Three categories of neurons:
1. Sensory neurons; carry information from sensory organs into the central nervous system.
2. Motor neurons; carry message out from the central nervous system to operate muscles and glands.
3. Interneurons exist entirely within the central nervous system and carry messages from one set of neurons to another. Interneurons collect, organize and integrate messages from various sources. They vastly outnumber the other two types.
The neurons parts:
1. Cell body; it contains the cell nucleus and other basic machinery common to all bodily cells.
2. Dendrites are thin, tube-like extensions that branch extensively and function to receive input for the neuron.
! In sensory neurons, dendrites extend from one end of the axon, rather than directly from he cell body.
3. The axon is another, thin tube-like extension from the cell body. Its function is to carry messages to other neurons, or in the case of motor neurons, to muscle cells. The axons of some neurons are surrounded by a casing called a myelin sheath. Myelin is a fatty substance produced by supportive brain cells called glial cells.
Action potentials
Neurons exert their influence on other neurons and muscle cells by firing off all-or-none impulses called action potentials. Action potentials are described as "all or none" because they either occur or don't occur; that is, they don't partially occur or occur in different sizes or gradations. A neuron can convey varying degrees of intensity in its message by varying its rate of producing action potentials.
Cell has a electrical charge across the membrane of -70mV, due to difference in charge of intracellulair and extracellular fluid. The inside of the cell (intracellulair) has a negative charge: soluble protein molecules (A-) and K+ (more concentrated inside the cell than outside). The outside of the cell has a positive charge: Na+ and Cl- ions.
Resting potential (-70mV) = charge across the membrane of an inactive neuron.
The action potential is initiated by a change in the structure of the cell membrane: thousands of channels that permit sodium (Na+) ions to pass through open up → Na+ moves in to cell → + charge in cell = deploarization phase.
As soon as deplorization occurs the channels that permitted Na+ to pass through close, but channels that permit K+ to pass through remain open. K+ are pushed outside of the cell →reestablish the resting potential = repolarization phase.
To main the original balance of K+ and Na+ across the membrane, membrane contains sodium-potassium pump (Na/K pump), that continuously moves sodium out of cell and potassium into it.
The axon membrane is constructed in such a way that depolarization to some critical value causes the sodium channels to open, thereby triggering an action potential. This critical value is referred to as the cell's threshold.
The speed at which an action potential moves down an axon is affected by the axon's diameter. Large diameter axons present less resistance to the spread of the electric current and therefore conduct action potentials faster than thin ones. Another feature that speeds up the rate of conduction in many axons is the presence of a myelin sheath. Each action potential skips down the axon, from one node to next, faster than it could move as a continuous wave.
[Side note: When you poke your finger with a pin, you feel the pressure of the pin before you feel the pain, you feel the pressure of the pin before you feel the pain. That is partly because the sensory neurons for pressure are large and myelinated, while those for pain are thin and mostly unmyelinated]
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| Synaptic transmission |
Synapse = the junction between each axon terminal and the cell body or dendrite of the receiving neuron
When an action potential reaches an axon terminal, it causes the terminal to release packets of a chemical substance called neurotransmitters. At an excitatory synapse, the transmitter opens sodium (Na) channels in the postsynaptic membrane → depolarization → increase the rate of action potentials triggered. At an inhibitory synapse, the transmitter opens either Cl- or K+ channels → hyperpolarization → decrease the rate of action potentials triggered.
Methods of mapping the brain
The methods of mapping the brain fall into three general categories:
1. Observing behavioral deficits that occur when a part of the brain is destroyed or is temporarily inactivated
2. Observing behavioral effects of artificially stimulating specific parts of the brain
3. Recording changes in neural activity that occur in specific parts of the brain when a person or animal is engaged in a particular mental or behavioral task
- TMS: Magnetic pulses cause a temporary loss in those neurons ability to fire normally. Because magnetic field affects only that part of the brain lying immediately below the skull, TMS can be used for mapping cerebral cortex only.
- EEG
- PET: injecting radioactieve substance into blood and measuring the radioactivity that has been emitted by each portion of the brain.
- fMRI