neuron structure

While not all neurons look the same, the basic structure of a motor neuron is represented in the image.

iagram of a neuron structure, with key features labelled. See d-link

Each specific part of the neuron plays an important role, allowing the neuron to send information throughout our entire body. Key terms listed below are labeled in blue on the neuron image above.

Neural communication depends on the ability of our neurons to respond to incoming stimulation and then pass signals to other neurons. Neurons send information electrochemically, which means half of this process is electrical and the other half is chemical. The chemicals cause an electrical signal.

How do neurons communicate to one another?

Our neurons are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions, such as a gate to a pool. This type of membrane is called semi-permeable or selectively permeable .

When a neuron is inactive, hanging out in our bodies not sending signals, it is called resting potential . There is a slightly negative charge inside the neuron, during resting potential because at rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. We call this polarization.

When a neuron decides to communicate and go to work, it is called action potential (the process by which a neuron fires). During action potential, an electrical signal passes along the axon and causes a release of neurotransmitters (chemicals) that transmit signals to other neurons. A depolarizing current creates this explosion of electrical activity. This means that a stimulus caused the resting potential to fire an action potential. This is what we call threshold . If the neuron does not reach threshold, then no action potential will fire.

In addition, when the threshold level is reached, an action potential will always fire. There are no large or small action potentials in a neuron – all action potentials are the same size. Therefore, the neuron either fires an action potential or does not. This is the ” all-or-none principle ”.

Action potentials are caused by an exchange of ions across the neuron membrane. A stimulus first causes the sodium channels to open. Since there are many more sodium ions on the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive for this brief moment and becomes depolarized. As this occurs, potassium channels open, and potassium rushes out of the cell, reversing the depolarization.

When the neuron fires, the depolarization of the cell membrane moves along the axon like a wave at a concert. When the next gates along the axon open, allowing positive sodium ions in, the previous gates close and begin to pump the positively charged sodium ions out of the axon and potassium ions back inside. As each section of the axon is depolarizing, the preceding section is going through the process of repolarization. This step is called the refractory period and the axon cannot fire again until it returns to resting potential.

The entire process is like falling dominoes all the way down the axon except these dominoes can set themselves back up as soon as they fall over.

What role do neurotransmitters play in communication between neurons?

Just like there are several different types of ice cream flavors, there are different neurotransmitters in the body! Just as each flavor affects our taste buds differently, various neurotransmitters do different things to our body. We cannot underestimate how important these chemicals, neurotransmitters, are to our body. Everything we do, we need a neurotransmitter to do it. Every time we think, move, laugh, or feel emotion; we are relying on our neurotransmitters.

Neurotransmitters are made in the cell body of the neuron and then transported down the axon to the terminal buttons (axon terminals). Molecules of neurotransmitters are stored in small “packages” called vesicles.

Neurotransmitters are released from one neuron at the presynaptic terminal button into the synaptic gap due to action potential. Neurotransmitters then cross the synapse where they may be accepted by the next neuron ( postsynaptic neuron) at a specialized site called a receptor. Neurotransmitters will bind only to specific receptors on the postsynaptic dendrite’s membrane that recognize them. The action that follows activation of a receptor site may be either depolarization (an excitatory postsynaptic potential) or an inhibitory postsynaptic potential. (This means once the neurotransmitter binds onto the receptor site of the dendrite, either action potential will take place, or it will not allow action potential to take place because it is an inhibitor.) Neurotransmission is then terminated by reuptake in which the neurotransmitter is taken back into the presynaptic terminal buttons that released it.

How do neurotransmitters influence behavior, emotion, and thoughts?
As you have learned, neurotransmitters act to either enhance or inhibit action potentials. Many substances such as drugs can alter the action of neurotransmitters in several ways. Substances such as toxins or drugs can either raise or lower the amounts of neurotransmitters released into the synapse, and change the reuptake process by either blocking reuptake or preventing it. Drugs that mimic or enhance the actions of neurotransmitters are known as agonists . Drugs that inhibit or block actions of neurotransmitters are antagonists.

A neurotransmitter’s effect is a function of the receptors to which it binds. The same neurotransmitter can be both excitatory and inhibitory, or produce different effects depending on the receptor site. Neurotransmitters can be broken into four categories: acetylcholine, monoamines, amino acids, and peptides.

Sensory neurons
Sensory neurons, also known as afferent neurons, carry information from our sensory receptors to our spinal cord or brain.

Your sensory neurons communicated how painful touching the hot stove was!

Motor neurons
Motor neurons, also known as efferent neurons, carry messages from the spinal cord and brain and distribute it to our muscles and glands.

Your motor neurons are what made your hands move away as fast as you could!

Interneurons
Interneurons connect and communicate between the afferent and efferent neurons. As you can imagine, this occurs rapidly. Motor neurons and sensory neurons refuse to communicate with each other, so they use the interneurons to help communicate back and forth to each other.

Central nervous system (CNS)
The central nervous system (CNS) includes our brain and spinal cord. These are the nerves that are encased in bone. The brain performs nearly all functions of the CNS. Behavior and mental processes are produced within specific locations in the brain that we will discuss in future lessons. The main function of the spinal cord is to receive sensory signals from the body and transmit them to the brain and then receive signals from the brain and relay them to the specific body parts. The spinal cord is also capable of reflex action.

Peripheral nervous system (PNS)
The peripheral nervous system (PNS) consists of all the other nerves in our body or all nerves that are not encased in bone. The PNS is divided into two categories, the Somatic and Autonomic Nervous Systems.

Somatic nervous system
The somatic nervous system controls all of our voluntary muscle movements. Every time we choose to move our body to dance, kick a soccer ball, write a haiku, or text message someone, we are using motor neurons located in the somatic nervous system.

Autonomic nervous system

The autonomic nervous system controls all of the automatic functions of our body such as our heart rate, lungs, and internal organs. Pretend you just consumed a pizza! The pizza enters into your stomach. Do you need to think about squirting stomach acid on the pizza? No! Do you press a switch to turn the food into energy your body can use? No! Did you even think about breathing while readying this? Although it would be really interesting if we could control every function of our body, they happen automatically, therefore named the autonomic nervous system. The autonomic system is broken down into two nervous systems: the sympathetic and parasympathetic.

Sympathetic nervous system
Whether it is a fire alarm ringing or someone attractive coming into view, we experience nervous symptoms. Whenever our body feels stress, the sympathetic nervous system automatically releases epinephrine to attempt to prepare ourselves.

When activated, the sympathetic nervous system will speed up your heart rate, increase your blood sugar and oxygen levels, dilate your pupils, and decrease your digestive process. Why does our body do this? It prepares the body for action to either fight to protect ourselves or run from danger. We call it the fight or flight response , which is activated by psychological states such as anxiety or unhappiness, as well as sexual arousal.

Chronic activation of the sympathetic nervous system is associated with ulcers and heart disease.

Parasympathetic nervous system

As soon as we know that the fire alarm was false, our heart starts to return to its normal rate, breathing slows, pupils contract, and digestive process resumes. This is a result of the parasympathetic nervous system returning your body to normal resting state after sympathetic activation.

In 1848, the case study of Phineas Gage’s accident led scientists to hypothesize that specific regions of the brain were responsible for our personality and behavior. Watch a video about the case of Phineas Gage. When the page loads, scroll down and click on Video 25: “Frontal Lobes and Behavior: The Story of Phineas Gage.” Also watch and take notes on Video 1: “Organization and Evaluation of Brain Function.”

Today’s neuroscientists no longer need to wait for injuries or accidents to study the brain. Recent technology has enabled neuroscientists to see inside the living brain. They can now surgically lesion tissue in specific brain areas in animals, or electrically, chemically, or magnetically stimulate the brain in order to study the effects of specific areas.

These brain-imaging techniques help neuroscientists to understand the relationships between specific brain regions and what functions they serve. Neuroscientists are also able to locate regions of the brain affected by neurological disorders, and develop new strategies to treat brain disorders.

Lesions
Scientists surgically remove or destroy tissue in a specific region of the brain to understand the function of the specific area.

Lesions are also conducted by neurosurgeons during brain surgery to remove tumors.

EEG
Researchers position electrodes on the scalp of subjects to record the waves of electrical activity that sweep across the brain’s surface. EEG or electroencephalogram measures brain activity to determine a relationship to cognitive or perceptual tasks.

CAT/ CT Scan

CAT/CT Scans or C omputerized A xial T omography Scans, are sophisticated x-rays of the brain. Cross-sectional 3-D images of the brain are taken and used to show the structure of the brain, but not activity or function. CAT/CT Scans are particularly useful in locating brain tumors damage to brain regions.

ET Scan
PET, positron emission tomography, scans depict brain activity by locating and measuring radioactivity after a person is given a radioactive form of glucose. The PET scan reveals areas of the brain that “light up” while using the glucose, allowing researchers to know which brain areas are most active during a specific activity.

MRI
The MRI, Magnetic Resonance Imaging, provides neuroscientists with the most detailed picture/ image of the brain. MRI scans use magnetic fields and radio waves to produce computer generated images that distinguish between the structures within the brain as well as different types of soft tissue.

fMRI
The fMRI, functional MRI, is a technique that shows blood flow and brain activity by comparing successive MRI scans. The fMRI reveals brain structure as well as functioning and activity when areas light up due to increased blood flow while a subject is performing different mental functions.

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