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Neuroanatomy

Neuroanatomy refers to the study of the parts and function of neurons.

Neurons are individual nerve cells.

The entirety of the human body’s neurons make up the nervous system, from the brain to the tips of the toes.

NeuronThe basic building block of the

nervous system -- a nerve cellNeurons perform three basic tasks

–Receive information

–Carry the information

–Pass the information on to the next neuron

The Basic Parts of a Neuron

A. Dendrites– Thin, branching fibers lined with

receptors at which the dendrite receives information from other neurons. The greater the surface area, the greater the amount of information. Some dendrites are covered with spines which greatly increase its surface area.

B. Cell Body/Soma–Contains the (C) nucleus and other parts of the cell needed to sustain life

D. Axon–Wire-like structure ending in the terminal buttons that extends from the cell body

E. Myelin Sheath– An insulating, fatty covering around

the axon that speeds neural transmissions. Axons that are myelinated appear white. Known as “white matter.”

F. Schwann Cells

Provide for the growth of the myelin sheath.

G. Nodes of Ranvier– Regularly spaced gaps in the myelin

sheath around an axon or nerve fiber. This is where depolarization takes place.

H. Terminal Buttons–The branched end of the axon that contains neurotransmitters

Neural Transmission

Synapse–The space between the terminal buttons on one neuron and dendrites of the next neuron

Chemicals contained in the terminal buttons that enable neurons to communicate.

Neurotransmitters fit into receptor sites on the dendrites of neurons like a key fits into a lock.

Neurotransmitters

Neurotransmitters

At the terminal buttons, neurotransmitters are released into the synapse and passed along to the dendrites of the next neuron.

If enough neurotransmitters have been sent, the next neuron will fire. If not, the message ends. This is called the all-or-nothing principle.

All-or-None Principle

The principle that if a neuron fires it will always fire at the same intensity

All action potentials are of the same strength.

A neuron does NOT fire at 30%, 45% or 90% but at 100% each time it fires.

Resting Potential

The state of a neuron when it is at rest and capable of generating an action potential

The neuron is set and ready to fire

Resting Potential

While in resting potential, a neuron is said to be “Polarized”

negative ions are within the cell. Surrounding the cell are positively charged ions.

The ions cannot mix while in this stage

A neuron has a pre-set level of stimulation that needs to be met or exceeded in order for it to pass the received impulses on to the next neuron. This is called a neuron’s threshold.

If the threshold has been met a chain reaction begins.

The cell becomes depolarized and allows positively charged ions into the axon at the nodes of ranvier.

Axon – inside and out

Action Potential This mix of positive and negative

ions causes an electrical charge to form (an action potential).

A brief electrical charge that travels down the axon of the neuron.

A neural impulse Considered an “on” condition of the

neuron

Refractory Period

The “recharging phase” when a neuron, after firing, cannot generate another action potential

Once the refractory period is complete the neuron can fire again

Refractory Period

Excess neurotransmitters are reabsorbed by the sending neuron – this is called re-uptake

The cell becoming polarized once again.

Neuron firing like a Toilet

1. Like a Neuron, a toilet has an action potential. When you flush, an “impulse” is sent down the sewer pipe

Neuron firing like a Toilet

2. Like a neuron, a toilet has a refractory period. There is a short delay after flushing when the toilet cannot be flushed again because the tank is being refilled

Neuron firing like a Toilet3. Like a Neuron, a toilet has a

resting potential. The toilet is “charged” when there is water in the tank and it is capable of being flushed again

4. Like a Neuron, a toilet operates on the all-or-none principle – it always flushes with the same intensity, no matter how much force you apply to the handle

Click here to see a neuron in action!

Click here to see a quick summary!

Depending on what type of neurotransmitter has been released, the next neuron will react differently.

….so, since the entire body is a connection of nerves, …

Inhibitory vs Excitatory

Inhibitory neurotransmitters decrease the likelihood of the firing action potential of a cell while

Excitatory neurotransmitters increase the likelihood of action potential

Acetylcholine (ACh)

ExcitatoryInvolved in muscle action, learning, and memory

Undersupply - Alzheimer’s disease

DopamineInhibitory: Pleasure, Reward and Motivation, Motor Control over Voluntary Movements

Excessive dopamine linked to schizophrenia;

SerotoninInhibitoryAffects mood, hunger, sleep, and arousal

Undersupply is linked to depression; Prozac and other anti-depressants raise serotonin levels

Epinephrine and Norepinephrine

Excitatory: Used for arousal in the flight/fight response, plays a role in learning and memory retrieval

Adrenaline Burst of Energy (small amounts in brain)

Undersupply can depress mood

GABAInhibitory: offsets excitatory messages (see Glutamate); helps regulate daily sleep-wake cycles

Undersupply linked to anxiety, seizures, tremors, and insomnia

GlutamateExcitatoryInvolved in memory, learning and movement

Oversupply can overstimulate the brain, producing migraines or seizures (epilepsy)

Endorphins

Inhibitory: Natural opiates that are involved in pain perception and positive emotions

released in response to pain and vigorous exercise

Drugs and Chemicals Interact with Neural

Transmission

Some drugs that people put into their bodies are classified as agonists.

Agonists may either speed up the neural process, cause an over-release or absorption of a neurotransmitter, or block the re-uptake process.

Prozac blocking the re-uptake of

Serotonin

Some agonists mimic the effects of a naturally occurring neurotransmitter

Dendrite of receiving Neuron

Agonist (like morphine – replacing natural endorphines)

If re-uptake is blocked, the lingering neurotransmitters in the synapse will continue to be absorbed until it is gone.

Therefore, a lingering feeling will occur

Examples of Agonists

Cocaine – blocks the re-uptake of dopamine

MDMA (Ecstasy) – blocks the reuptake of serotonin–Repeated use destroys serotonin producing cells

Some drugs that people put into their bodies are classified as antagonists.

Antagonists may slow or stop the transmission of a neurotransmitter, or they may bind themselves to receptors on a neuron’s dendrite, thus not allowing a message to be passed on.

Examples of Antagonists

Curare – a poison that stops the flow of Ach – causes paralysis

Antagonist (like

curare)

Neurotransmitter (such as Ach)

Dendrite of receiving Neuron

There are three types of neurons:–Afferent Neurons (Sensory Neurons)

–Interneurons–Efferent Neurons (Motor Neurons)

Types of Neurons

Types of NeuronsAfferents, or “sensory neurons”,

carry information from the body to the brain

Types of NeuronsInterneurons, found in the spinal

cord and the brain interpret incoming information

and determine the next course of action

Types of NeuronsEfferents, or “motor neurons”,

carry information from the spinal cord or the brain to the rest of the body in order to initiate behavior

The exceptions to the “general pathway” of neural activity are reflexes.

•Reflexes are controlled by the spinal cord without any conscious effort on behalf of the brain.

Reflexes are primitive responses

protect our bodies from danger

Reflex- Coughing

- Blinking

- Yawning (too much carbon dioxide in the blood)

….etc…….

Reflex

Sensory to spinal cord to motor… no brain processing involved….

Spinal Cord