How does the body send signals over long distances?
Chemical messengers, electrical or a combination of the two. Peripheral nerves are constructed for electrical transmission that leads to a chemical messenger activation.
Describe the cell body (soma).
Contains the nucleus, located mainly in the CNS.
Where are the cell bodies of sensory neurons located?
In the dorsal root ganglia (DRG).
Where are the cell bodies of motor neurons located
In the ventral horn.
What are dendrites, where are they located, and what are their functions?
Multiple branches close to cell body allow local cell to cell communication.
Where is the axon located and what are their functions?
Extends from the cell body, impulse transmission over long distance, defines receptive field.
Describe Schwann cells
Envelopes every axon, produces myelin.
May produce myelin sheaths around larger axons (>1 micron)
Describe the myelin sheath
Supporting cells wrap layers of their own PM around the axon forming an insulating layer.
Each supporting cell surrounds about 1 mm of axon-many cells required to cover whole axon.
How does myelination affect capacitance?
Decreases capacitance (decreases the loss of electrical signal).
Describe the nodes of Ranvier
Interruptions in the myelin sheath.
What are the functions of the nodes of Ranvier?
Allows axolemma to be exposed to the extracellular matrix-specifically the Na channel (the axonal membrane is uninsulated & therefore capable of generating electrical activity).
Allows for more rapid electrical conduction.
Largery axons transmit impulses at a faster rate (less Na channels to open).
Describe how nerve impulses travel along unmyelinated axons.
Travel as a uniform wave of depolarization d/t conformational changes in ion channels.
Describe how nerve impulses travel along myelinated axons.
The Na channels act like booster stations for the propagation of a passively conducted electrical current. This is a more rapid and efficient system.
Describe peripheral nerves
Mixed type w/ afferent & efferent fibers, a single nerve is both myelinated & unmyelinated.
Describe the anatomy of a nerve.
Describe the endoneurium
Surrounds each individual axon, non-neuronal glial cell.
Describe the perineurium
Contains the nerve fibers in fascicles, semipermeable, acts as a major barrier to diffusion of LAs (tight junctioned).
Describe the epineurium
Sheath covering the entire nerve, contains nutrient blood vessels.
All myelinated. Alpha, beta, gamma, delta.
What are the functions of a-fibers (alpha, beta, gamma, delta)?
Motor, tactile, proprioception, muscle tone, touch.
What are the functions of small A delta fibers?
Pain and temperature.
Describe B fibers
Myelinated, preganglionic autonomic
Describe C fibers (sC, dC)
Unmyelinated, postganglionic autonomic
What are the functions of C fibers (sC, dC)?
Pain and temperature.
Describe the phospholipid bilayer.
Impermeable to ions, electrical capacitance (acts as a capacitor plate)
Describe depolarization & action potential of a nerve.
Must hit threshold (point at which the cell will become fully depolarized)
Once threshold reached, Na channels open rapidly, the cell becomes positive (depolarized).
Describe repolarization & action potential if a nerve.
Na channel closes, K channels open (inside of cell is negative).
Describe the resting phase
Na & K channels are closed but K channels leak (100x more leaky than Na channels) Na K ATPase restores ionic gradients.
What is the Nernst potential
The electrical potential across a cell membrane that exactly opposes the net diffusion of a particular ion through the membrane following its concentration gradient.
Describe the hyperpolarization phase (refractory period)
The membrane potential becomes transiently more negative than the normal resting potential. Refractory period makes the axon unidirectional.
Describe membrane threshold
That point in the process of depolarization when continued stimulation leads to complete action potential.
Describe impulse firing threshold
Point where the balance of ionic currents reverses from ATP requiring polarization to gradient dependent depolarization.
What is normal resting potential of a neuron?
What are the 2 means of propagation?
Passive spread & active spread
Where does passive propagation occur?
In the dendrites & cell body-different kinds of Na channels depending on nerve cell type.
What are the different kinds of Na channels involved in passive propagation?
Ligand gated, gap junctions, temperature gated, mechano gated. Very few voltage gated.
What is passive propagation responsible for?
Sensory function, cell-to-cell communication & impulse modulation.
Describe active propagation.
Occurs in the axon.
Myelinated-node to node (more rapid & efficient).
Nonmyelinated- continuous wave.
In regards to active propagation, describe what occurs.
Once an action propagation is initiated in 1 region of a neuron, the depolarization "wave" will spread out sequentially to the rest of the neuron.