NERVOUS TISSUE

Histology 1- Neuron 2- Neuroglia: a- Astrocytes b- microglia c- Schwann cell d- Oligodendrocytes e- Ependymal cells

1- Neuron

Neuron:

Neurons: Roles generate impulses Integrate information (impulses) Transmit impulses

Neurons

Neurons: close up

Neuron classification

Neuroglia 90% of nervous tissue cells Do not have transmission properties Provide structural support and protect the integrity of the nervous tissue 5 types of cells: - Astrocytes - Microglia - Ependyma cells - Oligodendrocytes - Schwann cells

Astrocytes Provide physical support for neurons Control movements of nutrients and wastes to and from neurons Help recycle and process some neurotransmitters Play a role in the Blood-Brain Barrier (BBB) Play a role in synapse formation Maintain constant brain ECF

Blood-brain barrier Protects the brain from “external” influences Inability of some compounds to cross from the blood into the brain Due to tight junctions between endocytes of he capillary wall Maybe due to astrocytes Glucose, gases can pass Some medications and other compounds can not pass

Microglia Derived from macrophages Defense of the brain Clean up old, sick and dead cells

Ependymal cells Line the ventricles of the brain and form, with the blood vessels, the choroid plexuses Secrete the cerebrospinal fluid (CSF)

Schwann cells Found in the peripheral nervous system (PNS) only Form myelinated sheath

Myelin sheath and Schwann cell

Oligodendrocytes Present in the white matter of the CNS only One cell sends cytoplasmic extensions containing myelin toward several neurons Each extension wraps around a portion of the neuronal axon

Neuron physiology 1- Resting membrane potential 2- Action potential 3- Graded potential - Excitatory potential - Inhibitory potential

Neuron physiology 1- Resting membrane potential 2- Action potential 3- Graded potential - Excitatory potential - Inhibitory potential

Resting membrane potential Due to: a large number of sodium ions located outside of the cell A large number of potassium ions located inside the cell Large amount of proteins inside the cell gives it a negative charge while the outside is positively charged The difference between the 2 sides of the cell membrane is -70 mV

Resting membrane potential

Resting potential Na+ and K+ are constantly leaking across the cell membrane The Na+/K+ pump re-establishes the membrane potential

Neuron physiology 1- Resting membrane potential 2- Action potential 3- Graded potential - Excitatory potential - Inhibitory potential

Action Potential (AP) Wave of reversal of charges sweeping along the axonal membrane

Action Potential Initiated at the hillock of the neuron The hillock “sums up” the information coming from the dendrites and neuronal body. “Summing up” for a neuron means adding up all excitatory and inhibitory impulses. If the membrane potential (the sum) reaches threshold (-55 mV) or above), an AP is triggered

Action Potential The difference in potential activates voltage gated channels  channels that open and close as a function of the current, regional membrane potential

Action Potential

Action Potential: depolarizing phase

Action Potential All-or None effect: Once the threshold is reached, the AP is triggered. All APs are identical.

Action Potential: Refractory periods Absolute refractory period: all the depolarizing phase + most of repolarizing phase: Relative refractory period: end of repolarization until the Na/K pump reestablish resting state

Action Potential Note: the AP travels always in 1 direction, from the soma toward the axonal bulb The AP cannot travel toward the soma because of the presence of the refractory period: the voltage-gated sodium channels are still closed (because of the inner gate) while the neighboring channels are being activated and conduct the AP down the axon

Neuron physiology 1- Resting membrane potential 2- Action potential 3- Graded potential - Excitatory potential - Inhibitory potential

Graded Potentials - They are small changes in membrane potential across the cell membrane - They occur on the dendrites and the neuron soma - They are controlled by ligand-gated channels

Some compounds such as novocaine and tetrodotoxin (TTX) from the puffer fish bind to voltage gated sodium channels. What will be the consequences?

Ligand gated channels The ligand binding to these channels are various neurotransmitters present in the NS Ligand binding is specific and related to the receptor shape Receptors for ligand A usually cannot bind to ligand B

Excitatory post synaptic potential (EPSP) A sodium channel opens in response to the binding of a ligand on its receptor. Sodium ions move inside the cell, therefore depolarizing it The amount of sodium entering the cell is not enough for the membrane to reach threshold

EPSP Any time a positively charged ions (cations) move into the cell, the membrane depolarizes. Any other ECF cations will have the same effect Negatively charged ions (anions) from the ICF moving toward the ECF will have the same effect (excitation)

Inhibitory post synaptic potential (IPSP) Ligand gated potassium channels open in response to the binding of a ligand on its receptor. Potassium ions leave the cell  the membrane potential become more negative (-90mV)  it is moving away from the threshold it is even more difficult to depolarize the membrane  inhibition

IPSP Any positive charged ions moving from the cell toward the IF will have an inhibitory effect Any negatively charged ions moving from the IF toward the cell will also have an inhibitory effect (ex: chloride ions)

Summation The impulses arriving at the neuron are summed at the hillock. If the result produce a voltage greater than the threshold, an AP will be triggered

Temporal summation Two impulses must arrive at the same location at very close time intervals

Spatial summation Two impulses arrive at different locations at the same time

EPSP & IPSP & summation

Graded potentials Immediately after a graded potential, Na/K pumps reestablish resting membrane potential

Impulse conduction Saltatory conduction along the myelinated axons Continuous conduction along the unmyelinated axons

Saltatory conduction + Myelinated axon Myelin sheath Node of Ranvier Extracellular fl uid Direction of action potential propagation Intracellular – Axon hillock Saltatory conduction

Continuous conduction Unmyelinated axon + Extracellular fluid Resting Plasma membrane Intracellular fluid – Initiation Site of original action potential Site A Region of depolarization Site B Axon hillock Direction of action potential propagation Propagation Refractory state Site C repolarization (resting state) Site D continues (a) (b) (c) (d) Continuous conduction Along the axon In one direction only (due to the refractory period)

Factors affecting the speed of AP transmission State of myelination of the axon – Myelinated axon transmit much faster than unmyelinated axons The size of the axon: the larger the axon, the faster the AP

Readings: Chp. 7: p. 167-195 Chp. 8, p. 197-213 Clinical connections: p. 180, p. 193, p. 210, p. 212. Not expected: Toolbox, p.174, p. 178, p. 192,