Chapter 12: Nervous Tissue

Question 1

How are the endocrine system and the nervous system different?

Answer 1

The endocrine system communicates by chemical messenger (hormones). The nervous system communicates by employing electrical and chemical means to send messages from cell to cell.

Question 2

What is the 4 step function of the nervous system?

Answer 2

• Sense organs receive information about changes in the body
• They transmit messages to the brain and spinal cord (CNS)
• CNS processes this information and relates it to past experiences and determines its response
• CNS issues commands to muscles and gland cells to carry out a response

Question 3

What are afferent and efferent neurons? What fibers are they made of?

Answer 3

• Afferent neurons are sensory neurons (they lead into the CNS)
  
  • the somatic fibers come from skin, skeletal muscle, and joints
  • visceral fibers from internal organs
• Efferent neurons are motor neurons (they lead out of the CNS)
  
  • somatic fibers to skeletal muscles (somatic NS)
  • visceral fibers to smooth muscls, cardiac muscle, or glands (Autonomic NS)
• Remember, neurons are not the SAME

Question 4

What are the two subdivisions of the nervous system and their basic components?

Answer 4

• Central nervous system
  
  • Brain and spinal cord
• Peripheral Nervous System
  
  • rest of nervous system, composed of nerves and ganglia
    
    • Nerve-bundle of nerve fibers (axons) wrapped in fibrous connective tissue
    • Ganglion: knotlike swelling in a nerve where neuron cell bodies are concentrated

Question 5

Describe the motor division of the PNS

Answer 5

• Somatic (voluntary) nervous system
  
  • motor neurons to skeletal muscle tissue
  • only 1 motor neuron is used
  • somatic reflexes: involuntary muscle contractions
• Autonomic (involuntary) nervous system
  
  • motor neurons to smooth muscle and cardiac muscle, exocrine glands and endocrine glands
  • 2 motor neurons used
  • autonomic/visceral reflexes-involuntary responses

Question 6

What are the 2 principal cell types of the nervous system?

Answer 6

• Nerves
  
  • excitable cells that transmir electrical signals
  • functional units of the nervous system
• Neuroglia
  
  • supporting cells

Question 7

What are the universal properties of neurons?

Answer 7

• Excitability (irritability)
  
  • response to environmental changes called stimuli
  • produce an electrical signal
• Conductivity
  
  • conduct the electrical signal to other cells
• Secretion
  
  • when an electrical signal reaches the end of nerve fiber, secretes a neurotransmitter that influences the next cell

Question 8

What are the functional classes of neurons?

Answer 8

• Sensory (afferent) neurons
  
  • detect stimuli and transmit information to CNS
• Interneurons (association neurons)
  
  • connects motor and sensory pathways to CNS
  • makes decisions (integrating center)
  • about 90% of all neurons
• Motor (efferent) neuron
  
  • send signals out to muscles and gland cells (the effectors)

Question 9

What is the structure of a neuron (and the functions of its structures)?

Answer 9

• Soma
  
  • control center of neuron
  • nucleus with one nucleolus
  • no centrioles
  • extreme longevity
• Dendrites
  
  • branches that come off the soma
  • receives signals from other neurons
  • the more dendrites, the more info can be received
• Axon (nerve fiber)
  
  • transmits signals away from soma
  • only one (or none)
  • mostly unbranched
  • may be enclosed in myelin sheath
  • axoplasm = cytoplasm
  • axolemma = cell membrane
• Distal end has terminal arborization
• Synaptic knob (terminal button)
  
  • contains synaptic vesicles full of neurotransmitters

Question 10

What are the structural types of neurons?

Answer 10

• Multipolar neuron
  
  • one axon and multiple dendrites
  • most common (most neurons in CNS)
• Bipolar neuron
  
  • one axon and one dendrite
  • olfactory cells, retina, inner ear
• Unipolar neuron
  
  • single process leading away from the soma
  • sensory cells from skin and organs to spinal cord
• Anaxonic neuron
  
  • many dendrites but no axon
  • retina, brain, and adrenal gland

Question 11

What is axonal transport?

Answer 11

• A process that transports proteins to the axon or axon terminal to repair axolemma or transport organelles
• Two way passage
  
  • anterograde transport: movement down the axon away from the soma
  • retrograde transport: movement up the axon toward the soma
• Uses microtubules

Question 12

What are the numbers and functions of neuroglia cells?

Answer 12

• There are about 1 trillion in the nervous system
  
  • outnumber neurons by at least 10 to 1
• Protect neurons and help them function
• Bind neurons together and form framework for nervous tissue
• In fetus, guide migrating neurons to their destination
• If mature neuron is not in synaptic contact with another neuron, it is covered with glial cells

Question 13

What are the 4 types of neuroglia in the CNS?

Answer 13

• Ogliodendrocytes
  
  • form myelin sheaths in CNS
• Ependymal Cells
  
  • line internal cavities of the brain
  • secrete and circulate cerebrospinal fluid
• Microglia
  
  • WBCs that wander through CNS looking for debris and damage
• Astrocytes
  
  • most abundant glial cell in CNS
  • covers brain surface and most nonsynaptic regions of neurons in the gray matter (framework)
  • forms blood/brain barrier
  • convert glucose to lactate for neurons
  • secrete nerve growth factors
  • communicate electrically with neurons
  • absorb excess neurotransmitters and ions

Question 14

What is astrocytosis/ sclerosis?

Answer 14

When neuron is damaged, astrocytes form hardened scar tissues and fill in space.

Question 15

What are the two types of neuroglia cells in the PNS?

Answer 15

• Schwann cells
  
  • produce a myelin sheath around axons in PNS
  • Assist in regeneration of damaged fibers
    
    • axon ONLY can regenerate
• Satellite cells
  
  • surround / protect the neurosomas in ganglia of the PNS
  • provide electrical insulation around the soma
  • regulate the chemical environment of the neurons

Question 16

What is myelin sheath / myelination?

Answer 16

• Insulation around a nerve fiber (axon)
  
  • formed by ogliodendrocytes in CNS and Schwann cells in PNS
  • Consists of the plasma membrane of glial cells
    
    • 20% protein and 80% lipid
• Myelination: the production of myelin sheath
  
  • begins at 14 weeks of fetal development
  • proceeds rapidly during infancy
  • completed in late adolesence
  • dietary fat is important to CNS development

Question 17

What is myelin sheath in the PNS v. CNS?

Answer 17

• In the PNS
  
  • entire schwann cells spirals repeatedly around single nerve fiber
    
    • one hundred layers of membrane
    • no cytoplasm between membranes
  • Neurilemma: outermost coil of myelin sheath
    
    • contains nucleus and most of its cytoplasm
    • external to neurilemma is basal lamina and endoneurium
• In the CNS
  
  • an ogliodendrocyte myelinates several nerve fibers in its immediate vicinity
    
    • does not migrate around like Schwann cells
    • must push new layers of myelin under older ones, so spirals in toward nerve fiber
    • no neurilemma or endoneurium

Question 18

What is the structure of myelin?

Answer 18

• Myelin sheath is segmented
  
  • Nodes of ranvier: gaps between segments
  • Internodes: myelin covered segments
  • Initial segment: section between the axon hillock and the first glial cell
  • Trigger zone: the axon hillock and the initial segment
    
    • important in initating nerve signal

Question 19

What are brain tumors in relation to glial cells?

Answer 19

• Tumors are masses of rapidly dividing cells
  
  • in the nervous sytem, come from meninges, metastasis from other tumors (e.g. melanoma), and glial cells
• Gliomas grow rapidly and are highly malignant
  
  • Blood brain barrier decreases effectiveness of chemotherapy
  • treatment consists of radiation or surgery

Question 20

How do nerve fibers regenerate?

Answer 20

• Only the peripheral nerve fiber can regenerate and only if the soma is intact
• Steps:
  
  • fiber distal to injury degenerates
  • soma swells, ER breaks up
  • Axon stump spouts multiple growth processes
  • Schwann cells, basal lamina, and neurilemma spout regeneration tube
  • Once contact is reestablished, soma shrinks
    
    • nucleus returns to normal shape
    • atrophied muscle fibers grow

Question 21

What are the diseases of myelin sheath?

Answer 21

• Multiple sclerosis
  
  • ogliodendrocytes and myelin sheath in CNS deteriorate
  • Myelin replaced by hardened scar tissue (“sclerosis”)
  • Nerve conduction disrupted
    
    • ^{(double vision, tremors, numbness, speech defects)}
  • Onset between 20 and 40 years
  • Cause my be autoimmune
• Tay-Sachs disease
  
  • Hereditary disorder of Easter European infants
  • Abnormal accumulation of ganglioside disrupts conduction of nerve signals
    
    • blindness, loss of coordination, and dementia
  • fatal before age 4

Question 22

What is the conduction speed of nerve fibers?

Answer 22

• Speed at which a nerve signal travels along the surface of a nerve fiber
• Depends on two factors:
  
  • Diameter of fiber
    
    • larger fibers = more surface area = conduct signals faster
  • Presence or absence of myelin
    
    • myelin further speeds signal conduction
• Examples: slow signals are sent to gastrointestinal tract where speed is less of an issue
• Fast signals sent to skeletal muscles where speed improves balance and coordinated body movement

Question 23

What are the problems with nerve fiber generation?

Answer 23

• It takes up to 2 years
• some nerve fiber connect with wrong muscle fibers and some die
• Regeneration of damaged fibers in CNS does NOT happen

Question 24

Define the following: electric potential and electric current

Answer 24

• Electric potential: a difference in concentration of charged particles between one point and another
• Electrical current: a flow of charged particles from one point and another
  
  • in the body, currents are movements of ions
  • gated channels are opened or closed by various stimuli
  • enables cells to turn electrical currents on and off

Question 25

What is Resting Membrane Potential?

Answer 25

* Cell is positive outside the membrane and negative inside the membrane * potential energy difference at rest is -70 mV * cell is polarized * Unequal distribution of electrolytes between ECF and ICF * High concentration of Na outside and K inside * potassium has the greatest influence on RMP

Question 26

How do potassium and sodium affect RMP?

Answer 26

* Potassium has the greatest influence on RMP * highly permeable * leaks out until electrical charge of cytoplasmic ions attract it back in and equilibrium is reached * more concentrated in ICF * Sodium has some influence on RMP * not very permeable * more concentrated in ECF * some sodium leaks into cell (diffusion) * Sodium leakage makes RMP slightly less negative * Na+/K+ pump moves out 3 sodium for every 2 potassium * this exchange contributes about -3mV to RMP of -70 mV

Question 27

What are the 3 steps of changes in membrane potential?

Answer 27

* Depolarization (less negative) * Repolarization (returns to RMP) * Hyperpolarization (more negative) * will inhibit electrical impulse

Question 28

What are local (graded) potentials?

Answer 28

* short distances (die out quickly) * on dendrites and cell bodies * sodium gates open in response to chemicals, light, heat, or mechanical stimulation * size of signal depends on stimulus strength * membrane will either: * depolarize (leads to action potential) * hyperpolarize (prevents an action potential) * keeps you from twitching all the time

Question 29

What are the steps of an action potential?

Answer 29

* An action potential is a rapid up and down shift in the membrane voltage involving a sequence of steps * arrival of an axon hillock depolarizes membrane * depoloarization much reach threshold: critical voltage (-55 mV) to open voltage regulated gates * voltage gated sodium channels open * sodium enters and depolarizes cell * opens more channels (positive feedback) * as membrane potential rises above 0mV, sodium channels are inactivated and close * voltage peaks at +35mV * slow potassium channels open and outflow of K polarizes cell * Cell hyperpolarizes * RMP is restored as sodium leaks in and potassium is removed by astrocytes *

Question 30

What are the characteristics of an action potential?

Answer 30

* often called a "spike" (happens so fast) * only a think layer of cytoplasm next to cell membrane is affected * small amount of ions involved * follows an all-or-none law * threshold is or is not reached * Nondecremental (do not get weaker with distance) * Irreversible (cannot be stopped one started)

Question 31

What is the refractory period?

Answer 31

* The period of resistance to stimulation * During an action potential and for a few milliseconds after it is impossible to stimulate that region of a neuron to fire again * Absolute refractory period: cannot produce another AP * Relative refractory: can produce another AP but only with a superstimulus

Question 32

How do local anesthetics work?

Answer 32

* Novocaine and lidocaine * Nerve impulses cannot pass the anesthetized region * prevent voltage-gated sodium channels from opening

Question 33

What causes intesity of sensation with action potential?

Answer 33

* frequency of stimulus * number of neurons

Question 34

Compare action potentials in the nerve v. the muscle

Answer 34

* Location * nerve: only on the axon * muscle: entire sarcolemma * Resting Membrane Potential * nerve: -70 mV * muscle: close to -90mV * Duration * nerve impulse: 1/2 to 2 msec * skeletal musle: 1-5 sec * Cardiac and smooth muscle: 10-300 msec * Speed * fastest nerve impulse is 18 times faster than a muscle fiber

Question 35

What is continuous conduction in nerve tissue?

Answer 35

* Unmyelinated fibers have voltage gated channels along their entire length * Produce action potential along entire length of axon * baby steps v. giant steps * chain reaction continues until the nerve signal reaches the end of the axon * Myelinated fibers generate an action potenital in each Node of Ranvier * very fast * 'Saltatory Conduction'

Question 36

What are synapses?

Answer 36

* A presynaptic neuron may synapse with a dendrite, soma, or axon of postsynaptic neuron to form axodendritic, axosomatic, or axoaxonic synapses * A neuron can have an enormous number of synapses * ^{spinal motor neurons covered by about 10,000 synaptic knobs} * ^{In the cerebellum of the brain, one neuron can have as many as 100,000 synapses}

Question 37

What are the types of synapses?

Answer 37

* Electrical * spreads through gap junctions * faster * two-way transmission * can't make decisions * wake up in AM, some emotions * Chemical * one way transmission * from pre-synaptic to post-synaptic neuron * uses neurotransmitters * most thinking

Question 38

What is the structure of a chemical synapse (pre-synaptic v. post-synaptic)?

Answer 38

* Synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter * Postsynaptic neuron membrane contains proteins that function as receptors and ligand-regulated ion gates

Question 39

How do chemical synapses work?

Answer 39

* nerve impulses reach the axonal terminal of a presynaptic neuron and open calcium channels * neurotransmitter is released into synaptic cleft via exocytosis * neurostransmitter crosses synaptic cleft and binds to receptors on postsynaptic neuron * Postsynaptic brain permeability changes * causes excitatory (E-PSP) * post synpatic potential * or inhibitory (IPSP) effect

Question 40

What are the categories of neurotransmitters?

Answer 40

* More than 100 identified * Four major categories: * acetylcholine * in a class by itself * formed by acetic acid and choline * broken down by Acetylcholine Esterase (AChE) * amino acids * monoamines * synthesized from amino acids * broken down by Monoamineoxydase (MAD) * Include the catecholamines * epinephrine * norepinephrine * dopamine * histamine * ATP * serotonin * neuropeptides * chains of 2 to 40 amino acids

Question 41

How are neurotransmitters removed?

Answer 41

* Diffusion * move down concentration gradient away from synapse * Enzymatic degradation * E.g. AChE or MAD * Uptake by neuron or glial cells * neurotransmitter transporters

Question 42

What is neural integration?

Answer 42

* The ability to process, store, and recall information and use it to make decisions * Chemical synapses allow for decision making * brain cells are very well connected allowing for complex integration * ^{trade off: chemical transmission involves synaptic delay that makes info slower than it would be with no synapse}

Question 43

What are post synaptic potentials?

Answer 43

* Different neurotransmitters cause different types of postsynaptic potentials in the cells they bind to * A neurotransmitter might excite some cells and inhibit others, depending on the type of receptors the postsynaptic cells hae * ACh and norepinephrine work this way * ACh excites skeletal muscle but inhibits cardiac muscle because of different ACh receptors

Question 44

Define the following: summation (temporal and spatial)

Answer 44

* Summation: the process of adding up postsynaptic potentials and responding to their net effect * one neuron can receive input from thousands of other neurons * some produce ESPs and some ISPs * neuron's response depends on whether the net input is excitatory or inhibitory * occurs in trigger zone * Temporal summation: occurs when a single synapse generates ESPS so quickly that each is generated before the previous one fades * Spatial summation: occurs when ESPS from several different synapses add up to threshold at an axon hillock

Question 46

Define the following: facilitation and inhibition

Answer 46

* Facilitation: a process in which one neuron enhances the effect of another * Presynaptic inhibition: process in which one presynaptic neuron suppresses another one * opposite of faciliation

Question 47

What is strychnine poisoning?

Answer 47

* In spinal cord, Renshaw cells normally release an inhibitory transmitter (glycine) onto motor neurons preventing excessive muscle contraction * Strychnine binds to and blocks glycine receptors in the spinal cord * Causes a tetanus

Question 48

What is Alzheimer's disease?

Answer 48

* A disease that causes 100,000 deaths a year * affects 11% of population over 65 and 45% by age 85 * memory loss for recent evets, moody, combative, lose ability to talk/walk/ear * show deficiencies of acetylcholine and nerve growth factor (NGF) * Diagnosis confirmed at autopsy * atrophy of gyri folds in cerebral cortex * Neurofibrilliary tangles and senile plaques * formation of B-amyloid protein (from breakdown of plasma membrane) * Treatment * block AChE or NGF (inhibitors) * clear B-amyloid or halt its production (caused serious side effects)

Question 49

What is Parkinson's disease?

Answer 49

* Progressive loss of motor function beginning in 50s or 60s * no recovery * Degeneration of dopamine releasing neurons * Dopamine normally prevents excessive activity in motor centers (basal nuclei) * Involuntary muscle contractions * facial rigidity, slurred speech, pill rolling motion, illegible handwriting, slow gait * Treatment: drugs and physical therapy * dopamine precursor (L-dopa) crosses brain barrier * bad side effects on heart and liver * MAO inhibitor slows neural degeneration * surgical technique to relieve tremors