Neurons, Synapses and SignalingChapter 48

Warm Up Exercise

What types of cells can receive a nerve signal?

Nervous Organization

Neurons- nerve cells.

Brain- organized into clusters of neurons, called ganglia.

Central Nervous System- includes the brain and the spinal cord.

Peripheral Nervous System- all of the neurons extending from the brain and the spinal cord.

Types of Neurons

Sensory Neurons- transmit information from the senses to processing centers in the brain or ganglia.

Interneurons- neurons in the brain that analyze and interpret sensory input.

Motor Neurons- transmit signals for muscle and gland activity.

Neuron Structure and Function

Cell Body- contain the organelles and nucleus.

Dendrites- branched extensions that receive signals from other neurons.

Axon- extension from the cell body that transmits signals to other cells.

Synapse- junction between neurons.

Neurotransmitters- chemical messengers that pass information between neurons.

Glial Cells- supporting cells that insulate the axons of neurons and regulate fluid surrounding neurons.

Neuron

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Ion Pumps and Resting Potential

Membrane Potential- the difference in voltage between the inside and outside of the cell membrane.

Resting Potential- the membrane potential of a resting neuron is -60 to -80 mV. Formed by a high concentration of K+ ions inside the

cell, and high Na+ ions outside the cell.

Sodium-Potassium Pumps

Sodium-Potassium Pumps- maintain resting potential in the cell membrane. Transport 3 Na+ ions out for

every 2 K+ ions in. In addition to the Sodium-Potassium pump, ions diffuse across the concentration gradient. Many K+ channels are open, allowing for a large amount of K+ to move out of the cell, few Na+ channels are open allowing little flow inside, leading to a negative membrane potential inside.

Warm Up Exercise

Explain how a nervous response is transmitted through a series of neurons?

How does the sodium-potassium pump maintain a membrane gradient?

Ion Gated Channels

Ion Gated Channels- ion channels that open or close in response to stimuli. The opening and closing of ion gated channels alters

the membrane potential.

Hyperpolarization- an increase in the magnitude of the membrane potential.

Depolarization- a reduction in the magnitude of the membrane potential.

Graded and Action Potentials

Graded Potential- a shift in the membrane potential; is a response to hyperpolarization or depolarization

Action Potential- a massive change in membrane voltage, caused by depolarization. Action potentials can be regenerated to spread along

the axon at a constant magnitude.

Graded Potentials and Action Potentials

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Voltage Gated Ion Channels

Voltage-Gated Ion Channels- open and close when the membrane potential passes a particular level. Action potentials occur when depolarization increases

the membrane potential to a certain level, called the threshold.

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Warm Up Exercise

Explain what happens in hyperpolarization and depolarization? Which ions move in which direction?

Describe what happens in an action potential.

Action Potentials

Refractory Period- “downtime” when a second action potential cannot be initiated. Occurs because of the

inactivation of the sodium channels- during the falling phase and early part of the undershoot.

Action Potentials

Saltatory Conduction- how the action potentials jumps from node to node along the axon.

Communication With Other Cells

Electrical Synapses- contain gap junctions which allow electrical currents to flow from one neuron to the next.

Chemical Synapses- release a chemical neurotransmitter between cells.

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Chemical Synapse

Presynaptic neuron synthesizes neurotransmitter and packages in synaptic vesicles. The arrival of action potential at axon/synaptic terminal depolarizes plasma membrane, opening voltage-gated channels, which allow Ca2+ to diffuse into the synaptic terminal, which forces vesicles to fuse with membrane causing the release of neurotransmitter into the synaptic cleft. Neurotransmitters diffuse across the cleft and binds to and activates a specific membrane receptor (called a ligand-gated ion channel).

Ligand-Gated Ion Channels

Ligand-Gated Ion Channel- located in postsynaptic cell- binding of neurotransmitter to this receptor opens the channel and allows specific ions to diffuse across the postsynaptic membrane, resulting in a postsynaptic potential. Excitatory Postsynaptic Potential (EPSP)- occurs

when channel is permeable to both Na+ and K+. Causes depolarization.

Inhibitory Postsynaptic Potential (IPSP)- occurs when channel is permeable to either K+ or Cl-. Causes hyperpolarization.

Summation of Action Potentials

Temporal Summation- two EPSP’s occur at a single synapse in rapid succession- in this case the EPSP’s add together.

Spatial Summation- two EPSP’s produced simultaneously at different synapses on the same postsynaptic neuron- EPSP’s added together.

Exit Slip

In multiple sclerosis, myelin sheaths harden and deteriorate. How would this affect the nervous system function?

Warm Up Exercise

What is meant by the term saltatory conduction?

Explain the difference between an electrical and chemical synapse.

Discuss how a presynaptic cell transmits a chemical impulse once it receives the action potential near the axon terminal.

Neurotransmitters Acetylcholine- causes the opening of potassium

channels in cardiac muscle membrane. Leads to hyperpolarization, which reduces the rare at which the heart pumps (inhibitory).

Dopamine and Serotonin

Epinephrine and Norepinephrine

Organization of Human Brain

Cerebrum- the center for voluntary movement, learning, emotion, memory, etc. Divided into right and left hemispheres, connected by the

corpus callosum.

Cerebellum- coordinates movement and balance.

Organization of Human Brain

Diencephalon Thalamus

Hypothalamus

Pineal Gland

Pituitary Gland

Brainstem- receives signals from sensory neurons Midbrain

Pons

Medulla Oblongata

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