Nervous System: Overview and Divisions

Overview of the Nervous System

• The nervous system controls perception and experience of the world.
• The nervous system directs voluntary movement and is the seat of consciousness, personality, learning, and memory.
• It regulates homeostasis with the endocrine system, influencing respiratory rate, blood pressure, body temperature, sleep/wake cycle, and blood pH.

Anatomical Divisions of the Nervous System

• The central nervous system (CNS) and peripheral nervous system (PNS) are the anatomical divisions.
• The CNS includes the brain and spinal cord.
• The brain contains billions of neurons and is protected by skull bones.
• The spinal cord begins at the foramen magnum and runs through the vertebral foramina from the first cervical to the first or second lumbar vertebra.
• The spinal cord contains millions of neurons, fewer than the brain and enables the brain to communicate with most of the body below the head and neck.
• The PNS includes all nerves outside the skull and vertebral column.
• Nerves are bundles of neuron axons with blood vessels and connective tissue, carry signals to/from the CNS, and are classified by origin/destination.
• The PNS consists of 12 pairs of cranial nerves traveling to or from the brain and 1 pair of spinal nerves traveling to or from the spinal cord at each vertebrae level.

Functional Divisions of the Nervous System

• Sensory, integrative, and motor are the functional categories of the nervous system.
• The sensory (afferent) division of the PNS gathers information about the internal/external environments.
• Input is carried from receptors to the spinal cord and/or brain by spinal/cranial nerves.
• The somatic sensory division involves signals from skeletal muscles, bones, joints, skin, and organs of vision, hearing, taste, smell, and balance.
• The visceral sensory division transmits signals from viscera, including the heart, lungs, stomach, kidneys, and urinary bladder.
• Integrative functions analyze and interpret incoming sensory information to determine appropriate responses.
• Subconsciously, 99% of integrated sensory information is disregarded.
• The remaining sensory stimuli that the CNS responds to leads to motor response.
• Motor functions involve motor (efferent) division actions of the PNS, divided into somatic and autonomic divisions by organs that neurons contact.
• The motor/efferent division consists of motor neurons that carry out motor functions from the brain/spinal cord via cranial/spinal nerves.
• Effectors are organs that carry out effects of the nervous system.
• The somatic motor division transmits signals to skeletal muscle for voluntary control (aka voluntary motor division).
• The autonomic nervous system (ANS) or visceral motor division carries signals to thoracic and abdominal viscera and is critical for maintaining homeostasis.
• The ANS regulates the secretion of glands, contraction of smooth muscle, and contraction of cardiac muscle (aka involuntary motor division).

Neurons

• Neurons are excitable cells that send and receive signals as action potentials, typically consisting of a cell body, dendrites, and axons.
• The cell body (soma) is the metabolically active region that manufactures all proteins needed for the whole neuron with organelles supporting high biosynthetic activity.
• Free ribosomes and rough endoplasmic reticulum (protein synthesis) make up the Nissl bodies visible with the microscope.
• Cytoskeleton has microtubules that provide structural support and chemical transportation between the cell body and axon.
• Neurofibrils are intermediate filaments of the cytoskeleton that give structural support to neuron processes.
• Dendrites are short, branched processes that receive input from other neurons and transmit them toward the cell body as electrical impulses.
• Each neuron can have multiple dendrites.
• A single axon (nerve fiber) generates and conducts action potentials.
• The axolemma is the plasma membrane surrounding the axon and its cytoplasm (axoplasm).
• Slow axonal transport moves substances (cytoskeleton proteins) from the cell body through the axon at a rate of 1–3 mm/day.
• Fast axonal transport requires motor proteins and consumes ATP.
• Vesicles and membrane-bound organelles travel back toward (retrograde) or away from (anterograde) the cell body at a maximum rate of 200 mm/day and 400 mm/day respectively.
• Neurons have three main functional regions: the receptive region (dendrites and cell body), the conducting region (axon), and the secretory region (axon terminal).
• Multipolar neurons have a single axon and multiple dendrites, accounting for over 99% of all neurons.
• Bipolar neurons have one axon and one dendrite, with the cell body in between and are found in the eye and olfactory epithelium (nasal cavity).
• Pseudounipolar neurons have one fused axon extending from the cell body and dividing into two processes.
• One process carries sensory information from sensory receptors to the cell body and the other carries sensory information (pain, touch, and pressure) from the cell body to the spinal cord.
• Sensory (afferent) neurons carry information toward the CNS.
• Neuron cell bodies in the PNS receive information from sensory receptors and relay it via axons to the brain or spinal cord and are usually pseudounipolar or bipolar.
• Interneurons (association neurons) relay information within the CNS between sensory and motor neurons.
• Most neurons in the body are interneurons, are multipolar, and communicate with many other neurons.
• Motor (efferent) neurons carry information away from the cell body in the CNS to muscles and glands and are mostly multipolar.
• Nuclei are clusters of neuron cell bodies found in the CNS.
• Tracts are bundles of axons located in the CNS.
• Ganglia are clusters of neuron cell bodies found in the PNS.
• Nerves are bundles of axons located in the PNS.

Neuroglia

• Neuroglia (neuroglial) cells provide structural support and protection for neurons and maintain the environment.
• They can divide and fill in space when a neuron dies.
• Each type of neuroglial cell is specialized for a specific function, illustrating the structure-function core principle.
• Astrocytes, oligodendrocytes, microglia, and ependymal cells are the four types of neuroglia that reside in the CNS.
• Schwann cells and satellite cells are the two types of neuroglia that reside in the PNS.
• Astrocytes are large star-shaped cells.
• Astrocytes have many processes terminating in end-feet, anchoring neurons and blood vessels and helping maintain the brain's 3D structure.
• Astrocytes transport nutrients and gases between blood vessels and neurons and regulate the brain's extracellular environment.
• Astrocytes form the blood-brain barrier, a protective structure surrounding capillary endothelial cells and making them impenetrable to most polar compounds and proteins.
• Astrocytes repair damaged brain tissue through rapid cell division.
• Oligodendrocytes are also in the CNS with radiating processes with flattened sacs that wrap around axons of nearby neurons to form myelin.
• Microglia are small, scarce cells activated by injury to phagocytize within the CNS, ingesting disease-causing microorganisms, dead neurons, and cellular debris.
• Ependymal cells are ciliated cells that line hollow spaces within the CNS (brain and spinal cord) that manufacture and circulate cerebrospinal fluid.
• Schwann cells encircle axons in the PNS to provide myelination.
• Satellite cells surround neuron cell bodies in the PNS and provide supportive functions that are not well defined.

The Myelin Sheath

• Layers of plasma membrane from Schwann cells or oligodendrocytes in the PNS and CNS respectively, are formed by the myelin sheath.
• Neuroglial cells wrap multiple layers of membrane (myelin) around the axon to form the myelin sheath.
• The lipid content of myelin sheath insulates the axon (prevents ion movements) like rubber around a copper wire and increases the speed of action potential conduction.
• Myelinated axons conduct action potentials about 15–20 times faster than unmyelinated axons.
• The neurolemma is on the outer surface of myelinated axons in the PNS with the Schwann cell nucleus, organelles, and cytoplasm, not present in the CNS.
• Oligodendrocytes have multiple processes that myelinate multiple axons in the CNS.
• Schwann cells only myelinate one axon in the PNS.
• Myelination begins early in fetal development in the PNS and much later in the CNS. and very little myelin is in the brain of a newborn.
• Axons in both the CNS and PNS are longer than neuroglial cells.
• Multiple cells must provide a complete myelin sheath.
• Internodes are segments of axon covered by neuroglia.
• The node of Ranvier is the gap between adjacent neuroglia where the myelin sheath is absent.
• Small axons in the CNS and PNS are typically unmyelinated.
• White matter is composed of myelinated axons and appears white.
• Gray matter is composed of neuron cell bodies, unmyelinated dendrites, and axons, and appears gray.

Regeneration of Nervous Tissue

• Regeneration or replacement of damaged tissue is nearly nonexistent in the CNS.
• Regeneration is limited in the PNS; neural tissue can regenerate only if the cell body remains intact.

Gliomas and Astrocytomas

• Primary brain tumors originate in the brain; most are gliomas, caused by an abnormally high rate of division of glial cells.
• Predisposing conditions include exposure to ionizing radiation and certain diseases.
• Astrocytes are the most commonly affected cells; the tumor is called astrocytoma.
• Severity ranges from mild (good prognosis) to highly aggressive (very poor prognosis).
• Treatment varies with tumor type, age, and patient health, generally involving surgical removal with chemotherapy and possibly radiation therapy.