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.

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. 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 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.

 After a neuron fires its message, there is a brief period of time before it can fire again. This is called a neuron’s refractory period.  During the refractory period, excess neurotransmitters are reabsorbed by the sending neuron, called re-uptake, as well as the cell becoming polarized once again.

 In its resting state (resting potential or polarization), a neuron has a negative charge because mostly negative ions are within the cell. Surrounding the cell are positively charged ions. The ions cannot mix because it its resting stage, the cell membrane is semi-impermeable.

 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 or exceeded, a chain reaction begins.  With threshold being met, the cell becomes depolarized and allows positively charged ions into the axon at the nodes of ranvier. This mix of positive and negative ions causes an electrical charge to form (an action potential). At 120 meters per second, the action potential travels to the terminal buttons.

Axon – inside and out

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

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

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 Toilet 3. 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

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.

Click here to see a neuron in action! Click here to see a neuron in action!

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

Acetylcholine (ACh)  Enables muscle action, learning, and memory  Undersupply, as ACh- producing neurons deteriorate, marks Alzheimer’s disease

Dopamine  Reward and Motivation, Motor Control over Voluntary Movements  Excessive dopamine linked to schizophrenia; a lack of dopamine produces the tremors and lack of mobility (like in Parkinson’s disease)

Serotonin  Affects mood, hunger, sleep, and arousal  Undersupply is linked to depression; Prozac and other anti-depressants raise serotonin levels

Norepinephrine  Helps to control alertness and arousal  Undersupply can depress mood

GABA  Muscular movement  Undersupply linked to seizures, tremors, and insomnia

Glutamate  Involved in memory  Oversupply can overstimulate the brain, producing migraines or seizures

Endorphins  Natural opiates that are released in response to pain and vigorous exercise

Epinephrine  Adrenaline Burst of Energy (small amounts in brain)

Drugs and Chemical Interactions 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)

 After a neuron fires, if re- uptake is blocked, the lingering neurotransmitters in the synapse will continue to be absorbed by the receiving neuron 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

 Afferent Neurons are responsible for taking information from the senses TO the brain.

 Interneurons are located in the spinal cord and the brain.  Interneurons in the brain are responsible for sending messages along to the various areas of the brain for processing.

 Efferent Neurons responsible for taking information FROM the brain and the spinal cord, and back to the rest of the body.

 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 We cough, for example, when an irritant enters our windpipe and we need to expel it through our mouth. We sneeze when we need to clear our nasal air passages of irritants and allergens. We blink when danger threatens the sensitive tissues of the eye and when we need to moisten and clean the cornea. (This reflex occurs 900 times an hour!) We yawn when nerves in the brain stem find there's too much carbon dioxide in the blood. We cough, for example, when an irritant enters our windpipe and we need to expel it through our mouth. We sneeze when we need to clear our nasal air passages of irritants and allergens. We blink when danger threatens the sensitive tissues of the eye and when we need to moisten and clean the cornea. (This reflex occurs 900 times an hour!) We yawn when nerves in the brain stem find there's too much carbon dioxide in the blood.

Reflex Spinal Cord