Nervous System

• Nervous tissue consists of the brain, spinal cord, sensory organs, and connecting nerves.
• It senses the environment and processes incoming information.
• It influences and directs reactions to incoming information.
• Nerves encased in bone form the central nervous system, i.e. the brain and spinal cord.
• The central nervous system is responsible for processing information and directing actions.
• All nerves not encased in bone make up the peripheral nervous system
• The peripheral nervous systems main function is to carry messages to and from the central nervous system.
• Neural crest cells migrate to form ganglia throughout the trunk and in a variety of other tissues.
• The neural tube forms the brain and spinal cord, which makes up the central nervous system.
• The neural crest gives rise to neurons with cell bodies in the PNS and to Schwann cells, among other tissues.
• The nervous system includes the peripheral and central nervous systems.
• The peripheral nervous system is split into the autonomic and somatic nervous systems.
• The autonomic nervous system is split into sympathetic and parasympathetic divisions.
• The central nervous system contains the brain and spinal cord.
• The brain consists of the forebrain, midbrain, and hindbrain.
• The forebrain includes the telencephalon and diencephalon.
• The telencephalon includes the cerebral cortex, basal ganglia, hippocampus, and amygdala.
• The diencephalon includes the thalamus and hypothalamus.
• The midbrain consists of the mesencephalon.
• The mesencephalon includes the tectum and the cerebellum.
• The hindbrain consists of the metencephalon and myelencephalon.
• The metencephalon includes the pons and cerebellum.
• The myelencephalon includes the medulla.
• Nervous tissue is composed of neurons and neuroglia (nerve glue).
• Neurons generate and transduce electric impulses and are responsible for the nervous systems receptive, integrative, and motor functions.
• Neuroglia supports and protects neurons

Neuroglia cells:

• Astrocytes
• Schwann cells
• Oligodendrocytes
• Satellite Cells
• Microglia
• Ependymal cells

Neurons:

• All have a cell body which contains a cytoplasm, organelles, and is the metabolic and integrative center of neuron.
• All neurons have processes specialized for communication, including an axon and dendrites.
• Axon hillock is the origin of the axon.
• The initial segment is the part of the axon where action potential is initiated.
• Axon collaterals are branches of axons.
• Axon terminals are the effector portion, synaptic terminals, and terminal boutons.

Organelles in Neurons

• Cell membrane: Neuron's plasma membrane (plasmalemma), axon’s cell membrane is the axolemma.
• Nucleus: Prominent, centrally located, and the site of ribosomal synthesis.
• Nissl substance: Stacks of RER (rough endoplasmic reticulum) which are the site of protein synthesis, abundant in the cell body and proximal parts of dendrites, but absent in axon hillock and axons.
• Smooth endoplasmic reticulum.
• Golgi apparatus: Site of glycosylation, abundant, perinuclear location.
• Mitochondria: Abundant.
• Lysosomes.
• Pigments: Lipofuscin (aging pigment); melanin
• Microtubules (25 µm): Part of cytoskeleton that gives neurons their shape and is abundant in axons and dendrites and is responsible for axon transport.
• Neurofilaments (intermediate filaments, 10 um): Part of the cytoskeleton responsible for the shape of neurons and are abundant in axons and dendrites; also known as "Neurofibrils" and are bundles of neurofilaments seen with the light microscope; they are argyrophilic and visualized with silver stains.
• Microfilaments (f-actin, 5 µm): Part of the cytoskeleton, abundant in growth cones where they give contractile properties
• Vesicles: Vesicles in synaptic endings that contain neurotransmitters and are called synaptic vesicles.

Functional Classification of Neurons

• Sensory neurons (afferent nerves) carry signals from the outer parts of your body (periphery) into the central nervous system.
• Motor neurons (efferent nerves) carry signals from the central nervous system to the outer parts (muscles or glands) of your body.
• Interneurons connect various neurons within the brain and spinal cord.
• Interneurons facilitate or inhibit a motor response to a sensory stimulus.
• Receptors sense the environment (chemicals, light, sound, touch) and encode that information into electrochemical messages.
• These transformed messages are then transmitted by sensory neurons.

Morphological classification based on the number of processes:

• Multipolar neurons have one axon and two or more dendrites.
• Bipolar neurons have one axon and one dendrite.
• Unipolar (pseudounipolar) neurons have one process that divides close to the cell body into two long processes, an axon and a dendrite.
• 99% of neurons in the body are multipolar.
• Bipolar neurons are rare and occur in special sense organs of ear, nose and eye.
• Unipolar neurons begin as bipolar but processes fuse.
• They are primarily sensory neurons such as dorsal root ganglion neurons.
• Neurons communicate with other neurons and effector cells by means of synapses.
• Synapses are specialized junctions between neurons that transmit impulses from one neuron to another or to effector cells like muscle or gland cells
• Synapse is the site where two nerves communicate with each other.
• The presynaptic neuron is the neuron generating information toward the next neuron
• The postsynaptic neuron transmits information away from synapse
• Most synaptic communication is via chemical messengers, e.g. acetylcholine, serotonin, norepinephrine, dopamine, endorphins, GABA, glycine, glutamic acid, etc.

Synapse Components:

• Presynaptic component consists of synaptic vesicles (~40 nm) which are flat or round, clear or dense, and contain neurotransmitters or neuropeptides; and mitochondria. May contain pre-synaptic membrane densities (= active zones = sites of release).
• Synaptic cleft: (20-40 nm) May contain carbohydrate-containing material that serves to tightly adhere pre- to post-synaptic component. Postsynaptic component: membrane densities representing receptor sites may be present, can be dendrites or dendritic spines, cell body (soma), axon, or other target cells.
• Neurons communicate via chemical and electrical transmissions
• Chemical neurotransmission which makes up 99% of transmissions utilize nerve impulses traveling down the axon of the pre-synaptic cleft.
• Electrical neurotransmission makesup 1% of transmissions.
• synapses are classified by their position

The Three Types of Synapses

• Axosomatic synapse: an axon terminal ending on the soma of a neuron.
• Axoaxonic synapse: an axon terminal contacting another axon terminal.
• Axodendritic synapse: An axon ending on a dendrite.
• Axospinous synapse: An axon terminal facing a dendritic spine.
• The dendritic shaft contains microtubules.
• Dendritic spine cytoskeleton consists of actin filaments, and neurotransmitter receptors and ion channels localize in the postsynaptic density.
• Telodendria are the end branches of an axon
• The motor endplate (neuromuscular junction) is a specialized synapse.
• The motor endplate joins a motoneuron's axon terminal and a skeletal muscle fiber.
• It has the features of a synapse, but the postsynaptic component consists of muscle fiber
• There are deep invaginations of the sarcolemma (muscle membrane) at the junction

Neuromuscular Junction Diagram key:

• Presynaptic terminal
• Sarcolemma
• Synaptic vesicle
• Nicotinic acetylcholine receptor
• Mitochondrion
• Neuroglia are nerve glue and are made up of different cell types
• The term neuroglia, or "nerve glue," was coined in 1859 by Rudolph Virchow.
• Virchow conceived of the neuroglia as an inactive "connective tissue" holding neurons together in the central nervous system.
• The metallic staining techniques developed by Ramón y Cajal and del Rio-Hortega allowed the identification of the types of supporting cells in the CNS: oligodendrocytes, astrocytes, and microglia.
• The Schwann cell is the major neuroglial component in the peripheral nervous system (PNS).
• Astrocytes are star-shaped glial cells in the brain and spinal cord, that have small cell bodies and radial branched processes.
• They perform biochemical control of endothelial cells to form the blood-brain barrier

Astrocyte Functions:

• Clearing excess neurotransmitters.
• Stabilizing and regulating the blood-brain barrier.
• Promoting synapse formation.
• Provision of nutrients to the nervous tissue.
• Maintenance of extracellular ion balance.
• Regulation of cerebral blood flow.
• Repair and scarring the brain and spinal cord following infection and traumatic injuries.
• Astrocytes fence in neurons and oligodendrocytes.
• Astrocytes help isolate the brain parenchyma.
• They achieve this via long processes that project to the pia mater and the ependyma
• Glia limitans are formed through the connection of pia mater and ependyma.
• They enmesh synapses and dendrites and project processes to cell somas.
• Astrocytes form a syncytium via gap junctions and enables ion transfer and also allows for small molecules to diffuse across the brain parenchyma.
• Protoplasmic astrocytes have many processes and are large and star-shaped, attach to blood vessels
• Fibrous astrocytes are similar to protoplasmic astrocytes but have more filaments and glycogen. And lie in the white matter.
• Capillaries of the CNS form the blood–brain barrier, a protective layer that prevents exchange of solutes between the blood and brain.
• The difference with systematic circulation is the presence in certain brain interendothelial tight junctions that form the barrier.
• Finally, astrocytes induce and maintain these junctions.
• Microglia (Hortega cells) are macrophages of the CNS.
• Ramified microglia may transform into migratory, ameboid, and phagocytic cells.
• Microglia help phagocytose degenerating cells that undergo programmed cell death as part of normal development.
• They retain the ability to divide and have the immunophenotypic properties of monocytes and macrophages.
• Microglia secrete cytokines and growth factors for fiber tract development, gliogenesis, and angiogenesis.
• They also present antigens to T lymphocytes.
• Ependymal cells line ventricles and central cavities of brain and spinal cord and secrete cerebrospinal fluid (CSF)
• Their cilia help to circulate CSF
• Cell types include ependymocytes, choroid plexus epithelial cells, tanycytes, and, within the retina, Müller cells and retinal pigment epithelial cells.
• Oligodendrocytes have plump cell bodies with fairly dense cytoplasm and a darker nucleus and fewer, shorter processes than an astrocyte.
• Oligodendrocytes accomplish myelination in the CNS.
• They are the myelinating cells of the central nervous system (CNS)
• Myelination of the spinal cord begins in the ventral (motor) roots during month 4.
• Myelination of the association neocortex extends to 30 years of age.
• Myelin is an electrically insulating material to form the myelin sheath around the axon of a neuron
• Oligodendrocytes myelinate CNS axons while Schwann cells myelinate PNS axons.
• Nodes of Ranvier are uninsulated portions of axonal membrane between myelin sheaths
• Myelin segments are called internodes.
• Myelination increases speed of impulse propagation by saltatory conduction.
• Schwann cells accomplish myelination in the PNS, individual myelinating Schwann cells form a single internode
• Neurolemma (aka neurilemma or Schwann's Sheath) is the outermost nucleated cytoplasm layer of Schwann cells that encircle the axon of the neuron.
• It forms outermost layer of the nerve fiber in the peripheral nervous system.
• The neurolemma serves a protective function for peripheral nerve fibers.
• Non-myelinating Schwann cells are involved in maintenance of axons and are crucial for neuronal survival.
• Satellite glial cells are glial cells that cover the surface of nerve cell bodies in sensory, sympathetic and parasympathetic ganglia.
• Both satellite glial cells and Schwann cells are derived from the neural crest cells.
• Satellite Cells provide nutrients, remove metabolites, and have structural role (similar to astrocytes).