondria and spherical synaptic vesicles in the axonal end of the spherical terminal contain choline acetate. The axonal membrane of the spherical terminal is the presynaptic membrane, and the myometrium corresponding to the anterior membrane is the postsynaptic membrane. There are N receptors in the membrane. The postsynaptic membrane sags to form many deep grooves and wrinkles, increasing the synapse. Surface area of the rear membrane. The synaptic space between the presynaptic and posterior membranes is about 40-60 nm wide. It can be seen that the motor end plate is a neuromuscular junction. Acetylcholine released by synaptic vesicles acts on the N receptor of the post-synaptic membrane. It causes the myometrium to be excited and is transmitted to the entire muscle fibers through the transverse tubule system, causing the muscle fibers to contract. Each branch of the pre-charged motor neuron axon can dominate 100 to 2000 skeletal muscle fibers.

The axon of each motor neuron and the muscle fibers dominated by its branches are called a motor unit. The size of a motor unit is the number of fiber lines dominated by motor neurons. The number of motor units of each muscle in the human body is very different, usually the number of motor units of limb muscles is large; the number of small muscles such as hand and foot is small. For example, the human tibialis anterior muscle has 443 motor units and 562 muscle fibers per motor unit. The vermiform muscle of the hand has only 96 motor units, and the number of muscle fibers in each motor unit is 108. Muscle atrophy occurs when anterior horn motor neuron disease or peripheral nerve injury occurs, which will involve changes in the number of various types of muscle fibers and the units of each shoe.

Visceral motor nerve endings

(Visceral motor nerve ending) Visceral motor nerves are dominated by vegetative neurons, which usually pass through two neurons, with vegetative ganglia in the middle. The first neuron is called preganglionic neuron, the soma is located in the lateral horn of the spinal cord or the brainstem nucleus, and its axon is called the preganglionic fiber; the second neuron is called the postganglionic neuron, and the soma is located in the vegetative ganglia or plexus Inside, its axons are called postganglionic fibers. These postganglionic nerve fibers are distributed on the smooth muscle, heart muscle, and glandular cells of the viscera and blood vessels to form splanchnic motor nerve endings. These visceral efferent fibers are mostly myelinated nerve fibers with axons that are very thin and less than 1 m in diameter. The axon terminal structure is simple. After repeated branching, the terminal branch is beaded and swollen. Its enlarged end is called nerve ending nodule, which is attached to smooth muscle cells or glandular cells. The terminal branch is a bead-swelled part, called varicosity, with a diameter of 1 to 2 m. Bulk is the site where synapses are established with effector cells. The axon of a vegetative neuron is unbranched to form about 30,000 bulks. This structural form increases the location or surface area of neurons that release neurotransmitters. Under the electron microscope, it can be seen that the bulk contains many synaptic cisterns and mitochondria. The expanded axon membrane is a presynaptic membrane, and the corresponding effector cell membrane is a post-synaptic membrane, and there are neurotransmitter receptors on the membrane. The synaptic space is 15-20 nm. The gap between some bulges and effector cells is more than 80nm, even up to several micrometers, and there is no special structure of the post-synaptic membrane at the corresponding site, so this type of synapse is called distant synapse (distant synapse), or non-synaptic chemical transmission, produces effects in a diffuse form. The neurotransmitters of vegetative neurons are mainly choline acetate, norepinephrine and peptides. When nerve excitement sends nerve fibers along the viscera to the ends, it leads to the release of neurotransmitters, causing smooth muscle and myocardial contraction and glandular secretion. .