Endocrine System: Hormones, Glands and Reflex Arcs Explained

History:

Nicolas Staba Cabaye, while walking, saw a beautiful spinous flower and cut it. Dropping his Cabaye, he directed it toward the flower and the "short" one. He quickly felt a strong stab, and checking his wound with gravity, he saw the Staba pole, where his Cabaye had disappeared. Very scared, and with heavy bleeding, he rapidly went home, where his mother disinfected the wound with mineral water.

"The first beautiful flower observed and the 'short' one..."

Nervous impulses, through the visual nervous system, reach the brain, which analyzes the information via a short trip to efferent muscles.

The osteo-arthro-muscular system: contraction is caused by electrical impulses in the muscle fibers.

Nicolas's muscles moved and he could cut.

Nervous movement: nervous pulses are generated by external or internal stimuli and travel through the spinal medulla's sensory root nerves to the posterior horns, posterior cords and lateral sensory fibers, which conduct them to nervous centers, forming sensory pathways.

The nervous pulses, which as stimuli originate a response, travel to the higher nervous centers. From there, the signal descends through the anterior cord fibers, passes the medulla's anterior horns and exits through the motor roots of the spinal nerves, forming motor pathways.

This fits our definition: in the story, Nicolas observed the beautiful flower through the optic pathway, which sent the information to the brain. From there, the signal went to the muscles that allowed Nicolas to move and "cut".

The muscles

The muscles are responsible for moving the organs and structures formed by bones and articulations.

There are visceral muscles (smooth involuntary muscles) and, forming the heart, also cardiac muscles. Skeletal muscles are attached to bones and are part of the locomotor apparatus.

In our case, Nicolas used mainly the muscles of his lower extremities to walk, bend his legs and raise them again to reach, and his upper extremities to stretch his arm, close his hand and contract his arm to cut. He also used his abdominal muscles to bend his trunk.

The facial muscles were used when he looked in all directions, and when he made a grimace of pain at the first prick with the thorn on his hand.

The muscles

"Let's go, we feel a strong 'stab'..."

The sensory information received by a receptor in the skin is transmitted by the sensory branch to the spinal medulla. Neurons in the gray matter of the medulla respond, which send the signal to the muscle through the motor branch of a nerve, causing contractions that allow the movements.

Reflex arc

Motor responses: spinal medulla receptors, sensory nerve

Brain motor responses: nerve

Voluntary vs. reflex acts

The spinal medulla is a center that can act independently of will.

Voluntary acts are directed by the brain, while reflex acts, or involuntary acts, are directed by the medulla.

In a reflex circuit, the arc has 5 recognized components:

• Receptor: the sensory organ or sensory structure excited by the stimulus.

• Sensory transmitter: conducts the impulse from the sensory nerve to the nervous center. This is called the sensory pathway, and as it leads toward the nervous center it is called the centripetal pathway.

• The nervous center or modulator: produces a response according to the signal received.

• Motor transmitter: carries the elaborated response from the nervous center to the motor nerve. This is the motor pathway, which leads away from the central nervous system; this is called the centrifugal pathway.

• Effector: the responding organ that executes the modulator's command, e.g., a muscle that produces movement or a gland that secretes. In the patellar reflex, the effector is the extensor muscle of the leg, located in the thigh.

Voluntary vs. reflex acts

"Very scared..."

Endocrine glands

Adrenal glands

The adrenal glands, also called suprarenal glands, are located on the upper part of the kidneys. Their removal leads to death.

The two glands are distinct structures:

• The cortex or adrenal cortex, which is the part indispensable for life.

The adrenal cortex secretes several hormones:

Aldosterone, which regulates the metabolism of minerals, water and salt.

Cortisol, which participates in regulating carbohydrate metabolism.

• The medulla produces the adrenal medulla hormone adrenaline, which has about the same action as the sympathetic autonomic nervous system.

In general, it increases the amount of sugar in the blood.

In our story, when Nicolas was frightened by the reflex, his Cabaye stabbed him and his wound was very deep. In this case, the adrenal medulla is activated, and the action is produced by a hormone called "adrenaline".

Mechanism of muscle contraction

The contraction of all muscle fibers approaches one of the insertion points, thus producing a movement. The effect of muscle contraction depends on the direction of the muscle fibers, which constrict their insertion points.

Contraction of annular muscles produces closure or diminution of the light of the openings or ducts they surround. The contraction of the orbicular muscles closes the eyes and lips; the orbicular opening closes the mouth.

Contraction of cutaneous facial muscles, whose insertions are in the facial skin, produces mimic movements. The facial expression muscles depend on which ones come into action.

Skeletal muscles contract, approaching the bones on which they insert.

The physiological stimulus excites nervous fibers, and thus shrinks the muscle fibers. When the muscle fiber is disconnected from the nervous stimulus, it is unable to contract.

Muscular work energy comes fundamentally from glucose. The chemical reactions that produce energy consume glucose in the muscle to give rise to mechanical work (muscle contraction) and heat production.

The structure of the circuit that has the reflex action in its place is called the reflex arc.