Introduction to the Neuron Flashcards
Central nervous system
Brain and spinal nerves
Peripheral nervous system
Nerves outside of cranial and spinal
Autonomic nervous system
Responsible for many reflexes such as salivary flow. Divided into Parasympathetic and sympathetic
Synapses in a neuron
Each neuron has between 1,000 - 10,000 synapses
Diameter of a neuron
4 to 100 microns (one micron is 1/1000 of mm)
Velocity of neuron signal
1.2-250 mi/hr. The signal travels through neural networks via synapses
Definition of neuron
Electrically excitable cell that processes and transmits information by electrical and chemical signaling
Three tasks: 1. receive, 2. conduct, 3. transmit signals
Structure of the neuron
Cell body - contains organelles such as nucleus
Dendrites - transmit electrical impulses to the cell body
Axon hillock - located near the cell body nerve. Impulses originate there.
Axon - transmits impulse away from cell body
Myelin sheath - Myelin wrapped around the axon of neurons
Node of Ranvier - Permeable gaps in between myelin sheath to allow for exchange of ions
Pre-synaptic Terminal - end of the axon
Synaptic cleft - gap between pre and post synaptic terminal
Post-synaptic terminal - receiving portion of the adjacent neuron
Glia
Glia - non-neuronal cells. Astrocytes: star shaped glial cells that perform many functions such as support the blood-brain barrier, provide nutrients to the nervous tissue, maintain extracellular ion balance, and repair the brain and spinal cord after traumatic injuries
Action Potential
The electrical signal that is conducted along neuronal axons. Generated by special types of voltage-gated ion channels embedded in a cell’s plasma membrane.
Two kinds of action potentials
- generated by voltage-gated sodium channels in the axon (faster)
- generated by voltage-gated calcium channels in the pre-synaptic terminal (slower)
Resting membrane potential in neurons
-70 mV. There are largely negatively charged ions inside the cell. Neurons are excitable and can change their membrane potential if stimulated
What happens to transmit an action potential
- the ion channels open if the membrane potential is no longer near the resting potential of the cell, but increases to a precisely defined threshold value.
- channels open, allowing an inward flow of sodium ions, which changes the electrochemical gradient, which produces a further rise in membrane potential, which causes more channels to open. Process continues until all available ion channels are open; membrane potential is much higher
- inactivation of the ion channels when the polarity of the plasma membrane reverses. As the sodium channels close, sodium is actively transported out of the plasma membrane via na+/k+ pump (3 for 2)
- after the action potential, there is a transient negative shift called the afterhyperpolarization
Nodes of Ranvier
Areas where there is permeability to ion changes; only in a small area of the membrane
Polarization
Depolarization - potential difference approaches zero as membrane becomes permeable to Na
Repolarization - back to the resting potential
After-hyperpolarization - potential difference increases as positive charges leave cell and negative ones enter
The refractory period
Assures that impulse conduction happens only one way/unidirectional. Absolute refractory period is the interval which a second action potential simply isn’t going to happen, no matter the stimulus. The relative refractory period is the interval during which initiation of a second action potential is inhibited but not impossible
Saltatory conduction
Action potential moves along myelinated axons from one node of Ranvier to the next. Only unmyelinated regions of the axon depolarize; thus, the impulse moves 50x faster.
Myelin Sheath
in the central nervous system, myelinated axons form “white matter” (cell bodies and dendrites are gray). Demyelinating disorder - multiple sclerosis
Na+/K+ pump
the pump is an enzyme located in the plasma membrane. When an action potential is completed, the sodium potassium pumps three sodium ions out for every two pumped it. It takes 1 ATP for every translation
Calcium action potential
Transmitter is synthesized and stored in vesicles, and when terminal is depolarized, voltage-gated Ca channels open; this ion causes vesicles to fuse with presynaptic membrane and release neurotransmitters. Post-synapptic receives; a current there causes either an excitatory or inhibitory postsynaptic potential
Reuptake
Reabsorption of neurotransmitter by a neurotransmitter transporter of a pre-synaptic neuron - efficiency - not going to waste
Excitatory postsynaptic potential
Depolarize the postsynaptic neuron in order to continue transmitting the signal. Can summate or cancel out IPSP
Inhibitory postsynaptic potental
Hyperpolarizes the postsynaptic neuron so that nothing happens. Can summate or cancel out EPSP
Saltatory Conduction Overview
(1) Equilibrium = negative “resting” state (-70mv)
(2) Action potential transiently abolish the “resting” potential.
(3) Results in Na+ being able to enter the axon (along axon at Nodes of Ranvier)
(4) This rapid rise in Na+ renders the transmembrane positive (depolarization)
(5) This is followed by rise in K+ ions, which in turn close the Na+
(6) Na+/K+ pump remove NA+
(7) This occurs along each Node of Ranvier until the end of the axon
Calcium voltage-gated Review
(8) Action potential invades the presynaptic terminal
(9) Increases influx of Ca+
(10) Causes vesicles to fuse with presynaptic membrane (11) Transmitter is released into the synaptic cleft (12) Post-synaptic terminal receives neurotransmitter (13) Causes IPSP or EPSP
(14) Reuptake of neurotransmitter
Dopamine (DA)
A neurotransmitter that is produced in several areas in the brain, including the substantial nigra and the ventral tegmental area (both in the Midbrain). Problems with transmission of DA can lead to schizophrenia and parkinson’s disease. Plays major role in reward-driven learning - every reward increases dopamine transmission in the brain.
Serotonin (5-HT)
Neurotransmitter. Regulation of mood, appetite, sleep, cognitive functions. Antidepressants modulate serotonin at synapses, preventing reuptake, so more goes to the post-synaptic terminal. Influences moral judgment, behavior, aggression (inhibits)
