Neurons and Synapses Types of Neurons Sensory Motor Interneurons Key words: Types of neurons; sensory neurons; motor neurons; interneurons; afferent nerves; efferent nerves

Sensory Neurons INPUT From sensory organs to the brain and spinal cord. Brain Drawing shows a somatosensory neuron Vision, hearing, taste and smell nerves are cranial, not spinal Sensory Neuron Spinal Cord Key words: sensory neurons; afferent nerves; types of neurons

Motor Neurons OUTPUT From the brain and spinal cord To the muscles and glands. Spinal Cord Brain Sensory Neuron Motor Key words: Motor neurons; efferent nerves; types of neurons

Interneurons Interneurons carry information between other neurons only found in the brain and spinal cord. Spinal Cord Brain Sensory Neuron Motor Key words: interneurons; types of neurons

Structures of a neuron Key words: Neuron; sructures of neurons

The cell body Contains the cell’s Nucleus Round, centrally located structure Contains DNA Controls protein manufacturing Directs metabolism No role in neural signaling Key words: Cell body; soma; cell nucleus Interesting facts: The DNA in the nucleus of the cell has lost its ability to divide. therefore, when a neuron dies,for the most part, the adult brain cannot simply grow new neurons. (Note there are a few exceptions to this rule.) The relative inability to grow new neurons leads to two interesting questions: Q1: How do brain tumors (cancer) occur? A: Unlike neurons, glial cells can divide and grow new cells throughout one's lifetime. Most brain tumors are limited to glial cells, not neurons. Q2: If a person cannot grow new neurons, how does the brain change in order to accomodate new learning? A: One mechanism by which the brain adapts to help you learn new information involves the structure on the next slide: the dendrites.

Dendrites Information collectors Receive inputs from neighboring neurons Inputs may number in thousands If enough inputs the cell’s AXON may generate an output Key words: dendrite Interesting facts: - The word DENDRITE comes from the Greek word for tree. This may serve as a useful analogy in discussing the dendrites for several reasons: 1. The dendrites branch repeatedly from the cell body (to increase the surface area of the cell to better allow the cell to receive incoming information). These radiations from the cell body are often referred to as a dendritic tree. 2. In terms of function, the dendrites function similiarly to the roots of a tree. Just as the roots take water and other nutrients from the soil and carry them to other parts of the tree, the dendrites collect information and and spread it to other parts of the neuron.

Dendritic Growth Mature neurons generally can’t divide But new dendrites can grow Provides room for more connections to other neurons New connections are basis for learning Key words: dendrite Interesting facts: - The word DENDRITE comes from the Greek word for tree. This may serve as a useful analogy in discussing the dendrites for several reasons: 1. The dendrites branch repeatedly from the cell body (to increase the surface area of the cell to better allow the cell to receive incoming information). These radiations from the cell body are often referred to as a dendritic tree. 2. In terms of function, the dendrites function similiarly to the roots of a tree. Just as the roots take water and other nutrients from the soil and carry them to other parts of the tree, the dendrites collect information and and spread it to other parts of the neuron.

Axon The cell’s output structure One axon per cell, 2 distinct parts tubelike structure branches at end that connect to dendrites of other cells Key words: axon; action potentials Interesting facts: - The diameter of an axon may vary from approximately 1mm-20mm. - An axon may travel long distances to reach it's destination (longest axon is approximately 3 feet in humans and 10 feet in giraffes).

Myelin sheath White fatty casing on axon Acts as an electrical insulator Not present on all cells When present increases the speed of neural signals down the axon. Myelin Sheath Key words: myelin sheath; action potentials; axon Interesting facts: - The myelin sheath is NOT a part of the axon. The myelin sheath is actually formed of glial cells (oligodendricytes and Schwann cells) that wrap around the axon. - You may have often heard the brain referred to as either white matter or gray matter. The myelin sheath appears white in nature. Hence, the term white matter refers to areas of the brain that are myelinated. Gray matter refers to areas of the brain that are unmyelinated. - When you accidentally cut yourself, you often visually notice that you've cut yourself before you actually feel any pain from the cut. The reason for this is that visual information uses myelinated axons; whereas, pain information uses unmyelinated axons. - The loss of myelin is a significant factor in the disease multiple sclerosis (MS). When myelin is lost, the high-speed transmission of information is slowed down or blocked completely, which could lead the person with the inability to walk, write or speak.

How neurons communicate Neurons communicate by means of an electrical signal called the Action Potential Action Potentials are based on movements of ions between the outside and inside of the cell When an Action Potential occurs a molecular message is sent to neighboring neurons

Cell Membrane in resting state Ion concentrations Cell Membrane in resting state K+ Na+ Cl- A- Outside of Cell Inside of Cell Key words: ion concentrations; cell membrane; intracellular fluid; extracellular fluid; Na+; Cl-; K+ Slide ten represents a schematic of the typical concentrations of the intracellular and extracellular fluids. There are large concentrations of sodium and chloride ions concentrations of on the outside of the cell (relative to inside the cell). There are large concentrations of potassium ions and protein molecules on the insde of the cell (relative to concentrations on the outside of the cell).

The Cell Membrane is Semi-Permeable Cell Membrane at rest Na+ Cl- K+ A- Outside of Cell Inside of Cell Potassium (K+) can pass through to equalize its concentration Sodium and Chlorine cannot pass through Result - inside is negative relative to outside - 70 mv Key words: Cell membrane; semi-permeable; K+; Na+; Cl- The cell membrane is semi-permeable. That is, when the neuron is at rest, the cell membrane allows some ions (K+) to pass freely through the cell membrane, whereas other ions (such as Na+ and Cl-) cannot. Hit enter once and K+ ions will slowly pass through the cell membrane. After K+ animation is finished, hit enter again and animation showing that Na+ and l- ions cannot pass through the membrane will occur.

Resting Potential At rest the inside of the cell is at -70 microvolts With inputs to dendrites inside becomes more positive if resting potential rises above threshold an action potential starts to travel from cell body down the axon Figure shows resting axon being approached by an AP

Depolarization ahead of AP AP opens cell membrane to allow sodium (NA+) in inside of cell rapidly becomes more positive than outside this depolarization travels down the axon as leading edge of the AP

Repolarization follows After depolarization potassium (K+) moves out restoring the inside to a negative voltage This is called repolarization The rapid depolarization and repolarization produce a pattern called a spike discharge

Finally, Hyperpolarization Repolarization leads to a voltage below the resting potential, called hyperpolarization Now neuron cannot produce a new action potential This is the refractory period

Neuron to Neuron Axons branch out and end near dendrites of neighboring cells Axon terminals are the tips of the axon’s branches A gap separates the axon terminals from dendrites Gap is the Synapse Cell Body Dendrite Axon Key words: axon terminal .

Synapse Sending Neuron Synapse Axon Terminal axon terminals contain small storage sacs called synaptic vesicles key words: axon terminal; synaptic vesicles; neurotransmitters vesicles contain neurotransmitter molecules

Neurotransmitter Release Action Potential causes vesicle to open Neurotransmitter released into synapse Locks onto receptor molecule in postsynaptic membrane

Locks and Keys Neurotransmitter molecules have specific shapes Receptor molecules have binding sites When NT binds to receptor, ions enter positive ions (NA+ ) depolarize the neuron negative ions (CL-) hyperpolarize

Some Drugs work on receptors Some drugs are shaped like neurotransmitters Antagonists : fit the receptor but poorly and block the NT e.g. beta blockers Agonists : fit receptor well and act like the NT e.g. nicotine.

Summary 3 types of neurons The cell membrane Ion movements Action potentials Synapse Neurotransmitters Receptors and ions Agonists and antagonists