How can we classify neurons? (1)
By the shape (morphology) of the dendrites and soma
ex. pyramidal v.s. stellate neuron
- main dendrite comes up out of the apex
- dendrites come out at different places
How can we classify neurons? (2)
By the number of processes (bipolar vs. multipolar)
- unipolar (1 dendrite/axon)
- bipolar (1 dentrite, 1 axon)
- multipolar (many dendrites 1 axon)
How can we classify neurons?
By the presence of spines
- not all neurons have dendrite spines
Why are dendritic spines important?
- Increased surface (can receive more inputs, and increase regional space)
- Isolate chemical reactions (specific receptor processes are localized in separate spaces)
Dendritic spine morphology
- Head of spine + neck: houses the neurostransmitter
- spines can respond to different types of activation…different types of receptors
The axon: output of the neuron
- Axon hillock
- Axon terminal
- Important facts:
1) Each neuron has one and only one axon. It may
have several collaterals, and many terminals, but
only one axon leaves the cell body.
What are the implications of this?
*only one signal can leave the cell
2) Endoplasmic reticulum is absent.
What are the implications of this?
*no protein synthesis occurring in the axon
-> this must all be synthesized in the cell body
Axon terminal and synapse
- Synapse: space between neurons.
Electrical signals-> chemical signals
-Presynaptic (produce NT) vs. postsynaptic membrane (receive NT) - Vesicles (package NT) vs. receptors (receive NT)
How can we study the structure of neurons?
- Histology = the study of tissue structure
- ex. studying the mouse brain
Nissl stain
- binds to negatively charged nucleic acids:
DNA, RNA
Which structures will be stained by the Nissl stain? - cell body will be stained
- dendrite will be (close to endoplasmic reticulum, so some DNA/RNA)
Which structure will not be stained? - axons (No DNA/RNA)
Golgi stain
- stains the whole cell – both the cell body
(soma) and neurites (axon and dendrites) - silver nitrate. Stains only a small percentage of neurons
- Ramon y Cajal (1852-1934) used the Golgi stain to map out the circuitry of the brain
Neuron doctrine:
- neurons are individual cells
(as opposed to a continuous network like the
circulatory system) - The individual neuron is the structural and
functional unit of the nervous system
Why would you choose to use a Nissl stain rather than a Golgi stain
(and vice versa)?
- Nissl stain (Cell body count):
Cortex, olfactory bulbs,
Cortical layers - Golgi stain (Structure of the cell):
pyramidal cell, Purkinje cell,
interneuron, retinal cells
Immunohistochemistry
- Antibody stains recognize specific proteins in cells
- ex. GAD65+GAD67 marks all GABAergic neurons
Parvalbumin is restricted to a subtype of GABAergic neuron
More specifically in quizlet, for pictoral studying
Golgi-stained neurons in the visual cortex
Increased branching provides:
– increased surface for synaptic contacts,
– greater potential for interneuronal interaction,
– suggests a greater capacity for information
processing.
- Enriched environments sig. increase # of dendritic spines in the somatosensory cortex in adult rats placed under experimental conditions for 3 weeks.
*details: mostly seen in pyramidal cells
What are the mechanisms for dendritic spine growth?
- Activation of various postsynaptic receptors.
- Certain neurotransmitter systems (norepinephrine, dopamine, acetylcholine)
may modify the process of dendritic spine growth. - Local neurotrophins, transmitter release and synaptic protein synthesis promote synaptic
remodeling and changes in receptor expression or activation. - Medical angle: early mobilization can reduce a second stroke and can reduce mortality.
Hospitals now have specialized stroke units and patients undergo multidisciplinary
rehabilitation.
Glia: the “glue” of the brain
- insulate
- support and nourish neurons
*Astrocytes
Oligodendrocytes
Schwann cells
Microglia
Astrocytes:
- fill in the spaces between the neurons.
- regulate the extracellular concentration of various ions (such as potassium)
and neurotransmitters. - maintain the appropriate chemical environment for neurons.
Astrocytes and synaptic function
- Can release gliotransmitters that activate neuronal receptors
- Express receptors that are activated by neurotransmitters
- Activity of astrocytes can affect animal behavior – including sleep, drug-seeking
and hippocampal-dependent learning
Blood-brain barrier
- Protects the brain from chemicals in the blood, and also from infection.
- Endothelial cells surrounded by astrocytes.
Things that can cross the BBB:
- lipid soluble molecules such as steroids
- oxygen and carbon dioxide
- ethanol
- sugars and some amino acids
Things that cannot cross the BBB:
-large molecules (higher than 500 daltons molecular weight)
- Antibiotics (brain infections are difficult to treat!)
- Large proteins
- Ions and charged particles
Example: L-DOPA (the precursor to dopamine) can cross the BBB, but
dopamine cannot. So Parkinson’s patients are treated with
L-DOPA, not dopamine itself.
Oligodendrocytes:
- wrap around the axons in the
brain and form a myelin sheath.
High resistance = insulation of the axons. Gaps
in the myelin = nodes of Ranvier. - At these nodes, the axon is exposed.
- Most but not all axons are wrapped by glia
Schwann cells:
- similar to oligodendrocytes in that they wrap
around the axons, but these exist in the peripheral nervous system only.
Microgila:
- primarily scavenger cells, removing cellular debris, especially if cells are injured.
- can modulate local inflammation
- COVID-19 associated with widespread microglial activation
throughout the brain
