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Anatomy and Physiology > Ch.12 - Nervous Tissue > Flashcards

Ch.12 - Nervous Tissue Flashcards

1
Q

The Nervous System, Homeostasis, and Functions

A
  • Helps maintain homeostasis through the excitable characteristic of nervous tissue, which allows for generation of nerve impulses (action potentials) that provide communication with and regulation of most body organs
  • Helps to keep controlled conditions within limits that maintain life

Functions
- Sensory (input) function - detect internal/ external stimuli

  • Integrative (process) function - processes sensory info by analyzing it and making decisions for appropriate responses
  • Motor (output) function - the NS activates effectors (muscles and glands) through cranial and spinal nerves; stimulation of effectors causes muscles to contract and secrete hormones
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2
Q

The nervous tissue is responsible for

A

our perceptions, behaviors, and memories, and it initiates all voluntary movements

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3
Q

Neurology

A

the branch of medical science that deals with the normal functioning and disorders of the nervous system

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4
Q

Central Nervous System (CNS)

A
  • Consists of the brain and spinal cord
  • The source of thoughts, memories, and emotions
  • Skull contains: about 85 Billion neurons
  • Spinal cord contains: about 100 Million neurons
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5
Q

Peripheral Nervous System (PNS)

A
  • Consists of all nervous tissue outside the CNS
  • Consists of of cranial and spinal nerves with sensory (afferent) and motor (efferent) divisions, 12 pairs of cranial nerves, 31 pairs of spinal nerves, and sensory receptors (monitor changes in the internal/ external environment)
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6
Q

The PNS is divided into somatic, autonomic, and enteric nervous systems

A
  • Somatic Nervous System (SNS) - voluntary
  • Autonomic Nervous System (ANS) - involuntary; divided into sympathetic division, parasympathetic division, and enteric plexus
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7
Q

Nerve

A

A bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord

  • 12 pairs of cranial nerves
  • 31 pairs of spinal nerves
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8
Q

Neurons

A

have the property of electrical excitability, which the ability to respond to a stimulus and convert it into an Action Potential

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9
Q

Stimulus and Nerve Impulse

A
  1. Stimulus - any change in the environment that is strong enough to initiate a nerve impulse.
  2. Nerve Impulse (action potential) - an electrical signal that propagates along the surface of the membrane of a neuron
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10
Q

Nervous tissue comprises two types of cells

A

neurons and neuroglia

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11
Q

Neurons

A
  • Have the property of electrical excitability, which is the ability to respond to a stimulus and convert it into an action potential

Parts of a Neuron

  • Cell Body -
  • Nissil Bodies
  • Neurofibrils
  • Microtubules
  • Somatic Spines
  • Ganglion
  • Nerve Fiber
  • Dendrites
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12
Q

Neurotransmitter

A

a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland.

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13
Q

Structural neurons are classified according to the number of processes extending from the cell body

A
  1. Multipolar Neurons - have several dendrites and one axon; most neurons in the brain and spinal cord are multipolar as well as motor neurons.
  2. Bipolar Neurons - have one main dendrite and one axon; found in the retina of the eye, inner ear, and olfactory area of the brain.
  3. Pseudounipolor Neurons - have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body; cell bodies found in the ganglia of spinal and cranial nerves. They detect sensory stimulus.
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14
Q

Structurally neurons are classified according to the number of processes extending from the cell body

A
  1. Multipolar Neurons - have several dendrites and one axon; most neurons in the brain and spinal cord are multipolar as well as motor neurons.
  2. Bipolar Neurons - have one main dendrite and one axon; found in the retina of the eye, inner ear, and olfactory area of the brain.
  3. Pseudounipolor Neurons - have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body; cell bodies found in the ganglia of spinal and cranial nerves. They detect sensory stimulus.
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15
Q

Purkinje and Pyramidal Cells

A

Purkinje cells are found in the cerebellum and pyrmidal cells are found in cerebral cortex.

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16
Q

Functionally neurons are classified according to the direction in which the nerve impulse is conveyed

A
  1. Sensory (afferent) Neurons - located after sensory receptors; forms a nerve impulse that is carried into the CNS through cranial or spinal nerves; unipolar in structure
  2. Motor (efferent) Neurons - convey nerve impulses away from the CNS to effectors in the PNS through cranial or spinal nerves ; multipolar in structure
  3. Interneurons (association) - between sensory and motor neurons; multipolar in structure
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17
Q

Neuroglia

A

are specialized tissue cells that support neurons, attach neurons to blood vessels, produce the myelin sheath around axons, and carry out phagocytosis. They are smaller but more numerous than neurons.

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18
Q

There are 6 types of Neuroglia

A

Neuroglia in the CNS

  1. Astrocytes Function:
    - Microfilaments for strength
    - Blood brain barrier
    - Regulate growth
    - Maintain chemical environments
    - Learning and memory
    - 2 types: Protoplasmic and fibrous
  2. Oligodendrocyte - forms and maintains myelin sheath around CNS axons
  3. Microglial Cells - function as phagocytes, remove cellular debris formed during normal development of of the NS, and phagocytize microbes and damaged nervous tissue
  4. Ependymal Cells - produce, monitor, and assist in the circulation of cerebrospinal fluid; protects and nourishes the brain and spinal cord.

Neuroglia in the PNS
1. Schwann Cells - form myelin sheath around the PNS axons; myelinates a single cell; participate in axon regeneration

  1. Satellite Cells - surround the cell bodies of neurons of PNS ganglia; regulate exchanges of materials between neuronal cell bodies and interstitial fluid
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19
Q

Myelin Sheath

A

axons surrounded a multilayered lipid and protein covering. The sheath electrically insulates the axon and increases the speed of a nerve impulse conduction.

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20
Q

Neurolemma

A

the outer nucleated cytoplasmic layer of the Schwann Cell which encloses the myelin sheath and is found ONLY around axons in the PNS.

  • Aids in regeneration when an axon is injured by forming a regeneration tube that guides and stimulates regrowth of the axon
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21
Q

Myelin Sheath Gaps

A

gaps in the myelin sheath called neurofibril nodes or nodes of Ranvier, along the axon

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22
Q

Collections of Nervous Tissue

A
  1. Clusters of neuronal cell bodies
    - Ganglion - cluster of neuronal cell bodies (PNS)
    - Nucleus - cluster neuronal cell bodies (CNS)
  2. Bundles of axons
    - Nerve - groups of axons (PNS)
    - Tracts - groups of axons (CNS)
  3. Gray and white matter
    - Gray matter - contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia
    - White matter - composed of myelinated axons
    - Blood vessels present in both^
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23
Q

Excitable cells communicate with each other by

A
  1. Action potentials - allow communication over short and long distances (ex: nerve impulses can travel over long distance because they do not die out)
  2. Graded potentials - allow communication over short distances only due to decremental conduction (GP die out as they spread along a membrane)
  • The production of both potentials depends on the existence of a resting membrane potential and the presence of certain types of ion channels
  • They occur because ion channel in the membrane allow ion movement across the membrane that can change the membrane potential
24
Q

Membrane potential

A

an electrical voltage across the membrane; termed resting membrane potentials in excitable cells

25
Q

Current

A

is the flow of charged particles

26
Q

Electrochemical gradient

A

a concentration (chemical) difference plus an electrical difference

27
Q

Ion channels open and close due to the presence of gates. The electrical signals produced by neurons and muscle fibers rely four types of ion channels

A
  1. Leak channels - channel randomly opens and closes; found in dendrites, cell bodies, and axons of all types of cells
  2. Ligand-gated channels - chemical (neurotransmitters, hormones, particular ions) stimulus opens the channel; located in dendrites of some dendrites of sensory receptors and cell bodies of interneurons and motor neurons
  3. Mechanically gated channels - mechanical (in the form of vibration, touch, pressure, or tissue stretching) stimulus opens the channel.
  4. Voltage-gated channels - change in membrane potential opens the channel; participate in generation and conduction of nerve impulses in the axons of all types of neurons
28
Q

Resting Membrane Potential

A
  • typical value is -70mV (the negative sign indicates that the inside of the cell is negative relative to the outside
  • the membrane of a nonconducting neuron is (+) outside and (-) inside
  • membrane is said to be polarized (a cell that exhibits a membrane potential
29
Q

Factors that contribute to the resting membrane potential

A
  1. Unequal distribution of ions in the ECF and cystol and the selective permeability of membrane to sodium and potassium due to more potassium leak channels
  2. Most anions cannot leave the interior of the cell - they cannot follow potassium out of the cell because they are attached to nondiffusable molecules, such as large molecules proteins
  3. Electrogenic nature of the Na^+ - K^+ ATPases
30
Q

Graded Potentials

A

a small deviation from the resting membrane potential that makes the membrane either more polarized (Hyperpolarization - inside more negative) or less polarized (Depolarization - inside less negative)

  • Occur most often in dendrites and cell body of a neuron
  • Occur as the result of opening of ligand-gated or mechanically-gated
  • The signals are graded meaning they vary in amplitude (size) depending on the strength of the stimulus
  • Summation - process by which graded potentials add together
31
Q

Action Potential

A

a sequence of rapidly occurring events that decrease and eventually reverse membrane potential (depolarization) and then restore it to the resting state (repolarization)

32
Q

In a neuron, an action potential (AP) is called a:

A

nerve impulse which has two main phases:

  1. Depolarizing phase - negative membrane becomes less (-), reaches 0, then becomes (+)
    - Voltage-gated sodium channels open rapidly
    - inflow of sodium changes the membrane potential from -55 mV to +33m mV
    - steps 1-2
  2. Repolarizing phase - membrane potential is restored to the resting state of -70 mV
    - the slower opening of voltage-gated potassium channels and closing of previously open sodium channels leads to repolarization
    - steps 3-4
33
Q

Threshold

A

when depolarization reaches a certain level (about -55 mV in many neurons)

34
Q

Subthreshold stimulus

A

does NOT generate an AP due to weak depolarization

35
Q

Threshold stimulus

A

generates an AP; stimulus strong enough to depolarize the membrane to threshold

36
Q

Suprathreshold stimulus

A

generates an AP (often more than one AP); stimulus strong enough to depolarize the membrane above threshold

37
Q

All-or-none Principle

A

a nerve impulse occurs completely or it does not occur at all

ex: dominoes

38
Q

After-Hyperpolarizing Phase

A

the voltage-gated potassium channels remain open and the membrane potential becomes even more negative (about -90 mV) As the voltage-gated potassium channels close, the membrane potential returns to the resting level -70 mV.

39
Q

Refractory Period

A

the period of time during which an excitable cell cannot generate another nerve impulse in response to a normal threshold.

  1. Absolute Refractory Period - a very strong stimulus cannot initiate a second nerve impulse.
  2. Relative Refractory Period - can be triggered only by a suprathreshold stimulus.
40
Q

Propagation

A

an AP propagates or travels from point to point along the membrane; the traveling AP is a nerve impulse

41
Q

There are 2 types of propagation

A
  1. Continuous Conduction - involves step-by-step depolarization and repolarization of each adjacent area of the plasma membrane and occurs in unmyelinated axons and in muscle fibers
  2. Saltatory Conduction - the impulse jumps from neurofibril node to node along myelinated axons due to uneven distribution of voltage-gated channels
42
Q

Factors that affect the speed of propagation are

A
  1. Amount of myelination - Nerve impulses propagate more rapidly along myelinated axons than along unmyelinated axons
  2. Axon diameter - Larger diameter axons propagate nerve impulses faster than smaller ones due to their surface areas
  3. Temperature - Axons propagate nerve impulses faster when warmed and slower when cooled
43
Q

Classification of Nerve Fibers

A

A Fibers - Largest diameter axons (5-20 m) and myelinated; have a brief refractory period
(ex. sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations)

B Fibers - Axons with diameters of 2-3 m and myelinated; exhibit saltatory conduction (ex. conduct sensory nerve impulses from the viscera to the brain and spinal cord)

C Fibers - Smallest diameter axons (0.5-1.5 m) and unmyelinated; have the longest refractory period
(ex. pain, touch, pressure, heat, cold from the skin, and pain impulses from the viscera)

44
Q

Encoding of stimulus intensity

A

the intensity of a stimulus is coded in the rate of impulse production. For instance. frequency of nerve impulses, or AP.

45
Q

Comparison of electrical signals produced by excitable cells

A

Table 12.2

46
Q

Synapse

A

the site of communication between two neurons or between a neuron and an effector cell

  • Presynaptic neuron - a nerve cell that carries a nerve impulse towards a synapse
  • Postsynaptic cell - cell body that receives the signal
  • Postsynaptic neuron - carries a nerve impulse away from a synapse or to an effector cell
  • Effector cell - responds to the impulse at the synapse (muscles or glands)

Most synapses between neurons are axodendritic (from axon to dendrite) while others are axiosomatic (from axon to cell body) or axoaxonal (from axon to axon)

47
Q

Electrical Synapses

A
  1. AP that conduct directly between the plasma membranes of adjacent neurons through structures called Gap Junctions
  2. ionic current spreads directly from one cell to another through Gap Junctions

(a) Electrical Synapses allow for faster communication than chemical synapses because they conduct directly through gap junctions and (b) can synchronize (coordinate) the activity of a group of neurons or muscle fibers

48
Q

Chemical Synapses

A
  • There is only a one-way information transfer from a presynaptic neuron to a postsynaptic neuron that is separated by the synaptic cleft (filled with interstitial fluid)
  • Nerve impulses cannot conduct across the synaptic cleft so the presynaptic neuron converts that electrical signal (nerve impulse) into a a chemical signal (released neurotransmitter). After receiving a chemical signal, an electrical signal, or postsynaptic potential, is generated.
  • the time required for these processes at a chemical synapse (synaptic delay) is the reason that chemical synapses relay signals more slowly than electrical synapses
49
Q

A neurotransmitter causes either an excitatory or inhibitory graded potential

A
  1. A depolarizing postsynaptic potential is called an Excitatory post-synaptic potential because it brings the membrane closer to threshold by causing depolarization of the postsynaptic membrane (EPSP)
  2. A hyper polarizing postsynaptic potential is called an Inhibitory post-synaptic potential because it causes hyperpolarization of the postsynaptic membrane (IPSP)
50
Q

Structure of Neurotransmitter Receptors

A

Neurotransmitter receptors - neurotransmitter released from a presynaptic neuron bind to neurotransmitter receptors in the plasma membrane of a postsynaptic cell: classified as ionotropic or metabotropic

Ionotropic receptors - channels that directly bind to the neurotransmitter

Metabotropic receptors - proteins (G proteins) that bind to the neurotransmitter, but not the ion channel. Instead the binding protein initiates ion flow through another protein

51
Q

A neurotransmitter is removed in 3 ways

A
  1. Diffusion - some released neurotransmitter molecules diffuse away from the synaptic cleft
  2. Enzymatic degradation - certain neurotransmitters are inactivated (ex. acetylcholinesterase breaks down ACh)
  3. Uptake by cells - neurotransmitters are transported back into the neuron that released them (reuptake) or transported into neighboring neuroglia (uptake)
52
Q

Spatial and Temporal Summation of Postsynaptic Potentials

A
  • Spatial summation - summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time
  • Temporal summation - summation of a postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cells but at different times
53
Q

Neurotransmitters can be divided into 2 classes based on size

A
  1. Small-molecule neurotransmitters
    Acetylcholine (ACh)

Amino acids (found in CNS)

  • Glutamate - excitatory; most excitatory neurons in the CNS and 1/2 synapses in the brain communicate via glutamate
  • Aspartate - excitatory
  • Gamma-aminobutyric acid (GABA) - inhibitory; found only in CNS
  • Glycine - inhibitory

Biogenic acids may cause inhibition or excitation depending on the type of metabolic receptor at the synapse

  • Norepinephrine (NE) - plays a role in arousal, dreaming, and regulating mood; hormone
  • Epinephrine - smaller number of neurons in brain use this hormone
  • Dopamine (DA) - active during emotional responses, addictive behaviors, and pleasurable experiences
  • Serotonin - involved in sensory perception, temperature regulation, control of mood, appetite, and induction of sleep

ATP and other purines

Nitric oxide - excitatory; secreted in the brain, spinal cord, suprarenal glands, nerves of the penis, and has widespread affects throughout the body

Carbon monoxide - excitatory; produced int he brain and in response to neuromuscular and nueroglandular functions

  1. Neuropeptides - numerous and widespread in both CNS and PNS
    - Enkephalins - inhibit pain impulses by suppressing release of Substance P; may have a role in memory and learning, control of body temperature, sexual activity and mental illness
    - Endorphins - inhibit pain by blocking the release of Substance P; may have a role in memory and learning, control of body temperature, sexual activity and mental illness
    - Dynorphins - may be related to controlling pain and registering emotions
    - Substance P - found incensory neurons, spinal cord pathways, and parts of the brain associated with pain; enhances the perception of pain
    - Hypothalamic releasing and inhibiting hormones - produced by hypothalamus; regulate release of hormones by anterior and pituitary
    - Angiotensin II - stimulates thirst; As a hormone causes vasoconstriction and promotes release of aldosterone, which increases the rate of salt and water reabsorption by the kidneys
    - Cholecystokinin (CCK) - found in brain and small-intestine; As a hormone, regulates pancreatic enzyme secretion during digestion, and contraction of smooth muscle in gastrointestinal tract
    - Neuropeptide Y - stimulates food intake

Neurotransmitters can be modified by stimulating/inhibiting synthesis; blocking/enhancing neurotransmitter release; stimulating/inhibiting neurotransmitter removal; or blocking/activating the receptor site

54
Q

The CNS contains billions of neurons organized into complicated networks called Neural Circuits, there are 4 types

A
  1. Simple circuit - a presynaptic neuron stimulates a single postsynaptic neuron
  2. Diverging Circuit - a nerve impulse from a single presynaptic neuron causes the stimulation of increasing number of cells along the circuit
  3. Reverberating Circuit - a presynaptic cell cell causes the postsynaptic cell to transmit a series of nerve impulses (1-2-3…)
  4. Converging Circuit - the postsynaptic neuron receives nerve impulses from several different sources
  5. Parallel after-discharge circuit - A single presynaptic cell stimulates a group of neurons, each of which stimulates with a common postsynaptic cell
55
Q

Regeneration and Repair of Nervous Tissue

A
  • Plasticity - the capability to change based on experience
  • Regeneration - the capability to replicate or repair themselves; most mammalian lack this capability
  • A severed axon cannot be repaired or regrown

Neurogenesis in the CNS
- Neurogenesis -the birth of new neurons from undifferentiated stem cells occurs regularly in some animals. Current research is being done to find ways to promote neurogenesis and to encourage/promote regrowth in the CNS

Damage and Repair

  • Chromatolysis - nissil bodies break up into fine granular masses after about 1-2 days after injury to a process of normal peripheral neurons
  • Wallerian degeneration - degeneration of the distal portion of the axon and myelin sheath
  • Regeneration tube

Anatomy and Physiology (23 decks)

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