Neurophysiology Part 2 - The Neuron and Glial Cells PDF Summer 2024

Andre Azevedo, DVM, MSc Locum Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: List all the 7 parts that make up a neuron Describe the structural and functional classification of neurons List all types o...

Andre Azevedo, DVM, MSc Locum Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: List all the 7 parts that make up a neuron Describe the structural and functional classification of neurons List all types of glial cells and their function FYI  The nervous system (NS) has 2 categories of cells: NEURONS Greek neuron = nerve NEUROGLIA Greek glia = glue  Neurons are the major functional unit of the NS Nerve cells - specialized in information processing Do not divide once they reach maturity Injury leading to neuronal death will permanently change the structure and functions of the affected areas Which of the two has more neurons? CAT or DOG Which of the two has more neurons? HUMAN or ELEPHANT HOW MANY neurons? SPONGE 1. Neurons are composed by: DENDRITES – an information-receiving area of the cell membrane CELL BODY, SOMA or PERICARYON – contains organelles AXON – an information-carrying extension of the cell membrane AXON HILLOCK AXON HILLOCK or TRIGGER ZONE – axon origin; originates AP PRESYNAPTIC TERMINAL – end of axon; transmit information MYELIN SHEATH – enhances the speed of information transfer NODE OF RANVIER – gaps in the insulating myelin sheath  Neurons communicate via synapses Greek synapsis = connection Specialized contact areas with other neurons, muscle fibers or glands Action potentials travel along the axon at a speed that varies from 0.5 to 120 meters per second Larger axons are myelinated Smaller ones (< 1 μm in diameter) are not myelinated  The myelin sheath is a greatly modified plasma membrane Wrapped around the axon in a spiral fashion Originate from and are part of the: Schwann cells in PNS Oligodendrocytes in the CNS The periodic interruptions are the NODES OF RANVIER Critical to the functioning of myelin  Schwann cell Schwann cell cytoplasm forms a ring inside and outside of the sheath  The myelin sheath facilitates conduction Is an electrical insulator But its function of facilitating conduction has no exact analogy in electrical circuitry Allows SALTATORY CONDUCTION of the impulse Latin saltare = to jump Action potential jumps from node to node Depolarization is much more rapid than in unmyelinated fibers  Conduction velocity in myelinated fibers is proportional to the diameter Larger axons have longer internodes and faster conduction velocities In unmyelinated fibers, conduction is proportional to the square root of the diameter If nerves were not myelinated and equivalent conduction velocities were maintained, the human spinal cord would need to be as large as a good-sized tree trunk! Myelin facilitates conduction while conserving space and energy  Neurons can be classified according to its structure MULTIPOLAR MULTIPOLAR Most Mostcommon commontype type 11axon axonand andmany manydendrites; dendrites;the thelength length and andarragement arragementvary varyconsiderably considerably BIPOLAR Have 2 processes: 1 axon and 1 dendrite PSEUDO-UNIPOLAR (CALLED UNIPOLAR) The real unipolar is Have a single stem process that bifurcates common in insects to form 2 processes: One goes to the PNS the other to CNS  Neurons can also be classified according to its FUNCTION  SENSORY OR AFFERENT NEURONS Send information from receptors in sensory organs toward the brain/spinal cord Most of them are pseudo-unipolar dendrites b c receivinginfo  INTERNEURONS OR ASSOCIATION NEURONS Found in the brain/spinal cord (CNS), connecting motor and sensory neurons They are usually multipolar or bipolar neurons  MOTOR NEURONS OR EFFERENT NEURONS Send information from the brain/spinal cord to muscle/glands (effector organs) They are multipolar neurons Afferent and Efferent sounds the SAME: Sensory Afferent Motor Efferent  NEUROGLIA OR GLIAL CELL Smaller than neurons Does not have axons and dendrites Fill all the spaces in the NS not occupied by neurons or blood vessels More numerous than neurons in the nervous tissue Have the capacity to divide Does not produce action potential Does not participate directly in synaptic interactions and electrical signaling  Neuroglia or glial cell plays an important role in the NS Participate in the production of myelin sheaths of axons Modulate the growth of developing or damaged neurons Buffer extracellular concentrations of potassium and neurotransmitters Participate in the formation of contacts between neurons (synapses) Participate in certain immune responses of the nervous system  Glial cells are located: CENTRAL NERVOUS SYSTEM MICROGLIAL CELLS ASTROCYTES OLIGODENDROCYTES EPENDYMAL CELLS PERIPHERAL NERVOUS SYSTEM SCHWANN CELLS  Microglia cells Are the brain’s immune cells Act as macrophages – phagocytosis to clear debris Also known to release NO to prevent viral replication Protect the brain against injury and infection Play a role in the developing brain Help to destroy the unnecessary synapses  Astrocytes Star-shaped cells with numerous long-cell processes 50% of the glial cell population in CNS Provide structural and metabolic support for neurons Forms the outer and inner glial limiting membranes of the CNS Release neurotrophic factors important for neuronal survival Helps elongate axons and dendrites Participate in the repair process following tissue injury  Astrocytes Mantain a neuron’s working environment Control the levels of neurotransmitters around synapses Control the concentration of important ions (K, Na, Ca, Cl, HCO3) Astrocytes have ion channels and exchange ions with neighboring cells Provide metabolic support Close contact with capillary endothelium is important for glucose transport, regulation of the extracellular environment, glutamate metabolism, and maintenance of the blood-brain barrier Modulate how neurons communicate Release molecules that influence neuronal activity  Oligodendrocytes Provide support to axons of neurons in the Central Nervous System Has numerous processes that extend to adjacent axons to form myelin Myelinate most axons > 1 μm in diameter To speed conduction velocity The myelin sheath is the reason the brain has white matter Gray matter refers to areas with a high population of soma Ex: cerebral cortex  Ependymal cells Cover the ventricles of the brain, central canal of the CNS, and choroid plexus in the 4th ventricle of the medulla oblongata They are involved in creating cerebrospinal fluid (CSF)  Schwann cells Also called neurolemmocytes Arranged side by side along the axon Each cell forms internodes of the myelin sheath of various lengths (25-1000 μm) Provide support to axons of the Peripheral Nervous System Similar to oligodendrocytes Produce myelin in axons > 1 μm Guide regeneration of damaged axons of PNS  Wallerian degeneration Axonal degeneration distal to the lesion Regeneration is efficient in the PNS, not in the CNS Regeneration guided by Schwann cells FYI  FYI DOGS CATS https://veteriankey.com/spinal-nerves/ WHAT TYPE OF NERVOUS CELL ARE YOU? SCHWANN CELL ASTROCYTE The standout. Might feel The multitasker. Does a left out from the rest of ton of things at once and the group. Adaptable. still manages to get it all When things go wrong, done in the end. can fix the problem and regenerate. MICROGLIA EPENDYMAL CELL The defender and justice seeker. Won't stand for The forgotten one. Knows attacks on friends. Is its worth but feels like no obsessed with cleanliness one remembers it. and organization. NEURON OLIGODENDROCYTE The last cookie in the pack. The practical and swift Thinks it can do everything one. Wants things to move alone and, despite being quickly. Listens to audio at excellent at what it does, 2x speed. Neurons ultimately needs help from the whole group.