Cellular Anatomy Lecture 2 Psychology 210 Cellular Anatomy Lecture 2

Cells of the Nervous System Neurons Information processing and communicating nerve cells Glia Addressed later

What do you know about neurons coming into this class? How does a neuron communicate with another neuron? What type of signal is processed in a neuron? What are the parts of a neuron?

Parts of a Neuron 3 main parts Dendrites Soma (cell body) Axon Receive information Soma (cell body) Cellular maintenance Axon Transmits information

Dendrites Tree like processes Receives information from other neurons Dendritic spines Create more area for axons to transmit information Change depending on the amount of activity This is one of the suggested bases for learning and memory A neuron can have multiple dendrites

The soma Nucleus- contains DNA Nucleolus- constructs ribosomes Endoplasmic reticulum- contains ribosomes that construct proteins Golgi apparatus- “packages” proteins Mitochondria- produces energy

The Axon Transmits information to other neurons Myelin sheath insulation that aids in the speed of the neural transmission Axon terminal End of the axon Contains neurotransmitters Also called terminal buttons

More on the Axon Axon hillock Nodes of Ranvier Where the action potential begins Nodes of Ranvier Segments of bare axon not covered by myelin

Giant Squid Used to study axons in the 1930s-1940s Extra large axon visible to the eye Doesn’t have myelin, so it is extra large to add conductance

Characterization of neurons Sensory neurons Translates incoming sensory information into an electrical signal Afferent Neurons Conduct information towards the brain from the muscles and senses Interneurons Neurons in the spinal cord and the brain that both communicate a signal and process that signal Efferent Neurons Conduct information away from the brain to the muscles and senses Motor Neurons Translates electrical signal to the muscles and glands

Not all neurons look the same Named by the number of extensions from the cell body Unipolar One extension that branches into dendrites and an axon Bipolar Two extensions: one axon and one dendrite Multipolar Many extensions: one axon, many dendrites

Unipolar Neurons Found primarily in human embryos and invertebrates Sensory neurons that conduct impulses into the brain

Bipolar Neurons Found primarily as sensory neurons in vision, audition and olfaction

Multipolar neurons Found everywhere else Brain Peripheral autonomic nervous system Spinal cord

What happens in a nerve Neurons communicate information to other neurons The signal itself is an electrical signal within the neuron Action Potential From neuron to neuron, communication is chemical

The Resting Potential Two forces at work in a neuron Potassium Ions Two forces at work in a neuron Force 1: Equilibrium: the idea that the concentration of a molecule tries to remain constant throughout the medium (substance)

The Resting Potential Ex. Smells: the garbage begins to smell in one spot, but the smell slowly expands throughout the house The smell gets lighter and lighter as it expands throughout the house It tries to spread out as evenly as possible

Status of the main players (ions) Sodium (Na+): more of it outside of the neuron Potassium (K+): more of it inside of the neuron Chloride (Cl-): more of it outside of the neuron

Based on equilibrium… What does Na+, K+, and Cl- want to do? Are they “happy” where they are? What direction will they move if allowed?

Due to equilibrium… Na+ wants to move inside the neuron K+ wants to move outside the neuron Cl- wants to move inside the neuron

What about charge? Does charge play a role in things? According to charge, how might the ions react?

How do charges react to each other? Opposites attract Like charges repel

One more piece to the puzzle There are large molecules inside the neuron with negative charges

Now which way do the ions want to go? Potassium: Which way does equilibrium push? What about the charge?

Now which way do the ions want to go? Chloride: Which way does equilibrium push? What about the charge?

Now which way do the ions want to go? Sodium: Which way does equilibrium push? What about the charge?

At rest Resting potential -70mV Potassium can cross the membrane Sodium and Chloride cannot cross the membrane What will potassium want to do at rest?

Stimulation When stimulated by another neuron, some Na+ channels are opened K+ channels close

The action potential If the signal is strong enough, it makes it to the axon hillock If strong enough, an action potential is generated Threshold Begins an action potential

The action potential Depolarization: occurs when Na+ flows into the cell Due to Na+ channels opening

The action potential Now, there is more of a positive charge inside than outside (the Na stops coming in for the same reasons the K did originally, a balance of two forces) this represents the peak of the AP At the beginning of the AP, Na channels open and Na enters the cell

How’s potassium feeling now? Sodium moving inside the cell made the inside of the cell more positive This repels potassium to move outside

The action potential K+ channels open at the peak and K+ flows out of the cell This repolarizes the cell and even overshoots the resting potential of before Called hyperpolarization Then the Na/K pump brings the K back inside the cell and the Na back outside the cell 2 K for every 3 Na Uses energy (ATP)

The action potential At the peak, K+ channels open and K+ exits the cell Conditions are now back where they started in terms of potential and charge, but the ions are in opposite positions

The action potential The Sodium Potassium pump restores the original environment of the resting potential so that the neuron can fire yet again This is known as the refractory period

Graded Potentials Action Potentials are referred to as “all-or-none” Either get an action potential or not Inputs that don’t reach threshold: Graded Potentials Can add up across synapses/inputs to reach threshold

Saltatory Conduction Insulation is not perfect Signal loses strength down the axon Regeneration of the Action Potential occurs at each break in the myelin

At the Axon Terminal Vesicles containing neurotransmitters are present When an action potential reaches the axon terminal, Ca2+ channels open up and this causes the fusing of the vesicles to the membrane and the release of the neurotransmitters into the synaptic cleft

In the Synaptic Cleft Neurotransmitters cross the membrane and bind to receptors on the receiving neuron Depending on the neurotransmitter, it can either excite or inhibit the post-synaptic neuron Excitatory Postsynaptic Potential (EPSP)- the excitation of the receiving neuron Inhibitory Postsynaptic Potential (IPSP)- the inhibition of the receiving neuron

Neurotransmitters Various chemicals Can either excite the receiving neuron or inhibit it Acetylcholine- an excitatory NT typically found in the muscles GABA- an inhibitory NT typically found elsewhere in the nervous system

How transmission occurs Multiple synapses on each neuronal dendrite Some can excite while others inhibit Axon potentials can be created by multiple EPSPs from multiple neurons Called Spatial Summation

How transmission occurs One synapse can fire repeatedly on the same dendrite within a short temporal window (time period) Axon potentials can be created by multiple EPSPs from a single neuron Called Temporal Summation

Summary: One last thing Transmission within the neuron is electric Via action potentials Transmission between neurons is chemical Via neurotransmitters

Glial cells Astrocytes- structural support and blood brain barrier Oligodendrocytes- myelination of axons CNS Schwann cells- myelination of axons PNS Microglia- clean up