NERVOUS SYSTEM OVERVIEW

Medically Reviewed and Updated by Dr. Sony Sherpa (MBBS) - September 25, 2024

The nervous system is a vital component of the entire body, as without it, we would not be able to experience life at all.

It is the enabling factor that allows us to feel, interact with, and make sense of the world around us. Every experience, whether guided by pain, pleasure, or neutrality, is a product of how the nervous system receives, organizes, and responds to sensory information.

The nervous system of humans is more evolved than most organisms on the planet, bestowing us with the power of thought. Through the complex interactions of human neurons, we are not only able to experience our existence but are also able to participate in it, reflect upon it, learn from it, and make significant contributions toward outcomes moving forward.

The following overview briefly details all the components of a working nervous system and gives insight into how all the parts work together to create our lived experience.

Nerve Cells and Their Functions

Not all neurons are identical, nor do they all perform the same functions. The nervous system as a whole is responsible for receiving sensory information, processing it, and generating appropriate responses.

Different parts of the nervous system specialize in various aspects of these main functions. Some components are dedicated to receiving, processing, or responding to specific types of sensory information. Other parts play more generalized roles, integrating all three functions within particular contexts or systems.

Branches of the Nervous System

The nervous system comprises two main branches: the central and peripheral nervous systems, with each branch having many subdivisions that are categorized according to their primary functions. The central nervous system consists of the brain and spinal cord, while the peripheral nervous system consists of the neurons extending out to all other areas of the body.

A. The Central Nervous System

The central nervous system can be likened to the central processing unit (CPU) of a computer.

It comprises the brain and spinal cord, working to take in all sensory signals that neurons of the peripheral nervous system retrieve. This sensory information is then processed by the central nervous system before an appropriate response is enacted by giving the peripheral nervous system neural feedback.

1. The Brain

The brain is somewhat of a master organ, seated at the top of the body and governing the affairs of all the tissues below.

It is responsible for coordinating responses between tissues, the body, and the environment. The brain makes sense of all the nerve signals the peripheral nervous system generates through its contact with the outer environment and the internal tissues of the body.

Barriers That Protect the Brain

The skull protects the brain, which happens to be one of the body’s most sensitive organs. Inside the skull, the brain is surrounded by glands, known as the meninges, and lots of fluid, known as cerebrospinal fluid. These components further protect the brain, cushioning it from harm and absorbing shock upon impact. They also serve as the brain’s wing of the lymphatics system, allowing for toxins to be removed from the brain and for infections to be swiftly contained by the immune system. The brain is further protected from hazardous material in the blood thanks to the filtration power of the blood-brain barrier.[1]

Functional Components of the Brain

On the outside, the brain looks like a mass of wrinkly tissue. Through many decades of scrutiny, neuroscientists have managed to establish a basic framework for how this mass of gelatinous tissue manages to organize reality for each and every person.

The brain is a very complex organ with many compartments that all work together for optimal function. Parts of the brain have been divided both anatomically and according to function. Most anatomical regions correlate to one or more unique functions; however, many functions require the use of multiple parts of the brain.

In complex organisms across the planet, the brain can be divided into three simple segments: the hindbrain, midbrain, and forebrain. In humans, they comprise of the following:

• The Hindbrain consists solely of the cerebellum.
• The Midbrain resides in and forms a functional part of the brain stem.
• The Forebrain comprises the cerebral cortex and the diencephalon, which refers to the thalamus, hypothalamus, epithalamus, and subthalamus[2].

The cerebral cortex makes up ±80% of the human brain and comprises areas that are involved in exquisitely complex processes. This was the most recent part of the brain to develop in our evolution, distinguishing us from other known complex organisms on the planet.

The rest of the brain, resting underneath all the cerebral folds, pertains to survival instincts, involuntary responses, and regulating physiological mechanisms in the body. These parts of the brain include the brain stem, cerebellum, hypothalamus, thalamus, pituitary and pineal glands. Most complex organisms on the planet have these brain components, and they are ancient in their evolutionary origins.[3]

Brain Stem

The brain stem sits at the top of the spinal cord and controls very important functions necessary for basic survival. All functions of the brain stem pertain to tasks that we have no conscious control over, such as breathing, the beating of the heart, blinking, and blood vessel dilation. It is also involved in maintaining taste, touch, hearing, and our sense of balance.[4]

The brain stem connects the spinal cord to the rest of the brain and relays sensory signals back and forth between the two. It sorts through some of this sensory information and channels it to the appropriate brain compartments. Muscle movements and the five senses are separated through the different parts of the brain stem, which also control different bodily functions.

The brain stem can be divided into three sections:

• Midbrain. The midbrain rests slightly above the pons and is responsible for sorting through sensory and motor information. It generally relays information between the pons and spinal cord, as well as between the cerebral cortex and thalamus. Furthermore, the midbrain has control over some of the automatic reflexes of the eyes, head, and torso.
• Pons. The pons forms a slight bulge in the side of the brain stem and connects the bottom of the brainstem to the top. Pons is a word that translates to mean ‘bridge,’ literally bridging the medulla and the midbrain. The pons generally directs motor nerve signals toward the cerebellum via the midbrain. It also relays specific sensory information pertaining to touch to the higher regions of the brain.
• Medulla Oblongata. The medulla oblongata or medulla is the part of the brain stem responsible for controlling most of the reflexes that are involuntary, such as the heartbeat, swallowing, salivation, taste, hearing, and keeping our center balance. All the motor signals from either side of the body, left and right, cross over in the medulla and correspond to the opposite sides of the brain at large. Due to this wiring, our left brain controls the right side of the body while the right brain controls the left.

Cerebellum

The word cerebellum translates from Latin to mean ‘little brain’. The cerebellum is responsible for coordinating complex muscle movements. It is also involved in keeping our sense of balance and maintaining posture.

The cerebellum achieves coordination of muscle movements by relaying nerve impulses from the motor cortex (part of the cerebral cortex) to the spinal cord and vice versa. With the help of the thalamus, it is able to modify the neuronal instructions it receives back from the motor cortex. In this way, the cerebellum fine-tunes the neural signals preceding complex movement.

There is also evidence that motor memory in the cerebellum can store instructions for frequently practiced movements.

Diencephalon: Hypothalamus, Thalamus, Pineal and Pituitary

The hypothalamus and thalamus, together with the subthalamus and epithalamus, form a part of the brain known as the diencephalon. Together, the diencephalon and cerebral cortex form the human forebrain.

The diencephalon regulates many important functions of the body by monitoring the bloodstream and nervous impulses, emitting neuro-endocrine signals, and directing sensory and motor information from the brain stem to relevant areas of the brain.

Each part of the diencephalon has unique functions:

• The hypothalamus regulates temperature, hunger, thirst, metabolism, reproduction, emotional responses, water retention, aspects of the sleep-wake cycle, and smooth muscle contraction, including vasoconstriction, peristalsis, and bladder contraction. It also relays signals from the olfactory bulb, which is the unique part of the brain that receives sensory information pertaining to smell.[5]
• The Thalamus (also known as the dorsal thalamus) plays a role in maintaining our sense of consciousness, mood, motivation, and cognition, as well as regulating pain, pain perception, and arousal. Sensory impulses pertaining to each of the senses except smell are relayed through the thalamus to the appropriate brain areas. In this way, it monitors most of our senses and maintains homeostasis by responding with neuro-endocrine instructions that tend to either increase or decrease neuronal activity. It also processes motor signals relating to speech and language.[6]
• The Subthalamus (or ventral thalamus) rests beneath the thalamus and basically adds to and regulates aspects of its functionality. It specifically deals with movement regulation, and in addition to being part of the diencephalon, it forms part of the basal ganglia, which also regulates movement.[7]
• The Pineal Gland (also called the epiphysis cerebri) forms the most important part of the epithalamus, alongside the habenular complex[8]. The pineal gland is known as the “third eye” because it responds directly to light in order to regulate the sleep-wake cycle. Its main function is to produce melatonin, the sleep hormone, which acts to regulate many functions in the body indirectly through the circadian rhythm.[9]
• The Pituitary Gland is one of the main centers for endocrine regulation in the brain, receiving input from the rest of the diencephalon via the hypothalamus. In this respect, the hypothalamus is the main regulator of the pituitary gland. The pituitary secretes and regulates growth hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, oxytocin, thyroid-stimulating hormone, adrenocorticotropic hormone, antidiuretic hormone, dopamine and melanocyte-stimulating hormone – a testament to its importance in maintaining homeostasis. These hormones pertain to growth, hormonal fluctuations, metabolism, reproduction, bonding, thyroid function, adrenal function, and kidney activity.

Cerebral Cortex

The cerebral cortex is comprised of the left and right hemispheres of the brain and constitutes approximately 80% of the brain mass in humans.

Where other animals have a forebrain that is responsible for decoding sensory and motor signals directed from the hindbrain and midbrain, the human forebrain is capable of so much more than just central neuro-processing. Through this amazing feat of evolution, the cerebral cortex contains many more brain areas that allow us to reflect upon ourselves and our experiences.[10]

The many folds of the cerebral cortex increase the size of the human brain even more by virtue of increasing the surface area. Furthermore, humans have the ability to further increase the size of the brain through learning new things and practicing what one has learned. This has been demonstrated through observing the brains of artists, logicians, and musicians, who all have developed and increased the size of the relevant brain areas that correlate with their intellectual habits.

Parietal Lobe

The parietal lobe sits at the top of the cerebral cortex, extending from the middle (crown) of the head toward the back of the head, where it joins the occipital lobe. This part of the brain is responsible for integrating all the sensory information from the body and the occipital lobes, processing this information before issuing an appropriate response back to the rest of the body. Before we can understand what we are perceiving on a sensory level, our brain needs to process it in this area. Damage to this area can affect reading, writing, control over the eyes and fingers, speech, and many more important bodily functions that are usually taken for granted.

The somatosensory cortex is a very important part of the parietal lobe that deserves an extra special mention.

Somato-Sensory Cortex

The somatosensory cortex is the region of the brain that rests between the parietal lobe and frontal lobe. Technically, it forms part of the parietal lobe. However, it has been highlighted as a very important part of the brain and has been singled out for its functions. On the outside, this region rests just under or close to the crown of the head. It is here that the majority of motor information is sent, as well as processed sensory information from the rest of the parietal lobe.

One portion of the somatosensory cortex receives this information before translating it into a response, which is typically carried out in the adjacent primary motor area (a part of this sensory cortex). From the primary motor area, nervous system responses are sent back down via the brainstem and to the rest of the body.

Frontal Lobe

The frontal lobe is the predominant lobe hailed by modern man for our ability to logically rationalize and interfere with emotional responses using reason. Its position is in line with its name, being situated in the front of the head and eventually meeting up with the somatosensory cortex of the parietal lobe.

This area of the brain is also involved to some extent in processing motor information, as well as giving us our sense of identity or personality, intervening in our behavior, and contributing to our decisions through self-reflexivity.

Occipital Lobe

The occipital lobe is located at the back of the head, just above the cerebellum. It plays a crucial role in processing visual information from the eyes, allowing us to interpret what we see. Damage to the occipital lobe can lead to partial or complete loss of vision. Additionally, disruptions in this area can sometimes cause visual hallucinations, although other brain regions can also be involved in such phenomena.

Temporal Lobes

The temporal lobes rest on either side of the head, making up most of what is considered the left and right brain hemispheres. These lobes are largely responsible for processing the auditory signals from our ears and are quite sophisticated in their ability to distinguish between different sounds. Language, music, and “white noise” all get filtered through the temporal lobes.

Additionally to processing auditory signals, the temporal lobe is also responsible for many aspects of complex thought, visual memory, speech, and comprehension of language. The temporal lobes contain brain areas relevant to learning, memory, cognition, reason, and all functions that are typically associated with both left and right brains, such as art and mathematics. The temporal lobes are also involved in our ability to orient in the space around us.

Corpus Callosum

This is the area of the brain that connects the right and left hemispheres and integrates sensory and motor information from both sides.

Important Brain Areas in the Cerebral Cortex

Hippocampus

The temporal lobes, together with the frontal lobe, are also largely involved in emotional processing, decision-making, critical analysis, and problem-solving. One of the main areas that pertain to these functions would be the hippocampus, which is actually a surprisingly tiny section of the brain. However, without it, learning is not possible.

Amygdala

The hippocampus connects to the amygdala[11] and a few other brain areas. The amygdala is involved in the stress response and tends to hold information pertaining to negative situations that have taught us to avoid those inputs, such as when a child burns their hand for the first time.

Limbic System

With the addition of a few other areas throughout the entire brain, the hippocampus and adrenal cortex form part of the limbic system[12]. This system governs our ability to judge a situation for what it is and act appropriately. It also moderates motivation, social interaction, memory, and emotional reactions or impulses. Impairment of the limbic system tends to limit a person’s ability to perceive future consequences of their actions, reduce empathy, increase impulsive behavior, and impair visual depth perception.

Alcohol affects the limbic system, which is involved in emotion and decision-making, leading to impaired judgment and poor decision-making. This impairment is a key reason why driving under the influence of alcohol significantly increases the risk of accidents. Chronic alcohol use can cause severe damage to the limbic system, resulting in a reduced awareness of the consequences of one's actions.

2. Spinal Cord

The main purpose of the spinal cord is to relay signals between the brain and the peripheral nervous system. Some components of the peripheral nervous system are found inside the spinal cord; however, it is mostly dominated by central nervous system activity.

The spinal cord can be divided into four unique segments that correlate with the bone structure of the spinal column, including the cervical region, lumbar region, thoracic region, and sacral region[13]. From each section of the spinal column, nerve fiber bundles feed through to the rest of the body and relay sensory information between the brain and the peripheral nervous system. There are 31 pairs in total, with each pair being projected out on either side of the spine, correlating to a specific segment or spinal vertebrae. 

There are two main types of nerves projecting from the spinal cord: afferent and efferent nerves.

• Afferent nerves carry sensory information and extend out to muscles, skin, joints, and organs.
• Efferent nerves carry motor information and extend to bones, the heart, smooth muscle, glands, and secretory cells.

While both types exert a two-way communication with the spinal cord and brain, the afferent nerves tend to pick up sensory information and transmit it back, while the efferent nerves tend to exert motor information, informing tissues how and when to contract and/or function in response to sensory input.[14]

Through continuous two-way feedback between the brain, spinal cord, and peripheral nervous system, we are able to experience reality and interact with it.

B. The Peripheral Nervous System

The peripheral nervous system is basically a sophisticated extension of the spinal cord that protrudes out to all cellular tissues in the body.[15] Just like the spinal cord, the nerves of the peripheral nervous system can largely be divided into sensory afferent nerves and motor efferent nerves. Many of the nerves that run from the spinal cord extend long distances to reach the periphery of the body, like the hands and feet. The nerves running from the base of the spine can even reach beyond a meter, matching the length of a person’s legs.

The peripheral nervous system is divided into two parts: the autonomic nervous system and the sensory or somatic nervous system. The peripheral nervous system is involved in regulating vital bodily functions, such as breathing, the heartbeat, blood flow, digestion, movement, and immune responses.

1. Autonomic Nervous System

The autonomic nervous system largely controls the body's involuntary processes, which are important for its functioning. Processes of the autonomic nervous system are largely involuntary or unconscious, accounting for all muscle reflexes that we do not have conscious control over.[16]

The autonomic nervous system can be divided into three parts: the sympathetic and parasympathetic nervous systems and the enteric nervous systems.[17]

The sympathetic and parasympathetic nervous systems are complementary in their actions and tend to inhibit the activity of the other. The latter is responsible for the stress response, while the former is responsible for relaxation (the opposite of the stress response).

The enteric nervous system is capable of functioning independently of the other two parts of the autonomic nervous system. However, as it largely controls digestive processes, it is influenced by the activation of either of the other two systems.

Sympathetic Nervous System

The sympathetic nervous system is responsible for inducing a stress response, another name for which is the ‘fight or flight’ response. This part of the autonomic nervous system is much larger than its parasympathetic counterpart and innervates all organs along the alimentary canal. This includes the esophagus, stomach, digestive tract, liver, pancreas, spleen, and so on, all the way down to the adrenal glands, kidneys, bladder, and reproductive organs. Furthermore, the sympathetic nervous system affects the heart, muscles, skin, and nearly all other parts of the body.

The ‘Fight or Flight’ Response

During a stress response, the sympathetic nervous system acts largely to suppress the parasympathetic nervous system as well as the enteric nervous system.

Digestion stops, and blood flow is directed away from the middle of the body toward the periphery, such as the legs, feet, arms, and hands. The blood vessels also become constricted, with a state of hypertension being a prime result. The heart rate becomes elevated, sweating ensues, and a number of reactions can result, depending on the state of stress. Running, hiding, or freezing with fear are the flight components to the response whereas self-defense, facing the source of the fear, and getting angry constitute the fight portion.

All glucose reserves in the body are quickly made into energy to sustain muscle activity (increased contractions) during a stress response to allow for a quick escape or to provide stamina for a physical struggle. This energy is quickly expended, and thus, prolonged stress depletes the body’s energy reserves and has also been shown to induce damage in various tissues, such as in the heart.

The stress response serves to keep us alive when our lives are threatened. These days, it is rare that we need to activate our stress response for an extreme danger, such as a predator in the wild. Many people, nevertheless, are overly stressed for other reasons, and the stress response that ensues is often misappropriated in the modern setting (e.g., anger, panic, avoidance, depression, fear of future outcomes).

Sympathetic Responses and the Sleep-Wake Cycle

The sympathetic nervous system is permanently activated, even when no stress response is occurring. It plays a role in regulating our sleep-wake cycle, and without it, we would never be able to wake up, remaining stuck in a comatose state.

Sympathetic nervous activity promotes the secretion of adrenal “stress” hormones, collectively referred to as glucocorticoids. Small amounts of stress hormones are required to keep us awake during the day. Generally, cortisol spikes first thing in the morning and steadily declines throughout the day, reaching its lowest point during the evening. When we sleep, cortisol sees a slow and steady build-up until first thing in the morning, when it reaches its peak again, and we wake up to start the day (hopefully) feeling refreshed.

Stress, Cognition and Learning

Small quantities of elevated stress can also help one to perform mental tasks better. However, too much stress lowers our cognitive abilities, directing blood away from the brain and increasing our physical abilities as well as energy expenditure. Heightened stress directly inhibits our ability to learn new information and retain memory. See our article on how memory works for more information.

Sympathetic Immune Activity

The activity of the sympathetic nervous system extends to the lymphatics system, thymus, and spleen, all of which are physical components of the immune system. The sympathetic nervous system can regulate immune function by either promoting or suppressing inflammation. Furthermore, sympathetic activity is largely involved in our pain response and our perception of pain.

Parasympathetic Nervous System

The parasympathetic nervous system is the counterpart to the sympathetic nervous system and is largely responsible for suppressing the stress response. Parasympathetic nervous activity is also referred to as either ‘rest and digest’ or ‘feed and breed,’ alluding to its primary functions. This part of the autonomic nervous system is much smaller than the sympathetic component, only innervating the head, visceral organs, and genitalia while ignoring many muscles, bones, and skin. Actions of this system tend to be inverted to that of the sympathetic nervous system, calming heart rate, increasing digestion, and enhancing blood flow to the center of the body.

The Vagus Nerve

75% of the parasympathetic nervous system comprises the vagus nerve, which is attached to the entire digestive tract from beginning to end. The vagus nerve is responsible for regulating digestion, inducing optimal peristalsis (muscle contractions in the gut that move food along), and ensuring the right enzymes are released for breaking down food into cell-absorbable nutritional components. The vagus nerve also impacts the heart and lungs, lowering heart rate and helping us to breathe in a more relaxed manner. Furthermore, it is involved in taste, stimulation of saliva and hunger, regulating the mucosa of the throat, and picking up information from the outer ear such as temperature.

Parasympathetic Activation of the Immune System

The vagus nerve is also stimulated in response to pathogens in the gut or foods that may cause harm. It can activate the sympathetic nervous system and trigger an immune response when faced with such stimuli. In animal studies where the vagus nerve was eliminated, inflammatory, allergic, and asthmatic reactions were either completely attenuated or greatly reduced.

Enteric Nervous System

The enteric nervous system mostly serves fluid regulation in the gut as well as to ensure adequate muscle contractions during peristalsis and other digestive processes. Nerves of the enteric system only innervate the gut, with there being two main nerve complexes: the myenteric and the submucosal.

Myenteric nerves ensure that the gut contracts during peristalsis and work closely with the sympathetic and parasympathetic nervous systems, which instruct it when digestion is appropriate or not. The submucosal nerves control the osmotic movement of fluids and electrolytes across the digestive tract.

2. Somatic Nervous System

The somatic nervous system mostly innervates all muscles in the body, receiving sensory information and affecting motor responses that end up in muscle contraction and usage. Somatic nerves begin in the spinal cord and travel long distances to carry out their functions. Unlike the autonomic nervous system, the somatic nervous system does not innervate glands, organs, or the muscles that pertain to their functionality.[18]

The enteric nervous system also relies on sensory reception from the somatic nervous system to carry out its functions, as do other aspects of the peripheral nervous system. Sensory information travels first back to the brain before any actions can be carried out by the other branches of the peripheral nervous system.[19]

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