Chapter 12 Nervous System Cells

1. Chapter 12Nervous System Cells

2. Introduction • The function of the nervous system, along with the endocrine system, is to communicate • The nervous system is made up of the brain, the spinal cord, and the nerves

3. Organization of the Nervous System • Organized to detect changes in internal and external environments, evaluate the information, and initiate an appropriate response • Subdivided into smaller “systems” by location • 1) Central Nervous system (CNS) • 2) Peripheral nervous system (PNS)

4. Organization of the Nervous System • Central nervous system (CNS) • Structural and functional center of entire nervous system • Consists of the brain and the spinal cord • Integrates sensory information, evaluates it, and initiates an outgoing response

6. Organization of the Nervous System • Peripheral nervous system (PNS) • Nerves that lie in “outer regions” of nervous system • Cranial nerves—originate from brain • Spinal nerves—originate from spinal cord

8. Organization of the Nervous System • Afferent and efferent divisions • Afferent division—consists of all incoming sensory pathways • Efferent division—consists of all outgoing motor pathways

9. Organization of the Nervous System • “Systems” according to the types of organs they innervate • Somatic nervous system (SNS) • Somatic motor division—carries information to the somatic effectors (skeletal muscles) • Somatic sensory division—carries feedback information to somatic integration centers in the CNS

11. Organization of the Nervous System • “Systems” (cont.) • Autonomic nervous system (ANS) • Efferent division of ANS—carries information to the autonomic or visceral effectors (smooth and cardiac muscles and glands) • Sympathetic division—prepares the body to deal with immediate threats to the internal environment; produces “fight-or-flight” response • Parasympathetic division—coordinates the body’s normal resting activities; sometimes called the “rest-and-repair” division • Visceral sensory division—carries feedback information to autonomic integrating centers in the CNS

13. Cells of the Nervous System • Glia • Glial cells support the neurons • Five major types of glia : • Astrocytes • Microglia • Ependymal cells • Oligodendrocytes • Schwann cells

15. Cells of the Nervous System • Astrocytes • Star-shaped, largest, and most numerous type of glia • Cell extensions connect to both neurons and capillaries • Astrocytes transfer nutrients from the blood to the neurons • Form tight sheaths around brain capillaries, which, with tight junctions between capillary endothelial cells, constitute the blood-brain barrier (BBB)

17. Cells of the Nervous System • Microglia • Small, usually stationary, cells • In inflamed brain tissue, they enlarge, move about, and carry on phagocytosis

18. Cells of the Nervous System • Ependymal cells • Resemble epithelial cells and form thin sheets that line fluid-filled cavities in the CNS • Some produce fluid; others aid in circulation of fluid

19. Cells of the Nervous System • Oligodendrocytes • Smaller than astrocytes with fewer processes • Hold nerve fibers together and produce the myelin sheath

20. Cells of the Nervous System • Schwann cells • Found only in PNS • Support nerve fibers and form myelin sheaths (Figure 12-4) • Gaps in the myelin sheath are called nodes of Ranvier • Neurilemma is formed by cytoplasm of Schwann cell (neurilemmocyte) wrapped around the myelin sheath; essential for nerve regrowth • Satellite cells are Schwann cells that cover and support cell bodies in the PNS

23. Cells of the Nervous System • Neurons • Excitable cells that initiate and conduct impulses that make possible all nervous system functions

24. Cells of the Nervous System • Components of neurons • Cell body (perikaryon) • Ribosomes, rough endoplasmic reticulum (ER), Golgi apparatus • Provide protein molecules (neurotransmitters) needed for transmission of nerve signals from one neuron to another • Neurotransmitters are packaged into vesicles • Provide proteins for maintaining and regenerating nerve fibers • Mitochondria provide energy (ATP) for neuron; some are transported to end of axon

25. Cells of the Nervous System • Components of neurons (cont.) • Dendrites • Each neuron has one or more dendrites, which branch from the cell body • Conduct nerve signals to the cell body of the neuron • Distal ends of dendrites of sensory neurons are receptors • Dendritic spines—small knob-like protrusions on dendrites of some brain neurons; serve as connection points for axons of other neurons • Axon • A single process extending from the axon hillock, sometimes covered by a fatty layer called a myelin sheath (Figure 12-6) • Conducts nerve impulses away from the cell body of the neuron • Distal tips of axons are telodendria, each of which terminates in a synaptic knob

27. Cells of the Nervous System • Components of neurons • Cytoskeleton • Microtubules and microfilaments, as well as neurofibrils (bundles of neurofilaments) • Allow the rapid transport of small organelles • Vesicles (some containing neurotransmitters), mitochondria • Motor molecules shuttle organelles to and from the far ends of a neuron • Functional regions of the neuron • Input zone—dendrites and cell body • Summation zone—axon hillock • Conduction zone—axon • Output zone—telodendria and synaptic knobs of axon

28. Cells of the Nervous System • Classification of neurons • Structural classification—classified according to number of processes extending from cell body • Multipolar—one axon and several dendrites • Bipolar—only one axon and one dendrite; least numerous kind of neuron • Unipolar (pseudounipolar)—one process comes off neuron cell body but divides almost immediately into two fibers: central fiber and peripheral fiber

30. Cells of the Nervous System • Classification of neurons • Functional classification • Afferent (sensory) neurons—conduct impulses to spinal cord or brain • Efferent (motor) neurons—conduct impulses away from spinal cord or brain toward muscles or glandular tissue • Interneurons

31. Cells of the Nervous System • Reflex arc • A signal conduction route to and from the CNS, with the electrical signal beginning in receptors and ending in effectors • Three-neuron arc—most common; consists of afferent neurons, interneurons, and efferent neurons • Afferent neurons—conduct impulses to the CNS from the receptors • Efferent neurons—conduct impulses from the CNS to effectors (muscle or glandular tissue)

32. Cells of the Nervous System • Reflex arc • Two-neuron arc—simplest form; consists of afferent and efferent neurons • Synapse • Where nerve signals are transmitted from one neuron to another • Two types: electrical and chemical; chemical synapses are typical in the adult • Chemical synapses are located at the junction of the synaptic knob of one neuron and the dendrites or cell body of another neuron

33. Nerves and Tracts • Nerves—bundles of peripheral nerve fibers held together by several layers of connective tissue • Endoneurium—delicate layer of fibrous connective tissue surrounding each nerve fiber • Perineurium—connective tissue holding together fascicles (bundles of fibers) • Epineurium—fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve • Tracts—within the CNS, bundles of nerve fibers are called tracts rather than nerves

34. Nerves and Tracts • White matter • PNS—myelinated nerves • CNS—myelinated tracts • Gray matter • Made up of cell bodies and unmyelinated fibers • CNS—referred to as nuclei • PNS—referred to as ganglia

35. Nerves and Tracts • Mixed nerves • Contain sensory and motor neurons • Sensory nerves—nerves with predominantly sensory neurons • Motor nerves—nerves with predominantly motor neurons

36. Repair of Nerve Fibers • Mature neurons are incapable of cell division; therefore, damage to nervous tissue can be permanent • Neurons have limited capacity to repair themselves • Nerve fibers can be repaired if the damage is not extensive, the cell body and neurilemma are intact, and scarring has not occurred

37. Repair of Nerve Fibers • Stages of repair of an axon in a peripheral motor neuron • Following injury, distal portion of axon and myelin sheath degenerates • Macrophages remove the debris • Remaining neurilemma and endoneurium form a tunnel from the point of injury to the effector • New Schwann cells grow in the tunnel to maintain a path for regrowth of the axon • Cell body reorganizes its Nissl bodies to provide the needed proteins to extend the remaining healthy portion of the axon • Axon “sprouts” appear • When “sprout” reaches tunnel, its growth rate increases • The skeletal muscle cell atrophies until the nervous connection is reestablished • In CNS, similar repair of damaged nerve fibers is unlikely

38. Nerve Impulses • Membrane potentials • All living cells maintain a difference in the concentration of ions across their membranes • Membrane potential—slight excess of positively charged ions on outside of the membrane and slight deficiency of positively charged ions on inside of membrane (Figure 12-14) • Difference in electrical charge is called potential because it is a type of stored energy • Polarized membrane—a membrane that exhibits a membrane potential • Magnitude of potential difference between the two sides of a polarized membrane is measured in volts (V) or millivolts (mV); the sign of a membrane’s voltage indicates the charge on the inside surface of a polarized membrane

39. Nerve Impulses • Resting membrane potential (RMP) • Membrane potential maintained by a nonconducting neuron’s plasma membrane; typically –70 mV • The slight excess of positive ions on a membrane’s outer surface is produced by ion transport mechanisms and the membrane’s permeability characteristics • The membrane’s selective permeability characteristics help maintain a slight excess of positive ions on the outer surface of the membrane (Figure 12-15) • Sodium-potassium pump (Figure 12-16) • Active transport mechanism in plasma membrane that transports Na+ and K+ in opposite directions and at different rates • Maintains an imbalance in the distribution of positive ions, resulting in the inside surface becoming slightly negative with respect to its outer surface

40. Nerve Impulses • Local potentials • Local potentials—slight shift away from the resting membrane in a specific region of the plasma membrane (Figure 12-17) • Excitation—when a stimulus triggers the opening of additional Na+ channels, allowing the membrane potential to move toward zero (depolarization) • Inhibition—when a stimulus triggers the opening of additional K+ channels, increasing the membrane potential (hyperpolarization) • Local potentials are called graded potentials because the magnitude of deviation from the resting membrane potential is proportional to the magnitude of the stimulus

41. Action Potential • Action potential—the membrane potential of a neuron that is conducting an impulse; also known as a nerve impulse • Mechanism that produces the action potential (Figures 12-18 and 12-19) • An adequate stimulus triggers stimulus-gated Na+ channels to open, allowing Na+to diffuse rapidly into the cell, producing a local depolarization • As threshold potential is reached, voltage-gated Na+ channels open and more Na+ enters the cell, causing further depolarization • The action potential is an all-or-none response • Voltage-gated Na+ channels stay open for only about 1 millisecond before they automatically close • After action potential peaks, membrane begins to move back toward the resting membrane potential when K+ channels open, allowing outward diffusion of K+; process is known as repolarization • Brief period of hyperpolarization occurs and then the resting membrane potential is restored by the sodium-potassium pumps

42. Action Potential • Refractory period (Figure 12-20) • Absolute refractory period—brief period (lasting approximately half a millisecond) during which a local area of a neuron’s membrane resists restimulation and will not respond to a stimulus, no matter how strong • Relative refractory period—time during which the membrane is repolarized and is restoring the resting membrane potential; the few milliseconds after the absolute refractory period; membrane will respond only to a very strong stimulus

43. Action Potential • Conduction of the action potential • At the peak of the action potential, the plasma membrane’s polarity is now the reverse of the RMP • The reversal in polarity causes electrical current to flow between the site of the action potential and the adjacent regions of membrane and triggers voltage-gated Na+ channels in the next segment to open; this next segment exhibits an action potential (Figure 12-21) • This cycle continues to repeat • The action potential never moves backward, as a consequence of the refractory period • In myelinated fibers, action potentials in the membrane only occur at the nodes of Ranvier; this type of impulse conduction is called saltatory conduction (Figure 12-22) • Speed of nerve conduction depends on diameter and on the presence or absence of a myelin sheath

44. Synaptic Transmission • Two types of synapses (junctions) (Figure 12-23): • Electrical synapses occur where cells joined by gap junctions allow an action potential to simply continue along postsynaptic membrane • Chemical synapses occur where presynaptic cells release chemical transmitters (neurotransmitters) across a tiny gap to the postsynaptic cell, possibly inducing an action potential there

45. Synaptic Transmission • Structure of the chemical synapse (Figure 12-21) • Synaptic knob—tiny bulge at the end of a terminal branch of a presynaptic neuron’s axon that contains vesicles housing neurotransmitters • Synaptic cleft—space between a synaptic knob and the plasma membrane of a postsynaptic neuron • Arrangements of synapses • Axodendritic—axon signals postsynaptic dendrite; common • Axosomatic—axon signals postsynaptic soma; common • Axoaxonic—axon signals postsynaptic axon; may regulate action potential of postynaptic axon • Plasma membrane of a postsynaptic neuron—has protein molecules that serve as receptors for the neurotransmitters

46. Synaptic Transmission • Mechanism of synaptic transmission (Figure 12-25)—sequence of events is the following: • Action potential reaches a synaptic knob, causing calcium ions to diffuse into the knob rapidly • Increased calcium concentration triggers the release of neurotransmitter via exocytosis • Neurotransmitter molecules diffuse across the synaptic cleft and bind to receptor molecules, causing ion channels to open • Opening of ion channels produces a postsynaptic potential, either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP) • The neurotransmitter’s action is quickly terminated by either neurotransmitter molecules being transported back into the synaptic knob (reuptake) and/or metabolized into inactive compounds by enzymes and/or diffused and taken up by nearby glia (Figure 12-26)

47. Synaptic Transmission • Summation (Figure 12-28) • Spatial summation—adding together the effects of several knobs being activated simultaneously and stimulating different locations on the postsynaptic membrane, producing an action potential • Temporal summation—when synaptic knobs stimulate a postsynaptic neuron in rapid succession, their effects can summate over a brief period of time to produce an action potential

48. Neurotransmitters • Neurotransmitters—means by which neurons communicate with one another; there are more than 30 compounds known to be neurotransmitters, and dozens of others are suspected • Classification of neurotransmitters—commonly classified by these features: • Function—function of a neurotransmitter is determined by the postsynaptic receptor; two major functional classifications are excitatory neurotransmitters and inhibitory neurotransmitters; can also be classified according to whether receptor directly opens a channel or instead uses a second messenger mechanism involving G proteins and intracellular signals (Figures 12-29 and 12-30) • Chemical structure—mechanism by which neurotransmitters cause a change; there are four main classes; since the functions of specific neurotransmitters vary by location, they are usually classified according to chemical structure

49. Neurotransmitters • Small-molecule neurotransmitters (Figure 12-31) • Acetylcholine • Unique chemical structure; acetate (acetyl coenzyme-A) with choline • Acetylcholine is deactivated by acetylcholinesterase, with the choline molecules being released and transported back to presynaptic neuron to combine with acetate • Present at various locations; sometimes in an excitatory role and at other times, an inhibitory one

50. Neurotransmitters • Small-molecule neurotransmitters (cont.) • Amines • Synthesized from amino acid molecules • Two categories: monoamines and catecholamines • Found in various regions of the brain; affecting learning, emotions, motor control, etc. • Amino acids • Believed to be among the most common neurotransmitters of the CNS • In the PNS, amino acids are stored in synaptic vesicles and used as neurotransmitters