The Nervous System

(Controlling & Coordinating System)

A complex and yet unknown organ, the brain is the seat of consciousness, intellectual activity and emotions. It is also here that the various functions of the body are regulated and controlled, physical stimuli are felt and voluntary movements are activated. This centralizing and coordinating role is exercised throughout the body through a vast network of nerves that fill the dual motor and sensory function

Functions of the nervous system

• The nervous system and the endocrine system are the main control centers for maintaining the body's homeostasis
• Responsible for conscious thought and perception, emotions, personality, mind
• The nervous system has 3 main functions:

Sensory function: It's about control of voluntary effectors, in which the afferent neutron detects stimuli (internal or external changes) and send it on message to the brain or spinal cord

Integration function: processes information in the brain and spine

Motor function: control of involuntary effectors (smooth muscle, heart muscle, glands) and mediation of autonomous reflexes (heart rate, blood pressure, glandular secretion, etc.)

Cells in the nervous system: (nervous tissue)

Nerve tissue is made up of two main types of cells:

Neurons

                    (Cells that transmit nerve impulses)

At the base of the nervous system are neurons, cells that have a great peculiarity with the particularity of transmitting electrical signals and transmitting them to other cells (nerves, muscles, glands ...). Whether it is a motor, sensory or association, a neuron is always made up of a cell body and more or less numerous extensions. Among these extensions, we distinguish the dendrites, the branches responsible for receiving electrical impulses and the axon (enclosed by the myelin sheath are called myelinated axons that form white matter in the nervous system; while axons that do not have a myelin sheath are called axons unmyelinated which form gray matter in the nervous system), responsible for their transmission

The axon:
The specific structure of the neuron, the axon is an extension cell that is separated from the cell body in a cone implant, therefore it extends over a distance that varies between 1 mm (in the brain) and 1 m (in the leg). Most axons are covered in myelin, a white lipid substance. Wrapped in Schwann cells (or oligodendrocytes in the central nervous system), myelin forms a sheath divided into segments by constrictions, Ranvier's ganglia

The synapses:
The nerve message passes from one neuron to another in a region called "synapses". Most often, both neurons are not in direct contact and remain separated by a very thin slot, so the electrical signal must be converted into a chemical signal to allow transmission. When a nerve impulse reaches the pulsating terminal, the vesicles that contain them release neurotransmitters into the synaptic cleft. The moment these molecules come into contact with the postsynaptic neuron receptors, they generate an electrical signal

The different types of neurons:
Depending on their function, neurons are divided into three categories. Motor (or efferent) neurons guide nerve inflow to muscles and glands. Sensory (or related) neurons transmit messages from sensory receptors to nerve centers. Association neurons (or interneurons) connect two other neurons, the latter category represents 90% of all neurons in the body. Neurons can also be distinguished by their structure

The largest multipolar neurons have multiple dendrites and a long axon. These are essentially motor neurons and interneurons. Unipolar W neurons, which remain sensory neurons, have a unique extension which is divided into two branches. Finally, bipolar B neurons have two extensions

Neuroglial cells 

The accessory cells to fill the spaces and support neurons, supporting the cells of the nervous system, there is also a functional interdependence between neuronal cells and neurons

• Astrocytes: these cells like star-shaped, anchor neurons to blood vessels, its role lies to regulate the micro-environment of neurons and regulate the transport of nutrients and waste to and from neurons. Part of the "blood-brain barrier"

• Microglial cells: small ovoid cells, they are phagocytic to defend themselves against pathogens. They can also monitor the state of neurons

• Ependymal cells: cube-shaped or columnar cells, the central channel in the spinal cord. These cells cover the fluid-filled cavities of the brain and spinal cord, their function: production, transport and circulation of cerebrospinal fluid

• Oligodendrocytes: they similar to astrocytes cells and are ordered in rows along the nerve fibers, but have lesser processes, which its role in creating the myelin sheath in the central nervous system that isolates and protects axons

Divisions of the nervous system:

The nervous system is divided into two divisions:

1. Central nervous system (CNS): brain (located in the cranial cavity), the spinal cord (located in the vertebral cavity), which acts as the main control center for all bodily activities

2. The peripheral nervous system (PNS): cranial nerves (12 couples), spinal nerves (31 couples) that act as a link between the central nervous system and the body

PNS can be divided into sensory (afferent) nerves (send the body's nerve impulse to the central nervous system) and motor (efferent) nerves (send the CNS impulse to the effector organs), which are divided into:

• The somatic nervous system (SNS): controls and regulates the voluntary contraction of skeletal muscles
• The autonomic nervous system (ANS): regulates the involuntary control of the glands and smooth heart muscles

The ANS are divided into sympathetic and parasympathetic branches: they stimulate and inhibit the activities of the effector organs (except the digestive organs)

The central nervous system

                             (The control center of the nervous center)
The nervous system is the major communication and control network of the human body. It is up to him to control the movements of the organs and muscles, to process the sensory messages of the whole body and to guarantee mental and intellectual activities. These multiple functions are performed through the coordination of the peripheral nervous system, which brings together all the nerves of the body and the central nervous system

The brain:
                           (The heart of the nervous system)

Central element of the nervous system, the brain is housed in the cranial box, where it communicates with the rest of the body through the cranial nerves and the spinal cord. It consists of the brain stem, the cerebellum and in particular the brain, which represents almost 90% of its volume

The brain is composed of:

• Brain: conscious activity that includes perception, emotion, thought and planning
• Brain stem: medulla, bridge and midbrain (involuntary responses) and transmits information from the spine to the upper part of the brain
• Cerebellum: coordination center of movement and aspects of motor learning
• Thalamus: the switchboard of the brain, filters and then transmits the information to the various regions of the brain
• Hypothalamus: participates in the regulation of the activities of internal organs, controls the information of the autonomic nervous system, controls the pituitary gland and its hormones and regulates sleep and appetite

Cerebrum:
                   (An extraordinary complexity)
The human brain follows the different stages of animal evolution. Therefore, most primitive vital functions are provided by very deep, such as the hypothalamus. By covering this "reptile" brain, the limbic control system of more advanced functions: memory, emotions, learning. The cerebral cortex is the responsible for thinking, speaking, voluntary movements and conscious representation of sensations

Lobes of the Cerebrum:

• Frontal: the portion of the brain that is located just below the frontal bone of the anten, a motor area involved in movement and in the planning and coordination of behavior
• Parietal: the deep central groove marks the posterior edge of the frontal lobe, the caudal to which is the parietal lobe, under the parietal bone, sensory processing, attention and language
• Temporal: the tip of the "horn" is located just below the temporal bone (temple) of the skull, auditory perception, speech and complex visual perceptions
• Occipital: the posterior part of the brain under the occipital bone is the occipital lobe. visual center, plays a role in the processing of visual information

Brainstem:
Deeply anchored in the heart of the brain, the brain stem is an extension of the spinal cord, the structure of which preserves histologically (by wrapping the white matter in a nucleus of gray matter). Its three main parts, the spinal bulb, the bridge and the midbrain, contain ascending and descending nerve bundles that connect the brain and cerebellum to the rest of the body. The brain stem also plays a vital role in the innervation of the head, since 10 of the 12 pairs of cranial nerves are directly attached

Cerebellum:
Located in the back of the brain, the cerebellum is separated from the occipital lobes by a meningeal fold, the carp of the cerebellum. The two cerebellar hemispheres, delimited by a central projection, the worm, have a folded surface very different from that of the brain

The role of the cerebellum is very specific, which it controls the regulation and coordination of movements. For this, it continuously analyzes the messages sent by the sensory receptors and regulates the tension of the muscles by inhibiting the commands of the motor area of ​​the brain. The cerebellum, which is connected to the organs of balance, also regulates the height of the body by acting on the muscles involved

Spinal cord:

The spinal cord is the connecting center for reflexes, as well as the afferent (sensory) and efferent (motor) pathways for most of the body under the head and neck. The spinal cord begins at the level of the brain stem and ends approximately at the second lumbar vertebra. The spinal cord is the major conduit for neural messages from the skin of the body, joints and muscles to the brain and vice versa 

The spinal cord communicates with the body through the spinal nerves, which are part of the peripheral nervous system. Spinal nerves exit the spinal cord through the notches between each vertebra in the spine. Each spinal nerve joins the spinal cord through two branches, the dorsal root and the ventral root. Spinal nerves contain the fibers of sensory and motor neurons. A nerve does not contain cell bodies. They are found in the ganglion (sensory) or in gray matter (motor), gray matter is an area of ​​unmyelinated neurons in which cell bodies and synapses are produced. While the white matter is located around and surrounds the gray matter, which it consists the spinal tracts that allow to ascend and descend the spinal cord. The spinal tracts transfer information up or down the spinal cord to or from the brain. Tracts within the brain carry information from one place to another in the brain. The tracts are always part of the white matter, known as an area of ​​myelinated fiber traits , in which Myelination in the central nervous system differs from that of the nerves

Meninges: the central nervous system, that part of the nervous system enclosed in the skull and spine, are the three covers around the brain and spine and help cushion, protect and nourish the brain and spinal cord. It is protected by three membranes collectively named meninges. The three membranes are the dura (the outermost layer, very resistant), the arachnoid membrane (the intermediate layer and adheres to the dura). This meningeal layer has a cobweb-like appearance and texture.) And the pious mother (the "gentle mother" is a thin membrane that adheres tightly to the surface of the brain)

The peripheral nervous system:

                     (A network of sensory and motor paths)

All parts of the nervous system other than the brain and spinal cord (the spinal cord flows within the spine. Axons enter and exit the spinal cord through the dorsal and ventral roots, respectively. These roots join together to form the nerves spinal in that course through the body) include the peripheral nervous system (pNS). PNS has two parts: somatic nervous system SNP (voluntary) and visceral SNP or autonomic nervous system (ANS)

The central nervous system communicates with the whole body through 43 pairs of nerves: 12 pairs of cranial nerves connected directly to the brain and 31 pairs of spinal nerves connected to the spinal cord. This network, which forms the peripheral nervous system (SNP), branches out to reach the whole body

Nerve impulses are of two types: sensory and motor. In the first case, the nerve endings send messages to the central nervous system. In the other, the central nervous system orders a muscle to contract. Some nerves implement both types of tasks: these are mixed nerves

Cranial nerves:
In addition to the nerves that derive from the spinal cord and innervate the body, there are 12 pairs of cranial nerves (1. OLFACTORY 2. OPTICS 3. OCULOMOTOR 4. TROCLEAR 5. TRIGERMINAL 6. ABDUCT 7. FACIAL 8. VESTIBULUS-COCLEAR 9. GLOSOFARYNGAL 10. VAGUE 11. ACCESSORY 12. IPOGLYCAL) deriving from the brain stem and innervating (mainly) the head. Some of the cranial nerves are part of the central nervous system, some are part of the somatic SNP and others are part of the visceral SNP. Many cranial nerves contain a complex mix of axons that have different functions

Spinal nerves:
31 pairs along the spinal cord, the dorsal and ventral roots come together to form the spinal nerves. Spinal nerves contain sensory and motor fibers, like most nerves. Spinal nerves are given numbers that indicate the portion of the spine where they occur. There are 8 cervical nerves (C1-C8), 12 thoracic nerves (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5) and 1 coccygeal

Motor functions of the nervous system:
                     (How the body muscles are activated)
Thanks to its skeletal muscles, the human body can perform very varied and very precise movements. It is the motor cortex, an area of ​​the brain located in the back of the frontal lobes, which is assigned to these voluntary motor functions. The smooth muscles that ensure the contraction and relaxation of the internal organs depend on the autonomic nervous system, mainly directed by the hypothalamus. Finally, some muscle actions are not controlled by the brain but derive from the reflex stimulation of motor neurons in the spinal cord

Functional divisions of the Nervous System:

Functional divisions and structural divisions overlap,  the voluntary and autonomic nervous systems use parts of the central nervous system and the central nervous system

The Autonomic Nervous System (ANS) (involuntary)

Whether due to the contractions of the heart or the secretion of saliva, the action of the body's organs and visceral glands is controlled not consciously but through the autonomic nervous system. Visceral sensory axons give information, such as pressure and oxygen content in the blood in the arteries, it's about visceral function of the central nervous system. Visceral motor fibers order the contraction and relaxation of the muscles that make up the walls of the intestine and blood vessels (called smooth muscles), the rate of contraction of the heart muscle. It works in two different ways: the sympathetic system, which borrows the spinal cord from the medulla and the relay of a nodal chain, and the parasympathetic system, which largely uses the nerve bundles of the vagus nerve (Xe cranial nerve)

Somatic Nervous System (voluntary):

All the spinal nerves that supply the skin, joints and muscles that are under voluntary control are part of the somatic SNP. The somatic motor axons, which control muscle contraction, derive from motor neurons in the ventral spinal cord

The skeletal muscles can contract consciously, when translating the nervous message from the motor cortex neurons. The message reaches the brain stem, then descends to the spinal cord. Borrow a spinal nerve that stimulates muscle. The sensory receptors of the muscle emit a message in turn, intended to control movement. This signal returns to the cerebellum, which compares the movement performed with the movements learned and memorized from childhood. The cerebellum sends an inhibitory message to the muscle to control its action. At the same time, it acts on the motor cortex, thanks to the thalamus relay, to regulate its command

Dermatomes: are somatic or musculocutaneous areas that send signals to the brain through the fibers of specific spinal nerves

Myotome: a muscle or collects of muscles innervated by a single spinal segment

Disorders of the nervous system

• Epilepsy: a common and various sets of chronic neurological disorders described by convulsions

• Seizures: the physical results or changes in behavior that occur after an episode of abnormal electrical activity in the brain

• Alzheimer's disease: a degenerative brain disorder that leads to dementia, which is a gradual loss of memory, amnesia and ability to function

• Parkinson's disease: a brain disorder that causes tremors (tremors) and difficulty walking, moving and coordinating

• Cerebral palsy - A group of disorders that can involve brain and nervous system functions such as movement, learning, hearing, vision and thinking following damage to certain parts of the developing brain

• Herpes zoster (herpes zoster): a painful and blistering rash caused by the varicella zoster virus, the virus that causes chickenpox

• Glaucoma: a set of eye conditions that lead to damage to the optic nerve due to increased eye pressure

• Pink eye (conjunctivitis): is an inflammation or infection of the conjunctiva of the eye

• Multiple sclerosis: an autoimmune disorder,  that affects the central nervous system, brain and spinal cord