Nervous System Cells, Central and Peripheral Systems

Cells of the Nervous System

• The nervous system has two functional classes of cells: neurons and glial cells.
• Neurons are the main cells for information processing.
• Glial cells provide essential supporting functions.
• Neurons consist of a cell body, multiple dendrites which radiate outwards in distinctive patterns, and typically one axon with a series of axon terminals.
• Dendrites are the primary sites for gathering information, where other neurons form junctions called synapses.
• Axons primarily convey signals away to other neurons.

Peripheral and Central Nervous Systems

• The nervous system divides into the peripheral and central nervous systems.
• The CNS includes the parts encased by the skull and vertebral column.
• The PNS includes collections of neurons, dendrites, and axons which involve conveying information to and from the CNS.

Parts of the Peripheral Nervous System

• The PNS includes neurons existing outside the CNS, neurons with cell bodies in the PNS and processes in both, and axons of neurons with cell bodies in the CNS.
• Nerve cell bodies in the PNS gather along peripheral nerves in areas called ganglia.
• There are four functional categories of spinal nerve fibers: somatic and visceral afferent, somatic and visceral efferent.
• Sensory information travels to the CNS through primary sensory neurons (primary afferents) that have a cell body in a sensory ganglion, a peripheral process that gathers the information, and a central process that terminates in the CNS.
• Somatic and visceral efferents are distinct from each other.
• Motor neuron cell bodies for skeletal muscles are in the CNS, where axons reach the muscle through the PNS. Messages get sent to smooth muscle, cardiac muscle, and glands through two autonomic motor neurons, where the first cell body is a preganglionic neuron in the CNS and the second is a postganglionic neuron in an autonomic ganglion.

Parts of the Central Nervous System

• The CNS is divided into gray matter, where neuronal cell bodies, dendrites, and synaptic contacts are concentrated, and white matter, areas where axons travel between gray matter regions.
• A nucleus is an area of gray matter whose neurons have related functions.
• A cortex is an area of gray matter forming a layered surface.
• A tract refers to a collection of CNS axons with related functions.
• Tracts specify their origins and destinations.
• The CNS consists of the brain and spinal cord.
• The brain has three parts: the cerebrum, brainstem, and cerebellum.
• The Cerebrum is the largest part and has two massive cerebral hemispheres, separated by a fissure, and the diencephalon, located between the hemispheres and brainstem
• The diencephalon includes the thalamus, a cluster of nuclei inputs to the cerebral cortex, and the hypothalamus, a cluster of nuclei that control aspects of autonomic and hormonal function.
• The brainstem goes from the diencephalon to the spinal cord, with the midbrain, pons, and medulla.
• The cerebellum straddles the pons and medulla.

Neurons

• Neurons have a wide variety of sizes and shapes and use variations of common processes.
• Human brains contain an enormous number of neurons, that receive numerous synaptic inputs and project to many other neurons.
• Individual neurons connect to particular other neurons or body parts, giving specific functions to certain neuronal networks.
• Different CNS areas are organized via a modular construction, making each area very complex.

Functional Parts of Neurons

• Neurons collect information, process the information, conduct processed information, and transmit the information.
• Electrical signals conduct information within a neuron.
• Chemical messengers transmit signals between neurons.
• Dendrites receive information from other neurons via synapses.
• The relatively long axon conducts information away.
• Axon terminals transmit information beyond.
• Information processing is shared by the dendrites, cell body, and axon initial segment.
• Neurons are anatomically and functionally polarized, with electrical signals traveling unilaterally.

Convergence and Divergence

• Neurons receive a large amount of synaptic inputs and diverges to provide axon terminals to other neurons.

Organelles

• The cell body contains the usual organelles, with higher concentrations of some.
• The cell body and proximal dendrites have most of protein and membrane components.
• Neurons are delicate structures made of watery cytoplasm enclosed in a membrane.
• Neurons need a mechanical support system for structural integrity.
• In the CNS, the brain is suspended within in a watery bath. The cytoskeleton has microtubules, neurofilaments, and microfilaments.
• Microtubules, the thickest of the three types of filament, feature cylindrical assemblies of tubulin.
• The plus ends of axonal microtubules are directed away from the nucleus.
• Some dendritic microtubules are oriented bidirectionally.
• Microtubules give structural support and move things around within neurons.
• Axonal and dendritic transport enable neurons to move materials to the axonal terminals (anterograde), and return used components or convey signals to the cell body (retrograde).
• Kinesin moves materials in the anterograde direction.
• Cytoplasmic dynein drives retrograde movement.
• Molecules synthesized by a cell body move through slow transport.
• Neurofilaments are ropelike assemblies.
• Microfilaments are twisted pairs of actin filaments.
• Microfilaments are concentrated in the cytoplasm just under the neuronal cell membrane and also move growth cones.

Categories of Neurons

• Cell bodies range from 5 to 100 µm in diameter.
• Some axons measure a meter or more.
• Neurons are classified as unipolar, bipolar, and multipolar.
• The length and destination of an axon gives rise to a functional classification.
• Sensory neurons, primary afferents, convey information to the CNS.
• Motor neurons have axons ending on muscles, glands, or ganglionic neurons in the PNS.
• Interneurons are wholly within the CNS.
• Tract cells convey information from one area of the CNS to another.

Glial Cells

• Glial cells fill spaces among neurons in both the PNS and CNS.
• Glial cells provide support and stability for neurons.

Glial Cells of the Peripheral Nervous System

• PNS glial cells feature Schwann cells.
• Some PNS axons are in indentations in Schwann cells.
• Other axons are myelinated with Schwann cells wrapped spirally around a portion of the axon.
• Constrictions are between the sausages, corresponding to nodes of Ranvier.
• Myelin enables axons to conduct action potentials rapidly.

Glial Cells of the Central Nervous System

• CNS glial cells consist of ependymal cells, oligodendrocytes, astrocytes, and microglia.
• A layer of ependymal cells lines the ventricles.
• In critical locations, the layer is a secretory epithelium.
• Oligodendrocytes form myelin sheaths in the CNS.
• Each oligodendrocyte forms myelin segments on many axons.
• Astrocytes are plentiful in the CNS glia.
• Astrocytes have processes that fill spaces between neurons.
• Astrocytic processes pave the surfaces of CNS capillaries, providing metabolic support.
• Astrocytes cover neuronal cell bodies, synapses, and exposed axon areas, restrict extracellular fluid volume, regulate its composition, buffer effects of neuronal activity, and clear neurotransmitters.
• Microglia serve as an outpost of the immune system.
• In response to disease or injury microglia transform and clean cellular debris.