Neurons and their Actions Get ready to sense, receive, and react.

Things that make you go hmmm… Average number of neurons in the human brain= 100 billion Average number of neurons in an octopus brain= 300 billion Rate of neuron growth during development of a fetus (in the womb)= 250,000 neurons/minute Longest axon of a neuron= around 15 feet (Giraffe primary afferent axon from toe to neck) Velocity of a signal transmitted through a neuron= 1.2 to 250 miles/hour

A nerve cell; consists of many different parts. Neuron A nerve cell; consists of many different parts. OBJECTIVE 3| Describe parts of a neuron and explain how its impulses are generated.

Parts of a Neuron Cell Body: Life support center of the neuron. Dendrites: Branching extensions at the cell body. Receive messages from other neurons. Axon: Long single extension of a neuron, covered with myelin [MY-uh-lin] sheath to insulate and speed up messages through neurons. Terminal Branches of axon: Branched endings of an axon that transmit messages to other neurons.

Synapse Synapse [SIN-aps] a junction between the axon tip of the sending neuron and the dendrite or cell body of the receiving neuron. This tiny gap is called the synaptic gap or cleft. OBJECTIVE 4| Describe how nerve cells communicate. Synapse was coined by Lord Sherrington (1857-1952) who inferred it through behavioral experiments. Cajal (1852-1934) described the synapse based on his anatomical studies of the brain.

Types of Neurons Most communicate within the central nervous system (CNS - brain & spinal cord) with rare exceptions. There are three major types of neurons upon which information travels. The information travels from the Sensory Neurons to the Interneurons, and then finally to the Motor Neurons.

Sensory Neurons bring information from sensory receptors to the central nervous system. Brings information from the eyes, ears, etc., as well as from within the body like the stomach.

Interneurons neurons in the brain and spinal cord that serve as an intermediary between sensory and motor neurons. They carry info around the brain for processing.

Motor Neurons carry the information from the CNS to the appropriate muscles to carry out behaviors.

Follow the Neuron Path For example, if you hold your hand over a hot flame, the information about "heat" travels from your hand on the sensory neurons, to the Interneurons where it is brought to the appropriate brain region to process the information (now you know it is "hot") and make a decision about a corresponding action (too hot, let's move the hand). The information then travels on the Motor Neurons from the brain to the hand so that your muscles move the hand from the hot flame. See how easy that is?

Neural Impulse (A Neuron Firing) Defined as: the electrical and chemical transmission of information from one neuron to another. It takes the same path all the time - it is a process of conducting information from a stimulus by the dendrite of one neuron and carrying it through the axon and on to the next neuron. Let's look at all the elements that are involved in the neural impulse:

Change of the Neuron inside the neuron, the ions are mostly negatively charged. Outside the neuron, the ions are mostly positively charged. In this state (with mostly negative charge inside and positive charge on the outside) the neuron is said to be Polarized When a neuron fires (neuron impulse), that all changes… Just watch!

Ions  Definition: [n]  a particle that is electrically charged (positive or negative); an atom or molecule or group that has lost or gained one or more electrons We have positively (+) and negatively (-) charged particles called ions. Neural Impulse = Sodium (Na+) Potassium (K-)

Selectively Permeable Membrane The outer membrane of the neuron is not impermeable, but instead selectively allows some ions to pass back and forth. The way it selects is easy - it has pores that are only so big. So, only very small ions can fit through. Any large ions simply can't pass through the small pores.

Lights, Stimulus, Action The following slides go through the stages of a neuron firing, or a neural impulse, with other neurons or muscle fibers. You will want to write down these steps as a chain of events that goes in a circular motion.

Resting Potential while the neuron is Polarized, it is in a stable, negatively charged, inactive state The charge is approx. -70 millivolts, and it means that the neuron is ready to fire (receive and send information).

Stimulus eventually, some stimulation occurs (ex. hand to close to a flame), and the information is brought into the body by a sensory receptor and brought to the dendrites of a neuron.

Threshold a dividing line that determines if a stimulus is strong enough to warrant action. If the threshold is reached, an action potential will occur. For example, how hot does a stove have to be for a person to pull away. When you have reached that level of uncomfortable temperature, then you have reached the threshold.

All or None Law all-or-none law - a neural impulse will either occur or not. There is no in between. Once the threshold is reached, there is no going back, the neural impulse will begin and will go through the complete cycle.

Action Potential Once the stimulus reaches a certain threshold the neural membrane opens at one area and allows the positively charged ions to rush in and the negative ions to rush out. The charge inside the neuron then rises to approx. +40 mv. This only occurs for a brief moment, but it is enough to create a domino effect.

Absolute Refractory Period after the action potential occurs, there is a brief period during which the neuron is unable to have another action potential. Then the charge inside the neuron drops to about -90 mv (refractory period) before restoring itself to normal. Analogy: the absolute refractory period is like a gun. After you shoot it, you must reload it before you can shoot again. The charge dropping in the neuron is its way of reloading.

Repolarization the neuron tries to quickly restore it's charge by pumping out the positively charged ions and bringing back the negative ones. occurs fast enough to allow up to 1,000 action potentials per second.

The Firing of a Neuron

Speed of an Action Potential Neural impulses can travel from 10120 meters/sec, or 2-270 miles/hour. The speed of light is 186,000 (1.86 X 105) miles per second. The speed of sound, at sea-level, is 968 ft/sec. The speed of a garden snail is 0.03 mph. What should you learn from this? Neurons are slower than the energy neurons use, and faster than anything that uses neurons to move.

Neuron Activity (10 minutes) You have 8 minutes to organize yourselves as a neuron. You must all have a role as a part of the neuron (more than one person can make one part of a neuron). After 8 minutes of preparation you must show me through your movements a neuron firing. Before you fire you must identify which part of the neuron you represent.