There are different kinds of neurons in the nervous system. The efferent neuron has functions in generating and conducting an impulse to the next neuron or the effector organ, while afferent neurons pass the signals from the sensory organs to the nervous system. Interneurons pass information to the other nerve cells.

The skeletal system has several functions:
1. The skeleton provides the shape of the human body.
2. It has a great role in the movement of the human body along with the nervous and skeletal systems.
3. Bones also provide us protection. The perfect example is the thorax, which shields the lungs and the mediastinal structures.
4. The bones are also depo of the calcium and other ions. When the calcium blood level is low, the parathyroid gland secretes PTH which activates osteoclasts to dissolve bone tissue. That brings the calcium level back to normal.
5. The blood cells are produced in the bone marrow.

There are different types of muscle cells – skeletal, smooth and cardiac muscle. Skeletal muscle tissue is found in the skeletal muscles of the human body. The control over them is completely voluntary. Smooth muscle cells are found in the walls of internal organs and blood vessels, they are controlled by the autonomic nervous system. Cardiac tissue is present in the heart muscle. It is also controlled by the autonomic nervous system but it has its own electrical conduction mechanism.

The integumentary system has several functions. It provides a mechanical barrier from infectious agents and protects our body from injuries. The skin has sweat glands, that can remove waste and participate in the regulation of body temperature at the same time. The skin is a large sensory organ that constantly sends information to the central nervous system. When it is exposed to the sunlight, it can also produce vitamin D.

Endocrine glands sends messages to other organs by hormones. They secrete hormones in circulation, where they are attached to the plasma proteins. Then they travel to the target cells. The receptors for steroid hormones are located in the nucleus of the target cells, while receptors for the amino-acid hormones are located on the cell membrane surface.

The onset of puberty is within 9 to 14 years of age in males and between 8 to 13 years of age in females. It begins with the secretion of GnRH by the hypothalamus. The gonadotropin-releasing hormone affects pituitary gland to secrete FSH and LH. These hormones regulate the function of reproductive organs and stimulate the production of testosterone in testicles, as well as estrogen and progesterone in ovaries. The puberty ends when secondary sexual characteristics occur. At this point begin the period of adolescence.

The afferent neurons pass the sensory information to the central nervous system, where this information is processed. The signal now travels via efferent neurons from the brain for the voluntary, or spinal cord for reflex movements. When the impulse reaches the neuromuscular junction, it causes the release of neurotransmitter from the axon terminals. The neurotransmitter binds to the ion channels on the skeletal muscle, which induce depolarization and further contraction of the skeletal muscle fiber. Since one nerve innervates more than one muscle fiber, we can conclude that the signal from one efferent nerve can cause the skeletal muscle to contract and therefore to generate movement.

Endocrine and nervous systems are strongly connected through the hypothalamus and the pituitary gland. The hypothalamus is the part of the central nervous system, but this structure produces certain hormones which affect the secretion of the hormones of pituitary gland. Both of these systems pass the information and they are considered as regulatory systems. However, the nervous system causes the response from the effectory organs very fast, while the endocrine system needs more time but the reaction lasts longer.

Gastrulation and neurulation are phases in the embryo development. During gastrulation, three layers are formed – ectoderm, mesoderm, and endoderm. From these structures, all of the four tissues will develop. Neurulation represents the beginning of the nervous system development.