Neurons and Glia Chapter 2 Pg 32-57

Obstacles to Study  Cells are too small to see.  To study brain tissue with a microscope, thin slices are needed but the brain is like jello. Formaldehyde used to “fix” or harden tissue early in 19 th century.  Brain tissue is all the same color: Nissl stain revealed cell bodies – cytoarchitecture Golgi stain revealed parts of the neuron.

Golgi stain shows cell structure

Brodmann Areas  Different areas of the brain with different functions have different kinds of neurons.  Brodmann mapped the areas based on the kinds of cells found: Cytoarchitectonic method 52 functionally distinct areas identified by number.

Ramon y Cajal’s Principles  Neuron doctrine – neurons are like other cells.  Principle of dynamic polarization – electrical signals flow in only one, predictable direction within the neuron.  Principle of connectional specificity: Neurons are not connected to each other, but are separated by a small gap (synaptic cleft). Neurons communicate with specific other neurons in organized networks – not randomly.

Neuronal Circuits  Neurons send and receive messages.  Neurons are linked in pathways called “circuits”  The brain consists of a few basic patterns of circuits with many minor variations.  Circuits can connect a few to 10,000+ neurons.

Parts of the Neuron  Soma – the cell body  Neurites – two kinds of extensions (processes) from the cell: Axon Dendrites  All parts of the cell are made up of protein molecules of different kinds.

How Neurons Communicate  An all-or-nothing electrical signal, called an action potential, travels down the axon. The amplitude (size) of the action potential stays constant because the signal is regenerated. The speed of the action potential is determined by the size of the axon. Action potentials are highly stereotyped (very similar) throughout the brain.  At the end of the axon (terminal button), neurotransmitter is released, which may start an action potential in another neuron.

The Synapse  The synapse is the point of contact between neurons. Axon terminal button makes contact with some part of an adjacent neuron.  Synaptic vesicles containing neurotransmitter open when there is an action potential.  Neurotransmitter may enter the adjacent neuron – unused neurotransmitter is reabsorbed (reuptake).

Dendrites  Dendrites function as the antennae of the neuron, receiving input from other neurons.  Dendrites are covered with synapses.  Each synapse has many receptors for neurotransmitters of various kinds.  Dendritic spines – specialized dendrites that isolate reactions at some synapses.

Dendritic Spines

How to Tell Axons from Dendrites  Dendrites receive signals – axons send them.  There are hundreds of dendrites but usually just one axon.  Axons can be very long (> 1 m) while dendrites are < 2 mm.  Axons have the same diameter the entire length – dendrites taper.  Axons have terminals (synapses) and no ribosomes. Dendrites have spines (punching bags).  Don’t be fooled by the branches – both have them.

Ways of Classifying Neurons  By the number of neurites (processes): Unipolar, bipolar, multipolar  By the type of dendrites: Pyramidal & stellate (star-shaped)  By their connections (function) Sensory, motor, relay interneurons, local interneurons (Golgi Type II neurons)  By neurotransmitter – by their chemistry

Parts of the Soma (Cell Body)  Nucleus – stores genes of the cell (DNA)  Organelles – synthesize the proteins of the cell  Cytosol – fluid inside cell  Plasmic membrane – wall of the cell separating it from the fluid outside the cell.

Organelles Mitochondria – provide energy Microtubules – give the cell structure Rough endoplasmic reticulum – produces proteins needed to carry out cell functioning Ribosomes – produce neurotransmitter proteins Smooth endoplasmic reticulum – packages neurotransmitter in synaptic vesicles Golgi apparatus – Part of the smooth endoplasmic reticulum that sorts proteins for delivery to the axon and dendrites

Kinds of Cells  Neurons (nerve cells) – signaling units  Glia (glial cells) – supporting elements.  Miscellaneous other cells: Ependymal cells – form the lining of the ventricles, also aid brain development Microglia – remove debris left by dead or degenerating neurons and glia.  Veins, arteries, and capillaries in the brain.

Functions of Glia  Separate and insulate groups of neurons  Produce myelin for the axons of neurons  Scavengers, removing debris after injury  Buffer and maintain potassium ion concentrations  Guide migration of neurons during development  Create blood-brain barrier, nourish neurons

Kinds of Glia  Oligodendrocytes – surround brain & spinal cord neurons and give them support. In white matter, provides myelination In gray matter, surround cell bodies  Schwann cells – provide the myelin sheath for peripheral neurons (1 mm long).  Astrocytes – absorb potassium, perhaps nutritive because endfeet contact capillaries (blood vessels), form blood-brain barrier.