Neuron Anatomy- the structure of a neuron reflects their function.  One part of the cell receives incoming signals. One part generates outgoing signals

 

Neurons Can Communicate - Because of their unique anatomical design, and because they are excitable, neurons can communicate.  They can communicate with each other, with muscles and with glands.

 

 

Neurons Have Three Structural Characteristics- All neurons have a cell body, a receptive portion, and a transmitting portion

 

           

 

            1.  Dentrites

Multiple branching, extend from cell body

                       

            Receive signals coming

 in from other cells and

 send them toward the

 axon

 

            2.  Cell body or soma-

                        Located centrally- it has the nucleus

 

The  cell body is the main nutritional and metabolic region of the neuron.  Like the dendrites, it receives signal from the other cells and sends tem toward the axon

 

            3.  Axons

                        A thin single fiber extends from the cell body

           

                        The axon generates an action potential, an

 outgoing signal also called an nerve impulse,

and conducts it to the next cell.  The axon is

the transmitting or conductive region of the

 neuron

 

           

                        Bundles of axons are called tracts in the CNS

 

Bundles of axons are called nerves in the PNS

 

Ganglion- bundles of nerves outside the CNS that looks like a lump

 

Informational Flow in Neurons is Directional

 

 

 

 

Signals are received at Synapses

 

The cell body and dendrites have a large surface area to receive communication with other neurons.  Signals are received from other neurons from synapses, the junctions between neurons.

 

Axon can vary in length

 

Some axons can be very short only 1 or 2 mm in length - communicating only with cells in their immediate vicinity.

                       

Axons can be very long - more than a meter - and can communicate over long distances.  For example, some axons of some spinal cord neurons can reach all the way to muscle of the big toe.  In such neurons, the axons makes up most of the volume of the cell.

 

            In general, the longest axon have the largest cell bodies

 

Signal are sent out from Axons

 

Each neuron has a single axon. Which arises from the cell body at a region called the axon hillock.

 

            Axons can branch, forming axon collaterals.

 

            At their ends, axons can branch into thousands of axon terminals.

           

            The action potential or outgoing signal is generated at the axon hillock and conducted along the axon to the axon terminals.

 

Some axons are myelinated

 

Some axons are covered with an insulating material called the myelin sheath, which is produced by the support cells of the nervous system.

 

Schwann cell- support cells of the peripheral nervous system- produce myelin sheath.

 

As the Schwann cell wraps around and around the axon in the process of myelination, its cytoplasm is squeezed out.  The tightly wound cell membrane becomes the actual insulation.

                       

 

Many Schwann cells insulate a single axon

 

It takes many schwann cells to insulate a single axon.  Neighboring schwann cell do not actually touch, so that there are gaps in the myelin sheath.  These gaps are called the Nodes of Ranvier, are essential for conduction of the action potential.

 

Multiple myeloma is an autoimmune disease in young adults.  It causes slow destruction of the myelin sheath, resulting in patchy, sporadic destruction.  Short circuiting of the neuron results, causing temporary blindness, double vision, and an inability to control muscles.  This leads to uncoordination, imbalance, muscle weakness and paralysis.  The autonomic nervous system may also be affected, resulting in the loss of control of the urinary bladder and the large intestine.

 

 

 

The Action Potential -nerve impulse

 

Resting Potential of a neuron -70mV;with Na+ mainly on the outside and K+ on the inside

 

Depolarization- the action potential begins at the axon hillock where Na+ voltage gated channels are greatest.  Signals from the dendrites and cell body cause the axon hillock to depolarize.  Sodium rushes into the cell when voltage gates are open. The inside of the cell becomes less negative

 

Threshold- For an action potential to actually occur, the electrochemical gradient caused by Na rushing in, must be greater than 15 mV.  Weak stimuli to cause an action potential.  Thus we say that an action potential is an all-or none event.  Action potentials always have the same amplitude and duration.

 

Action potential- If threshold is reached an nerve impulse spreads along the entire axon.  It spreads as a wave along the axon.

 

Repolarization- K+ leaves the cell by voltage gated channels, causing the cell to become more negative, back to -70mV.  Although the voltage is the same as resting, the ions are not the same as resting.

 

Hyperpolariztion-K+ continues to leave the cell by voltage gated channel slowly, causing the cell to become even more negative, more negative than resting potential.

 

Absolute Refractory Period- period when another action potential cannot happen. This is when many Na+ channels are inactive and will not open no matter what voltage is applied.  This prevents action potential to happen only one at a time.

 

 

Resting Potential-  For the cell to go back to resting potential, the sodium-pump must be activated using ATP.  The correct levels of Na+ will be on the outside and the correct levels of K+ will be on the inside of the cell.  An action potential can now be generated again.

 

 

            Conduction Velocity is dependent on diameter and myelination of the axon.

 

                        Diameter of the axon causes the speed of the action potential to increase

                        Myelination causes the speed of the action potential to increase

 

            Saltaltory Conduction

 

Conduction actually appear to jump to the Nodes of Ranvier, increasing the speed of conduction.  There are more voltage gated channels at each Node of Ranvier.

 

Reflex arc- Rapid, involuntary responses such as jerking your hand from a hot

  stove, knee jerk reaction, blinking when object is close to eyes,  or pin prick.