Nervous System Organization

• The nervous system controls and communicates throughout the body.
• The nervous system monitors sensory input from inside and outside the body.
• The nervous system integrates sensory input.
• It produces motor output, which is a response to stimuli that activates effector organs.

Organization of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord and acts as the integration and command center.
• The peripheral nervous system (PNS) contains paired spinal and cranial nerves, which carry messages to and from the spinal cord and brain.

Peripheral Nervous System Divisions

• The sensory (afferent) division carries impulses to the brain from skin, skeletal muscles, and joints via sensory afferent fibers.
• The sensory division transmits impulses from visceral organs to the brain via visceral afferent fibers.
• The motor (efferent) division transmits impulses from the CNS to effector organs.

Motor Division

• The somatic nervous system enables conscious control of skeletal muscles.
• The autonomic nervous system (ANS) regulates smooth muscle, cardiac muscle, and glands.
• The autonomic nervous system has sympathetic and parasympathetic divisions.

Histology of Nerve Tissue

• The two principal cell types of the nervous system are neurons and supporting cells.
• Neurons are excitable cells that transmit electrical signals.
• Supporting cells surround and wrap neurons.

Astrocytes

• Astrocytes are abundant, versatile, and highly branched glial cells that cling to neurons and their synaptic endings and cover capillaries.
• Functionally, astrocytes support and brace neurons, anchor them to their nutrient supplies, guide migration of young neurons, and control the chemical environment.

Microglia and Ependymal Cells

• Microglia are small, ovoid cells with spiny processes and act as phagocytes that monitor neuron health.
• Ependymal cells range in shape from squamous to columnar, lining the central cavities of the brain and spinal column.

Oligodendrocytes, Schwann Cells, and Satellite Cells

• Oligodendrocytes are branched cells that wrap CNS nerve fibers.
• Schwann cells (neurolemmocytes) surround the PNS fibers.
• Satellite cells surround neuron cell bodies within ganglia.

Neurons (Nerve Cells)

• Neurons are the structural units of the nervous system.
• Neurons consist of a body, axon, and dendrites.
• They are long-lived, amitotic, and have a high metabolic rate.
• The neuron's plasma membrane is involved in electrical and cell-to-cell signaling.

Axon Structure

• Axons are slender processes of uniform diameter that arise from the hillock.
• Long axons are called nerve fibers.
• Typically, a neuron has only one unbranched axon.
• Rare branches, if present, are called axon collaterals.
• The axonal terminal is a branched terminus of an axon.

Axon Function

• Axons generate and transmit action potentials and secrete neurotransmitters from the axonal terminals.
• Movement along axons occurs in anterograde (toward the axonal terminal) and retrograde (away from the axonal terminal) directions.

Myelin Sheath

• The myelin sheath is a whitish, fatty (protein-lipoid) segmented sheath surrounding most long axons.
• The function of the sheath is to protect the axon, electrically insulate fibers from each other, and increase the speed of nerve impulse transmission.

Myelin Sheath and Neurilemma Formation

• Myelin sheaths are formed by Schwann cells in the PNS.
• A Schwann cell envelopes an axon in a trough, encloses it with its plasma membrane, and forms concentric membrane layers that make up the myelin sheath.
• The remaining nucleus and cytoplasm of a Schwann cell is called the neurilemma.

Nodes of Ranvier

• Nodes of Ranvier, also known as neurofibral nodes, are gaps in the myelin sheath between adjacent Schwann cells.
• They are the sites where axon collaterals can emerge.

Unmyelinated Axons

• In unmyelinated axons, a Schwann cell surrounds nerve fibers without coiling. They partially enclose 15 or more axons.

Regions of the Brain and Spinal Cord

• White matter consists of dense collections of myelinated fibers.
• Gray matter consists of mostly soma and unmyelinated fibers.

Neuron Classifications: Structural

• Neurons are classified structurally as multipolar, bipolar, or unipolar based on the number of processes.
• Multipolar neurons have three or more processes.
• Bipolar neurons have two processes: an axon and a dendrite.
• Unipolar neurons have a single, short process.

Neuron Classifications: Functional

• Functional classifications include sensory (afferent), motor (efferent), and interneurons (association neurons).
• Sensory (afferent) neurons transmit impulses toward the CNS.
• Motor (efferent) neurons carry impulses away from the CNS.
• Interneurons (association neurons) shuttle signals through CNS pathways.

Synapses

• A synapse is a junction that mediates information transfer from one neuron to another neuron, or to an effector cell.
• The presynaptic neuron conducts impulses toward the synapse.
• The postsynaptic neuron transmits impulses away from the synapse.

Types of Synapses

• Axodendritic synapses occur between the axon of one neuron and the dendrite of another.
• Axosomatic synapses occur between the axon of one neuron and the soma of another.
• Other types include axoaxonic (axon to axon), dendrodendritic (dendrite to dendrite), and dendrosomatic (dendrites to soma) synapses.

Electrical Synapses

• Electrical synapses are less common than chemical synapses.
• Electrical synapses correspond to gap junctions found in other cell types and are important in the CNS for arousal from sleep, mental attention, emotions and memory, and ion and water homeostasis.