Anatomy of the Nervous System

The purpose of the nervous system is to transmit signals between the brain and the rest of the body. The nervous system regulates everything we do, voluntary and involuntary.

Each and every sensory and cognitive function you’re using right now to interpret the information in this article – sight, memory, perhaps even movement, if you’re taking notes – is controlled by the nervous system.

There are two major divisions of the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS).

The central nervous system is made up of the brain, the spinal cord, and the retina and controls essentially all the functions that keep you alive and allow you to experience life.

All sensory processes, regulation of organ systems like the cardiovascular system, and higher cognitive functions are rooted in the CNS.

The function of the peripheral nervous system is, essentially, to receive and distribute information to help the CNS make decisions on how to respond to the surrounding environment.

Main Functions of the Nervous System

One of the core functions of the nervous system is to regulate processes of the following categories:

Motor: regulated by efferent neurons, the motor system encompasses the movement of all muscle tissues, both voluntary and involuntary.

Sensory: sensory information is interpreted through hearing, sight, spatial awareness, temporal awareness, temperature, taste, touch, and smell.

Largely regulated by afferent neurons, sensory information is sent to the CNS, which makes decisions based on these neurotransmissions, and directs organ systems accordingly.

Automatic: this mainly refers to the processes which are not regulated by the brain, but by a pathway called the reflex arc. Reflexes are typically motor reactions that need to happen extremely fast, so going through the brain would actually be a disadvantage.

Instead, sensory receptors of the PNS are stimulated then pass signals to a given motor neuron via the spinal cord. This signal then activates the target muscle or organ to engage in the necessary movement.

Divisions of the Nervous System

There are over 100 trillion neural connections in the average human brain, all in constant communication via synapses.

These synapses only take a fraction of a millisecond to transmit a given electrical impulse through the spinal cord – it’s estimated to go at a speed of 268 mph!

The way these synapses are arranged and connected depends on the exact sub-system they fall under.

Cranial nervous system

These are the nerves that connect the brain to the eyes, ears, mouth, and other sensory organs of the head. Twelve pairs of nerves make up the cranial nervous system.

Peripheral nervous system.

This sub-system consists of sensory neurons, ganglia (clusters of neurons), and nerves that connect the central nervous system to arms, hands, legs, and feet. Thirty-one pairs of nerves make up the peripheral nervous system.

Autonomic nervous system

This is a collection of nerves that connect the central nervous system to the lungs, heart, stomach, intestines, bladder, and sex organs.

Central nervous system

This is comprised of the brain, spinal cord, and retina. The “command center” of the body.

Building Blocks of the Nervous System

The building block of the nervous system is the neuron. (Fun fact: The human brain contains approximately 100 billion neurons. That’s more than 14x the number of human beings currently on planet Earth!)

The anatomy of a neuron may differ slightly based on its function but the structures that comprise it remain the same.

Another cell type central to the functioning of the nervous system (pun intended) is the glial cell. Glial cells

  1. Help support and hold neurons in place
  2. Protect neurons
  3. Create myelin which helps to move nerve impulses
  4. Repair neurons and help restore neuron function
  5. Trim out dead neurons
  6. Regulate neurotransmitters

Neuron Anatomy

The neuron is composed of the following parts (NICHD, 2018):

  • Nucleus
  • Cell body
  • Dendrite: responsible for receiving information via synapses for the cell to process and send through the axon, through the axon terminal, to the synapse to be passed on again
  • Axon: the “corridor” through which sensory information is passed to the dendrite to be sent to another neuron. (The term “nerve” doesn’t refer to a neuron, even though it seems like it should. What it actually refers to is a collection of multiple axons is a collection of axons that work together as a collective. In the same way that a grouping of asparagus stalks makes one bunch of asparagus, a bundle of axons makes one nerve. The different types of nerves are cervical, thoracic, lumbar and sacral nerves.)
  • Myelin sheath: a fatty tissue that insulates the axon by preventing depolarization. This allows electrical impulses to travel across the axon uninterrupted.
  • Node of Ranvier: these are the gaps in the myelin sheath. Their function is to speed up propagation of action potentials along the axon via saltatory conduction. (Because these gaps are not myelinated, the action potentials appear to jump between nodes like the water fountains at Disneyworld.) Saltatory conduction also helps to conserve energy by decreasing the required movement of ions by 100x.
  • Axon terminal: the end of the axon, the last stop before electrical impulses are sent through the synapse. This structure converts electrical impulses into chemical signals which, when released, are then called neurotransmitters. Neurotransmitters pass through the synapse to the next dendrite and are then converted back into an electrical impulse to repeat this process until it reaches the proper organ.

Types of Neurons

There are four different types of neurons. Their anatomy is determined by what types of information they’ll need to propagate, to what organs, and in what region of the body.

In fact, there are so many different types of neurons in the brain alone that they are not yet all described. This is because neurons in the brain will differ based on what part of the target neuron they’re communicating to (dendrite vs. axon), express different genes, express varying electrical impulses, and several more distinctions.

The types of neurons are below:

  • Unipolar
  • Bipolar
  • Pseudounipolar
  • Multipolar

Neurons in the spinal cord are sensory and motor neurons. Sensory neurons are activated by sensory input from the environment (touch, taste, smell, sound, sight). The input of information from the external environment can be physical or chemical and correspond to all five senses. Most sensory neurons are pseudounipolar.

Motor neurons in the spinal cord are a part of CNS and connect to muscles, glands, and organs throughout the body. They transmit impulses from the spinal cord to skeletal and smooth muscle tissues based on information gathered by the sensory neurons. These are typically multipolar.

The lower motor neurons extend from the spinal cord to the muscles and the upper motor neurons travel from the brain and spinal cord to the distal parts of the body.

Lastly, there are interneurons. These serve as the connections between spinal, motor, and sensory neurons and communicate with each other by forming networks throughout the body, the structure of which differs based on need and organ system. These neurons are multipolar as well.

The Central Nervous System

The higher functions of the nervous system also referred to as the “executive functions,” are controlled by the brain. These include:

Cognition: thinking, learning, memory, language, insight, creating and planning goals. Cognition is primarily controlled in the frontal lobe and sets humans apart from all species on earth in its evolutionary development.

Emotions: controlled from many different structures, emotions can influence where in the brain memories are stored and the way they’re recalled. Emotions are controlled from many different organ systems as well, the CNS being only one. The hippocampus and amygdala are two of the nervous system structures involved in the regulation of emotions.

Consciousness: largely focused in the cerebellum, consciousness is a cognitive skill that is still very poorly understood by scientists, but central in what separates humans from many species.

Anatomy of the CNS

The CNS is made up of the brain, spinal cord, and retina.

The brain’s major components are the cerebrum, cerebral hemisphere, brain stem, and cerebellum.

Anatomical Structure Hindbrain/Midbrain/Forebrain
Cerebrum Forebrain
Cerebral hemispheres (left and right) Forebrain
Brain stem Midbrain, Hindbrain
Cerebellum Hindbrain

The spinal cord is the point of transition between the CNS and the PNS. This is because the nerves of the peripheral nervous system use the spinal cord to communicate directly to the brain in order to control the rest of the body.

Pairs of nerves extend from the sides of the vertebral column to travel throughout the distal areas of the body. Attached to these nerves are ganglia, which contain the soma of neurons.

The neurons which constitute the spinal cord are known as afferent and efferent neurons. Afferent neurons carry information to the central nervous system, while efferent neurons carry information away from the central nervous system to the peripheral nervous system based on the communication received.

The Peripheral Nervous System

The PNS is more straight-forward than the CNS, thank goodness. It is made up of the pairs of nerves that branch off from the spinal cord and extend throughout the body.

There are twelve pairs of cranial nerves and 31 pairs of spinal nerves, all of which serve to form the networks of communication between the brain and all other organs in the body.

References

  1. Queensland Brain Institute. (2017, December 15). The spinal cord. Retrieved from https://qbi.uq.edu.au/brain/brain-anatomy/spinal-cord
  2. Queensland Brain Institute. (2018, March 26). Types of neurons. Retrieved from https://qbi.uq.edu.au/brain/brain-anatomy/types-neurons
  3. Queensland Brain Institute. (2018, July 17). Central Nervous System: brain and spinal cord. Retrieved from https://qbi.uq.edu.au/brain/brain-anatomy/central-nervous-system-brain-and-spinal-cord
  4. Toro, R. (2013, August 5). Diagram of the human nervous system (infographic). Retrieved from https://www.livescience.com/27975-human-body-system-the-nervous-system-infographic.html
  5. Zimmermann, K. A. (2018, February 14). Nervous system: Facts, function & diseases. Retrieved from https://www.livescience.com/22665-nervous-system.html