The nervous system is a complex network of organs and nerve fibers in animals that work in tandem to sense environmental changes and coordinate its actions by transmitting signals to and from different parts of their body.
The nervous system is found in almost every multicellular organism, including us, but differs widely in the level of complexity. Sponges, placozoans, and mesozoans are the only multicellular organisms that have no or simplified nervous system.
The nervous system’s complexity, especially in humans, makes it one of the most extensively researched fields in the medical sciences. Below, we have compiled some of the most intriguing facts about the nervous system you probably don’t know.
Some Interesting Short Facts
1. The nervous system is made up of made up of two parts; the central nervous system and the peripheral nervous system.
2. The spinal cord work in tandem with the brain to transmit signals to and from the brain to different parts of the body. However, in a few cases, the spinal cord can act independently. In other words, your spinal cord can send signals directly to the muscles, without the consent of your brain.
3. The human spinal cord is Divided Into 31 Nerve Segments, 12 of them are thoracic, eight cervical, five lumbar, five sacral, and one coccygeal. In comparison, a cow has 37, and a horse has 42 spinal cord segments.
Segments of the Spinal Cord | Image Courtesy: Cancer Research UK
4. The electrical impulses conducted by the nerve cells travel at a maximum speed of 100 meters per second.
5. The average weight of the brain of an adult is about 1400g or 3 pounds. However, it requires about 20 percent of your total blood and oxygen supply.
6. Perhaps the longest neurons in an average human body are actually a part of the sciatic nerve (largest nerve), running from the toe to the spine.
7. The Difference Between Central And Peripheral Nervous System
In vertebrate, the nervous system is divided into two parts; the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is composed mainly of the brain and spinal cord.
The peripheral nervous system, on the other hand, is made up of nerves and neuron cells. In other words, anything outside the brain and spinal cord is part of the peripheral nervous system.
As we know, the brain exercises the foremost control over other organs in a human (or animal) body. It is tasked with processing sensory and motor information and triggering a coordinated response in most cases via the spinal cord.
The peripheral nervous system, which consists of nerves and ganglia, connects different organs, limbs, and other parts of the body to the CNS. Its objective is to relay information to and from the brain and spinal cord.
8. The PNS Has Two Segments; the Somatic Nervous System and Autonomic Nervous System
The vast network of the peripheral nervous system is divided into two sections, the somatic nervous system and the autonomic nervous system, each with a particular set of functions.
The somatic nervous system is responsible for the voluntary movements of the body. It consists of sensory nerves that carry sensory information from different parts of the body to the central nervous system, and motor nerves send feedback signals from the CNS to the muscles.
The autonomic nervous system controls involuntary muscles and influences physiological and reflex actions such as coughing, sneezing, heart rate, urination, and pupil response. The autonomic nervous system can be further divided into three parts; sympathetic, parasympathetic, and enteric nervous systems.
9. Nerve Tissue First Appeared in Life Forms About 600 Million Years Ago
Body of a simple bilaterian with nervous system | Image Courtesy: Wikimedia Commons
Humans and most animals on earth are bilaterians, species that are bilaterally symmetrical, or in simple words, have mirror-like opposite sides (left-right, and front-back).
The scientific understanding is that bilaterians are evolved from early wormlike organisms that emerged during the Ediacaran period (Ediacaran biota) about 550-600 million years ago.
The simplest bilaterian animal has a tube-shaped body with a hollow gut that runs straight from mouth to anus. Its nervous system consists of a single nerve cord with a ganglion, a group of neuron cells, near the mouth, called “brain.”
10. There are Different Types of Neurons
Neurons or nerve cells in the human body exist in various shapes and sizes. They can be broadly categorized based on their function and structure or morphology. The earliest known classification of neurons was done by Italian pathologist Camillo Golgi, who grouped them into two types; one with long axons (type I) and one with small axons (type II).
Neurons are most widely categorized based on their function, including sensory neurons, motor neurons, and interneurons. Based on structure, neurons can be categorized as following,
Different types of neurons based on their structure. 1. unipolar, 2. bipolar, 3. multipolar, and 4. pseudounipolar | Image Courtesy: Jonathan Haas/Wikimedia Commons
Unipolar: A single neurite (either dendrite or axon) extends from the cell body.
Bipolar: Any neuron with two extensions, one axon, and one dendrite, from the cell body. Most sensory neurons in humans are bipolar neurons.
Multipolar: Neurons with two or more dendrites, but only one axon. The majority of neurons in the central nervous system are multipolar neurons. They are also be found peripherally.
Pseudounipolar: A pseudonipolar neuron has only one axon, which is split into two branches. One end of the neuron is connected to the peripheral nervous system, and the other with the central nervous system.
11. A Typical Neuron Is Made Up of Four Different Parts
Image Courtesy: BruceBlaus
There are different types of neurons in a human body, as we have already learned, and they come in different shapes and sizes. However, a typical neuron or nerve cell comprises four major parts; soma, axon, dendrites, and axon terminal.
The soma region of a neuron contains the nucleus, where most of the protein synthesis takes place. An axon is an extension of the nerve cell that transmits electrical impulses to connected neurons and muscles. At the other end of the axon lies axon terminal(s) that contains synapses, a specialized structure that allows a neuron to transmit electrical signals to other neurons.
12. The Nervous System is Susceptible To Numerous Diseases
As compared to the peripheral nervous system, the CNS has a more rigid defense mechanism that prevents any physical and chemical damages or disorders. However, the diseases associated with the central nervous system are much more severe and destructive.
A general term for any damage to a peripheral nerve is neuropathy or peripheral neuropathy. It can be further classified based on the number of affected nerves (mononeuropathy or polyneuropathy), the type (autonomic, motor, or sensory), etc. Apart from physical injury, peripheral neuropathy is caused by acute vitamin deficiency, heavy metal poisoning, and various genetic and metabolic conditions.
A PET scan comparing the brain metabolism of a healthy individual with that of a drug addict | Image Courtesy: National Institutes of Health
A central nervous system disease signifies damage or injury to the brain and spinal cord. The CNS disorders can range from addiction (usually to drugs) to ADHD, Autism, and depression. More severe conditions such as encephalitis, epilepsy (seizures), and multiple sclerosis can be fatal.
In certain neurological conditions, individuals experience a gradual loss of neuron function before its complete death leading to dementia, disorientation, movement difficulty, and tremor. Such diseases are classified as neurodegenerative disorders.
13. The Optic Nerve Is Part Of The Central Nervous System
The optic nerve, which relays visual information from retinas to the brain, is classified as one of the twelve paired cranial nerves and a peripheral nervous system component. However, the structural nature of the optic nerves makes it an integral part of the central nervous system.
The optic nerve fibers are covered with oligodendrocyte myelin sheath, a fatty insulating substance that allows faster transmission of electrical impulses. It is present in the brain and spinal cord.
A schematic of the visual system in Humans | Image Courtesy: Miquel Perelló Nieto
Speaking of the optical nerve, its positioning or pathway inside the human brain is also quite fascinating. The visual information received from the retinas is first processed through a relay center known as the lateral geniculate nucleus (inside the thalamus) before reaching the visual cortex in either hemisphere of the brain.
The visual cortex located on the right hemisphere receives information from the eye’s left visual field (as shown in the diagram). Similarly, the left visual cortex receives signals from the right visual field.
14. The Curious Case of Mirror Neurons
A newborn monkey imitates tongue expression
In the late 1980s and early 1990s, a team of neurophysiologists at the University of Parma researched macaque monkeys to study brain neurons that specialize in motor functions. In a moment of surprise, the team discovered that a particular group of brain neurons in those monkeys activates, not just when they perform a certain activity — for instance, picking up food, but also when someone imitates that action in front of them.
Later studies put forward the idea that Broca’s area, a small region in the frontal lobe associated with language comprehension and various cognitive tasks, is homologous to the ventral premotor cortex in monkeys, where such neurons were first discovered.
It led to the speculation about mirror neuron’s involvement in the hand, mouth, and facial gestures in humans and other primates. Moreover, many experts in the field linked the mirror neuron system to explain human empathy and autism.
Recent studies have confirmed the existence of mirror neurons over a much wider network in the human brain than previously anticipated. It includes the premotor cortex, the primary somatosensory cortex, and the inferior parietal cortex.
15. The Human Brain Is Made Up of 86 Billion Neurons
A popular belief regarding the number of neurons present in a human brain is that it equals the total number of stars in the Milky Way, which is about 100 billion. For a considerable period of time, neurologists have more or less agreed with such analogy though neither of the prepositions is empirically supported.
However, according to a recent study, the total number of neurons in the human brain could be around 86 billion; that’s considerably fewer than previously anticipated quantities. Researchers obtained this number by tallying the total amount of neurons in a single part of the brain then extrapolate the final number based on the brain’s volume.