Nervous System: Neurons and Glial Cells

• The nervous system is the most complex system, composed of neurons and glial cells.
• Neurons are interconnected, forming a complex system for processing information.
• Nerve tissue is distributed throughout the body.
• The nervous system consists of two major divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS is composed of the brain and spinal cord.
• The PNS consists of cranial, spinal, and peripheral nerves, as well as ganglia outside the CNS
• The two cells in nerve tissue are neurons and glial cells.
• Neurons have long processes.
• Glial cells have short processes, support and protect neurons, participate in neural activities, neural nutrition, and defense of cells in the CNS.
• Neurons respond to stimuli by altering the ionic gradient across membranes.
• Excitable cells can rapidly change their electrical potential in response to stimuli
• Neurons react to stimuli with membrane depolarization that spreads across the neuron's membrane.
• Action potentials are the nerve impulses capable of traveling long distances along neuronal processes, transmitting signals to other neurons, muscles, and glands.
• The nervous system continuously stabilizes intrinsic conditions of the body and maintains behavioral patterns.

Development of Nerve Tissue

• The nervous system develops from the ectoderm in the third week of development.
• The neural plate forms from the ectoderm and folds upward to create neural folds.
• The neural folds fuse to form the neural tube, which gives rise to the CNS.
• Neural crest cells separate from the neuroepithelium and differentiate into cells of the PNS and other non-neuronal cell types.

Neurons

• Neurons are the functional units of the CNS and PNS.
• Neurons have three main parts: the cell body, dendrites, and axon.
• The cell body (perikaryon or soma) contains the nucleus and organelles, and serves as the synthetic or trophic center.
• Dendrites are elongated processes specialized to receive stimuli at synapses.
• The axon is a long process specialized to generate and conduct nerve impulses.
• Neurons are classified by the number of processes extending from the cell body.
• Multipolar neurons have one axon and two or more dendrites.
• Bipolar neurons have one dendrite and one axon.
• Unipolar neurons have a single process that bifurcates.
• Anaxonic neurons have many dendrites but no true axon.
• Sensory neurons are afferent.
• Motor neurons are efferent.
• Interneurons form complex functional networks in the CNS.
• neuronal perikarya occur in the gray matter
• axons concentrated in the white matter
• In the PNS, cell bodies are in ganglia and axons are bundled in nerves.

Parkinson Disease

• Parkinson disease involves muscular tremors, reduced facial muscle activity, loss of balance, and postural stiffness.
• It is caused by the loss of dopamine-producing neurons in the CNS substantia nigra.
• Treatment involves L-dopa.

Cell Body (Perikaryon or Soma)

• The neuronal cell body contains the nucleus and surrounding cytoplasm, producing cytoplasm for the processes.
• Neuronal cell bodies have many nerve endings and a large, euchromatic nucleus with a nucleolus.
• Cytoplasm of perikarya contains polyribosomes and RER, named Nissl bodies.
• Golgi apparatus is located in the cell body.
• Mitochondria are throughout the cell
• Microtubules, actin filaments, and neurofilaments are abundant
• Cross-linked neurofilaments are referred to as neurofibrils.
• Some contain pigments

Dendrites

• Dendrites are short processes emerging and branching off the soma that receive signals.
• Dendrites are covered with synapses.
• Dendritic spines are dynamic membrane protrusions along dendritic branches, serving as initial processing sites for synaptic signals.
• Spine morphology depends on actin filaments and changes with synaptic modification and are of key importance in neural plasticity that occurs during embryonic brain development

Axons

• Most neurons only have one axon, and it may be longer than its dendrites.
• The plasma membrane of the axon is called the axolemma
• The contents are known as axoplasm.
• Axons originate from the axon hillock
• Initial segment of the axon has concentrated ion channels that generate the action potential.
• Axons undergo terminal arborization.
• Interneurons and motor neurons have collaterals.
• Axon branches end with a terminal bouton that contacts another cell at a synapse.
• Axoplasm contains mitochondria, microtubules, neurofilaments, and transport vesicles.
• severed axons degenerate quickly
• Molecules are transported bidirectionally within axons.
• Anterograde transport moves molecules via kinesin from the perikaryon to the synaptic terminals.
• Retrograde transport carries macromolecules via dynein from the periphery to the cell body.
• Anterograde and retrograde transports occur at rates of 50-400 mm/d.
• A slower anterograde stream moves the axonal cytoskeleton itself at a rate of a few millimeters per day.

Nerve Impulses

• Nerve impulse, or action potential is an electrochemical process.
• Initiated when electrical threshold is met
• Action potential is propagated as a wave of membrane depolarization due to voltage-gated Na+ and K+ channels.
• ATP-dependent Na-K pumps and membrane proteins maintain a potential difference across the axolemma of about -65 mV.
• During membrane depolarization, voltage-gated Na+ channels open, making the axoplasm positive.
• After membrane depolarization, the voltage-gated Na+ channels close and voltage-gated K+ channels open.
• Depolarization stimulates adjacent portions of the axolemma to depolarize and return to resting potential.
• After a refractory period, the neuron repeats the process and generates another action potential.
• Impulses prompt neurotransmitter discharge with promotes action potentials.

Local Anesthetics

• Local anesthetics interfere with sodium ion influx.

Synaptic Communication

• Synapses are where nerve impulses are transmitted from one neuron to another.
• Synapses convert an nerve impulse into a chemical signal that affects the postsynaptic cell.
• Synapses act by releasing neurotransmitters that act on specific receptor proteins.
• The presynaptic axon terminal contains mitochondria and synaptic vesicles.
• The postsynaptic cell membrane contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse.
• The synaptic cleft separates these presynaptic and postsynaptic membranes.
• At the presynaptic region, nerve impulse opens calcium channels, triggering neurotransmitter release by exocytosis or similar mechanisms.
• The released neurotransmitter molecules diffuse across the synaptic cleft and bind receptors, producing either an excitatory or an inhibitory effect.
• Neurotransmitters from excitatory synapses cause postsynaptic Na+ channels to open and initiate a depolarization wave
• Inhibitory neurotransmitters open Cl- or other anion channels, causing influx of anions and hyperpolarization of the postsynaptic cell.
• Interplay between excitatory and inhibitory effects allows synapses to process neuronal input.
• Impulses from presynaptic neurons to postsynaptic cells are modified at the synapse by connections with other neurons.
• Response is determined by the summation of activity at hundreds of synapses.
• Three common morphological types of synapses occur between neurons of the CNS.
• The chemical transmitter used at neuromuscular junctions and some synapses of the CNS is acetylcholine.
• Within the CNS, other major categories of neurotransmitters: amino acids, monoamines, and small polypeptides.
• After release, transmitters are removed quickly by enzymatic breakdown, by glial activity, or by endocytotic recycling.

Medical Application Pertaining to Neurotransmitters

• Synaptic levels need regulation
• Selective serotonin reuptake inhibitors (SSRIs) augment levels of serotonin at CNS synapses by inhibiting reuptake.

Glial Cells & Neuronal Activity

• Glial cells support neuronal survival and are more abundant than neurons.
• In the CNS, glial cells surround both neuronal cell bodies and the processes of axons and dendrites.
• Glial cells substitute for cells of connective tissue, supporting neurons and creating optimal microenvironments.
• The fibrous intercellular network of CNS tissue is the neuropil
• There are six major kinds of glial cells, four in the CNS: oligodendrocytes, astrocytes, ependymal cells, and microglia
• Two in the PNS: Schwann cells and satellite cells of ganglia
• Oligodendrocytes extend processes that wrap around CNS axons, forming myelin sheaths
• Myelin electrically insulates the axon, facilitating rapid transmission of nerve impulses.
• Astrocytes have radiating, branching processes with bundles of GFAP.
• Astrocytes connect with synapses with slender processes
• Astrocytes support movements of differentiating neurons during CNS development
• Fibrous processes called perivascular feet cover capillary endothelial cells and modulate blood flow and nutrient movement
• Protoplasmic processes contribute towards glial limiting membrane

Astrocytoma

• Brain tumors that is derived from fibrous astrocytes an is distinguished by GFAP expression
• Fibrous astrocytes are abundant in white matter
• Protoplasmic astrocytes predominate in the gray matter

Ependymal Cells:

• These line the ventricles of the brain and the central canal of the spinal cord.
• In some locations, they have cilia that facilitate CSF movement and mircovilli.
• They have apical junctional complexes but no basal lamina.
• Basal cells possess extended branching process

Microglia:

• Small cell bodies with long, branched processes that protect immune system of the brain
• Microglial cells originate from circulating blood monocytes, belonging to the same family as macrophages
• Nuclei cells can be recognized by their small, dense, slightly elongated structure

Alzheimer Disease

• Alzheimer disease (type of dementia), affects neuronal perikarya and synapses within the cerebrum.
• Functional defects are due to neurofibrillary tangles (accumulations of tau protein associated with microtubules)
• Neuritic plaques (dense aggregates of B-amyloid protein around the outside of neurons).

Multiple sclerosis (MS):

• autoimmune mechanism that interferes with the activity of neurons producing various neurologic problems
• T lymphocytes and microglia phagocytose and degrade myelin debris, in progression of the disease
• With disease progression, the actions of the cells mentioned exceed myelin production and capacity to repair the myelin sheaths

Schwann Cells

• Are found only in the PNS differentiated from of the neural crest
• Schwann produce myelin sheathes but unlike those in oligodendrocytes it can only from myelin form only one axon