Chapter 11 Fundamentals of NS and Nervous Tissue

Question 1

Nervous system functions

Answer 1

Master rapid control and communicating system of body

Question 2

Nervous System:

Two major anatomical components

Answer 2

*Central Nervous System (CNS):
brain and spinal cord

*Peripheral Nervous System (PNS): 
all nerves that enter and exit from CNS
ganglia – clusters of nerve cell bodies

Question 3

Nervous System:

Three major physiological divisions

Answer 3

Sensory division:
detects environmental changes (internal and external)

Integrative division:
processes and stores sensory information and decides if output needed

Motor division
generates movements and glandular secretions

Question 4

PNS divided into two functional subdivisions:

• Sensory, or afferent
• Motor, or efferent

Answer 4

Sensory, or afferent – convey impulses into CNS

* somatic sensory nerves – from skin, skeletal muscle and joints
* visceral sensory nerves – from organs

Motor, or efferent – convey impulses from CNS
*somatic motor nerves – to skeletal muscle
*autonomic nervous system (ANS) – involuntary system:
visceral motor nerves – smooth muscle, cardiac muscle, glands
sympathetic and parasympathetic divisions

Question 5

Neuroglia Cells:

Small support cells of nervous system

Answer 5

Astrocytes
Microglial cells 
Ependymal cells
Oligodendrocytes 
Satellite cells 
Schwann cells

Question 6

Neuroglia Cells:

Astrocytes

Answer 6

Astrocytes – in CNS

* make exchanges between capillaries and neurons 
* control chemical environment
* guide migration of young neurons and formation of synapses

Question 7

Neuroglia Cells:

Microglia Cells

Answer 7

Microglial cells – in CNS = resident phagocytic cells

Question 8

Neuroglia Cells:

Ependymal Cells

Answer 8

Ependymal cells – in CNS – line central cavities and spinal cord – cilia help circulate CSF

Question 9

Neuroglia Cells:

Oligodendrocytes

Answer 9

Oligodendrocytes – in CNS – processes wrap around fibers forming myelin sheath

Question 10

Neuroglia Cells:

Satellite Cells

Answer 10

Satellite cells – in PNS – similar function as astrocytes in CNS

Question 11

Neuroglia Cells:

Schwann Cells

Answer 11

Schwann cells – in PNS – surround nerve fibers forming myelin sheath – important in regeneration of damaged fibers

Question 12

Neurons

Answer 12

•	structural units of nervous system
•	have extreme longevity
•	high metabolic rate
•	post-mitotic
•	morphology (shape) varies but contains the same basic components: 
	*Soma or cell body 
	*Processes – extend from cell body
	*Dendrites 
	*Axon

Question 13

Neurons:

Soma or cell body

Answer 13

Soma or cell body – contains nucleus – most located within CNS:
nuclei = clusters of nerve cell bodies in CNS
ganglia = clusters of nerve cell bodies in PNS

Question 14

Neurons:

Processes

Answer 14

Processes – extend from cell body

* tracts = bundles of processes in CNS
* nerves = bundles of processes in PNS

Question 15

Neurons:

Dendrites

Answer 15

Dendrites – receptive or input region – carry information toward cell body

Question 16

Neurons:

Axon

Answer 16

Axon - one per neuron - carries action potential to other nerve cells or to effectors
**axon hillock initial section of axon where axon leaves cell body
• usual site where action potential generated
**axon collaterals – occasional branch off length of axon
**axon terminals - contain neurotransmitter vesicles (chemical packets)that transmit information to other nerve cells at junctions called synapses

Question 17

Neurons:

Morphology types

Answer 17

Multipolar – 99% of neurons – have multiple dendrites plus one axon

Bipolar – one dendrite and one axon from cell body – found in retina of eye and olfactory mucosa

Unipolar – short process from cell body into T-like central and peripheral processes – primary sensory neurons

Question 18

Myelin Sheath

Answer 18

Myelin = lipid material formed from membranes of cells or processes of cells that covers axons

Schwann cells in PNS, oligodendrocytes in CNS

* *Acts as an insulator
* *Nodes of Ranvier = gaps that occur between myelin producing cells and only place where ion exchange can occur  
* *Increases velocity of action potential up to 50x faster than unmyelinated neurons
* *Conserves energy

Question 19

Resting Membrane Potential (RMP)

Answer 19

Voltage = measure of potential energy generated by separated electrical charges
All cells have a negative resting membrane potential
*ICF of the cell is negatively charged with respect to the ECF
Most cells maintain resting membrane potential within a narrow range
**Excitable cells = nerve and muscle cells have a more polarized (negative) RMP than non-excitable cells
(-40 to -90 mV)

Question 20

Factors contributing to Resting Membrane Potential

Answer 20

Differences in ionic distribution and permeability of membrane to various ions
K+ high inside cell, Na+ high outside cell
conductance to K+ large - leakage of out of cells results in negative charge within cells

Large negatively-charge proteins trapped within cell - influence ion distribution

Electrogenic pump (Na+-K+ ATPase)
	pumps 3 Na+ out,  2 K+ in - produces net negative charge within cells

Question 21

Changing RMP

Answer 21

Stimulus alters ion permeability - e.g. opening ligand-gated channel

Causes local reduction in membrane potential:
**Depolarizing stimulus - cell less negative/ less polar, moves membrane potential closer to zero
**Hyperpolarizing stimulus
cell more negative/more polar, moves membrane potential more negatively

Question 22

Changing RMP:

Graded potential

Answer 22

Graded potential = stimulus that produces a local response

**produces non-propagated potential where size of potential decreases exponentially with distance from initiation site

Question 23

Action potential

Answer 23

Action potential – initiated if stimulus large enough & threshold point reached
an “all-or-none” response
**positive feedback occurs and polarity of cell reverses
self-propagating electrical impulse depolarizes adjacent membrane
AP carried along whole length of cell membrane without decrement
**constant amplitude, shape and speed for a given excitable cell type
size and shape differ from one excitable tissue to the next (e.g. neurons vs cardiac muscle)
**AP duration for neurons and skeletal muscle ~ 4 ms

Question 24

Phases of an AP

Answer 24

Phases of an AP
involves changes in conductance of Na+ and K+ ions via voltage-gated channels
**resting state - sodium gates inactive and Na+ not moving
some potassium leak channels are open and K+ moving freely according to its equilibrium
depolarizing stimulus is applied

	**depolarization – local depolarization current open voltage-gated Na+ channels
	threshold point (15 to 30 mV change from RMP) = point of no return - AP initiated
	positive feedback mechanism initiated – triggers opening of critical number of fast activating voltage-gated (fVG) Na+ channels
	Na+ entry pulls the membrane potential towards Na+ equilibrium potential (lasts ~1 ms)
	self-limiting as fVG Na+ channels have automatic  inactivation gates 

**repolarization
fVG Na+ channels closing - rapid decrease in Na+ conductance (entry)
slow activating voltage-gated (sVG) K+ channels open  
increase in K+ conductance - leaves cell 
returns membrane potential toward RMP

**hyperpolarization stage = slower phase of AP
sVG K+ channels slow to close
membrane potential  pulled toward K+ equilibrium potential 
membrane potential more negative than resting levels 
sVG K+ channels close and cellular pumps restore proper ion distribution

Question 25

Phases of an AP: | Resting state

Answer 25

Phases of an AP involves changes in conductance of Na+ and K+ ions via **voltage-gated channels **resting state - sodium gates inactive and Na+ not moving some potassium leak channels are open and K+ moving freely according to its equilibrium depolarizing stimulus is applied

Question 26

Phases of an AP: | Depolarization

Answer 26

``` **depolarization – local depolarization current open voltage-gated Na+ channels threshold point (15 to 30 mV change from RMP) = point of no return - AP initiated positive feedback mechanism initiated – triggers opening of critical number of fast activating voltage-gated (fVG) Na+ channels Na+ entry pulls the membrane potential towards Na+ equilibrium potential (lasts ~1 ms) self-limiting as fVG Na+ channels have automatic inactivation gates ```

Question 27

Phases of an AP: | Repolarization

Answer 27

**repolarization fVG Na+ channels closing - rapid decrease in Na+ conductance (entry) slow activating voltage-gated (sVG) K+ channels open increase in K+ conductance - leaves cell returns membrane potential toward RMP

Question 28

Phases of an AP: | Hyperpolarization

Answer 28

**hyperpolarization stage = slower phase of AP sVG K+ channels slow to close membrane potential pulled toward K+ equilibrium potential membrane potential more negative than resting levels sVG K+ channels close and cellular pumps restore proper ion distribution

Question 29

Refractory Periods

Answer 29

Refractory Periods = minimum time after an action potential has been generated before the membrane can respond again **Absolute refractory period **Relative refractory period

Question 30

Absolute refractory period

Answer 30

Absolute refractory period (~1 ms duration) from beginning of action potential until repolarization is ~2/3 complete attributable to activation state of fast activating voltage-gated sodium channel if all open – can’t open more once closed – have a delay period before they can re-open no stimulus, regardless of strength, can initiate another action potential during this period

Question 31

Relative refractory period

Answer 31

Relative refractory period follows the absolute refractory period and lasts until RMP is re-established requires a stronger stimulus than normally required to reach threshold sufficient number of fast-activating voltage-gated Na+ channels capable of re-opening but must overcome the hyperpolarizing effect of the still open voltage-gated K+ channels

Question 32

Propagation

Answer 32

Propagation: nerve Impulse = wave of action potentials along an axon an action potential is a local response occurring at one specific area on the membrane AP initiated at one point in the membrane triggers APs in neighbouring areas adjacent areas affected by influx of + ions during depolarization self-propagating “domino-like effect”

Question 33

Coding of Stimulus Intensity

Answer 33

Coding of Stimulus Intensity: once generated, AP’s independent of stimulus strength CNS determination of stimulus strength – via **frequency of AP’s –strong stimuli generate more AP/s

Question 34

Conduction Velocity

Answer 34

Conduction Velocity in a neuron, large diameter fibers (axons) have a faster conduction rate due to lower resistance to conduction myelin = lipid material that covers axons: acts as an insulator prevents loss of current and flow of ions between ECF and ICF Nodes of Ranvier = gaps that occur between myelin producing cells and only place where ion exchange can occur AP appears to jump from node-to-node = saltatory conduction increases velocity of action potential up to 50x faster than unmyelinated neurons conserves energy (uses less ATP) Na+-K+ ATPase activity is required to restore RMP since ions only cross membrane at the nodes, less pump activity is required multiple sclerosis (MS) = demyelinating disease

Question 35

Synapse

Answer 35

Synapse: place where information is transmitted from one cell to another = synaptic transmission Electrical synapse: allows direct passage of ions from one cell to another through structure called gap junctions found in cardiac and smooth muscle tissues Chemical synapse: pre-synaptic neuron – at axon terminal converts electrical signal (action potential) to chemical signal chemical signal = neurotransmitter (NT) synaptic cleft = physical gap that neurotransmitter diffuses across post-synaptic membrane – receives NT that binds to specific receptor Types of chemical synapses based on location: axodendritic = axon terminal to dendrite axosomatic = axon terminal to soma axoaxonal = axon terminal to axon

Question 36

Electrical synapse

Answer 36

Electrical synapse: allows direct passage of ions from one cell to another through structure called gap junctions found in cardiac and smooth muscle tissues

Question 37

Chemical synapse

Answer 37

Chemical synapse: pre-synaptic neuron – at axon terminal converts electrical signal (action potential) to chemical signal chemical signal = neurotransmitter (NT) synaptic cleft = physical gap that neurotransmitter diffuses across post-synaptic membrane – receives NT that binds to specific receptor

Question 38

Types of chemical synapse

Answer 38

Types of chemical synapses based on location: axodendritic = axon terminal to dendrite axosomatic = axon terminal to soma axoaxonal = axon terminal to axon

Question 39

Chemical Synapse: | Pre-synaptic Neuron

Answer 39

Presynaptic Neuron arriving action potential causes a depolarization of axon terminal activates voltage-gated Ca++ channels on the plasma membrane **Ca++ influx allows neurotransmitter vesicles to fuse with pre-synaptic membrane exocytosis of neurotransmitter into synaptic cleft pumps remove Ca++ from axon terminal

Question 40

Chemical Synapse: | Synaptic Cleft

Answer 40

Synaptic Cleft neurotransmitter diffuses across cleft = synaptic delay ~0.5msec

Question 41

Chemical Synapse: | Post-synaptic membranes

Answer 41

Post-synaptic membranes **NT binds to receptors initiating signal transduction temporarily opens ion channels producing a graded, non-propagated post-synaptic potential depending on neurotransmitter may be excitatory or inhibitory **excitatory post-synaptic potential (EPSP) depolarizes membrane and increases chance of AP **inhibitory post-synaptic potential (IPSP) hyperpolarizes membrane and decreases chance of AP

Question 42

Chemical Synapse: | NT effects are terminated

Answer 42

NT effects are terminated reuptake of NT by astrocytes or presynaptic terminal – NT destroyed or stored degrading enzymes - degrade NT - associated with post-synaptic membrane or in synaptic cleft diffusion of NT away from synapse

Question 43

Modification of Synaptic Events (Fig. 11.18)

Answer 43

Modification of Synaptic Events: a single EPSP cannot induce an AP in the postsynaptic cell **summation – adding together postsynaptic potentials: EPSPs and IPSPs are electrically equivalent but in opposite direction to get AP in post-synaptic neuron, the changes from more than one stimulus must be added together **temporal summation = summing of post-synaptic potentials arriving in rapid sequence produces a step-wise change in the post-synaptic potential **spatial summation adds all EPSPs + IPSPs arriving at the same time on the post-synaptic neuron sum total of effects of NTs released from more than one pre-synaptic neuron

Question 44

Strength of individual synapses can vary as a function of their use or activity

Answer 44

Strength of individual synapses can vary as a function of their use or activity: Patterns of synaptic activation can change the response to subsequent activation may remain for short (millisecond) or long (min to days) duration may result in potentiation (facilitation) underlie learning and memory

Question 45

Neurotransmitters (NT) (Table 11.3)

Answer 45

Neurotransmitters: over 50 postulated neurotransmitters *Acetylcholine (ACh)- most important low mw NT and found in all motor neurons synthesis enzyme = choline-acetyltransferase substrates = acetyl CoA and choline **degraded in synaptic cleft by acetylcholinesterase (AChE) bound to outside of postsynaptic membrane *Biogenic amines = derived from amino acids catecholamines = tyrosine derivatives include: dopamine, norepinephrine and epinephrine indolamines = serotonin – derived from tryptophan; histamine – derived from histidine broadly distributed throughout most of brain and spinal cord promotes learning and memory controls mood (through activation of reward centers) promotes sleep induction

Question 46

Neurotransmitters (NT)

Answer 46

Amino acids – glutamate, aspartate, glycine, GABA Peptides – substance P, endorphins, enkephalins, dynorphins Purines – ATP, adenosine Gases – nitric oxide (NO) - highly permeant & diffuses out of axon terminal - increases cGMP in target cell Function determined by receptor : *excitatory – cause depolarization glutamate *inhibitory – cause hyperpolarization GABA, glycine *both – depending on receptor present on tissue ACh, NE Action: direct versus indirect *direct – binding of NT directly gates ion channel producing rapid reponse **inotropic receptors (Fig. 11.19) *indirect – binding of NT promote broader, longer-lasting effects by acting through intracellular second messengers **metabotropic receptors (Fig. 11.20) = G-protein linked receptors NT binds receptor - G-protein becomes activated activate/inactivate different enzymes which regulate 2nd messengers 2nd messengers (cAMP, cGMP, DAG (diacylglycerol), IP3 (inositol triphosphate), Ca++ : gate ion channels activate other proteins that produce physiological responses increase ICF Ca++

Question 47

Neural integration

Answer 47

Neural integration • organized into **neuronal pools: postsynaptic neurons in center of discharge zone more likely to reach threshold and generate AP’s than those in the edges (they become facilitated – no AP but closer to threshold) **serial processing – one neuron stimulates the next, which stimulates the next… e.g. reflexes **parallel processing – inputs segregated into many pathways important for higher level processing (integration) **circuits = patterns of synaptic connections in neuronal pools