Chapter 12: Nervous Tissue

Question 1

What are the two main subdivisions of the nervous system?

Answer 1

CNS - consists of brain and spinal cord

Peripheral nervous system (SNS, ANS, ENS) - consists of all nervous tissue outside the CNS (nerves, ganglia, enteric plexuses, sensory receptors

Question 2

Define nerve

Answer 2

Bundle of hundreds-thousands axons plus associated connective tissue and blood cells (PNS)

Question 3

How many cranial and spinal nerves are there?

Answer 3

12 pairs of cranial nerves energy from the brain and 31 pairs of spinal nerves emerge from the spinal cord

Each nerve follows a defined path and serves a specific regions of the body

Question 4

Define ganglion

Answer 4

Ganglia are small masses of nervous tissue, consisting primarily of neuron cell bodes (PNS)

Question 5

Define enteric plexuses

Answer 5

Extensive networks of neurones located in the walls of organs of the GI tract. The neurons within them help regulate the digestive system

Question 6

Define sensory receptors

Answer 6

Monitors changes in the external or internal environment (touch, olfactory, photoreceptors, etc.)

Question 7

What are the subdivisions of the PNS?

Answer 7

Somatic nervous system (SNS) consists of

1. sensory neurons that convey info to SNS from somatic receptors for special senses
2. motor neurons that conduct impulses from CNS to skeletal muscles only (voluntary)

Automatic nervous system (ANS) consists of

1. sensory neurons that convey info to SNS from automatic receptors mainly in visceral organs
2. motor neurons that conduct nerve impulses from CNS to smooth & cardiac muscle, and glands (involuntary) - consists of 2 subdivisions (SD & PSD)

Enteric nervous system (ENS) is the ‘brain of the gut’ (involuntary)
- monitor changes in GI tract as well as stretching of its walls. governing contractions of smooth muscles, secretions, and activities of CI endocrine cells (hormones)

Question 8

What are the subdivisions of the motor part of the automatic nervous system?

Answer 8

Sympathetic - fight or flight

Parasympathetic - rest and digest

Question 9

What are the 3 basic functions of the nervous system?

Answer 9

Sensory function - detect internal stimuli and inform the brain and spinal cord

Integrative function - processes sensory info and makes decisions for appropriate responses

Motor function - elicited appropriate motor responses by activating effectors through cranial and spinal nerves

Question 10

What does a neuronal cell body contain?

Answer 10

• Nucleus surrounded by cytoplasm that includes typical cellular organelles
• Nissil bodies - free ribosomes and prominent clusters of rough ER

Question 11

What is the structure of a dendrite?

Answer 11

Usually short, tapering, and highly branched

Question 12

Define axon hillock

Answer 12

A cone shaped elevation of the cell body where the axon meets

The part of the axon closet to the axon hillock is the initial segment

Question 13

Define trigger zone

Answer 13

The junction of the axon hillock and initial segment where nerve impulses arise and from which they travel from

Question 14

Protein synthesis occurs in an axon

True or false

Answer 14

False - there is no rough ER present

Question 15

Define axon collaterals

Answer 15

Side branches from the axon, typically occur at a right angle which end by dividing into many fine processes called axon terminals

Question 16

Define varicosities

Answer 16

A variation of axon terminals (other than synaptic end bulbs) that are a strong of swollen lumps

Question 17

What are slow and fast axonal transport?

Answer 17

These are transport systems that carry material from the cell body the the axon thermals and back

Slow - moves materals about 1-5 mm per day (unidirectional towards axon terminal), supplying new axoplasm (cytoplasm) to developing or regenerating axons and replenishes axoplasm in growing and mature axons

Fast - moves materials 200-400mm per day, using proteins that function as motors to move materials along the surfaces of microtubules of the cytoskeleton (bi-directional), supplying organelles/synaptic vesicles, and taking vesicles to be recycled

Question 18

What are the 3 structural classifications of neurons?

Answer 18

Multipolar neuron - have several dendrites and one axon - most common in the brain and spinal cord and motor neurons

Bipolar neurons - one main dendrite and one axon - found in the eye, inner ear, and olfactory area

Unipolar neuron - dendrites and one axon that are fused together to form a continuous process that emerges from the cell body, the begin as bipolar neurons in the embryo and fuse to become a single process functioning as sensory receptors

Question 19

What are the 3 functional classifications of neuronal cells?

Answer 19

Sensory (afferent) - either contain sensory receptors at distal ends or are located just after sensory receptors that carries informaition into the CNS (usually unipolar)

Motor (efferent) - convey AP away from CNS to effectors in the PNS through cranial or spinal nerves (usually multipolar)

Interneurons - mainly located within SNS between sensory and motor neurons the process incoming sensory info and elicit a motor response (usually multipolar)

Question 20

Define neuroglia

Answer 20

Make up ~1/2 the volume of CNS that actively participate in the activities of nervous tissue

Smaller than neurons and do not generate or propagate AP and they can multiply and divide

They fill spaces where neurons were in times of injury

Question 21

List the 2 types of neuroglia located in the PNS

Answer 21

Schawann cells

Satellite cells

Question 22

List the 4 types of neuroglia located in the CNS

Answer 22

Astrocytes
Oligodendrocytes
Microglia
Ependymal cells

Question 23

Describe astrocytes and their function

Answer 23

Star shaped cells that are the largest and most numerous type of neuroglia

Makes contact with blood capillaries, neurons and pie matter, functioning to:

• support neurons with microfilaments
• wrap capillaries to isolate them from harmful substances and secretes chemicals to create a BBB
• secrete chemicals that regulate growth, migration, and interconnection in embryos
• maintain appropriate environment for nerve impulses
• play a role in learning/memory by influencing formation of neural synapses

Question 24

Describe oligodendrocytes and their function

Answer 24

resemble astrocytes but are smaller and fewer processes

Processes are responsible for forming and motioning the metering sheath

Question 25

Describe microglia and their function

Answer 25

Small cells with slender processes that give off numerous spine-like projections which function as phagocytes

Question 26

Describe ependymal cells and their function

Answer 26

Cuboidal to columnar cells arranges in a single layer that passes microvilli and cilia, lining the ventricles and central canal if spinal cord process, monitors, and assist in circulation of CSF

Question 27

Describe schwann cells and their function

Answer 27

encircle PNS axons forming the myelin sheath also participate in axon regeneration, which is more easily accomplished in the PNS

Question 28

Describe satellite cells and their function

Answer 28

flat cells surround cell bodies of neurons of PNS ganglia , providing structural supporting regulates the exchanges of materials between neuronal cell bodies and ISF

Question 29

How is the myelin sheath formed in the PNS?

Answer 29

Each schwann cell wraps about 1mm of a single axons length by spiring 100 times around the axon to create the myelin sheath

Question 30

Define neurlemma

Answer 30

The outer nucleated cytoplasmic layer of the schwann cell, which encloses the myeline sheath When injured, the neurolemma aids in regeneration by forming a regeneration tube that guides and stimulates growth of the axon

Question 31

Define nodes of Ranvier

Answer 31

Gaps in the myelin sheath Found in both PNS and CNS (just fewer)

Question 32

How is the myelin sheath formed in the CNS?

Answer 32

oligodendrocytes put forth about 15 broad, flat processes that spiral around CNS axons, forming a sheath

Question 33

Axons in the CNS display little regrowth after injury True or False

Answer 33

True - thought to be due in part to the absence of a neurolemma

Question 34

A childs myelination is the same from childhood to adulthood true or false

Answer 34

False - myelination increases as we age

Question 35

What is the difference between a ganglion and a nucleus?

Answer 35

Ganglion - cluster of neuronal cell bodies in PNS Nucleus - cluster of neuronal cell bodies in CNS

Question 36

What is the differences between a tract and a nerve?

Answer 36

Tract - bundle of axons located in CNS Nerve - bundle of axons located in PNS

Question 37

White vs. Gray matter

Answer 37

White matter is composed of primarily myelinated axons Gray matter contains neuronal cell bodies, dendrites, unmyelinated tons, axon terminal, and neuroglia In the spinal cord, white matter surrounds of inner core of gray patter that looks like and H

Question 38

Graded potentials vs. action potentials

Answer 38

Graded potentials are used for short-distance communication only ex. develops in a sensory receptors in the fingers Action potentials allow for communication over long distances ex.travels along axon into the CNS and causes release of NT at a synapse with an interneuron

Question 39

muscle AP vs nerve AP

Answer 39

Muscle AP is an AP in a muscle fiber Nerve AP occurs in a neuron (nerve cell)

Question 40

Define membrane potential

Answer 40

Electrical potential differences across the membrane In excitable cells, this voltage is termed the resting membrane potential The flow of ions along the membrane due to the potential is termed current

Question 41

Define electrochemical gradient

Answer 41

When ion channels are open, they allow specific ions to move across the membrane down their electrochemical gradient - a concentration (chemical) difference plus an electrical difference

Question 42

Define leak channels

Answer 42

Gates of leak channels randomly alternate between open and closed positions

Question 43

Define ligand-gated channel

Answer 43

Gates open and close in response to binding of a ligand (chemical) stimulus

Question 44

Define mechanically-gated channels

Answer 44

Gates open or closes in response to mechanical stimulation in the form of vibration, touch, pressure, or tissue stretching

Question 45

Define voltage-gated channel

Answer 45

Gate opens in response to a change in membrane potential These participate in the generation of action potentials in the axons of all types of neurons

Question 46

Define polarized cell

Answer 46

A cell that exhibits a membrane potential

Question 47

What are the 3 factors that contribute to the resting membrane potential

Answer 47

Resting potential - equal negative ions along the inner membrane as there is positive ions along the outer membrane 1. Unequal distributions of ions in ECF and cytosol - since the membrane has more K+ leak channels than Na+ leak channels, K+ ions leave the cell faster than the Na+ ions enter the cell, making the inner more negative and the outer more positive 2. Inability of most anions to leave cell - trapped anions cannot follow K+ out of the cell as they are attached to nondiffusable molecules such as ATP 3. Electrogenic nature of the Na+-K+ ATPases - expels 3 Na+ ions for every 2+ ions imported contributing to the negativity f the resting potential

Question 48

Define graded potential

Answer 48

Small deviation from the resting membrane potential that makes the membrane either more polarized (inside more neg) or less polarized (inside less neg) When the response makes the membrane more polarized it is termed hyper polarizing graded potential When the response makes the membrane less polarized it is termed depolarizing graded potential

Question 49

When do graded potentials occur?

Answer 49

When a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cells membrane they mainly occur in the dendrites and cell body of a neuron as this is where these channels are abundant

Question 50

Define decremental conduction

Answer 50

Graded potentials die out as the stream along the membrane as the charges are lost across the membrane through leak channels

Question 51

Define summation

Answer 51

Graded potentials add together, become stronger and lasting longer The result is a larger depolarizing graded potential

Question 52

What is the neuronal threshold?

Answer 52

An action potential occurs in the membrane of a neuron when depolarization reached approx -55mV

Question 53

# Define: Subthreshold stimulus Threshold stimulus Suprathreshold stimulus

Answer 53

Subthreshold stimulus: a weak depolarization that cannot bring the membrane potential to threshold Threshold stimulus: a stimulus that is just strong enough to depolarize the membrane to threshold Suprathreshold stimulus: a stimulus that is strong enough to depolarize the membrane above threshold

Question 54

Describe the all-or-none principle

Answer 54

Once threshold is reached an action potential is generated The stimulus does influence the amplitude of the AP, but if it is depolarized above threshold, it will fire more frequently

Question 55

Describe the following stages in an action potential: Resting phase Depolarizing phase Repolarizing phase After hyperpolarizing phase

Answer 55

Resting - All Na+ and K+ channels are closed and there is small buildup of negative charges along the inside of the cell and positive charges along the outside of the cell Depolarizing - membrane potential reaches threshold, Na+ channels open, moving into the neuron, creating a positive charge inside the cell causing the membrane to depolarize Repolarizing - Na+ channels close and K+ channels open and membrane begins to depolarize as K+ ions leave the neurone and negative charges buildup along inside the cell After hyper polarizing - As K+ ion channels remain one and the membrane potential become even more negative (-90 mV) then at rest. The ion channels then close and membrane potential returns to resting of -70mV

Question 56

What is a refractory period? Differentiate between absolute and relative refractory periods

Answer 56

Refractory period - cell cannot generate another AP in response to a normal threshold stimulus Absolute - Even strong stimuli cannot initiate an AP as the Na+ channels are inactivated and cannot reopen before returning to the resting states Relative - a second AP can be initiated by a larger than normal stimulus as K+ channels are still open but Na+ channels have reached their resting state

Question 57

Define propagation

Answer 57

APs keep their strength as it spreads along the membrane

Question 58

Define continious and saltatory conduction

Answer 58

Continuous: step-by step depolarization and depolarization of each adjacent segment of the plasma membrane (occurs in unmyelinated axons and muscle fibres) Saltatory: special mode of AP propagation that occurs along myelinated axons, because of the uneven distribution of voltage-gated channels (jumping from nodes of ranvier)

Question 59

What 3 major factors affect the speed of propagation?

Answer 59

1. amount of myelination - APs propagate more rapidly in myelinated axons 2. Axon diameter - larger diameter axons propagate APs faster due to large surface area 3. Temperature - warmer axons propagate faster

Question 60

Differentiate between the following nerve fibers: A fibers B fibers C fibers

Answer 60

A fibers: largest diameter, myelinated, have a brief absolute refractory period B fibers: myelinated, exhibit saltatory conduction, have a somewhat larger absolute refractory period than A fibers C fibers: smalled diameter, unmyelinated, have the longest absolute refractory period

Question 61

How can your sensory systems detect stimuli of differing intensities if all nerve impulses are the same size?

Answer 61

Frequency of action potentials - the greater intensity of stimuli, the more frequent the APS Total recruited neurons - the greater intensity of stimuli, the more sensory neurons activated

Question 62

Compare graded potentials and action potentials on the following characteristics: ``` Origin Channels Conduction Amplitude Duration Polarity Refractory period ```

Answer 62

Origin: GP (dendrites/cell body) AP (trigger zones) Channels: GP (ligand or mechanically gates) AP (voltage for Na+ and K+) Conduction: GP (decremental, short distance) AP (propagate, long distance) Amplitude: GP (1-50 mV depending on stimulus) AP (all or none ~100mV) Duration: GP (longer) AP (shorter) Polarity: GP (hyper polarizing or depolarizing) AP (depolarizing, followed by re-polarizing, and then resting phase) Refractory period: GP (not present, summation can occur), AP (present, summation cannot occur)

Question 63

Define axodendritic and axoaxonic synapses

Answer 63

``` Axodendritic = from axon to dendrite Axoaxonic = from axon to axon ```

Question 64

How are APs conducted at electrical synapses?

Answer 64

APS conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions, which act as tunnels to connect the cytosol of the two cells directly This allows for faster communication and synchronization of a group of neurons/muscle fibres

Question 65

How are APs conducted at chemical synapses?

Answer 65

Membranes of pre- and post-synaptic neurones in a chemical synapse are separated by a synaptic cleft The presynaptic nuron releases NTs that diffuses and binds to the post synaptic neuron, creating a postsynaptic potential (graded potential) It turns an electrical signal into a chemical signal that is received by postsynaptic neuron and turned back into an electrical signal

Question 66

Define synaptic delay

Answer 66

The time requires for processes at a chemical signal is about 0.5msec and is the reason chemical synapses are slower than electrical synapses

Question 67

Describe how a typical chemical synapse transmits a signal (7 steps)

Answer 67

1. AP arrives an synaptic end bulb of presynaptic axon 2. Depolarizing phase of AP causes an influx of Ca2+ ions 3. Increased Ca2+ in the neuron triggers exocytosis of synaptic vesicles 4. Nts diffuse across cleft and bind with postsynaptic receptors 5. Binding opens the channels and allows particular ions to flow across membrane 6. Ion influx changes the voltage acrsss the membrane, termed postsynaptic potential (either depolarizing/EPSP or hyperpolarizing/IPSP) 7. Depolarizing postsynaptic potential can trigger an AP when it reached threshold

Question 68

Describe the two types of neurotransmitter receptors

Answer 68

Ionotropic - contains NT binding site and an ion channel (ligand-gated channel); in absence of the NT the channel is closed (mostly excitatory) Metabotropic - contains NT binding site but lacks ion channel but is coupled to a separate ion channel called a G protein; when a NT binds, the G protein opens (or closes) the channel or indirectly activates second messenger which opens or closes it (mostly inhibitory)

Question 69

How can NTs be removed from the synaptic cleft?

Answer 69

Diffusion - diffuse away where it can no longer exert an effect Enzymatic degradation Uptake - NT taken back into presynaptic cell (reuptake), or transported into neighbouring neuroglia (uptake)

Question 70

Distinguish between spatial and temporal summation

Answer 70

Spatial - 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 of postsynaptic potentials in response to stimuli that occur in the same locations in the membrane of a postsynaptic cell at different times

Question 71

Define neursecretory cells

Answer 71

certain neurons in the brain that also secrete hormones

Question 72

Define acetylcholine NT

Answer 72

small molecule NT released by many PNS neurons and by some CNS neurons Excitatory when minded with ionotropic receptors but inhibitory when in it binds to metabotropic receptors

Question 73

Describe the functions of the following amino acids: Glutamate GABA & glycine

Answer 73

Glutamate (excitatory) - most excitatory neurons in CNS and 1/2 in PNS communicate via glutamate; binds to ionotropic to cause influx of Na+ causing a EPSP GABA & glycine (inhibitory) - GABA is found only in the CNS; binding at ionotropic receptors opens Cl- channels

Question 74

Describe the functions of the following biogenic amines: norepinephrine epinephrine dopamine serotonin

Answer 74

NE - arousal, dreaming, and regulating mood; serves also as a hormone epinephrine - also serves as a hormone dopamine - emotions, addictions, and pleasurable experiences; regulates skeletal muscle tone and some aspects of movement serotonin - concentrated in raphe nucleus and involved in perception, temp regulation, control of mood, appetite, and induction of sleep

Question 75

Define catecholamines

Answer 75

Have an amino group and a catechol ring composed of 6 carbons and 2 adjacent hydroxyl groups They are synthesizes from the amino acid tyrosine and inactivation occurs via reuptake where they are recycled or destroyed Ex. DA, NE, Epi

Question 75

Define catecholamines

Answer 75

Have an amino group and a catechol ring composed of 6 carbons and 2 adjacent hydroxyl groups They are synthesizes from the amino acid tyrosine and inactivation occurs via reuptake where they are recycled or destroyed Ex. DA, NE, Epi

Question 76

Describe the function of nitric oxide and carbon monoxide

Answer 76

NO - an important excitatory NT in brian, spinal cord, adrenal garden, & neves to penis; formed on demand & acts immediately & briefly as it is a highly reactive free radical CO - excitatory Nt produced in brain; formed on demand as needed; might protect against excess neuronal activity

Question 77

Describe the the following neuropeptides: Enkephalins Endorphins

Answer 77

Enkephalins - inhibit pain impulses by suppressing release of substance p Endorphins - inhibit pain by blocking release of substance P Both may have role in memory and learning, body temp control, sexual activity and mental illness * use the same receptors as opioids * substance P is released by neurons that transmit pain related input from peripheral pain receptors

Question 78

Describe the various types of neural circuits in the nervous system: ``` Simple series circuit Diverging circuit Converging circuit Reverberating circuit Parallel after-discharge circuit ```

Answer 78

A circuit is a functional group of neurons that process specific types of information Simple series - a single presynaptic neuron stimulates a single postsynaptic neuron, and so on Diverging circuit - a single presynaptic neuron stimulates several post synaptic neurones at the same time Converging circuit - several presynaptic neurons stimulates one postsynaptic neuron Reverberating circuit - incoming impulse stimulates the 1st neuron, which stimulate the 2nd, which stimulates the 3rd, and so on. Branches from later neurons synapse with earlier one sending impulses back through the circuit again and again Parallel after-discharge circuit - a single presynaptic neuron stimulates a group of neurons, each which synapses with a common postsynaptic cell

Question 79

Describe plasticity and neurogenesis

Answer 79

Plasticity is the capability to change based on experience which can include sprouting of new dendrites, protein synthesis, changes in synaptic connections Regeneration is the capability to replicate or repair themselves *Despite plasticity, in the CNS little or no repair of damage to neurons occur

Question 80

Define neurogenesis

Answer 80

The birth of new neurons for undifferentiated stem cells There are neurogenesis is some areas of the brain (hippocampus) but not present in other regions and the spinal cod due to: - inhibitory influences from neuroglia - absence of growth-stimulating cues that were present during fetal development

Question 81

What is the role of the neurolemma in regeneration?

Answer 81

Axons and dendrites that are associated with a neurolemma in the PNS may undergo repair if the cell body is intact, the Schwann cells are functional, and scar tissue formation does not occur too rapidly. Chromatoylsis: 24-48 hrs after injury the Nissil bodies break up into granular masses Wallerian degeneration: 3-5 days after the part of the axon distal to the damaged region becomes swollen and breaks up into fragments; the myelin sheath also deteriorates, however, the neurolemma remains The Schwann cells on either side multiple and grow toward each other forming a regeneration tube across the injured area

Question 82

Briefly outline the following disorders: ``` Multiple sclerosis (MS) Epilepsy ```

Answer 82

MS - a progressive destruction of myelin sheaths surrounding neurons in the CNS which slows and then short-circuits propagation of nerve impulses Epilepsy - short, recurrent attacks of motor, sensory, or psychological malfunction, initiated by abnormal, synchronous electrical discharges from millions of neurons in the brain, perhaps from abnormal reverberating circuits