Fundamentals of the Nervous System and Nervous Tissue "Biology gives you a brain. Life turns it into a mind." -Jeffrey Eugenides

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

Nervous System Figure 11.1

Organization of the Nervous System 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

Peripheral Nervous System (PNS): Two Functional Divisions 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

Motor Division: Two Main Parts Somatic nervous system Conscious control of skeletal muscles Autonomic nervous system (ANS) Regulates smooth muscle, cardiac muscle, and glands Divisions – sympathetic and parasympathetic

The Cells of the Nervous System 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 Figure 2.1: Estimated numbers of neurons in humans. Because of the small size of many neurons and the variation in cell density from one spot to another, obtaining an accurate count is difficult. (Source: R. W. Williams & Herrup, 1988) Fig. 2-1, p. 30

Supporting Cells: Neuroglia 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

Supporting Cells: Neuroglia 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,

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

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

Astrocytes Figure 11.3a

Microglia and Ependymal Cells 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, Schwann Cells, and Satellite Cells 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 Figure 2.10: Shapes of some glia cells. Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons in the central nervous system; Schwann cells have a similar function in the periphery. The oligodendrocyte is shown here forming a segment of myelin sheath for two axons; in fact, each oligodendrocyte forms such segments for 30 to 50 axons. Astrocytes pass chemicals back and forth between neurons and blood and among neighboring neurons. Microglia proliferate in areas of brain damage and remove toxic materials. Radial glia (not shown here) guide the migration of neurons during embryological development. Glia have other functions as well. Fig. 2-10, p. 35

Figure 2.11: How an astrocyte synchronizes associated axons. Branches of the astrocyte (in the center) surround the presynaptic terminals of related axons. If a few of them are active at once, the astrocyte absorbs some of the chemicals they release. It then temporarily inhibits all the axons to which it is connected. When the inhibition ceases, all of the axons are primed to respond again in synchrony. (Source: Based on Antanitus, 1998) Fig. 2-11, p. 36

Neurons (Nerve Cells) 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 2.4: Neurons, stained to appear dark. Note the small fuzzy-looking spines on the dendrites. Fig. 2-4, p. 32

Neurons (Nerve Cells) Figure 11.4b

Figure 2.2: An electron micrograph of parts of a neuron from the cerebellum of a mouse. The nucleus, membrane, and other structures are characteristic of most animal cells. The plasma membrane is the border of the neuron. Magnification approximately x 20,000. (Source: Micrograph courtesy of Dennis M. D. Landis) Fig. 2-2, p. 31

The Cells of the Nervous System 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

Nerve Cell Body (Perikaryon or Soma) 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

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

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

Axons: Structure 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

Axons: Function 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

Myelin Sheath 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 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.

Myelin Sheath and Neurilemma: Formation 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

Myelin Sheath and Neurilemma: Formation Figure 11.5a–c

Nodes of Ranvier 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

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

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

Regions of the Brain and Spinal Cord White matter (diencephalon) – dense collections of myelinated fibers Gray matter – mostly soma and unmyelinated fibers

White matter 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

White matter 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 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.

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

Neuron Classification 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

Blood brain barrier 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.

BBB

Fig. 2-12, p. 37 Figure 2.12: The blood-brain barrier. Most large molecules and electrically charged molecules cannot cross from the blood to the brain. A few small, uncharged molecules such as O2 and CO2 cross easily; so can certain fat-soluble molecules. Active transport systems pump glucose and amino acids across the membrane. Fig. 2-12, p. 37