FUNDAMENTALS OF THE NERVOUS SYSTEM AND NERVOUS TISSUE

1. FUNDAMENTALS OF THE NERVOUS SYSTEM AND NERVOUS TISSUE

2. Nervous System • Has three overlapping functions: • 1.It uses its millions of sensory receptors to monitor changes occurring both inside and outside the body • The gathered information is called sensory input • 2.It processes and interprets sensory input and decides what should be done at each moment—a process called integration • 3.It causes a response, called motor output, by activating effector organs

3. NERVOUS SYSTEM’S FUNCTIONS

4. Organization of the Nervous System • Divided into two principal parts: • Central nervous system (CNS): consist of the brain and spinal cord, which occupy the dorsal body cavity • Integrating and command center of the nervous system • Interprets sensory input and dictates motor responses based on past experience, reflexes, and current conditions • Peripheral nervous system (PNS): the part of the nervous system outside the CNS • Consists mainly of the nerves (bundles of axons) that extend from the brain and spinal cord • Spinal nerves carry impulses to and from the spinal cord • Cranial nerves carry impulses to and from the brain • Serve as the communication lines that link all parts of the body to the CNS

5. ORGANIZATION OF THE NERVOUS SYSTEM • The peripheral nervous system (PNS) has two functional subdivisions: • 1.The sensory, or afferent, division of the peripheral nervous system consist of nerve fibers that convey impulses to thecentral nervous system from sensory receptors located throughout the body • Somatic afferent fibers: sensory fibers conveying impulses from the skin, skeletal muscles, and joints • Visceral afferent fibers: sensory fibers that transmit impulses from the visceral organs (organs within the ventral body cavity)

6. NERVOUS SYSTEM ORGANIZATION

7. ORGANIZATION OF THE NERVOUS SYSTEM • The peripheral nervous system (PNS) has two functional subdivisions: • 2.Motor, or efferent, division of the peripheral nervous system carries impulses from the central nervous system to effector organs, which are muscles and glands: • These impulses activate muscles to contract and glands to secrete; that is, they effect (bring about) a motor response • Two main parts: • The somatic nervous system (SNS) consists of somatic nerve fibers (axons) that conduct impulses from the CNS to skeletal muscles, and allow conscious control of motor activities • Referred to a the voluntary nervous system because it allows us to consciously control our skeletal muscles • Theautonomic nervous system (ANS) consists of visceral motor nerve fibers that regulate the activity of smooth muscle, cardiac muscle, and glands • Referred to as the involuntary nervous system • Has two functional subdivisions: which typically work in opposition to each other—what one subdivision stimulates, the other inhibits • Sympathetic • Parasympathetic

8. NERVOUS SYSTEM ORGANIZATION

9. NERVOUS SYSTEM ORGANIZATION

10. Histology of Nervous Tissue • Nervous tissue is made up of just two principal types of cells: • 1.Supposting cells: • Smaller cells that surround and wrap the more delicate neurons • 2.Neurons: • The excitable nerve cells that transmit electrical signals

11. Neuroglia • Smaller cells that are associated with neurons: provide a supportive and protective network for neurons • Also called glial cells • Like neurons, most glial cells have branching processes (extensions) and a central cell body • They outnumber neurons in the CNS by about 10 to 1 • Make up about ½ the mass of the brain • Six types: • Four in the CNS (central nervous system) • Two in the PNS (peripheral nervous system)

12. Neuroglia (Glial) in the CNS • Astrocytes (a) are glial cells of the CNS that regulate the chemical environment around neurons and exchange between neurons and capillaries • Most abundant and versatile glial cells • Radiating processes cling to the neurons and nearby capillaries anchoring and bracing them to their nutrient supply (blood capillaries) • Mop up leaked potassium ions and recapture (recycle) neurotransmitters • Held together by gap junctions that enable them to signal one another (and perhaps neurons) via intracellular calcium pulses

13. CELLS OF THE NERVOUS SYSTEM

14. Neuroglia (Glial) in the CNS • Microglia (b) are glial cells of the CNS that monitor health and perform defense functions for neurons: • Protective role is important because cells of the immune system are denied access to the CNS (under normal circumstances) • Long thorny processes that touch nearby neurons • Transform into a special type of macrophage that phagocytizes microorganisms or neuronal debris

15. CELLS OF THE NERVOUS SYSTEM

16. Neuroglia (Glial) in the CNS • Ependymal cells (c) are glial cells of the CNS that line the central cavities of the brain and spinal cord and help circulate cerebrospinal fluid: • Permeable barrier between the cerebrospinal fluid that fills the central cavities (of the brain and spinal cord) and the tissue fluid bathing the cells of the CNS • Many are ciliated • Helps to circulate the cerebrospinal fluid that cushions the brain and spinal cord

17. CEREBROSPINAL FLUID

18. CELLS OF THE NERVOUS SYSTEM

19. Neuroglia (Glial) in the CNS • Oligodendrocytes (d) are glial cells of the CNS that wrap around neuron fibers, forming myelin sheaths—insulating covering • Also branched (fewer processes)

20. Neuroglia in the PNS (Peripheral Nervous System) • Satellite cells (e) are glial cells of the PNS whose function is largely unknown • They are found surrounding neuron cell bodies within ganglia • Name comes from the resemblance to the moons (satellites) around a planet

21. Neuroglia in the PNS (Peripheral Nervous System) • Schwann cells (e), or neurolemmocytes, are glial cells of the PNS that surround nerve fibers, forming the myelin sheath • Function similar to oligodendrocytes of CNS • Vital to regeneration of peripheral nerve fibers

22. CELLS OF THE NERVOUS SYSTEM

23. NEURON

24. NEURONS (NERVE CELLS) • Structural units of the nervous system • Specialized cells that: • Conduct messages in the form of electrical impulses throughout the body • Have extreme longevity: • Function optimally for a lifetime (over 100 years) • Largely amitotic: • No ability to divide • Exceptions: • Olfactory epithelium: smell • Hippocampus: region of brain involved in memory • Have an exceptionally high metabolic rate requiring oxygen and glucose • Cannot survive for more than a few minutes without oxygen

25. NEURONS (NERVE CELLS)CELL BODY • The neuron cell body, also called the perikaryon or soma, is the major biosynthetic center containing the usual organelles except for centrioles • Most are located in the CNS, where they are protected by the bones of the skull and vertebral column • Clusters of cell bodies in the CNS are called nuclei, whereas those that lie along the nerves in the PNS are called ganglia

26. NEURONS (NERVE CELLS) • Processes: • Armlike extensions from the cell body of all neurons • CNS (brain and spinal cord): contain bothneuron cell bodies and their processes • Bundles of neurons in the CNS are called tracts • PNS (peripheral): consist mainlyof neuron processes • Bundles of neurons in the PNS are called nerves

27. NEURONS (NERVE CELLS)DENDRITE • Dendrites are cell processes that are the receptive regions of the cell • Provide an enormous surfacearea for receiving signals from other neurons or the environment • Bristle with thorny appendages having bulbous or spiky ends called dendritic spines • Convey incoming messages toward the cell body • These electrical signals are NOT nerve impulses (action potentials) but are short-distance signals celled graded potentials

28. NEURONS (NERVE CELLS)AXON • Initial region of the axon arises from a cone-shaped area of the cell body called the axon hillock and then narrows • Each neuron has a single axon that generates and conducts nerve impulses away from the cell body to the axon terminals • Short or long • Example: axons of the motor neurons controlling the skeletal muscles of your toes extend from the lumbar region of your spine to your foot (3-4 feet) • Long axons are called nerve fibers • Conducting component of the neuron • Generates nerve impulses and transmit them, typically away from the cell body

29. NEURONS (NERVE CELLS)TERMINAL BRANCHES • Each neuron has only one axon, but axons may have occasional branches, called axon collaterals • Extend from the axon at more or less right angles • Axon usually branches profusely at its end (terminus) • 10,000 or more terminal branches per neuron is not unusual

30. NEURONS (NERVE CELLS)TERMINAL BRANCHES • When the impulse reaches the axonal terminals, it causes neurotransmitters, signaling chemicals stored in vesicles there, to be released into the extracellular space • Either excite or inhibit neurons (or effector cells) • Axon plasma membrane (axolemma)

31. NEURONS (NERVE CELLS)MYELIN SHEATH • The myelin sheath is a whitish, fatty, segmented covering that protects, insulates, and increases conduction velocity of axons • Myelinated fibers (axons bearing a myelin sheath) conduct nerve impulses rapidly • Unmyelinated fibers (axons without a myelin sheath) conduct impulses slower • Associated only with axons • Dendrites are always unmyelinated

32. NEURONS (NERVE CELLS)MYELIN SHEATH • Formed in the PNS by Schwann cells • Wrapped around the axon (jelly roll fashion) • Initially loose but as the wrapping gets tight the cell cytoplasm is gradually squeezed from between the membrane layers • Many concentric layers of Schwann cell plasma membrane enclose the axon (like gauze wrapped around an injured finger) • This tight coiling is the myelin sheath

33. NEURONS (NERVE CELLS)MYELIN SHEATH • Plasma membranes of myelinating cells contain much less protein than the plasma membranes of most body cells • Channel and carrier proteins are absent, a characteristic that makes myelin sheaths exceptionally good electrical insulators

34. NEURONS (NERVE CELLS)NEURILEMMA • The nucleus and most of the cytoplasm of the Schwann cell and up as a bulge just external to the myelin sheath • This portion of the Schwann cell which includes the exposed part of its plasma membrane, is called the neurilemma • Adjacent Schwann cells along an axon do not touch one another, so there are gaps in the sheath • These gaps, called nodes of Ranvier (neurofibril nodes), occur at regular intervals (1 mm) along the myelinated axon • It is at this nodes that axon collaterals can emerge from the axon

35. NEURON

36. SCHWANN AXON

37. BRAIN • Regions of the brain and spinal cord containing dense collections of myelinatedfibers are referred to as whitematter and are primarily fiber tracts • Gray matter contains mostly nerve cell bodies and unmyelinated fibers

38. Classification of NeuronsStructural • There arethree structural classes of neurons: classified according to the number of processes extending from their cell body • Multipolar neurons: • Have three or more processes (many dendrites and single axon) • 99% of neurons belong to this class • Major neuron type in the CNS • Bipolar neurons have a single axon and dendrite • Found only in some of the special sense organs, where they act as receptor cells • Retina of the eye and olfactory mucosa • Unipolar neurons have a single process extending from the cell body and divides T-like into proximal and distal branches • Mainly in ganglia in the PNS, where they function as sensory neurons

39. Classification of NeuronsFunctional • There arethreefunctional classes of neurons: classified according to the directionin which the nerve impulse travels relative to the central nervous system • Sensory, or afferent, neurons conduct impulses toward the CNS from receptors • Except for the bipolar neurons found in some special sense organs, virtually all sensory neurons are unipolar, and their cell bodies are located in sensory ganglia outside the CNS • Some can be very long: • Example, fibers carrying sensory impulses from the skin of your toes travel for more than a meter before reaching their cell bodies in a ganglion close to the spinal cord • Motor, or efferent, neurons conduct impulses from the CNS to effectors (muscles and glands) of the body periphery • Multipolar, except for some neurons of the autonomic nervous system • Cell bodies are located in the CNS • Interneurons, or association, neurons conduct impulses between sensory and motor neurons • Most are confined within CNS • In CNS integration pathways • 99% of the neurons of the body • Almost are multipolar, but there is considerable diversity in both size and fiber-branching patterns

40. NEUROPHYSIOLOGY • Neurons are highly responsive to stimuli: • When adequately stimulated, an electrical impulse is generated and conducted along the length of its axon • This response, called the action potential (nerve impulse), is always the same, regardless of the source or type of stimulus

41. Basic Principles of Electricity • Opposite charges attract each other, energy must be used (work must be done) to separate them • Comingtogether of opposite charges liberates energy that can be used to do work • Voltageis a measure of the amount of difference in electrical charge between two points, called the potential difference • Measured in volts or millivolts ( 1 mV = 0.001 V ) • Greater the difference in charge between two points, the higher the voltage • The flow of electrical charge from point to point is called current, and is dependent on voltage and resistance (hindrance to current flow) • Used to do work • The amount of charge that moves between the two points depends on two factors: voltage and resistance • Resistance: hindrance to charge flow • Provided by substances through which the current must pass • Substances with high electrical resistance are called insulators • Substances with low electrical resistances are called conductors

42. Basic Principles of Electricity • Ohm’s Law: relationship between voltage, current, and resistance • Current (I) = Voltage (V) / Resistance (R) • Current (I) is directly proportional to voltage (V) • Greater the voltage (potential difference), the greater the current • Current (I) is inversely related to resistance (R) • Greater then resistance, the smaller the current

43. Basic Principles of Electricity • In the body, electrical currents are due to themovement of ions (rather than free electrons)across cellular membranes • There are NO free electrons in a living system • There is a slight difference in the numbers of positive and negative ions on the two sides of cellular plasma membranes (there is a charge separation), so there is a potential across those membranes • The resistance to current flow is provided by the plasma membrane

44. The Role of Membrane Ion Channels • The plasma membrane has many ion channels made of proteins • Some of these channels are passive, or leakage, channels which are always open • Some are active, or gated that change shape or open/close in response to the proper signal • Chemically gated, or ligand-gated, channels open when the appropriate chemical (in this case, neurotransmitter) bind • Voltage-gated channels open and close in response to changes in the membrane potential • Mechanically gated channels open in response to physical deformation of the receptor (sensory receptors for touch and pressure) • Each type of channel is selective as to the type of ion (or ions) it allows to pass • Example: a potassium ion channel allows only potassium ions to pass

45. GATED CHANNELS

46. The Role of Membrane Ion Channels • When gated channels are open, ions diffuse quickly across the membrane following their electrochemical gradients, creating electrical currents and voltage changes across the membrane according to the rearranged Ohm’s law equation: • Voltage (V) = current (I) X resistance (R) • Ions move along chemical gradients when they diffuse passivelyfrom an area of their higher concentration to an area of lower concentration • Ions move along electrical gradients when they move toward an area of opposite charge • It is ion flows along electrochemical gradients that underlie all electrical phenomena in neurons

47. The Resting Membrane Potential • The potential difference between two points is measured with a voltmeter • When one microelectrode of a voltmeter is inserted into the neuron and the other rests on the neuron’s outside surface, a voltage across the membrane of approximately -70 mV is recorded • Minus (-) sign indicates that the cytoplasmic side (inside) of the membrane is negatively charged relative to the outside • This potential difference in a resting neuron (Vr) is called the restingmembrane potential, and the membrane is said to be polarized • Values vary from -40 mV to -90 mV • The resting potential exists only across the membrane; that is, the bulk solutions inside and outside the cell are electrically neutral

48. MEASURING MEMBRANE POTENTIAL

49. The Resting Membrane Potential • The neuron cell membrane is polarized, beingmore negatively charged inside than outside • The degree of this difference in electrical charge is the resting membrane potential • The resting membrane potential is generated by differences in ionic makeup of intracellular and extracellular fluids, and differential membrane permeability of the plasma membrane to those ions

50. The Resting Membrane Potential • Cell cytosol (inside cell) contains a lower concentration of Na+ and ahigher concentration of K+ than the extracellular (outside) fluid • In the extracellular fluid, the positive charges of sodium and other cations (+ ion) are balanced chiefly by chloride ions (Cl-) • Negatively charged (anionic) proteins (A-) help to balance the positive charges of intracellular cations (primarily K+ ions) • Although there are many other solutes (glucose, urea, and other ions) in both fluids, potassium plays the most important role in generating the membrane potential