The Nervous System: Structure and Function

• The nervous system dictates how humans perceive and experience the world.
• It directs voluntary movement and is the seat of consciousness, personality, learning, and memory.
• It regulates homeostasis along with the endocrine system; this includes respiratory rate, blood pressure, body temperature, the sleep-wake cycle, and blood pH.

Structural Divisions of the Nervous System

• The nervous system divides anatomically into the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS consists of the brain, protected by the bones of the skull, and spinal cord.
• The spinal cord begins at the foramen magnum and continues through the vertebral foramina of the first cervical to the first or second lumbar vertebra.
• The PNS consists of nerves, which are bundles of axons, blood vessels, and connective tissue in the body outside the protection of the skull and vertebral column.
• There are 12 pairs of cranial nerves that travel to and from the brain.
• There are 31 pairs of spinal nerves that travel to and from the spinal cord.

Functional Divisions of the Nervous System

• The tasks the nervous system performs include sensory, integrative, and motor functions.
• Sensory functions involve gathering information about the internal and external environments of the body.
• Sensory input is gathered by the sensory or afferent division of the PNS, further divided into somatic and visceral sensory divisions.
• The somatic sensory division consists of neurons that carry signals from skeletal muscles, bones, joints, and skin.
• It also transmits signals from the organs of vision, hearing, taste, smell, and balance.
• It is sometimes called the special sensory division.
• The visceral sensory division consists of neurons that transmit signals from viscera (organs) such as the heart, lungs, stomach, kidneys, and urinary bladder.
• Sensory input from both divisions is carried from sensory receptors to the spinal cord and/or brain via spinal and cranial nerves.
• Integrative functions analyze and interpret incoming sensory information.
• Motor functions are actions performed in response to integration.
• Motor output is performed by the motor or efferent division of the PNS, which can be subdivided into somatic and autonomic divisions based on the organs the neurons contact.
• Organs that carry out the effects of the nervous system are called effectors.

Nervous tissue and neurons

• Neurons are excitable cells responsible for sending and receiving signals via action potentials.
• Most neurons have three parts: the cell body, dendrites, and axon.
• The cell body (soma) is the most metabolically active region of the neuron and manufactures all needed proteins and houses many organelles.
• Dendrites are short, branched processes that receive input from other neurons as electrical impulses and transmit them toward the cell body; each neuron may have multiple dendrites.
• Each neuron has one axon, also called a nerve fiber, that can generate and conduct action potentials.
• The axon has distinct regions including the axon hillock, telodendria, axon terminals or synaptic bulbs, and axolemma.
• The neuron has three main functional regions: receptive, conducting, and secretory.
• Neurons are classified structurally into three groups
• Multipolar neurons have a single axon and multiple dendrites.
• Bipolar neurons, with one axon and one dendrite with a cell body between them, are in the eye and the olfactory epithelium in the nasal cavity.
• Pseudounipolar neurons have one fused axon extending from the cell body that divides into two processes; these are sensory neurons carrying information related to pain, touch, and pressure.
• Neurons are classified functionally into three groups
• Sensory (afferent) neurons in the PNS carry information from sensory receptors toward the CNS (brain or spinal cord).
• Interneurons (association neurons) relay information within the CNS between sensory and motor neurons, making up most neurons in the body.
• Motor (efferent) neurons carry information from the cell body in the CNS to muscles and glands.
• Structural groups of neuron components in the CNS (nuclei, tracts) and the PNS (ganglia, nerves) group together.
• Neuroglia (neuroglial cells) provide structural support and environmental protection for neurons.
• Astrocytes, oligodendrocytes, microglia, and ependymal cells reside within the CNS.
• Schwann and satellite cells reside in the PNS.

Myelin Sheath, White Matter, and Gray Matter

• The myelin sheath comprises repeating layers of the plasma membrane of Schwann cells (PNS) or oligodendrocytes (CNS).
• Myelination is the process of forming the myelin sheath from the plasma membranes of the neuroglial cell that surrounds the axon.
• Gaps in the myelin sheath are called Nodes of Ranvier.
• Myelin increases the speed of action potential conduction.
• White matter in the brain and spinal cord is composed of myelinated axons that appear white.
• Gray matter is composed of neuron cell bodies, unmyelinated dendrites, and axons that appear gray.

Nervous Tissue Regeneration

• Regeneration or the replacement of damaged nervous tissue is nearly non-existent in the CNS and limited in the PNS.
• Neural tissue can only regenerate if the cell body remains intact.