Key terms

Nervous systemThe body system that collects, processes, and responds to information using electrical signals
NeuronA nerve cell; the basic unit of the nervous system
Glial cellA cell that supports and protects neurons
Central nervous systemPart of the nervous system containing the brain and spinal cord
Peripheral nervous systemPart of the nervous system containing associated nerves that are not part of the brain or spinal cord
Endocrine systemThe body system that regulates cells and organs using chemical substances called hormones
HormoneChemical messenger that acts as a regulatory substance
GlandOrgan that secretes chemical substances, such as hormones

The nervous system

The nervous system uses electrical impulses to collect, process and respond to information about the environment.

Nervous system cells

The unique structure of neurons makes them specialized for receiving and transmitting electrical impulses throughout the body. Neurons are supported by glial cells, which surround, protect, and insulate them.
Diagram of a neuron
Image credit: Wikimedia, CC BY-SA 3.0
All neurons have several features in common, including a cell body, dendrites, and an axon. These structures are important for transmitting neural impulses, electrical signals that allow neurons to communicate with one another.
Neurons are specialized, depending on their required functions:
  • Sensory neurons carry impulses from sense organs, such as the eyes or ears.
  • Motor neurons carry impulses to muscles and glands.
  • Interneurons transfer signals between sensory and motor neurons, as well as in between other interneurons.
In a resting neuron, there is a separation of ions in the cell regulated by sodium-potassium pumps. If a neuron receives a large enough signal, the resting potential changes, producing an electrical impulse called an action potential. Once an impulse begins, it moves down the axon until it reaches the axon terminal.

Parts of the nervous system

The nervous system is made up of two parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
Diagram of the central nervous system (CNS) and peripheral nervous system (PNS)
Image modified from Wikimedia, CC BY-SA 4.0
The CNS is made of the brain and spinal cord. Commands to the body originate in the brain and the spinal cord connects the brain with the rest of the nerves in the body.
The PNS can be broken down into two parts:
  • The somatic nervous system (SNS) regulates voluntary activities such as muscular movement. It also controls reflexes, such as pulling your hand away from the hot surface of a stove.
  • The autonomic nervous system (ANS) regulates activities that are not under conscious control and has two divisions that are opposite of one another: the sympathetic and parasympathetic nervous systems. The sympathetic nervous system prepares the body for “fight-or-flight” responses, while the parasympathetic nervous system is active during restful periods.

The endocrine system

Like the nervous system, the endocrine system is a regulatory system. However, instead of using electrical impulses for signaling, it produces and uses chemical signals called hormones, which travel through the bloodstream and control the actions of cells and organs.
Diagram of the major endocrine glands (includes both male and female glands)
Image modified from Wikimedia, CC BY 3.0

Common hormones and glands

HormoneGland produced inRole
Thyroid hormoneThyroidRegulates metabolism
Adrenaline (epinephrine)Adrenal glandInvolved in "fight or flight" response
CortisolAdrenal glandInvolved in "fight or flight" response, regulates metabolism and immune responses
EstrogenOvariesSexual and reproductive development, mainly in women
TestosteroneTestes, sometimes adrenal glands or ovariesSexual and reproductive development, mainly in men
InsulinPancreasBlood sugar regulation, fat storage
GlucagonPancreasBlood sugar regulation

Regulation of the endocrine system

The endocrine system is regulated by negative feedback mechanisms that work to maintain homeostasis. The concentration of hormones, and how they effect other body systems, are controlled in this manner.
For example, blood glucose regulation is controlled by insulin and glucagon, hormones produced by the pancreas.
When blood glucose increases, the pancreas releases insulin, which stimulates the uptake of glucose from the blood. This prevents blood glucose from getting too high.
When blood glucose concentration drops, the pancreas releases glucagon, which stimulates the breakdown of glycogen and releases glucose into the blood. This raises blood glucose back to normal levels.
Negative feedback loop of blood glucose. As blood glucose increases, insulin is released, causing the blood glucose to drop.
When the blood glucose drops, glucagon is released, resulting in an increase in blood glucose.
Blood glucose is regulated by a negative feedback loop between insulin and glucagon, two opposing hormones made by the endocrine system.

Common mistakes and misconceptions

  • While neurons are often the most recognized nervous system cell, they are not the only ones! In fact, glial cells outnumber neurons by almost 10 times. Glial cells (also known as neuroglia) support the neurons, guiding and protecting them, and provide myelin sheaths around their axons.
Image from Wikimedia, CC BY 3.0
  • Hormones are not just part of the reproductive system. Although there are reproductive hormones, such as testosterone and estrogen, most hormones are not involved in or regulated by the reproductive system. Many hormones control regular body functions, such as hunger or sleepiness.
  • Although both the endocrine system and the nervous system are both regulatory, there are a few fundamental differences. For one, the endocrine system uses chemical signaling (hormones, produced by glands) while the nervous system uses electrical signaling (neural impulses). The signal transmission of the nervous system is fast because neurons are interconnected, but the functions are more short-lived. Signal transmission in the endocrine system is slow, since hormones must travel through the bloodstream, but the responses tend to last longer.