Central Nervous System

Introduction

The central nervous system translates sensory stimuli received via our sensory organs of sight, sound, smell, taste, and touch, as well as from the internal organs. By coordinating and consolidating sensory information and actions, our brain assumes the role of being the center for our thoughts, the interpreter of our external environment, and the controller of the body movements.

Anatomy and Structure of the Central Nervous System

The central nervous system is comprised of the brain and the spinal cord. While the brain is protected by the skull, the spinal cord is protected by the vertebrae. Additionally, both of these structures of the central nervous system are enclosed and protected by the meninges.

The Brain

The brain is the most vitally important organ in the human body, and it’s one of its major consumers of oxygen. The gray and white matter of the brain share the needed portion of the oxygen in uneven parts.

Whereas the gray matter consumes 94% of the total oxygen supply needed by the brain, the white matter consumes only 6% of the remaining supplies.

Even though it’s symmetrically divided into two hemispheres, the brain isn’t symmetrical when it comes to the distribution of the neurotransmitters located in the gray matter. Namely, the left hemisphere of the brain contains more neurons than the right hemisphere of the brain. This, in part, explains the fact that the majority of people are right-handed since the left brain hemisphere controls the right side of our body.

The Brain Stem

The brain stem performs the most vital, automatic functions of the human body. Basically, it relays all the information and signals from the brain to other parts of the body via the spinal cord. Also labeled as the medulla, this structure of the cerebral cortex is one of the parts of the brain that connects the cerebrum with the spinal cord.

This part of our brain controls the bodily functions that we are not normally aware of by sending signals to the involuntary muscles (muscles whose activity is independent). Those are the muscles found around our heart, lungs, neck, legs, as well as abdomen.

Even though it’s small, the brain stem is actually a very powerful part of our brain. It’s so powerful, it regulates one of the most essential processes of our body – breathing. But the list of its ‘super-powers’ does not end here. It also includes swallowing, eye-movement, chewing – all of the automatic body processes, in fact.

White and Gray Matter of the Brain

Gray matter covers the cortical structures, but it’s also present in the brain, spinal cord, brain stem, cerebellum, and the medulla oblongata. It serves as a center of several specialized regions of the brain that regulate and control memory, attention, thought, language, consciousness, perception, etc.

Spinal Cord

The spinal cord is a long and thin tube-like structure that serves as a communication path between the brain and the body. It’s a center for coordinating many reflexes of the body.

This structure of the central nervous system is also the location of groups of spinal interneurons that are responsible for the control of the motor commands related to rhythmic movements, such as walking.

Cranial Nerves

Conventionally, there are 12 pairs of cranial nerves that stem directly from the central nervous system. Each cranial nerve has a different function for either sense or movement as well as a name that reflects its function. They are numbered according to their exit location in the skull:

 

  • Olfactory nerve (CN I) – responsible for the sense of the smell.
  • Optic nerve (CN II) – Responsible for vision.
  • The oculomotor nerve (CN III) – responsible for the coordination of eye movement.
  • Trochlear nerve (CN IV) – responsible for the coordination of eye movement.
  • Trigeminal nerve (CN V) – responsible for the sensations on our facial skin and for control of the muscles responsible for chewing.
  • Abducens nerve (CN VI) – responsible for eye movement.
  • The facial nerve (CN VII) – responsible for the control of facial expression.
  • Vestibulocochlear nerve (CN VIII) – responsible for both hearing and balance.
  • Glossopharyngeal nerve (CN IX) – responsible for salivation, oral sensation, and taste.
  • Vagus nerve (CN X) – responsible for heart rate and digestion.
  • The accessory nerve (CN XI) – responsible for the sternocleidomastoid muscle.
  • The hypoglossal nerve (CN XII) – responsible for the movement of the tongue.

Cerebellum

The ‘little brain’, or the cerebellum, helps with the transfer of signals from other parts of the brain to the motor neurons in order to control and coordinate the movements of our body. These motor movements include balance, movement coordination, i.e. walking, swaying of arms, body posture, feet flexing, etc. It’s positioned between the brain stem and the cerebrum.

Since it receives information from the sensory systems, the cerebellum also plays a significant role in cognition, i.e. learning, speaking, processing emotions, maintaining attention, etc. If the cerebellum suffers an injury, we won’t be able to control and coordinate our movements, nor maintain balance.

Diencephalon

The diencephalon plays a very important role in the regulation of sleep, alertness, and wakefulness. It also manages our emotions, memories, appetite, blood pressure, and sleep, and has a role in childbirth.

This cerebral cortex structure consists of 4 elements: the epithalamus, thalamus, subthalamus, metathalamus, and the hypothalamus. It also enfolds a cavity of the brain labeled as the third ventricle. The third ventricle of the diencephalon is filled with cerebrospinal fluid.

Cerebrum

The cerebrum is responsible for both the regulation and control of the most complex mental processes: language and the processing of information. The subcortical structures of the cerebrum include the hippocampus, basal ganglia, and the olfactory bulb.

Being the largest structure of the central nervous system, the cerebrum supports the initiation and coordination of movement, regulation of temperature, touch, vision, hearing, judgment, reasoning, problem-solving, emotions, as well as learning.

Disorders of the Central Nervous System

The disorders of the central nervous system can occur due to various reasons, such as:

  • Problems with the blood supply in the brain;
  • Deterioration of brain cells;
  • Abnormal development of the central nervous system in the womb;
  • Infection of brain tissue;
  • Cell damage;
  • The aging process.

 

  • Stroke – When the blood supply in the brain is obstructed by damage to the blood supplying arteries or a blood clot, the neuron cells start to die out. In turn, the destruction of neuron cells leads to dysfunction of the cortical structures and a stroke.
  • Blockage of the blood vessels – Longer periods of high blood pressure or diabetes may cause damage to the tiny blood vessels of the brain structures. The blockage of arteries that cause a stroke can occur as a result either of localized blockages in tiny blood vessels deep within the brain or due to a blockage triggered by a fragment of material that has reached the brain through the blood supply network.
  • Thrombus – The blood flow in the brain slows down when fatty deposits accumulate on the neuron cell walls. These accumulations turn into blood clots or thrombuses that obstruct the supply of oxygenated blood. When the brain tissues are deprived of blood and nutrients, a stroke is inevitable.
  • Internal bleeding of the brain – The intracerebral hemorrhage is one of the main causes of stroke. It mostly affects patients that suffer from hypertension. Namely, the high values of blood pressure strain the blood supplying arteries in the brain, which leads them to rupture and result in bleeding.
  • Multi-infarct dementia – Dementia can occur due to clots that block the small blood vessels whose function is to supply the brain. By this, they trigger infarcts in small areas of brain tissue.
  • Meningitis – Inflammation of the meninges is usually triggered by a virus or bacterium. Meningitis typically occurs due to the Meningococcus bacteria. The symptoms include headache, fever, nausea, vomiting, stiff neck, as well as discomfort caused by a bright light. The outermost layer, the dura mater, is often less affected than the other two meninges of the brain – the arachnoid and the pia mater.

Fun Facts

Did you know?

  1. Vertebrae refers to a complex structure composed of bone and hyaline cartilage.
  2. Our brain (the brain stem included) grows and develops by exercising, just like any other muscle in the human body. We’re not talking only about physical exercises, but of brain training as well. This includes all cognitive processes – learning languages, logical tasks, arts, memorizing patterns, dance choreographies, etc.
  3. The brain stem is made of a complex net of neurons that help us wake up.
  4. The trigeminal cranial nerve is the largest of our cranial nerves and has both sensory and motor functions.
  5. Smoking negatively affects the gray matter of the brain. It causes loss of the gray matter volume of the smoker’s brain.
  6. The thalamus is responsible for our consciousness by keeping us awake, active, and alert.
  7. The epithalamus is involved in the regulation of our feelings and emotions.
  8. The corpus callosum is the largest white matter structure, comprising more than 200 million axonal projections.
  9. There are around 1000 billion neurons in the human brain.
  10. Because of their appearance, the basal ganglia are also called “islands” of gray matter.