Dr. Gerald S. Hecht
Assistant Professor of Psychology
College of Sciences
webmaster@psiwebsubr.org

Neuron structure

• CELL BODY:
(or soma) contains the nucleus, plus some ribosomes (where neurotransmitters are made) and mitochondria.

• DENDRITES:
Branch-like structures which extend from the cell body-- dendrites receive information (BOTHsensory information from the "outside world"AND neural information from other neurons)  and carry the information into the CELL BODY.

• DENDRITIC SPINES:
Short outgrowths on some dendrites which contain specialized proteins called RECEPTORS (see below).

• AXONS:
long, thin fibers which are the information sending part of the neuron - sending out periodic electrical impulses toward other neurons, glands, or muscles (these impulses are called action potentials). Neurons have only one axon or none at all. The state of axons when they are not sending impulses is called the resting potential.

• AXON TERMINAL: A "bulb-like"swelling at end of an axon which plays a role in the chemical communication that takes place between neurons.

• PRESYNAPTIC NEURON: The neuron that is transmitting a chemical signal from its axon terminals
• POSTSYNAPTIC NEURON: The neuron whose dendrites are receiving the chemical signal.

  

The Resting Potential of Neurons

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At rest, there are 2 sources of POTENTIAL ENERGY accumulating around the neuron:

1. THECONCENTRATION GRADIENT  

2. THE ELECTROSTATIC GRADIENT

• CONCENTRATION GRADIENT:
During the resting potential, a difference in the concentration of chemicals is established between the inside of the axon and the outside. This is because the axon membrane is selectively permeable (it allows certain chemicals to cross but not others). The concentration of  sodium (Na+) ions is 10 times greater outside the axon than inside. During the resting potential, Chloride channels in the axon membrane remain open allowing chloride ions to pass through; Sodium channels remain closed, however and Sodium begins to accumulate outside the axon.

• ELECTROSTATIC GRADIENT: The highly concentrated Sodium (Na+) ions outside the axon carry a positive electrical charge and are therefore attracted to the negatively charged Chloride (Cl-) ions inside the neuron—but can’t in because the Sodium channels are closed. The +and – charges are attracting each other and thus the axon is now a 70mV (millivolt) battery (a battery is a device which creates an electrical gradient—a separation of + and – charges). The voltage of the axon is always stated relative to inside... therefore the resting potential of a neuron is -70mV

The Action Potential of Neurons

The actual voltage across axon membranes during the resting potential can fluctuate somewhat...

• Hyperpolarization
(increased polarization): occurs when the negative charge inside the axon increases from incoming IPSP’s (e.g., -70mV becomes –80mV)

• Depolarization
(decreasing polarization toward 0mV): occurs when the negative charge inside the axon decreases from incoming EPSP’s (e.g., -70mV becomes –55mV).

However, If the axon membrane is depolarized beyond a critical level known as the Threshold... the resting potential collapses and an ELECTRICAL SIGNAL is propagated down the axon toward the AXON TERMINAL

• Threshold:
A critical level of depolarization of the axon membrane that triggers the action potential

• Action Potential: (Nerve impulse):
Neuronal stimulation which results in a SUDDEN DEPOLARIZATION of Tremendous Amplitude which propagates down the entire axon to the terminal ("peak" or "spike") before returning to its resting potential.

• All-or none law:
If threshold is met or exceeded, a neuron will generate an action potential at a specific magnitude (size and shape); if threshold is not met, no action potential will occur.

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When the potential across an axon membrane reaches threshold, Na+ channels open and allow these ions to enter (the Na+ gates are voltage dependent; this causes the membrane potential to depolarize past zero to a reversed polarity (e.g., -70mV becomes +50mV at the highest amplitude of the action potential)

• When the action potential reaches its peak, Na+ channels close and the Na+ ion pump "throws" the Na+ back outside the axon behind the channel… this ensures that the action potential will travel in one direction only… towards the axon terminals.

•  The neuron, (much like a toilet) has a refractory period-- it cannot emit another action potential until the Na+ ion pump restores the polarity to a level approximating the resting potential.

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The action potential always begins at the axon hillock (between the cell body and the beginning of the axon)
• 
  
The action potential is regenerated due to Na+ ions "jumping" down the axon, activating each voltage dependent Na+ channel-- one after the other.

The Concept of the Synapse

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Neurons are separated from one another by a narrow gap known as asynapse.
• Neurons:

1. Synthesize neurotransmitters (chemicals used by neurons to "talk to each other").
2. Deliver these chemicals to the axon terminal for storage in synaptic vesicles until released into the synapse.

• The release of neurotransmitters into the synapse depends on an action potential traveling down the axon to the terminal.

1. Neurotransmitters
, once released into the synapse, bind to receptors and alter the activity of the postsynaptic neuron.
2. Neurotransmitters
can be prevented from reaching the receptors by

1. REUPTAKE:
"recycled" back into the presynaptic neuron
2. ENZYMATIC DEGRADATION:
"digested" (lysed) by enzymes.
3. DIFFUSION: swept away by extracellular fluid

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