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Nervous Tissue

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Title: Nervous Tissue


1
Nervous Tissue
2
Functions of the Nervous System
  • Along with the endocrine system, the nervous
    system helps to maintain homeostasis in the body
  • Homeostasis is the state of physiological balance
    that the body attempts to maintain
  • The nervous system regulates body activities by
    responding rapidly using nerve impulses, or
    action potentials

3
Functions of the Nervous System (continued)
  • The endocrine system responds slowly by releasing
    chemical messengers called hormones
  • The specific nervous system functions can be
    grouped as follows
  • Sensory functiondetects stimuli, changes in the
    internal or external environment
  • Integrative functionprocesses and analyzes
    sensory information so that an effective response
    can be made
  • Motor functionresponds to the stimuli

4
Organization of the Nervous System
  • Central Nervous System (CNS)consists of the
    brain and spinal cord
  • Peripheral Nervous System (PNS)includes all
    nervous tissue outside the CNS, such as cranial
    nerves (12 pair) and spinal nerves (31 pair)

5
Organization of the Nervous System (continued)
  • The PNS is subdivided further into the following
  • Somatic Nervous System (SNS)consists of neurons
    that cause skeletal muscles to respond voluntary
  • Autonomic Nervous System (ANS)consists of
    neurons that cause smooth muscle, cardiac muscle,
    and glands to respond involuntary
  • Enteric Nervous System (ENS)sometimes included
    within the ANS consists of neurons that control
    responses within the digestive, or
    gastrointestinal tract involuntary

6
Organization of the Nervous System (continued)
  • The ANS is further subdivided into the following
  • Sympathetic Divisioncontrols fight-or-flight
    responses
  • Parasympathetic Divisioncontrols
    rest-and-digest responses

7
Cells within the Nervous System
  • Neuronslike muscle cells, they are excitable
    (can produce action potentials/nerve impulses in
    response to stimuli)
  • Neurogliasupport, nourish, and protect the
    neurons

8
Neurons
  • Cell Bodycontains the nucleus, cytoplasm, and
    typical cellular organelles
  • Dendritesmultiple processes that receive
    impulses and conduct them to the cell body
  • Axonsusually single processes that carry
    impulses away from the cell body to an effector
    (another neuron, a muscle fiber, or a gland)

9
Types of Neurons (Structural)
  • Multipolar Neuronsusually have several dendrites
    and one axon most neurons in the brain and
    spinal cord are this type have many processes
  • Bipolar Neuronshave one main dendrite and one
    axon found in the retina of the eye, the inner
    ear, and in the olfactory area of the brain have
    two processes
  • Unipolar Neuronshave one main process

10
Types of Neurons (continued)
  • Myelinated neuronsneurons whose axons are
    surrounded by a myelin sheath, a multilayered
    lipid and protein covering the myelin insulates
    the axon and increases the speed of impulse
    conduction gaps in the myelin sheath are called
    nodes of Ranvier
  • Unmyelinated neuronsneurons whose axons do not
    have a myelin sheath

11
Types of Neurons (Functional)
  • Sensory neuronsneurons that carry sensory
    information to the spinal cord and/or brain
    afferent neurons
  • Interneuronsneurons within the CNS (brain and
    spinal cord) that process sensory information and
    decide the appropriate response most neurons in
    the body are of this type have short axons also
    called association neurons
  • Motor neuronsneurons that carry response
    information from the brain and/or spinal cord to
    effectors efferent neurons

12
A Reflex Arc
  • The most basic conduction pathway through the
    nervous system is a reflex arc
  • It consists of 5 components
  • Receptor
  • Sensory neuron
  • CNS integrating center (usually consisting of one
    or more interneurons either in the brain or
    spinal cord)
  • Motor neuron
  • Effector

13
Neuroglia
  • Make up about half the volume of the CNS
  • Nerve glue that holds nervous tissue together
  • Generally smaller than neurons but more numerous
  • Neuroglia are not able to transmit nervous
    impulses
  • They can multiply and divide in the mature
    nervous system, though
  • Most brain tumors involve neuroglia

14
Types of Neuroglia
  • Central Nervous System
  • Astrocytes
  • Oligodendrocytes
  • Microglia
  • Ependymal cells
  • Peripheral Nervous System
  • Schwann cells
  • Satellite cells

15
Astrocytes
  • Star-shaped cells with many processes
  • Help maintain appropriate chemical environment
    for the generation of nerve impulses nourish
    neurons take up excess neurotransmitters assist
    in brain development help form the blood-brain
    barrier

16
Oligodendrocytes
  • Smaller than astrocytes, with fewer processes
  • Produce myelin sheaths within the CNS

17
Microglia
  • Small cells with few processes
  • Protect CNS cells from disease by phagocytosis of
    the invading microbes
  • Clean up dead or injured nerve tissue

18
Ependymal Cells
  • A single layer of epithelial cells
  • Line the ventricles of the brain and the central
    canal of the spinal cord, spaces filled with
    cerebrospinal fluid
  • Produce cerebrospinal fluid

19
Schwann Cells
  • Flattened cells that surround PNS axons
  • Form myelin sheaths around axons of the PNS by
    wrapping around them many times
  • Help in regeneration of PNS axons

20
Satellite Cells
  • Flattened cells arranged around the cell bodies
    of neurons in ganglia, groups of cell bodies
    outside of the CNS
  • Support neurons in PNS ganglia

21
Gray vs. White Matter
  • Gray Matterregions of the brain and spinal cord
    that appear gray in color consists of neuronal
    cell bodies, dendrites, unmyelinated axons, and
    neuroglia
  • White Matterregions of the brain and spinal cord
    that appear white in color consists of many
    myelinated axons and some unmyelinated axons

22
Membrane Potentials
  • The plasma membrane of each body cell has a
    membrane potential, a voltage difference that
    exists between the inside and the outside of the
    membrane due to various concentrations of charged
    atoms, or ions, in each location
  • Muscle cells and neurons are excitable these
    cells have action potentials when responding to
    stimuli and resting potentials when not responding

23
Membrane Potentials (continued)
  • A membrane potential occurs partly because the
    plasma membrane contains ion channels (proteins)
    which allow various ions, or charged atoms, to
    pass into/out of a cell
  • Cells vary in their number and types of ion
    channels
  • Ion channels may be open at some times and
    closed at other times
  • Ions move into/out of open ion channels by the
    process of diffusion
  • In diffusion, substances move from an area of
    greater concentration to an area of lesser
    concentration
  • Diffusion is a passive process, which means that
    the cell does not use ATP energy when it occurs

24
Membrane Potentials (continued)
  • In addition to ion channels, the plasma membranes
    of cells contain proteins that transport ions
    into/out of cells by active transport, which does
    require a cell to use ATP energy
  • The most common example of active transport is
    the sodium-potassium pump, which consists of
    proteins located in the plasma membrane
  • K ions are pumped into a cell and Na ions out
    of a cell against the concentration gradient
    (from lesser to greater concentration)

25
Membrane Potentials (continued)
  • Three Na ions are pumped out of the cell to
    every two K ions that are pumped in
  • In addition, the cytoplasm of a cell contains
    many negatively charged anions, such as those
    attached to large proteins
  • As a result, the cytoplasm just inside the plasma
    membrane of a cell is negative compared to the
    extracellular fluid just outside the membrane,
    which is positive
  • In neurons, the typical value of the resting
    membrane potential is -70 millivolts (mV)

26
Membrane Potentials (continued)
  • A cell that has a membrane potential is said to
    be polarized most body cells are polarized, but
    the millivolt values vary
  • The membrane potential of excitable cells
    (neurons and muscle cells) can change in response
    to a stimulus
  • A stimulus may cause ion channels to open or
    close differently than under normal (resting)
    conditions

27
Graded Potentials vs. Action Potentials
  • A graded potential is a small change in membrane
    potential in a localized area of the plasma
    membrane
  • An action potential is a change in membrane
    potential that propagates the length of the
    plasma membrane also called a nerve impulse

28
Action Potentials
  • The events of an action potential occur in two
    phases depolarization and repolarization
  • During depolarization, the negative membrane
    potential decreases toward 0 mV and eventually
    becomes positive (about 30 mV)
  • During repolarization, the resting membrane
    potential is restored (back to its negative value
    of -70 mV)

29
Action Potentials (continued)
  • During an action potential, ion channels open
    first to allow Na ions to move into the cell
  • This causes depolarization as the membrane
    potential changes from -70 mV to 30 mV
  • Next, ion channels open to allow K ions to move
    out of the cell
  • This causes repolarization as the membrane
    potential changes back from 30 mV to -70 mV
  • If K continues to flow out of the cell, the
    membrane potential may reach -90 mV and
    hyperpolarization occurs
  • Eventually, the resting membrane potential is
    restored

30
Action Potentials (continued)
  • Neurons, like muscle cells, go through a
    refractory period after generating an action
    potential
  • The refractory period is a short period of time
    during which an excitable cell cannot respond to
    a second stimulus

31
All-or-None Principle
  • Action potentials arise according to the
    all-or-none principle
  • When depolarization reaches threshold level,
    which is about -55 mV, an action potential occurs
  • The action potential is always the same size, no
    matter how strong the stimulus is
  • The frequency of a stimulus is the main factor
    that varies in the conduction of a nerve impulse

32
Continuous vs. Saltatory Conduction
  • Continuous conductionthe step-by-step
    depolarization and repolarization of the plasma
    membrane described previously occurs in muscle
    fibers and unmyelinated axons
  • Saltatory conductiona special type of nerve
    impulse conduction that occurs in myelinated
    axons the impulse leaps from one node of Ranvier
    to another, resulting in much faster conduction
    of an impulse

33
Speed of Conduction
  • Myelinated axons conduct impulses faster than
    unmyelinated axons
  • Axons with large diameters conduct impulses
    faster than axons with small diameters

34
Transmission at Synapses
  • When an action potential is propagated from a
    neuron to a skeletal muscle cell, the signal must
    be transmitted across a type of synapse called a
    neuromuscular junction
  • In this situation, the impulse is transmitted
    electrically along an axon and chemically
    across a synapse (because of the neurotransmitter
    acetylcholine)

35
Transmission at Synapses (continued)
  • As an action potential is transmitted from one
    neuron to another, it usually must cross a
    different type of synapse, but the basic process
    is similar to that involving the neuromuscular
    junction
  • The neuron that sends the impulse is called the
    presynaptic neuron the neuron that receives the
    impulse is called the postsynaptic neuron
  • The two neurons are separated by a synaptic
    cleft, which is a tiny space filled with
    interstitial fluid

36
Transmission at Synapses (continued)
  • When a nerve impulse arrives at a synaptic end
    bulb of a presynaptic neuron, ion channels in
    that neurons plasma membrane open and allow Ca2
    ions to flow into the cell
  • This increase in Ca2 in the presynaptic neuron
    triggers exocytosis of some of the synaptic
    vesicles, causing the release of neurotransmitter
    molecules into the cleft each vesicle contains
    thousands of molecules of neurotransmitter

37
Transmission at Synapses (continued)
  • The neurotransmitter molecules diffuse across the
    cleft and bind to receptors in the postsynaptic
    neurons plasma membrane
  • This opens ion channels in the postsynaptic
    neuron and allows specific ions to move into that
    cell from the synaptic cleft

38
Transmission at Synapses (continued)
  • As the ions flow into the postsynaptic neuron,
    the membrane potential there changes
  • If threshold level (-55 mV) is reached,
    depolarization occurs hyperpolarization may also
    occur
  • The neurotransmitter is then removed from the
    synaptic cleft, so that it will not continuously
    influence the postsynaptic neuron
  • This occurs through the action of enzymes,
    reuptake by the neuron that released the
    substance, or by the diffusion of the
    neurotransmitter away from the cleft

39
Excitatory and Inhibitory Postsynaptic Potentials
  • If a neurotransmitter depolarizes the
    postsynaptic membrane, an excitatory postsynaptic
    potential (EPSP) results
  • If a neurotransmitter hyperpolarizes the
    postsynaptic membrane, an inhibitory postsynaptic
    potential (IPSP) results

40
Summation of Postsynaptic Potentials
  • Most CNS neurons receive input from many synapses
  • Integration of these inputs is called summation
  • When summation results from the buildup of
    neurotransmitter released by several presynaptic
    end bulbs at the same time, it is called spatial
    summation
  • When summation results from the buildup of
    neurotransmitter released by a single presynaptic
    end bulb two or more times in rapid succession,
    it is called temporal summation

41
Summation (continued)
  • A single postsynaptic neuron receives both
    excitatory and inhibitory information at the same
    time
  • If the total summation effect is excitatory and
    threshold level is reached, an EPSP results and
    creates a nerve impulse
  • If the total summation effect is inhibitory, an
    IPSP results and no impulse is generated
  • If the total summation effect is excitatory but
    threshold level is not reached, an EPSP results
    however, an impulse is not generated until a
    subsequent stimulus causes depolarization

42
Neurotransmitters
  • Many substances act as neurotransmitters
  • Neurotransmitters can be excitatory or inhibitory
  • A neurotransmitter can even be excitatory in some
    locations and inhibitory in others
  • Examples include the following
  • Acetylcholine (Ach)
  • Gamma aminobutyric acid (GABA)inhibitory
  • Norepinephrine (NE)
  • Epinephrine
  • Dopamine (DA)
  • Serotonin
  • Nitric oxide (NO)
  • Endorphinsthe bodys natural painkillers

43
Regeneration and Repair of Nervous Tissue
  • Human neurons have very limited ability to repair
    themselves or regenerate
  • In the PNS, repair may generally occur if the
    cell body and Schwann cells are not damaged
  • In the CNS, little or no repair may occur
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