It consists of the brain and spinal cord, which integrate and coordinate the entire body’s activities.
Through these physical organs, thought, emotion and sensation are experienced, and movement is organized.
Long- and short-term metabolism and homeostasis are regulated through close interaction with the endocrine system.
While the central nervous system is functionally composed of neurons, other types of cells, such as glial cells, play essential supportive functions.
Some cranial nerves, such as the optic and olfactory nerves, are also part of the central nervous system.
Central nervous system functions
The primary function of the central nervous system is integration and coordination.
The central nervous system receives input from various sources and cohesively implements an appropriate response to stimuli.
For example, to walk, the central nervous system needs visual and intellectual clues such as the texture of the surface, its inclination, the presence of obstacles, etc.
Based on these stimuli, the central nervous system alters skeletal muscle contraction.
Once babies learn to walk, this happens involuntarily, no longer requiring conscious thought or concentration.
A similar process is required to receive complex stimuli and generate a coordinated response for various activities, be it balancing a bicycle, holding a conversation, or developing an immune response.
The central nervous system, especially the brain, is considered the physical seat for most higher-order mental functions, with neural connections that form the basis for thinking and memory retention.
The brain plays a vital role in developing speech, language, and communication, involving an association of abstract symbols and sounds with concrete objects and emotions.
Motivation, ambition, reward, and satisfaction are also mediated through neural connections in the central nervous system.
At the same time, the brain’s limbic system also controls the most basic emotions and drives, such as pleasure, fear, anger, hunger, thirst, drowsiness, and sexual desire.
Additionally, involuntary reflexes are mediated by the spinal cord, providing protection and quickly preventing injury.
The central nervous system directly or indirectly influences almost all internal organ systems, whether related to respiration, digestion, excretion, circulation, or reproduction.
The key to the work of the central nervous system is integration, as it receives information from various sources and creates a cohesive response.
This is particularly important for animals in complex social structures, such as humans. For example, meeting an old friend and catching up on coffee can seem like a relaxing event.
However, the central nervous system needs to be full of activity to facilitate a successful interaction.
It begins when you see the friend and recognize him: your brain correlates the neurochemical signals received from the optic nerve with the image you have in memory.
It continues with the collection of shared experiences and the slip into the vocabulary of an earlier time.
Some research suggests that the central nervous system may even associate different body languages with different sets of people or events.
You may come across phrases that haven’t been in your vocabulary for years or change your accent and posture slightly without being aware of it.
The central nervous system recovers memory and correlates with current events (your friend’s sight and conversation) to generate an emotional and physiological response.
You can end up with the brain directing skeletal muscles to walk to a coffee shop, instructing the vocal cords to issue an invitation, and even using your understanding of cultural markers to determine whether a hug or handshake would be a fitting end to the gathering.
Anatomy of the central nervous system
Invertebrates, the brain, and the spinal cord are enclosed in bony cavities, with the brain residing within the skull and the spinal column protecting the spinal cord.
The brain consists of two large hemispheres demarcated by a thick band of nerve fibers called the corpus callosum.
Each hemisphere can be divided into four lobes: the frontal, parietal, temporal, and occipital lobes.
Each of these lobes has a relatively different function, which is related to higher levels of cognition (frontal lobe), somatosensory input (parietal lobe), auditory stimuli (temporal lobe), or visual stimuli (occipital lobe).
Localization of function to different lobes was initially discovered in patients with brain damage.
The outer layer of the brain or cerebral cortex can be divided according to its sensory, motor, and association areas.
The primary sensory cortex receives sensory information from the body and specialized sense organs.
The motor areas are involved in controlling and executing voluntary motor activities.
The association areas are necessary for perception, abstract thinking, and the association of new sensory inputs with memory.
These boundaries of the cerebral cortex are generally represented bilaterally in both hemispheres.
The cerebellum is smaller than the brain, comprises two lobes, and is located behind the brain stem.
It is involved in coordinating different muscle groups to produce smooth movements, control posture, and balance.
Neurons in the inner ear associated with balance transmit information to the cerebellum, receiving auditory and visual communication.
The brain stem comprises three parts: the midbrain, the pons, and the medulla oblongata.
The medulla controls most involuntary actions, while the midbrain and pons are associated with sensory functions, arousal, and motivation.
The brainstem connects the brain to the spinal cord.
The spinal cord is approximately 17 inches long, tapering along the spinal column in humans, starting near the occipital bone and ending in the lumbar region of the spine.
It connects the brain with most parts of the body and, at the same time, contains independent neural networks for the generation of patterns and the execution of reflexes.
It can be divided into 31 segments, each giving rise to a pair of spinal nerves.
The spinal nerves carry sensory and motor signals between the body and the spinal cord.
The central nervous system is composed entirely of neurons, their axons and dendrites, and the supporting cells of the central nervous system.
The spinal cord is a cylindrical part of the central nervous system, approximately 40-45 cm long and 1.5 cm wide, located in the vertebral canal.
Macroscopically, the central nervous system comprises gray and white matter.
Gray matter contains most of the bodies of neurons, dendrites, the initial unmyelinated portions of axons, astrocytes, and microglial cells.
While the main components of white matter are myelinated axons, lipids in myelin sheaths account for the white appearance of value and the myelin that produces oligodendrocytes.
The gray matter is located inside the spinal cord and is divided into anterior and posterior. Lateral columns, all three joined by the central gray commissure, where the central canal is located.
The central canal is lined by a single layer of columnar ependymal cells, a subtype of neuroglial cells that support the central nervous system.
Functionally related groups of nerve cell bodies are called nuclei.
The white matter
The white matter consists of ascending and descending nerve fibers grouped in the anterior, lateral, and posterior funiculi.
The posterior funiculi are separated by the medial posterior septum and the anterior funiculi by the medial anterior fissure.
The white matter is devoid of neuronal cell bodies, but microglial cells are present.
The aggregates of neuronal cell bodies in the white matter are called nuclei.
In the cerebrum and cerebellum cortex, gray matter overlaps the central medullary mass of white matter.
In the spinal cord, the situation is the opposite; the gray matter is located centrally, in cross-sections, is H-shaped, and resembles a “butterfly” surrounded by white matter.
The cerebral hemispheres
The cerebral hemispheres consist of a coiled cortex of gray matter, with a thickness of about 3 mm and a total surface area of 1.2 to 2.6 m 2.
Which covers the central medullary mass of the white matter, which carries fibers between different parts of the cortex and other parts of the central nervous system.
The surface area of the crust is increased by its convolutions, which are separated by fissures.
The cerebral cortex consists of neurons, nerve fibers, and glia.
The cerebral cortex
La corteza cerebral the neocorteza consists of six layers.
In humans, the primitive three-layered arrangement persists only in the olfactory cortex and the cortical part of the limbic system in the temporal lobe.
Most of the neurons in the cerebral cortex are arranged vertically, and the most abundant neurons are efferent pyramidal cells.
Efferent pyramidal cells are the giant pyramidal cells found in layer V of the motor cortex regions; they are called Betz cells.
The six layers of the cerebral cortex are:
- Molecular layer: consists of a few nerve cells.
- Outer granular layer: relatively thin layer consisting of numerous small, densely packed neurons.
- The pyramid or outer pyramidal layer is composed of median pyramidal nerve cells.
- Inner granular layer: This contains small, irregularly shaped nerve cells.
- Pyramidal or inner pyramidal layer: includes large pyramidal cells.
- Multiform layer: composed of small spindle-shaped and polymorphic nerve cells.
The cerebellum coordinates muscle activity and maintains posture and balance.
The cerebral cortex forms a series of deeply convoluted folds or follicles supported by the branching of the central medulla of the white matter.
The cerebral cortex consists of three distinct layers:
The molecular layer: contains two main types of neurons, stellate cells and basket cells, which are dispersed between dendritic branches and numerous thin axons that run parallel to the long axis of the folia.
Ganglionic layer or Purkinje cell layer: The coating comprises a single row of large Purkinje cells.
In typical micrographs, only pear-shaped cell bodies are seen; the vast branching pattern of dendrites in the molecular layer can be made visible only by unique staining methods.
The axons of Purkinje cells provide the only efferent pathway to the deep cerebellar nuclei. Thus Purkinje cells constitute the only outlet for all motor coordination in the cerebellar cortex.
Granular layer: the layer is densely populated by small granule cells with dark staining nuclei and little cytoplasm.
Each cell emits four to five dendrites, making synaptic contact with the afferent mossy fibers.
The axons of the granule cells rise vertically into the molecular layer, where they bifurcate at a T junction; the ramifications run parallel to the long axis of the cerebellar folio and synapse with the dendrites of the Purkinje cells.
Scattered throughout the granule cells are the Golgi cells; their dendrites branch into the molecular layer, and their axons synapse with the dendrites of the granule cells.
The route to the cerebellar cortex includes mossy fibers and climatic fibers.
Mossy fibers enter the granular layer and form synapses with the granule cells.
Contacts between mossy fibers and granule cell dendrites occur within glomeruli structures.
Golgi cell terminals also infiltrate these structures.
The scaling fibers reach the molecular layer, where a thread “climbs” over the dendrites of the Purkinje cell surrounding them.
The blood-brain barrier
The blood-brain barrier is a protective structure that provides tight control over the passage of substances that move from the blood to the tissues of the central nervous system.
The barrier is essential to guarantee the specific nature of the neuronal microenvironment.
The main structural component of the barrier is the capillary endothelium, where the endothelial cells are sealed together by tight junctions, and thus capillaries of a continuous type are formed.
In addition, the basement membrane of the capillaries is surrounded by the perivascular feet of the astrocytes. Therefore the normal functioning of the astrocytes is essential for the integrity of the blood-brain barrier.
The blood-brain barrier is ineffective or absent in specific brain parts, such as the choroid complex (where cerebrospinal fluid is produced), the posterior pituitary, and the neurohypophysis (where hormones are released).
The meninges, the cerebrospinal membranes, invert the brain and spinal cord, the optic nerve, and the first portions of the cranial and spinal nerve roots.
There are three cerebrospinal membranes: the pia mater, the arachnoid, the intermedial, and the dura mater, the outermost.
The three cerebrospinal membranes consist of connective tissue.
They are primarily of mesodermal origin; the dura and arachnoid covering the telencephalon and diencephalon are probably derived from the neural crest.
It consists of fibrous connective tissue and contains many sensory nerves and blood vessels.
The deep surface is separated from the arachnoid by the subdural space, which contains fluid.
The dura that surrounds the brain and spinal cord are continuous through the foramen magnum.
An epidural space separates the dura that surrounds the spinal cord from the inner surface of the periosteal-lined vertebrae.
The epidural space contains blood vessels (veins) and fat.
The dura that surrounds the brain attaches itself to the inner periosteal lining of the skull bones.
So, there is usually no epidural space, but that space can arise in case of epidural hemorrhage.
The thin layer of connective tissue on the inner and outer surface is lined with a single layer of flattened cells.
The cobweblike appearance is caused by numerous trabeculae connecting the arachnoid and pia mater.
The trabeculae are made up of loose connective tissue, and the space is covered by these trabeculae (the space between the pia mater and the arachnoid is called the subarachnoid space).
The subarachnoid space contains cerebrospinal fluid that slowly seeps through the trabeculae.
When the pia mater closely follows the surface of the brain and spinal cord, the arachnoid does not immerse itself in the sulci on the surface of the brain, and therefore the size of the subarachnoid space varies considerably.
Significant expansions of the subarachnoid space are called cisternae.
The pia mater
A thin layer of connective tissue that completely covers the surface of the brain and spinal cord.
The pia mater is lined with an epithelial layer towards the subarachnoid space.
It is firmly attached to nervous tissue by numerous astrocyte foot processes.
The pia mater is richly vascularized, and arteries and veins enter the brain and spinal cord of the pia mater.
It contains numerous nerve fibers.
Cerebrospinal fluid, produced in the four ventricular cavities of the brain, flows between the pia mater and the arachnoid, protecting against pathogens and mechanical support to the entire central nervous system.
Special glial cells called ependymal cells produce cerebrospinal fluid.
Central nervous system disorders
The central nervous system can be attacked by pathogens: bacteria (bacterial meningitis ), viruses (viral encephalitis), fungi (fungal meningitis, abscesses), or parasites (toxoplasmosis, cysticercosis).
Alternatively, the central nervous system could be a secondary site for infection in the advanced stages of a disease of a different organ, such as tuberculosis or syphilis.
The meninges covering the central nervous system are particularly susceptible to infection, especially when head trauma allows pathogens from other organs to access these delicate tissues through the cerebrospinal fluid.
The central nervous system is also particularly susceptible to changes in the vascular networks that supply critical nutrients, glucose, and oxygen.
Blockages in blood vessels or burst capillaries can lead to strokes due to the death of neuronal cells.
Depending on the location of the injury and the type of medical care received, the individual could suffer loss of sensory, motor, cognitive or associative functions.
Some people lose language skills (aphasia), others lose memory, while others may lose the full range of voluntary movements (paralysis).
Neurons have a limited capacity for regeneration and plasticity.
Therefore, ailments that lead to the accumulation of waste or unfolded proteins within the body’s cells are particularly debilitating for the nervous system.
Conditions such as Alzheimer’s and Parkinson’s disease are progressive neurodegenerative disorders.
Symptoms become more debilitating with age. While there is an apparent genetic factor in some of these conditions (Huntington’s disease) in most other neurodegenerative diseases, genetic and environmental factors appear to be involved.
The cause of Alzheimer’s disease is not yet known, although autopsies of patients who have suffered from the condition often reveal protein plaques in the brain.
There could be the implication of neurotransmitter deficiency, aggregates of specific proteins, changes in the vascular structure of the brain, enlargement of the cerebral ventricles, and a reduction of the active tissue in the cerebral cortex.
Parkinson’s disease involves a progressive loss of motor ability, beginning with fine motor skills and changes in posture and balance.
Over time, all deliberate movements become difficult. The primary region of the brain affected by the disease is the substantia nigra, a part of the midbrain.
As in Alzheimer’s, the definitive cause of Parkinson’s disease is not known.
The central nervous system may also be affected by cancerous growths and tumors.
Symptoms will depend on the location of the growth, size, malignancy, and site of origin.
Therefore, they could cause headaches, loss of cognitive ability, hearing loss, and changes in motor control and autonomic functions.