Ch 48: Nervous System

2016

Ch 48: Nervous SystemFrom Topic 6.5Nature of science: Cooperation and collaboration between groups of scientists—biologists are contributing to research into memory and learning (4.3).Essential idea: Neurons transmit the message, synapses modulate the message.Understandings:• Neurons transmit electrical impulses.• The myelination of nerve fibres allows for saltatory conduction.• Nerve impulses are action potentials propagated along the axons of neurons.• Neurons pump sodium and potassium ions across their membranes to generate a resting potential.• An action potential consists of depolarization and repolarization of the neuron.• Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.• A nerve impulse is only initiated if the threshold potential is reached.• Synapses are junctions between neurons and between neurons and receptor or effector cells.• When presynaptic neurons are depolarized they release a neurotransmitter into the synapse.Guidance:• The details of structure of different types of neuron are not needed.• Only chemical synapses are required, not electrical, and they can simply be referred to as synapses.Applications and skills:• Application: Secretion and reabsorption of acetylcholine by neurons at synapses.• Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.• Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.• Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.Utilization:• An understanding of the workings of neurotransmitters and synapses has led to the development of numerous pharmaceuticals for the treatment of mental disorders.Aim 8: The social effects of the abuse of psychoactive drugs could be considered, as could the use of the neurotoxin Botox for cosmetic treatments.

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.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 48.9

• Step 2: Threshold.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 48.9

• Step 3: Depolarization phase of the action potential.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 48.9

• Step 4: Repolarizing phase of the action potential.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 48.9

Moving Potential

• 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

Saltatory Conduction

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