Microanatomy & Neurotransmission Flashcards
Soma (L. body)
Core region
Processes information.
Dendrites (L. tree branches)
Branching extensions;
- Receive information;
- # of dendrites = amount of incoming information.
Dendritic Spines
Protrusions from a dendrite that serves as point of contact with other axons.
Axon (L. axle)
- Carries information to other neurons;
- White matter.
Myelin Sheath
- Insulates axons;
- Signal travels further, faster, stronger.
Axon Hillock
- Point at which the axon leaves the soma (cell body).
Axon Collateral(s)
- Point at which axon branches out;
- Allows message to be sent in multiple directions simultaneously.
Terminal Button
- Stops extremely close to dendritic spine of another neuron;
- Does not touch other neurons.
Synapse
- Junction between one neuron and the other;
- Space between terminal button & dendritic spine.
What are Neurons
- Carry out brain’s major functions;
- Many different types;
- Can be very specialized.
What are Glia Cells
- Aid and modulate activity of neurons;
- Maintenance, nourishment, metabolism, synthesis & clean-up.
What are the two main types of nerve cells
Neurons
Glia Cells
What are the types of neurons
Sensory Neurons
Interneurons
Motor Neurons
Sensory Neurons
Brings information to the brain (afferent).
Interneurons
Associate sensory & motor neurons.
Motor Neurons
Carry information (motor instructions) from brain into spinal cord and muscles (efferent).
What are the subtypes of sensory neurons
Bipolar neurons;
Somatosensory neurons.
Somatosensory neurons.
E.g. multipolar cell.
Bipolar neurons;
E.g. retinal bipolar cell.
What are the subtypes of Interneurons
Stellate cell (star shaped):
Pyramidal cell (pyramid shaped):
Purkinje cell:
Purkinje cell:
Output cell; Extremely branched dendrites.
Pyramidal cell (pyramid shaped):
Long axon with multiple sets of dendrites.
Stellate cell (star shaped):
Very small, many dendrites extending around entire cell body.
Motor Neurons
Extensive dendritic networks to collect information from multiple sources.
Large cell bodies to process information.
All outgoing information must pass through motor neurons to reach target muscles.
Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN)
What are the Subtypes of glia cells
Ependymal cell
Astrocyte
Microglia
Oligodendroglia
Schwann cell
Glia Cells
Glia (L. glue) cells provide insulation, nutrients and support to all neurons.
They are like neuron parents.
Ependymal cell
located on walls of ventricles, produce CSF;
Astrocyte
provides structural support, regulates blood brain barrier;
Microglia
immune function, engulfs foreign substances;
Oligodendroglia
insulates axons in the CNS;
Schwann cell
insulates axons in the PNS.
Glial Cells & Neuronal Repair
Microglia & Schwann cells play a role in repairing damaged neurons in the PNS:
- Microglia remove debris;
- Schwann cells form path for new axons to follow & insulate new axons.
Axons
- Carries information;
- Connects neurons to each other; - White matter.
Axons tend to project in bundles
- Nerve when outside the CNS;
- Tract within the CNS.
How do neurons communicate?
Neurotransmission occurs in two different steps.
1. Electrical; 2. Chemical.
For one neuron to communicate with another neuron, it must use both electrical and chemical signals (mostly, there are always exceptions).
Electrical Communication
Each neuron has a resting membrane potential. This occurs because the inside of the cell is
negatively charged, relative to the outside of the cell.
- Large negatively charged proteins (A-) – inside;
- Sodium ions (Na+) – outside;
- Potassium ions (K+) - inside;
- Chloride ions (Cl-) – outside.
What is our membrane potential
Our entire existence as a species is dependent on this membrane potential being at -70mV.
sodium-potassium pumps
We rely entirely on sodium-potassium pumps to keep this balance.
- Exchanges 3 Na+ (outbound) for 2 K+ (inbound);
- Use up ~2/3 of a cell’s energy expenditure.
Hyperpolarization
membrane potential is exaggerated, so difference between inside and outside are greater;
Depolarization
membrane potential is diminished,
so difference between inside and outside are lessened.
How can membrane potentials change?
Channels and pores on the cell membrane surface allow ions in and out of the cell;
- Different stimulations will open different pores/channels and allow ions to enter/exit the cell.
The Action Potential
An action potential is a brief (~1ms), but very large, reversal in polarity of an axon’s membrane.
Inside the cell (i.e. intracellular) becomes positive, relative to the outside (i.e. extracellular);
This change is abruptly reversed, thanks to the Na+/K+ pumps, and the resting membrane potential (-70mV) is restored.
What is reversal of membrane polarity caused by
Reversal of membrane polarity is due to an influx of Na+ and efflux of K+. Threshold potential is -50mV
- Stimulation of cell is required to depolarize a membrane to -50mV;
- Once threshold is met, no further stimulation is required, and
depolarization continues until the inside of the
cell reaches +30mV (or +40mV), relative to the outside.
How does a neuron reach the action potential threshold?
Each neuron will receive excitatory and inhibitory input from pre-synaptic cells.
- Excitatory Post Synaptic Potentials (EPSP) - Inhibitory Post Synaptic Potentials (IPSP)
The sum of these inputs (potentials) will ultimately decide the fate of the post-synaptic cell.
Spatial summation
EPSPs&IPSPs that happen close together in space will be summed;
Temporal summation
EPSPs & IPSPs that happen close together in time will be summed.
Voltage-gated Ion Channels
gated channels that opens/closes in relation to membrane potential.
- Na+ channels are sensitive to small changes in membrane potential, so any ion fluctuations will cause them to open;
- Voltage gated K+ channels are attuned to -50mV. Once this threshold is met, they all open.
- The entire axon of a neuron is filled with voltage gated ion channels (especially at the axon hillock), and therefore an action potential propagates the entire length of an axon: from cell bodyàterminal.
Chemical Communication
Chemical communication between two neurons occurs via trading of neurotransmitters:
What is a neurotransmitter
- Chemical released by a neuron onto a target site;
- Causes excitatory or inhibitory effectàthe only two responses a cell can have;
- Some debate over what constitutes of neurotransmitter (i.e. criteria).
How are neurotransmitters released
Neurotransmitters are released into the
synaptic cleft – small space that separates two neurons.
What is step one of chemical communication
An electrical signal arrives and triggers the release of neurotransmitters in to the synaptic cleft.
What is step two of chemical communication
The neurotransmitters bind to receptors on the post- synaptic neuron that triggers an action potential.
What is step three of chemical communication
The neurotransmitter is re- cycled back into the pre- synaptic cell by reuptake transporters.
What are the four steps of chemical transmission
Synthesis
Release
Receptor Action:
Inactivation (X4):
Synthesis
Synthesized from DNA/mRNA and stored in vesicles located in the axon terminal.
Release
Transportedtopre-synapticmembrane,released in response to action potential;
Receptor Action:
- Activate target receptors on post-synaptic membrane;
- Depending on the receptor, the post-synaptic cell can be excited or inhibited.
Inactivation (X4):
Must be inactivated or will continuously stimulate(or inhibit) post-synaptic neuron;
- Modes of inactivation are i)glial celluptake,ii) enzymatic degradation, iii) diffusion and/or iv) reuptake.
What are the classes of neurotransmitters
Monoamines NT’s
Amino Acid NT’s
Peptide NT’s
Transmitter Gases
Monoamines NT’s
Dopamine (DA), Norepinephrine (NE), Epinephrine (aka Adrenaline); Serotonin (5-HT); and Histamine.
Amino Acid NT’s
Glutamate, GABA, glycine and D-serine;
Peptide NT’s
- Somatostatin, Substance P, Opioid peptides;
Transmitter Gases
Nitric oxide, carbon monoxide.
