Nervous System: Structure and Function

• Nervous tissue's role involves facilitating communication and control throughout the body

Neuron Structure

• Neurons consist of a cell body (soma), dendrites, an axon, and synapses

Nerve Impulse Conduction

• Nerve impulse conduction involves:
  • Resting membrane potential
  • Action potential
  • Synaptic transmission

Nervous System Divisions

• The central nervous system (CNS) and peripheral nervous system (PNS) are differentiated by their components and functions

Anatomic Divisions of the Nervous System

• CNS includes the brain (cerebrum and cerebellum) and spinal cord
• PNS includes cranial nerves, spinal nerves, peripheral nerves, and ganglia

Functional Divisions of the Nervous System

• Sensory division (afferent) includes somatic and visceral components
• Motor division (efferent) has somatic and autonomic components

Peripheral Nervous System Subsystems

• Somatic nervous system (SNS)
• Autonomic nervous system (ANS) includes the sympathetic and parasympathetic nervous systems

Nerve Cells

• Neurons
• Glial cells

Neuron Functions

• The cell body (perikaryon) contains the nucleus and most organelles and serves as the synthetic and trophic center
• Dendrites are numerous elongated processes that extend from the cell body and receive stimuli
• The axon is a single long process that ends at synapses

Neuron Structural Classification

• Multipolar neuron
• Bipolar neuron
• Unipolar neuron
• Anaxonic neuron

Neuron Functional Classification

• Sensory (afferent) neurons transmit sensory information from receptors to the CNS
• Motor (efferent) neurons carry signals from the CNS to muscles or glands to initiate actions
• Interneurons act as connectors between sensory and motor neurons and are vital for processing and integrating information

Neuron Signaling

• Electrical signaling involves neurons generating action potentials (nerve impulses) in response to stimuli
• Those impulses travel along the axon to the axon terminals
• Chemical signaling features the action potential triggering neurotransmitter release (e.g., dopamine, serotonin, acetylcholine) at the synapse
• Released neurotransmitters bind to receptors on the postsynaptic cell, initiating a response

Glial Cells

• Glial cells of the central nervous system include ependymal cells, microglia, oligodendrocytes, and astrocytes
• Glial cells of the peripheral nervous system include Schwann cells and satellite cells

Glial Cells: Oligodendrocytes

• Oligodendrocytes get their name from the Greek words oligos (small, few), dendron (tree), and kytos (cell)
• Oligodendrocytes are located in the central nervous system
• These cells produce myelin sheaths that insulate axons in the CNS, increasing the speed of electrical impulse transmission
• Predominant glial cells in white matter
• Not visible in routine microscope

Glial Cells: Astrocytes

• Astrocytes are named from the Greek words astro (star) and kytos (cell)
• Astrocytes are located in the central nervous system
• They provide structural/metabolic support to neurons (especially at synapses) and aid in repair processes
• Astrocytes contain bundles of intermediate filaments made of glial fibrillary acid protein (GFAP), a unique marker

Glial Cells: Ependymal Cells

• Ependymal cells line the ventricles and central canal of the CNS
• These cells aid in the production and movement of CSF
• Ependymal cells appear as columnar or cuboidal
• Some ependymal cells have apical ends containing cilia (aid movement of cerebrospinal fluid) and microvilli (for absorption)
• These cells do not have a basal lamina, but basal ends are elongated and extended branching processes adjacent to a neurophil

Glial Cells: Microglia

• Microglia reside in the central nervous system
• These cells derived from monocytes and act as antigen-presenting cells of the CNS
• Microglia are involved in inflammatory responses, repair, and phagocytosis
• Evenly distributed throughout regions of gray and white matter
• Nuclei are recognizable in routine H&E stains

Glial Cells: Schwann Cells

• Schwann cells are also known as neurolemmocytes
• They are derived from the embryonic neural crest
• Schwann cells are located in the peripheral nervous system
• Functionally, they exert a trophic and supportive effect on neurons, insulating, nourishing, and regulating their microenvironments

Glial Cells: Satellite Cells

• Satellite cells are derived from the embryonic neural crest
• Located in the Peripheral Nervous System
• Act trophically and provide support to neurons, insulating/nourishing them and regulating their microenvironments

The Central Nervous System

• Relatively soft and easily damaged by injuries affecting the protective skull or vertebral bones
• Regions show areas of white and gray matter

CNS: White and Gray Matter

• Gray matter has dendrites, astrocytes, and microglia, but is lacking myelin (unmyelinated)

• White matter contains myelin-producing oligodendrocytes, astrocytes, is rich in myelin (myelinated)

• Gray matter is the thick cortex or surface layer of the cerebrum and cerebellum, while white matter resides in the deeper regions of both

CNS: Cerebellar Cortex Layers

• Molecular layer (outer layer) contains lots of neurophil and scattered neuronal cell bodies
• Purkinje cells (middle layer): thin and feature large neurons, these are the principal neurons of the cerebellar cortex
• Granular layer (inner layer) contains very small, densely packed neurons

CNS: Spinal Cord

• Gray matter features posterior (dorsal) and anterior (ventral) horns
• White matter contains ascending and descending tracts

CNS: Meninges

• Three meningeal layers: dura mater, arachnoid, pia mater
• Dura mater (tough mother) features dense, fibrous connective tissue, 2 layers that include a periosteal and meningeal layer
• Dura separated from the vertebral column by the epidural space
• Arachnoid (spider web-like) features 2 components
  • sheet of connective tissue in contact with the dura mater
  • system of loosely arranged trabeculae composed of collagen and fibroblast
• The subarachnoid space is space between arachnoid dura/pia maters and is filled with cerebrospinal fluid (CSF)

CNS: Blood Brain Barrier

• A functional barrier that controls tightly in most tissues over the passage of substances moving from blood into the CNS tissue
• The main component of BBB is the capillary endothelium
• Functions to protect neurons and glia from bacterial toxins, infectious agents, and other exogenous substances
• Helps maintain the stable composition of ions in the interstitial fluid

CNS: Choroid Plexus

• Highly vascular tissue folded/projecting into the large ventricles of the brain
• Each villus contains a thin layer of well-vascularized pia mater covered by cuboidal ependymal cells
• Main function is to remove water from the blood and release it as CSF
• Plays a key role of maintaining the homeostasis of the central nervous system by regulating the CSF composition and volume

PNS: Nerve Fibers

• Contain axons enclosed within sheaths of glial cells specialized to facilitate axonal function
• Axons are sheathed by schwann cells in peripheral nerves
• Sheaths may or may not form myelin around axons

PNS: Classification of Nerve Fibers

• Myelinated Fibers
• Unmyelinated Fibers

PNS: Ganglia

• These contain cell bodies of neurons, satellite cells, are supported by connective tissue and surrounded by a capsule
• Ganglia serve as relay stations to transmit nerve impulses

PNS: Ganglia Classification

• Sensory ganglion
• Autonomic ganglion

PNS: Sensory Ganglia

• Contain the cell bodies of sensory neurons and transmit sensory information from the periphery to the CNS
• Neurons are pseudounipolar and relay information to the spinal cord via synapses with local neurons
• Located near dorsal roots of spinal nerves (dorsal root ganglia) and cranial nerves

PNS: Autonomic Ganglia

• Named from the Greek "autos" (self) and "nomos" (law)
• These are bulbous dilations in autonomic nerves with multipolar neurons
• Affect activity of smooth muscle, secretion of some glands, heart rate, and homeostasis
• Locations include near or inside organs for parasympathetic ganglia and close to the spinal cord in the sympathetic chain (sympathetic ganglia)