Nervous System.

Introduction.

Cutting the Stone (Hieronymus Bosch)

The nervous system and the endocrine system are the main systems to control and regulate the body.

The nervous system is responsible for receiving stimuli from the environment or interior parts of the body (detected by the sense organs and propioceptors), processing this information and deciding the appropriate response. The information must be conducted, first from the sense organs and propioceptors to the integration centres (Central nervous system), and then from the integration centres to the effectors, which are always muscles or glands.

Due to this, the nervous system has sensitive, integrative and motor functions.

Although the endocrine system is the other one which is responsible for controlling the body, its functions are different. The responses coordinated by the nervous system are quicker (they take place in milliseconds), but are not so lasting.

The endocrine system, however, coordinates slower responses (they take place in seconds), but are lasting (growth, for instance, is coordinated by the endocrine system and lasts for years).

Anatomy of the Nervous System.

Parts of the Nervous System.

The nervous system can be divided according to two different criteria.

First, we can divide the nervous system according to anatomical criteria. And we have two different anatomical parts: the central nervous system and the peripheral nervous system. The central nervous system can be also divided into spinal cord and brain, where the integration centres can be found. The peripheral nervous system transmits information from the receptors to the central nervous system and from the central nervous system to the effectors.

Second, we can divide the nervous system according to functional criteria, so we can find the somatic nervous system and the autonomous nervous system. The somatic nervous system carries out conscious or voluntary actions (all the movements of skeletal muscles). The autonomous nervous system carries out non conscious actions (such as digestion, arterial pressure, etc.).

Third, we can find  two divisions into the autonomous nervous system, called sympathetic and parasympathetic nervous system. The autonomous organs receive (generally) innervation from both divisions, and one of them activates the organ whereas the other one inactivates the organ. In other words, sympathetic and parasympathetic divisions have usually opposite functions (there are some exceptions, there are organs that have only sympathetic or parasympathetic innervation, and there are organs where both divisions have the same function). The sympathetic division is mainly related to actions that promote energy consumption, whereas parasympathetic is mainly related to actions that promote energy saving. For instance, sympathetic nerves increase the heart beat rate and parasympathetic nerves reduce the heart beat rate.

In this lesson we are going to study the nervous system according to anatomical parameters, first analysing the central nervous system and then the peripheral nervous system.

All the central nervous system is covered by three protective layers called meninges. We talk about cranial meninges to the protective layers of the brain and spinal meninges to the protective layers of the spinal cord.

The outer layer is called dura mater and is attached to the bone surface (cranial bones and vertebrae). The second layer, lying under the dura mater, is vaguely similar to a spider web and is called arachnoid. The third one, attached to the nervous tissue, is called pia mater. Between the arachnoid and the pia mater there is a space filled by a liquid called cerebrospinal fluid. This liquid not only provides protection to the nervous system, but also nutrients. 

Meninges

Central Nervous System: Spina Cord.

The spinal cord is a thin bundle made up of nervous tissue, covered and protected by the vertebral column. The spinal cord transports information from the afferent nerves to the brain and from the brain to the efferent nerves. So the spinal cord is the place where the afferent nerves arrive at, and the place where efferent nerves leave from. It is also the place where spinal reflexes take place.

The spinal cord is cylindrical, with a slightly ellipsoidal cross-section. In the interior, there is an H-shaped dark coloured zone (similar also to a butterfly), made up of grey matter (mainly non myelinated neurones). This is the part where integrative actions are carried out. The peripheral part of the cord is made up of white matter, what it means mainly myelinated neurones. This white zone is responsible for transmitting  the information upwards (to the brain) and downwards (to the efferent nerves). The four prolongations of the central grey zone are called dorsal horns (there are two anterior and two posterior dorsal horns).

These four dorsal horns divide the peripheral white zone into four funiculi (called anterior, posterior and left or right lateral funiculi). As we said, these funiculi are responsible for transmitting the information.

The spinal cord is crossed by a central duct called central canal. There are, besides, two fissures, one of them in the anterior part, the other one in the posterior part. The anterior fissure is wider and more important than the anterior one, and is called anterior medial sulcus (or fissure). The posterior fissure is narrower and is called posterior medial sulcus.

Spinal Cord: Section.

The spinal cord is not only related to the transmission of information, but also to the spinal reflexes. Spinal reflexes are simple, but very fast responses to external stimuli. These responses are carried out extremely quickly because the information is not transmitted to the brain, they are coordinated in the grey zone of the cord. They allow immediate responses to serious injuries, accidents or problems that must be solved quickly, such us burns, hits or abrupt muscle contractions.

Central Nervous System: The Brain.

The brain is the main organ where the information is analysed. After the analysis, this organ decides the responses and send orders to the effector organs. Furthermore, this is the place where are generated our sensations, feelings or emotions and where our memories are stored.

The brain has three parts:

• Brain Stem:
• Medulla oblongata.
• Pons.
• Midbrain.
• Cerebrum.
• Cerebellum.

Let’s analyse these parts one by one:

• Brainstem:
• Medulla oblongata: it is the lower part of the brainstem, located just at the end of the spinal cord. All the nervous funiculi form the spinal cord crosses the medulla to the brain. Part of the medulla tract crosses from the left to right and from the right to the left in the decussation of the pyramids. In the medulla oblongata are also located the the cardiovascular centre, that control the heart beat rate, strength of the heart contraction and the arterial pressure, and the respiratory centre, that control the breathing rate.
• Pons: the pons is located between the medulla oblongata and the midbrain. It is only two centimetres long, and connects the medulla oblongata with to the rest of the brain, and som parts of the brain in a parallel way. In the pons are located some important nuclei that control sleep and warning, and the pneumotaxic centre that controls inhalation and exhalation during breathing.
• Midbrain: This is the upper part of the stem-brain. In the midbrain there are some important nuclei related to coordination and movement of the eyes.
• Cerebrum: The cerebrum is a large neural mass, divided into two hemispheres linked by the corpus callosum. The surface of the cerebrum is not smooth, it has many folds that increase its  surface area. We are going to analyse the most important parts of the cerebrum: 
• Lobes: Each hemisphere of the cerebrum is divided into four lobes, called frontal, parietal, temporal and occipital lobe.
• Cerebral cortex: The cerebral cortex is the superficial part of the cerebrum. It is made up of grey matter, what it means that the neurones are not myelinated. The cerebral cortex can be divided into different areas according to their function. The motor area is the part of the cortex in charge of the control of skeletal muscles. The sensory area is the part of the cortex in charge of analysing the information from sense organs and receptors. Finally, the association areas are responsible for the coordination between the sensory and the motor areas.
• Basal Ganglia: located in basal zones of the hemispheres, they control some automatic movements of skeletal muscles, such as the movement of the arms when we walk or run (in order to keep balance).
• Thalamus: Located between the midbrain and the upper part of the cerebrum, the thalamus is control the transmission of information from lower to upper parts of the brain. It is also related to some specific sensations, such as the pain.
• Hypothalamus: Located under the thalamus, it is the main controller of homeostasis and chemical balance. It is the main control centre of the endocrine system (secretion of hormones).
• Lambic System: It is surrounding the thalamus. It is the main emotional centre of the brain. It is related to primary reactions, such us hate, love, aggression or attack-defence reactions. It is also related to the memory (this is why emotional facts are lasting remembered and some of them are absolutely unforgettable).
• Cerebellum: It is in the lower posterior part of the cranium. It is divided into two hemispheres, and has also an anterior and a posterior lobe in each hemisphere. The cerebellum compares the movements that our body is doing with the movements that it must be doing according to the orders of the motor area of the cortex. The cerebellum detects mistakes in the movements and sends information to the motor cortex in order to correct them. So, it does not control the muscles directly, but send the information to the part of the cerebrum that controls the muscles. Furthermore, it is related to some automatic movements to control our balance (the movement of our arms that helps us not to keep our balance, for instance). Finally, it also controls some repetitive movements that we can carry out in a semi-automatic way (writing, for example).

Anatomy of the Central Nervous System

Peripheral Nervous System.

The nerves transmit information from the sense organs to the central nervous system and from the central nervous system to the muscles and glands. The nerves part of the receptors in the sense organs to the spinal cord or directly to the brain, or of the spinal cord or the brain to the effector organs. The nerves connected to the spinal cord are called spinal nerves. The nerves connected to the brain are called cranial nerves.

There are twelve pairs of cranial nerves and thirty one pairs of spinal nerves. The spinal nerves exits of the spinal cord through the gap between the vertebrae. Each pair of nerves control a corporal band surrounding their vertebral zone. There are eight pairs of cranial nerves, twelve pairs of dorsal nerves, five pairs of lumbar nerves, five pairs of sacral nerves and one pair of coccygeal nerves.

The neurones that made up the nerves are myelinated to improve and accelerate the transmission of information. 

Dermatomes.

Autonomous Nervous System.

As we analysed before, the autonomous nervous system carries out autonomous or involuntary actions. There are two divisions in the autonomous nervous system: sympathetic and parasympathetic.

The sympathetic division is made of nerves that part of the dorsal and lumbar zones of the spinal cord. Just after leaving the cord, the sympathetic nerves form ganglia called sympathetic ganglia. All these ganglia make a ganglia chain located adjacent to the vertebral column. The information is transmitted from these ganglia to the effector organs.

The parasympathetic nerves part of the brainstem or of sacral zones of the spinal cord. The ganglia associated to these nerves are not located near the vertebral column, but near or inside the effector organ innervated.

As we studied before, when both sections of the autonomous nervous system control an organ, they usually carry out opposite actions. So, if one of the sections activates an organ, generally the other one deactivates it. The parasympathetic section is usually related to actions that preserve energy, whereas the sympathetic section is related to actions that consume energy. For instance, the sympathetic system increases the heartbeat and breathing rates, whereas parasympathetic system lowers heartbeat and breathing rates.

Physiology of the Nervous System.

The functioning of the nervous system is mainly based on the ability of the neurones to send signals one another when they are in contact by synapsis. The synapsis is the communication established between adjacent neurones. It is chemical communication and the chemical messenger transmitted from one neurone to another is called neurotransmitter.

The chemical messenger received by a neurone by the synapsis is quickly transmitted to the whole cell by an electrical process. It is related to the ionic differences between the inner and the outer part of the neurone. The inner part of the neurone is richer in anions, whereas the outer part of the cell is richer in cations. Due to this, there is a polarisation of the membrane, negative in the interior and positive in the exterior, that leads to an electrical potential of -60mV. After receiving the chemical signal from another neurone, the cell open ionic channels that allow the free transport of some anions and cations. The membrane depolarises, transmitting the impulse form the synapsis to the whole neurone. The signal will be also transmitted by synapsis to all the neurones connected with the activated neurone.

When a receptor detects one stimulus, it transmits the signal to an associated neurone. This neurone transmits the information to the nerves, connected to the spinal cord. The spinal cord transmits the information to the brain (the spinal reflexes are an exception), where it is analysed to find out the required response. The first place where the information is analysed is the cerebral cortex, but different parts of the brain will work to decide what to do. After that, the motor cortex will send the order to the muscle that must be moved and the cerebellum will analyse if the order is being carried out properly.

Or maybe the brain decides that the response must be executed by the autonomous nervous system, that will send the information to inner organs and glands. For instance, if the stimulus is related to an increase of our body temperature, the autonomous nervous system will order the sweat gland to release sweat to the skin.