.................. THE ROLE OF RELATION I. THE HUMAN NERVOUS SYSTEM

Activities to do. Read the explanations on the nerve cells and perform the first Multiple Choice Test. Then read the explanation of the human nervous system and perform the second Multiple Choice Test, the Mute Picture, the Connect pictures with names and the Crossword.

 
1 . The role ot relation. Is a function is based on capturing the variations in the environment (called stimuli), its evaluation and issuing the appropriate responses.

2 . Systems and organs involved in the function of relation. They are:

2.1) The receptors. They are called the "organs of sense". They are an isolated organs with a close contact with the nervous system. For example the eyes, nose, tongue, etc.

2.2) The nervous system. It is the system that basically consists of nervous tissue, which is a tissue made up of nerve cells or neurons.

2.3) The endocrine system. It is formed by the endocrine glands, ie glands that secrete chemicals (hormones) into the blood, which cause specific effects on cells.

2.4) The effectors. They are the organs and systems responsible for conducting the responces. They can be of two types: the movements and the secretions. The types of effectors are:

• The locomotor system. It is composed of the skeletal system and the muscular system. They are responsible for performing the movements.
• The exocrine glands. Are the glands that secrete substances to the outside, such as sweat glands, or to the inside of the digestive tube, such as gastric glands.

3 .The neurons and the nerve impulse transmission. The neurons are cells specialized in the transmission of information due to the fact that its membrane can generate weak electrical currents that move from one end to the other, called the nerve impulse. The neurons that conduct nerve impulses to the central nervous system are called sensory, and that which lead the nervous impulse from the central nervous system to muscles and glands are called motor.

The motor neurons present cell body (soma) in which lay the nucleus and organelles, a long extension called axon and numerous small extensions called dendrites. The sensory neurons present a cell body and two axons.

The axon is also called the nerve fiber.May be covered by a number of cells that form the myelin sheath , which has a white color. These axon bundles form the white matter of the nervous system. The cell bodies and the axons without the myelin sheath are called gray matter.

The neurons are connected to each other without touching (this is called a synapse). The receptors in the neuron stimulate the nerve impulse to move through the axon to the synaptic button, where it causes the generation of a small synaptic vesicles which contain substances called neurotransmitters, which cross the synaptic fissure and are captured by the dendrites of the next neuron , generating insode of it a new electric current, and successively, up to the organs effectors. This is called the nerve impulse transmission.

5 . The human nervous system. The nervous system has two parts, the central nervous system (CNS) and the peripheral nervous system (PNS).

5.1) The Central Nervous System (CNS). Is formed by the brain and the spinal cord. Both of those organs are protected by bones (skull and vertebral column respectively) and covered with three protective membranes, called meninges. There is the absorbing liquid, called the cerebrospinal fluid, which can be found between the innermost membrane and the next one. The CNS is responsible for receiving and interpreting sensory impulses and for generating motor impulses .

5.2) The Peripheral Nervous System (PNS). It is a set of nerves that connects the central nervous system (the brain and the spinal cord) with the various parts of the body. The nerves are wire-shaped structures formed by axon bundles of many neurons. The most thicker ones present the protective outer membrane. It is then a structure similar to the domestic power lines. The nerves can be classified according to three criteria:

1) Depending on the direction in which the nerve impulses are transmited. There are three different types of nerves: the sensory nerves (which lead the nerve impulse to the central nervous system), the motor nerves (which lead the nerve impulse to the muscles and glands) and the mixed nerves (which lead the nervous impulse in both directions).

2) Depending on the location where they come from. There are two types of nerves: cranial nerves which exit from the skull and spinal nerves which exit from the spinal cord.

a) Cranial nerves. They is only 12 pairs of them (12 to the left and 12 to the right). Some are sensitive, other motors and other mixed. They control the muscles of the head and neck, except one, called the vagus nerve that controls many organs.

b) Spinal nerves. There is 31 pairs of them. They are all of the mixed type. The sacral region, because of its shape, is called a "tailbone". Each spinal nerve is formed by the combination of nerve fibers from the dorsal and ventral roots. The dorsal root is sensory and presents the ganglion called a dorsal root ganglion or spinal ganglion, formed of the bodies of neurons that receive information from the skin and organs. The ventral root is motor, which mean that it carries the information to the skin and organs.

3) Depending on whether they coordinate the involuntary or voluntary acts. There are two different types of nerves: the nerves of the Autonomic Nervous System and the nerves of the Voluntary Nervous System.

a) The Autonomic Nervous System or Vegetative. Is the one that controls in an involuntary way the funcions of the internal organs (heart, lungs, stomach, intestine and urinary bladder), the blood pressure, the sweat and urine production and the body temperature. Is controlled by the hypothalamus and the spinal cord. Its nerves are almost entirely made of the unmyelinated fibers. There are two different types:

• The Parasympathetic Nervous System. It is dominant in the moments of relaxation. It is made of the cranial vagus nerve and it shares the spinal nerves of the sacral region.
• The Sympathetic Nervous System. It is dominant in the moments of stress. Its nerves share the rest of the spinal nerves. The nerve fibers of this system are partially separated from the rest of the spinal nerves and form two ganglia chains located on both sides of the spine.

 

b) The Voluntary (Somatic) Nervous System. Is the one that controls the entire or part of the voluntary actions of our body. Those can be conscious actions, such as grasping an object we want, or unconscious, such as moving forward the right leg when walking. It is controlled by the brain. Its nerves are enterily made of the myelinated fibers.

6 . Parts of the Central Nervous System (CNS). There are two: the brain and the spinal cord.

a) The brain. It is a mass of neurons of about 1.5 kg of weight that is made, on the outside, of the gray matter, formed basically by the nerve cell bodies, and, inside, of the white matter cformed by the axons. The brain presents profound inbounds (fissures) that are delimited by the lobe areas (convolutions). The cranial nerves are the nerves that come out from different areas of the brain. The brain is divided into the six following parts:

• Cerebrum. It is the largest part and in it lie the memory, the ability to think and, therefore, to use the meaningful language and have creative skills. It has a deep fissure that divides it into two cerebral hemispheres.
• Limbic system. It lays in the deep center of the brain (corpus callosum). It receives the emotions (hunger, thirst, fear, aggression and sexual desire) and is involved in the actions of the response.
• Thalamus. It works by selecting the informations that go to the brain.
• Hypothalamus. It regulates the autonomic nervous system. It also affects the pituitary gland through two ways: by neurons and by secreting hormones.
• Cerebellum. It is controlling the muscles responsible for maintaining posture and body balance.
• Medulla oblongata. It lays under the cerebellum. It is responsible of the autonomic reflex control of heart and breathing rate, swallowing, vomiting and blood pressure.

b) Brain stem. It presents the gray matter on the inside and white matter on the outside (on the contrary to the brain). The spinal nervs, that come out of it, innervate the muscles, glands and organs in the closest area. It performs two functions: in gray matter the spinal reflexes occur (see next chapter) and in white matter the transmission of nerve impulses between the brain and the rest of the body is made.

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