The Central Nervous System (CNS)

The central nervous system (CNS) consists of the brain, the spinal cord, and the retina. It is the command center of the body, powering everything that we do and interpreting everything around us.

This system continues developing throughout the entire human lifetime, constantly creating and eliminating neurons to keep the standard nervous system functioning refreshed.

The central nervous system works by taking in the information collected by the peripheral nervous system and redistributing information based on that. It controls everything from a sneeze to the rate of your heartbeat.

The CNS contains about 86 billion nerve cells in total, this is more than 12x the amount of people on planet Earth right now! All of them are connected by trillions of nerve fibers – axons and dendrites – that facilitate the transmission of information throughout the entire nervous system.

The Brain

Humans have the largest brain of all vertebrates relative to body size (Lewis, 2018). Despite what you may have heard, though, size doesn’t actually matter.

Many have believed for quite a long time now that brain size is a direct correlate to intelligence in animal species, however, there is not much scientific evidence to support this idea. What is more accurate to say is that the ratio of brain to body size is a more dependable measure of intelligence.

In addition to having the largest brain relative to body size, humans also have more neurons per unit volume than several other species. A major reason for this is the amount of surface area on the human brain. The surface area in the brain is created by all of the folds in the tissue. The more folds, the more surface area.

The human brain weighs about 3.5 pounds and makes up approximately 2% of the average adult body weight and controls almost everything we do. “Wait, almost?” you ask, incredulously. Yes, almost… kind of.

The brain does not directly control movements like reflexes – these actions pass through your spinal cord at most because of the speed with which they need to happen. Here is what the brain is made up of and what it does control, though.

Structures and Functions of the Human Brain

The entire CNS is made up of “gray matter” and “white matter.” Gray matter is made up of neurons, the cell type that forms the basis for all types of nerve cells.

The largest part of the human brain is cerebrum, which is derived from the forebrain, or the prosencephalon. The cerebrum is divided into two hemispheres which are connected by the corpus callosum. This region of the brain governs the sensory, motor, and cognitive functions of the brain.

The left hemisphere of the cerebrum contains important regions that play an integral role in speech and language. It is also associated with mathematics and retrieval of information.

The right brain plays a key role in visual and auditory processing, spatial skills and artistic ability. But, all of these functions involve both sides of the brain so the popular notion of being a “left- or right-brained” is poorly supported by evidence. Sorry to kill the vibe.

The outermost layer of the cerebrum, known as the cerebral cortex is considered to be “the hub of thought,” and consists of the following structures:

  • Frontal lobe: controls emotions, problem-solving, learning, memory, and more.
  • Parietal lobe: governs spatial orientation and navigation by integrating information gathered by numerous sensory systems
  • Temporal lobe: processes auditory stimuli and assists in language; includes the hippocampus and amygdala which play primary roles in memory and emotion, respectively
  • Occipital lobe: regulates visual processing

Corpus callosum. Yes, it definitely sounds like a spell you’d learn at Hogwarts, but, believe me, it’s not (I tried it). This is a collection of nerve fibers that connects the left and right hemispheres of the brain.

Oddly enough, the left brain controls all the muscles on the right side of the body and vice versa. Left- or right-handedness is the result of one hemisphere being dominant over the other.

The brainstem is derived from the midbrain (mesencephalon) and hindbrain (rhombencephalon) and is the connection between the spinal cord and the brain. It consists of the medulla oblongata, pons, and midbrain. The major responsibilities of the brainstem are to

  • Relay information between the brain and body
  • Supply cranial nerves to the face and head (nerves extend from this region of the brain to these areas)
  • Perform critical functions in controlling the heart, respiration, and consciousness.

Between the cerebrum and the brainstem are the thalamus and hypothalamus. The hypothalamus relays sensory and motor signals to the cortex and is involved in regulating consciousness, sleep, and alertness. The hypothalamus connects the nervous system to the endocrine system via the pituitary gland

The cerebellum, which develops from the hindbrain is essential to motor control in that it partially regulates coordination and balance. This structure may have some cognitive functions as well.

Protection and Foundational Composite of the CNS

The organs of the nervous system are very well-protected and securely supported by tissues and organ structures throughout the body.

As they should be! You don’t want your nervous bits swinging around every time you make a move, do you? Of course not!

To get the utmost protection, parts of the CNS are given up to three distinct forms of protection. Both the brain and spinal cord are first protected by layers of membrane called the meninges. The brain is further encapsulated by the skull, and the spinal cord by the vertebrae.

Third, there is the cerebrospinal fluid: this is incredibly important in that it provides a buffer that reduces the force of any potential impact between the brain and skull, or between the spinal cord and vertebrae.

The brain is further encapsulated by the skull, and the spinal cord by the vertebrae. Third, there is the cerebrospinal fluid: this is incredibly important in that it provides a buffer that reduces the force of any potential impact between the brain and skull, or between the spinal cord and vertebrae.

It has already been mentioned that the CNS is made up entirely of two major categories of nerve cells: grey matter and white matter. Gray matter is comprised of neuron cell bodies and their associated dendrites, glial cells, capillaries (they provide the blood supply through the brain tissue, which is what makes it pink!).

Gray matter is found primarily in the outer layers of the brain, and in the spinal cord. You can identify it in the spinal cord, as this is what forms the inner “butterfly” shape you can see in a cross-section of the spinal cord.

White matter, on the other hand, makes up areas of the CNS which have the majority of axons. White matter is underneath gray matter in the brain, while in the spinal cord, it is the external layer surrounding the gray core.

The Spinal Cord

The spinal cord only weighs about 35g and is only about 1cm in diameter. It is protected by the 26 vertebrae and can be divided into five regions, altogether having 31 pairs of nerves extend from the sides of the vertebral column into the distal portions of the body. The five regions of the spinal cord are as follows:

  1. Cervical (neck): 8 pairs of nerves
  2. Thoracic (chest): 12 pairs of nerves
  3. Lumbar (abdominal): 5 pairs of nerves
  4. Sacral (pelvic): 1 pair of nerves
  5. Coccygeal (tailbone): 1 pair of nerves

Gray matter in the spinal cord is what constitutes the buttery silhouette. Each “wing” is referred to as a “horn.

These horns are a mass gathering of sensory neurons, and all of it is surrounded by white matter, functioning as the myelin sheath does to an axon. (So you can think of the spinal cord as the Mother of All Neurons, with every nerve of the peripheral nervous system acting as the dendrites, the spinal cord as the axon and the brain as the nucleus.)

The Retina

The retina is the neural portion of the eye. It originates from an “out-pocketing” of the diencephalon called the optic vesicle which undergoes invagination to form the optic cup.

Specifically, it is the inner wall of this optic cup that gives rise to the retina, while the outer wall gives rise to the pigment epithelium. The pigment epithelium is what makes your eye blue, brown, etc. as it contains the melanin.

The melanin isn’t just for looks, either. It functions to reduce backscattering of light that enters the eye, making it easier (and possible) to see.

The pigment epithelium also plays an important role in the maintenance of photoreceptors as well, renewing photopigments and phagocytosing the photoreceptor disks. This is done at an incredibly high rate, essential to maintaining a sense of vision. 

Photoreceptors send electrical impulses to the retina, the neurons of which then sends this information to the brain via the optic nerve.

There are five types of neurons in the retina:

  • Photoreceptors
  • Bipolar cells
  • Ganglion cells
  • Horizontal cells
  • Amacrine cells

The photosensitive nerve cells in the retina are the rods and cones. Both have an outer segment that is composed of membranous disks that contain photopigment, allowing for the intake of color-specific auditory stimuli.

All of these structures, led by the brain, spinal cord, and retina, and connected by trillions of synapses, regulate the core processes our body needs to keep us alive and breathing.

References

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. The Retina. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10885/