-Jeffrey Eugenides

• The master controlling and communicating system of the body
• Functions
– Sensory input – monitoring stimuli
– Integration – interpretation of sensory input
– Motor output – response to stimuli

Figure 11.1

• Central nervous system (CNS)
– Brain and spinal cord
– Integration and command center
• Peripheral nervous system (PNS)
– Paired spinal and cranial nerves
– Carries messages to and from the spinal cord and brain

• Sensory (afferent) division
– Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain
– Visceral afferent fibers – transmit impulses from visceral organs to the brain
• Motor (efferent) division
– Transmits impulses from the CNS to effector organs

• Somatic nervous system
– Conscious control of skeletal muscles
• Autonomic nervous system (ANS)
– Regulates smooth muscle, cardiac muscle, and glands
– Divisions – sympathetic and parasympathetic

• The human nervous system is comprised of two kinds of cells:
– Neurons: excitable cells that transmit electrical signals
– Glia: Supporting cells – cells that surround and wrap neurons
• The human brain contains approximately 100 billion individual neurons.
• Behavior depends upon the communication between neurons.

Fig. 2-1, p. 30

• The supporting cells (neuroglia or glial cells):
– Provide a supportive scaffolding for neurons
– Segregate and insulate neurons
– Guide young neurons to the proper connections
– Promote health and growth

• Glial cells make up 90 percent of the brain's cells. Glial cells are nerve cells that don't carry nerve impulses.
• The various glial (meaning "glue") cells perform many important functions, including: digestion of parts of dead neurons, manufacturing myelin for neurons, providing physical and nutritional support for neurons,

• Most abundant, versatile, and highly branched glial cells
• They cling to neurons and their synaptic endings, and cover capillaries

• Functionally, they:
– Support and brace neurons
– Anchor neurons to their nutrient supplies
– Guide migration of young neurons
– Control the chemical environment

Figure 11.3a

• Microglia – small, ovoid cells with spiny processes
– Phagocytes that monitor the health of neurons
• Ependymal cells – range in shape from squamous to columnar (Ciliated)
– They line the central cavities of the brain and spinal column
– Their apical surfaces are covered in a layer of cilia, which circulate CSF around the central nervous system. Their apical surfaces are also covered with microvilli, which absorb CSF. Ependymal cells are a type of Glial cell and are also CSF producing cells

Microglia and Ependymal Cells
Figure 11.3b, c

• Oligodendrocytes – branched cells that wrap CNS nerve fibers
• Schwann cells (neurolemmocytes) – surround fibers of the PNS
• Satellite cells surround neuron cell bodies with ganglia

Oligodendrocytes, Schwann Cells, and Satellite
Cells
Figure 11.3d, e

Fig. 2-10, p. 35

Fig. 2-11, p. 36

• Structural units of the nervous system
– Composed of a body, axon, and dendrites
– Long-lived, amitotic, and have a high metabolic rate
• Their plasma membrane function in:
– Electrical signaling

Fig. 2-4, p. 32

Figure 11.4b

Fig. 2-2, p. 31

• The membrane refers to the structure that separates the inside of the cell from the outside environment.
• The nucleus refers to the structure that contains the chromosomes.
• The mitochondria are the structures that perform metabolic activities and provides energy that the cells requires.
• Ribosomes are the sites at which the cell synthesizes new protein molecules

• Contains the nucleus and a nucleolus
• Is the major biosynthetic center
• Is the focal point for the outgrowth of neuronal processes
• Has no centrioles (hence its amitotic nature)
• Has well-developed Nissl bodies (rough
ER)
• Contains an axon hillock – cone-shaped area from which axons arise

• Armlike extensions from the soma
• Called tracts in the CNS and nerves in the
PNS
• There are two types: axons and dendrites

• Short, tapering, and diffusely branched processes
• They are the receptive, or input, regions of the neuron

• Slender processes of uniform diameter arising from the hillock
• Long axons are called nerve fibers
• Usually there is only one unbranched axon per neuron
• Rare branches, if present, are called axon collaterals
• Axonal terminal – branched terminus of an axon

• Generate and transmit action potentials
• Secrete neurotransmitters from the axonal terminals
• Movement along axons occurs in two ways
– Anterograde — toward axonal terminal
– Retrograde — away from axonal terminal

• Whitish, fatty (protein-lipoid), segmented sheath around most long axons
• It functions to:
– Protect the axon
– Electrically insulate fibers from one another
– Increase the speed of nerve impulse transmission

• Myelin is about 40 % water; the dry mass of myelin is about 70 - 85 % lipid (cholesterol and phospholipid)) and about 15 - 30 % proteins.
• Some of the proteins that make up myelin are myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), and proteolipid protein (PLP).
• The primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.

• Formed by Schwann cells in the PNS
• A Schwann cell:
– Envelopes an axon in a trough
– Encloses the axon with its plasma membrane
– Has concentric layers of membrane that make up the myelin sheath
• Neurilemma – remaining nucleus and cytoplasm of a Schwann cell

Figure 11.5a
–c

• Are gaps in the myelin sheath formed by spaces between successive oligodendrocytes (in CNS) or Schwann cells (in PNS) along the length of the axon.
• Nodes of Ranvier contain Na+ ion channels, and are sites where action potentials are generated by membrane depolarizations.
• They are the sites where axon collaterals can emerge

• A Schwann cell surrounds nerve fibers but coiling does not take place
• Schwann cells partially enclose 15 or more axons

• Both myelinated and unmyelinated fibers are present
• Myelin sheaths are formed by oligodendrocytes
• Nodes of Ranvier are widely spaced
• There is no neurilemma

• White matter (diencephalon) – dense collections of myelinated fibers
• Gray matter – mostly soma and unmyelinated fibers

• Situated between the brainstem and cerebellum, the white matter consists of structures at the core of the brain such as the thalamus and hypothalamus
• Certain nuclei within the white matter are involved in the expression of emotions, the release of hormones from the pituitary gland, and in the regulation of food and water intake

• The nuclei of the white matter are involved in the relay of sensory information from the rest of the body to the cerebral cortex, as well as in the regulation of autonomic
(unconscious) functions such as body temperature, heart rate and blood pressure.

• Grey matter – closely packed neuron cell bodies form the grey matter of the brain.
• The grey matter includes regions of the brain involved in muscle control, sensory perceptions, such as seeing and hearing, memory, emotions and speech.

• Structural:
– Multipolar — three or more processes
– Bipolar — two processes (axon and dendrite)
– Unipolar — single, short process

• Functional:
– Sensory (afferent) — transmit impulses toward the CNS
– Motor (efferent) — carry impulses away from the CNS
– Interneurons (association neurons) — shuttle signals through CNS pathways

Comparison of Structural Classes of Neurons
Table 11.1.1

Comparison of Structural Classes of Neurons
Table 11.1.2

Comparison of Structural Classes of Neurons
Table 11.1.3

• The blood-brain barrier is a mechanism that surrounds the brain and blocks most chemicals from entering.
• Because neurons in the brain generally do not regenerate, it is vitally important for the blood brain barrier to block incoming viruses, bacteria or other harmful material from entering.

Fig. 2-12, p. 37