HUMAN NERVOUS SYSTEM: PARTS AND FUNCTIONS (WITH PICTURES) - NEUROPSYCHOLOGY - 2025

The human nervous system controls and regulates most of the body's functions, from the capture of stimuli through sensory receptors to the motor actions that are carried out to give a response, through the involuntary regulation of internal organs.

In humans it is made up of two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord.

Human nervous system, divided into central nervous system and peripheral nervous system

The peripheral nervous system is made up of nerves, which connect the central nervous system to every part of the body. The nerves that transmit signals from the brain are called motor or efferent nerves, while the nerves that transmit information from the body to the central nervous system are called sensory or afferent.

At the cellular level, the nervous system is defined by the presence of a type of cell called a neuron, also known as a "nerve cell." Neurons have special structures that allow them to send signals quickly and accurately to other cells.

Connections between neurons can form neural networks and circuits that generate perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.

Malfunction of the nervous system can occur as a result of genetic defects, physical damage from trauma or toxicity, infection, or simply aging.

The structure of the nervous system of human beings is composed of two well differentiated parts / subsystems; on the one hand, there is the central nervous system and on the other the peripheral nervous system.

The peripheral nervous system

Peripheral nervous system.

At a functional level, within the peripheral nervous system the autonomic nervous system (ANS) and the somatic nervous system (SNSo) are differentiated. The autonomic nervous system is involved in the automatic regulation of internal organs. The somatic nervous system is responsible for capturing sensory information and allowing voluntary movements, such as waving or writing.

The peripheral nervous system is mainly composed of the following structures: ganglia and cranial nerves .

- Autonomic nervous system

Autonomic nervous system.

The autonomic nervous system (ANS) is divided into the sympathetic system and the parasympathetic system. The autonomic nervous system is involved in the automatic regulation of internal organs.

The autonomic nervous system, together with the neuroendocrine system, is in charge of regulating the internal balance of our body, lowering and raising hormonal levels, the activation of the viscera, etc.

To do this, it carries information from the internal organs to the central nervous system through the afferent pathways, and transmits information from the central nervous system to the glands and muscles.

It includes the cardiac muscles, the smooth of the skin (which innervates the hair follicles), the smooth of the eyes (which regulates the contraction and dilation of the pupil), the smooth of the blood vessels and the smooth of the walls of the organs internal (gastrointestinal tract, liver, pancreas, respiratory system, reproductive organs, bladder…).

The efferent fibers are organized into two different systems, called the sympathetic and parasympathetic systems.

The sympathetic nervous system is mainly responsible for preparing us to act when we perceive an outgoing stimulus, activating one of the automatic responses, which can be flight, freezing or attack.

The parasympathetic nervous system for its part maintains the activation of the internal state in an optimal way. Increasing or decreasing its activation as necessary.

- Somatic nervous system

Somatic nervous system.

The somatic nervous system is in charge of capturing sensory information. For this, it uses the sensory sensors distributed throughout the body that distribute information to the central nervous system and thus transport the orders of the central nervous system to the muscles and organs.

On the other hand, it is the part of the peripheral nervous system associated with the voluntary control of body movements. It consists of afferent nerves or sensory nerves, and efferent nerves or motor nerves.

Afferent nerves are responsible for transmitting sensation from the body to the central nervous system (CNS). The efferent nerves are responsible for sending orders from the central nervous system to the body, stimulating muscle contraction.

The somatic nervous system consists of two parts:

• Spinal nerves: they emerge from the spinal cord and are made up of two branches: a sensory afferent and another an efferent motor, so they are mixed nerves.
• Cranial nerves: send sensory information from the neck and head to the central nervous system.

Both are explained below:

Cranial nerves

There are 12 pairs of cranial nerves that arise from the brain and are responsible for transporting sensory information, controlling some muscles and regulating some internal glands and organs.

I. Olfactory nerve. It receives the olfactory sensory information and carries it to the olfactory bulb, located in the brain.

II. Optic nerve. It receives visual sensory information and transmits it to the brain centers of vision through the optic nerve, passing through the chiasm.

III. Internal ocular motor nerve. It is responsible for controlling eye movements and regulating the dilation and contraction of the pupil.

IV. Trochlear nerve. It is responsible for controlling eye movements.

V. Trigeminal nerve. It receives somatosensory information (such as heat, pain, textures…) from the sensory receptors of the face and head and controls the muscles of chewing.

SAW. External ocular motor nerve. Control eye movements.

VII. Facial nerve. It receives gustatory information from the receptors of the tongue (those located in the middle and anterior part) and somatosensory information from the ears and controls the muscles necessary to make facial expressions.

VIII. Vestibulocochlear nerve. Receive auditory information and control balance.

IX. Glossopharyngeal nerve. It receives taste information from the back of the tongue, somatosensory information from the tongue, tonsils, and pharynx, and controls the muscles needed to swallow (swallow).

X. Vagus nerve. It receives sensitive information from the glands, digestion and heart rate and sends information to the organs and muscles.

XI. Spinal accessory nerve. It controls the muscles of the neck and head that are used for its movement.

XII. Hypoglossal nerve. Control the muscles of the tongue.

Spinal nerves

The spinal nerves connect the organs and muscles to the spinal cord. The nerves are responsible for carrying information from the sensory and visceral organs to the spinal cord, and transmitting orders from the spinal cord to the skeletal and smooth muscles and glands.

These connections are what control reflex acts, which are performed so quickly and unconsciously because the information does not have to be processed by the brain before issuing a response, it is directly controlled by the spinal cord.

In total, there are 31 pairs of spinal nerves that exit bilaterally from the spinal cord through the space between the vertebrae, called invertebral foramina.

Central Nervous System

Central nervous system: brain and spinal cord.

The central nervous system is made up of the brain and spinal cord.

At the neuroanatomical level, two types of substances can be distinguished in the central nervous system: white and gray. The white matter is formed by the axons of neurons and the structural material, while the gray matter is formed by the neuronal bodies, where the genetic material is found, and the dendrites.

This distinction is one of the bases on which the myth that we use only 10% of our brain rests, since the brain is made up of approximately 90% white matter and only 10% gray matter.

But, although gray matter is apparently composed of material that only serves to connect today, it is known that the number and the way in which the connections are made significantly affect the functions of the brain, since if the structures are in perfect condition, but there are no connections between them, they will not work properly.

- Brain

The brain is in turn composed of multiple structures: cerebral cortex, basal ganglia, limbic system, diencephalon, brain stem and cerebellum.

Cerebral cortex

The cerebral cortex can be anatomically divided into lobes, separated by furrows. The most recognized are the frontal, the parietal, the temporal and the occipital, although some authors postulate that there is also the limbic lobe.

Frontal lobe (orange), parietal lobe (pink), occipital lobe (purple), temporal lobe (green).

The cortex is divided into two hemispheres, the right and the left, so that the lobes are present symmetrically in both hemispheres, with a right and a left frontal lobe, a left and right parietal lobe, and so on..

The cerebral hemispheres are divided by the interhemispheric fissure, while the lobes are separated by different sulci.

The cerebral cortex can also be categorized based on functions in the sensory cortex, association cortex, and frontal lobes.

The sensory cortex receives sensory information from the thalamus, which receives information through sensory receptors, except for the primary olfactory cortex, which receives information directly from sensory receptors.

Somatosensory information reaches the primary somatosensory cortex, located in the parietal lobe (in the postcentral gyrus).

Each sensory information reaches a specific point in the cortex, forming a sensory homunculus.

As can be seen, the brain areas corresponding to the organs do not follow the same order with which they are arranged in the body, nor do they have a proportionate size relationship.

The largest cortical areas, compared to the size of the organs, are the hands and lips, since in this area we have a high density of sensory receptors.

Visual information reaches the primary visual cortex, located in the occipital lobe (in the calcarine fissure), and this information has a retinotopic organization.

The primary auditory cortex is located in the temporal lobe (Broadman's area 41), being responsible for receiving auditory information and establishing a tonotopic organization.

The primary taste cortex is located in the frontal operculum and anterior insula, while the olfactory cortex is located in the piriform cortex.

The association cortex includes the primary and secondary. The primary association cortex lies adjacent to the sensory cortex and integrates all the characteristics of perceived sensory information such as color, shape, distance, size, etc. of a visual stimulus.

The secondary association cortex is located in the parietal operculum and processes the integrated information to send it to more “advanced” structures such as the frontal lobes, and for these structures to put it in context, give it meaning and make it conscious.

The frontal lobes, as we have already mentioned, are responsible for processing high-level information and integrate sensory information with motor acts that are performed to act in a manner consistent with the perceived stimuli.

In addition, it performs a series of complex, typically human tasks, called executive functions.

Basal ganglia

The basal ganglia are found in the striatum and mainly include the caudate nucleus, the putamen, and the globe pallidus.

These structures are interconnected and, together with the association and motor cortex through the thalamus, their main function is to control voluntary movements.

Limbic system

The limbic system is made up of both subcortical structures, that is, they are located below the cerebral cortex. Among the subcortical structures that comprise it, the amygdala stands out and, among the cortical ones, the hippocampus.

The amygdala is almond-shaped and is made up of a series of nuclei that emit and receive input and output from different regions.

Brain tonsil in light blue

This structure is related to multiple functions, such as emotional processing (especially negative emotions) and its effect on learning and memory processes, attention and some perceptual mechanisms.

The hippocampus or hippocampal formation, is a cortical area shaped like a seahorse (hence its name hippocampus from the Greek hypos: horse and campus: sea monster) and communicates bidirectionally with the rest of the cerebral cortex and with the hypothalamus.

This structure is especially relevant for learning, since it is in charge of consolidating memory, that is, of transforming short-term or immediate memory into long-term memory.

Diencephalon

Human diencephalon in red

The diencephalon is located in the central part of the brain and is made up mainly of the thalamus and hypothalamus.

The thalamus is made up of several nuclei with differentiated connections, being very important in the processing of sensory information since it coordinates and regulates the information that comes from the spinal cord, the trunk and the diencephalon itself.

So all sensory information passes through the thalamus before reaching the sensory cortex (except for olfactory information).

The hypothalamus is made up of several nuclei that are widely related to each other. In addition to other structures of both the central and peripheral nervous systems, such as the cortex, the trunk, the spinal cord, the retina and the endocrine system.

Its main function is to integrate sensory information with other types of information, for example, emotional, motivational information or previous experiences.

Brain stem

Brain stem in red

The brain stem is located between the diencephalon and the spinal cord. It is composed of the medulla oblongata, pons, and midbrain.

This structure receives most of the peripheral motor and sensory information and its main function is to integrate sensory and motor information.

Cerebellum

Cerebellum (cauliflower shape)

The cerebellum is located at the back of the skull, behind the trunk, and is shaped like a small brain, with the cortex on the surface and the white matter inside.

It receives and integrates information mainly from the cerebral cortex and the brain stem. Its main functions are the coordination and adaptation of movements to situations, as well as maintaining balance.

- Spinal cord

Spinal cord and brain.

The spinal cord runs from the brain to the second lumbar vertebra. Its main function is to connect the central nervous system with the peripheral nervous system, for example, taking motor orders from the brain to the nerves that innervate the muscles so that they give a motor response.

In addition, it can trigger automatic responses when receiving some type of highly relevant sensory information such as a prick or a burn, without that information passing through the brain.

References

1. Dauzvardis, M., & McNulty, J. (nd). Cranial nerves. Retrieved on June 13, 2016, from Stritch School of Medicine.
2. Redolar, D. (2014). Introduction to the organization of the nervous system. In D. Redolar, Cognitive Neuroscience (pp. 67-110). Madrid: Médica Panamericana SA