Topic 6.5 Neurons and Synapses 2016

Parts of the Nervous System Central nervous system (CNS) Brain and spinal cord Both contain fluid-filled spaces which contain cerebrospinal fluid (CSF). The central canal of the spinal cord is continuous with the ventricles of the brain. White matter is composed of bundles of myelinated axons Gray matter consists of unmyelinated axons, nuclei, and dendrites. Peripheral nervous system (PNS) Everything outside the CNS. Nervous System: https://www.youtube.com/watch?v=x4PPZCLnVkA

Peripheral Nervous System (PNS)

Neuron Anatomy

Neuron Anatomy

Membrane Potential Membrane Potential: a term used to illustrate there is an electrical potential difference between the inside of the cell and the surrounding extracellular fluid. -70 mV is the resting membrane potential of a neuron, which means that the inside of the cell is negative compared to the outside. Membrane Potential: http://www.sumanasinc.com/webcontent/animations/content/electricalsignaling.html

Normal Levels Sodium-Potassium Pump: is used in establishing the membrane potential in neurons (1) it makes the [Na] high in the extracellular space and low in the intracellular space (2) it makes the [K+] high in the intracellular space and low in the extracellular space (3) it creates a negative voltage in the intracellular space compared to the extracellular space.

Hyperpolarization Gated K+ channels open  K+ diffuses out of the cell  the membrane potential becomes more negative

Depolarization Gated Na+ channels open  Na+ diffuses into the cell  the membrane potential becomes less negative

Action Potential Action Potential: All or Nothing Depolarization If graded potentials sum to  -55mV a threshold potential is achieved. This triggers an action potential. Axons only Action Potential: http://www.sumanasinc.com/webcontent/animations/content/action_potential.html Action Potential w/ Graph: http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html

Action Potential Diagram

Step 1: Resting State. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Step 2: Threshold. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Step 3: Depolarization phase of the action potential. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Step 4: Repolarizing phase of the action potential. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Moving Potential

Saltatory Conduction In myelinated neurons, only unmyelinated regions of the axon depolarize. Signal jumps from one node to the next, making the impulse travel 100x faster than on a unmyelinated neurons. Khan Academy: https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/v/saltatory-conduction-neurons Saltatory Conduction: http://wps.aw.com/bc_goodenough_boh_3/104/26721/6840613.cw/content/index.html Fig. 48.11

Synapses Electrical Synapses. Action potentials travels directly from the presynaptic to the postsynaptic cells via gap junctions.

Chemical Synapses More common than electrical synapses. Postsynaptic chemically-gated channels exist for ions such as Na+, K+, and Cl-. Depending on which gates open the postsynaptic neuron can depolarize or hyperpolarize.

Fig. 48.12

Routes of Nerve Transmission