LEARNING GOAL 2.5: DESCRIBE THE PROCESS OF NEUROTRANSMISSION Neurotransmission

Neurotransmitters Neurotransmitters are chemicals produced in neurons that provide messages throughout the nervous system Each neurotransmitter has a specific effect on the body – some affect sleep, others affect memory, or mood, or movement, cognition, temperature, etc. Abnormal levels of different neurotransmitters are associated with many harmful effects – depression, insomnia, epilepsy, eating disorders, Alzheimer’s – because they prevent signals from being transmitted (as in memory loss) or they overly excite neurons (as in seizures)

Neurons Neuron: nerve cell  Responsible for sending signals throughout the nervous system  Approx. 86 billion neurons in the human brain  Contrary to popular belief, neurons can regenerate if damaged Held in place by glial cells  Insulate and provide nutrients to neurons; clean up dead neurons

Anatomy of a Neuron

Dendrites (“little tree”) Bond with neurotransmitters and transmit their signal to the cell body Cell body (soma)  Keeps neuron alive; determines whether to transmit signals onward to other neurons Axon  Transmits messages to other neurons, muscles, or glands Myelin sheath  Fatty layer that insulates neurons (prevents nearby signals from interfering)

Neurotransmission Click the image to see a simulation! Synapse: region where the presynaptic neuron’s axon terminal almost touches the postsynaptic neuron’s dendrite

Neurotransmission, Step by Step 1. The first neuron (called the presynaptic neuron) “decides” to send a signal through the nervous system. In order to do this, an electrical signal called an action potential moves down the neuron, cell body to axon. 2. The presynaptic neuron’s axon then releases a particular type of neurotransmitter. 3. The neurotransmitters float around in the open space between neurons, called a synapse.

Neurotransmission, Step by Step 4. Some of the neurotransmitter molecules then bond with receptor sites on the dendrites of a second neuron (called the postsynaptic neuron). Note that each type of neurotransmitter can bond only with certain receptor sites (like a key in a lock). 5. If this happens enough times, the postsynaptic neuron may also “fire,” at which point an action potential moves down the postsynaptic neuron and the process repeats itself.

Chemical or Electrical? Note that neurotransmission has two distinct and alternating stages: 1. An electrical signal, called an action potential, travels through a single neuron. 2. A chemical signal, in the form of neurotransmitters, travels between two neurons.

An All or Nothing Process Each neuron has a minimum threshold of signals it needs to receive in order to fire.  If that threshold is surpassed, the neuron will fire and release all of its neurotransmitters.  If that threshold is NOT surpassed, the neuron doesn’t fire and thus releases none of its neurotransmitters at all.  A neuron will NOT release just some of its neurotransmitters – neurotransmission is all or nothing.

A Brief Investigation We’re going to examine how well established your neural pathways are…  Pair up with someone sitting near you. Choose one person to be the Participant and the other to be the Researcher.  The Participant exposes his/her toes and covers his/her eyes.  The Researcher gently but firmly pokes ONE of the Participant’s toes (2 nd, 3 rd, or 4 th ).  The Participant guesses which toe was just poked. The Researcher records whether the Participant guessed right.  Repeat for a total of ten times.

How We Learn, in One Sentence “Neurons that fire together, wire together.” As certain synapses are used more often, they develop more myelin, which strengthens their connection and makes it easier to activate that same neural pathway in the future.

Check Your Understanding 1. Fill in the blanks: Neurotransmission occurs when the ______ of one neuron releases chemicals called __________, which bond with ___________ in the __________ of another neuron. 2. If dopamine molecules bond with neuron #1 a thousand times and with neuron #2 three thousand times, does that mean that neuron #2 will release three times as many neurotransmitters as neuron #1? Explain.