Chapter 12: Nervous Tissue

List the components of the CNS vs. the PNS

CNS:

• brain

• spinal cord

PNS:

• cranial nerves

• spinal nerves

• enteric plexuses (in small intestine)

• sensory receptors (in skin)

Trace the organization of the nervous system

Describe the 3 major functions of the nervous system

1. SENSORY → detect changes thru sensory receptors

2. INTEGRATIVE → analyze incoming sensory info, store some aspects, & make decisions regarding appropriate behaviours

3. MOTOR → respond to stimuli via effectors

What are neurons?

• electrically excitable cellular structures

• nerve impulse = action potential

What is considered threshold potential?

-55mV

What is considered resting membrane potential?

• voltage across cell membrane when inactive → -70mV

• makes inside (-) relative to inside (more K+ inside, more Na+ outside)

Identify the three classifications of a neuron

1. Multipolar neuron

2. Bipolar neuron (one dendrite)

3. Pseudounipolar neurons

Differentiate the 2 types of dendrititc branching

1. Purkinje cell

2. Pyramidal cell

Describe the 3 functional classifications of neurons

1. Sensory/Afferent Neurons → conveys info to CNS

2. Interneurons/Association neurons → process sensory info & elicit motor response

1. Motor/Efferent neurons → conveys actions potential from the CNS

Which type of neuron are sensory, interneuron, & motor neurons usually?

sensory → pseudounipolar

interneuron → multipolar

motor neuron → multipolar

What are neuroglia?

• NOT electrically excitable

• make up about half the volume of the nervous system

• can multiply & divide

• 6 kinds total (4 in CNS, 2 in PNS)

List and describe the function of the 4 neuroglia in the CNS

1. ASTROCYTES → support neurons by maintaining chemical environment (Ca2+ & K+)

2. OLIGODENDROCYTES→ produce myelin in CNS

3. MICROGLIA → participate in phagocytosis

4. EPENDYMAL CELLS → form & circulate CSF (cerebrospinal fluid)

List and the describe the function of the 2 neuroglia in the PNS

1. SATELLITE CELLS → support neuroglia in PNS; in ganglia

2. SCHWANN CELLS → produce myelin in PNS

What is myelin sheath?

multilayered lipid & protein covering around some axons that insulates them & increases the speed of nerve & impulse conduction

Excitable cells communicate w/ each other via ___ or ___

action potentials; graded potentials

What is the difference between action and graded potentials?

• action potential → allow communication over short & long distances

• graded potential → allow communication over short distances only

What does the production of an AP or GP depend upon?

the existence of a resting membrane potential & the existence of certain ion channels

List the 5 types of ion channels

1. Leak channels

2. Ligand-gated channels

3. Mechanically-gated channels

4. Voltage-gated

Describe leak channels

• randomly alternate between open & closed

• K+ channels = more numerous than Na+ channels

Describe ligand-gated channels

• respond to chemical stimuli (ligand binds to receptor; e.g. acetylcholine)

Describe mechanically-gated channels

• respond to mechanical vibration or pressure stimuli

Describe voltage-gated channels

• respond to direct changes in membrane potential

The membrane of a non-conducting neuron is postive outiside and negative inside. This is determined by:

1. unequal distribution of ions across the plasma membrane & the selective permeability of the neuron’s membrane to Na+ & K+

2. Most anions cannot leave the cell

3. Na+/K+ pumps

Contrast hyperpolarizing and depolarizing graded potential

hyperpolarizing graded potential → below RMB (-70mV); more polar = more (-)

depolarizing graded potential → above RMB; less polar = more (+)

A graded potential occurs in response to the ______

opening of a mechanically-gated or ligand-gated ion channel

The amplitude of a graded potential depends on the _____. Graded potentials can be ___ to become larger in amplitude.

stimulus strength; added together

List and describe the 2 phases of action potentials

1. Depolarization → voltage-gated Na+ channel activation gates are open

2. Repolarization → voltage-gated K+ channels are open; Na+ channels are inactivating

(together = “absolute refractory period”)

Stimulus causes depolarization to ___

threshold

What occurs during relative refractory period?

voltage-gated K+ channels are still open; Na+ channels are in resting state

Action potentials can only occur if the membrane potential reaches ___

threshold

List the phases of changes in ion flow through voltage-gated hcnanels during the depolarizing and repolarizing phases of a nerve impulse

1. Resting state

2. Depolarizing phase

3. Repolarization phase begins

4. Repolarization phase continues

Describe what occurs during resting state

• all voltage-gated Na+ & K+ channels are closed

• axon plasma membrane is at resting membrane potential

• small buildup of negative charges along inside surface of membrane & an equal buildup of positive charges along outside surface of membrane

Desrcibe what occurs during the depolarizing phase

• when membrane potential reaches threshold, the Na+ channel activation gates open

• as Na+ ions move thru these channels into the neuron, a buildup of (+) charges forms along the inside surface of membrane

• membrane becomes depolarizaed

Describe what occurs during the repolarizing phase

• Na+ channel inactivation gates close & K+ channels open

• membrane becomes repolarized as some K+ ions leave the neuron

• few (-) charges begin to build up along the inside surface of membrane

Describe what occurs when the repolarization phase continues

• K+ outflow continues; as they leave the nuron, more (-) charges build up along inside surface of membrane

• K+ outflow eventually restores RMP

• Na+ channel activation gates close & inactivation gates open

• return to resting state when K+ gates close

Compare the origin of graded potentials vs. action potentials

GP → mainly in dendrites & cell body

AP → at trigger zones and propagate along axon

Compare the types of channels for GP vs. AP

GP → ligand-gated or mechanically-gated ion channels

AP → voltage-gated channels for Na+ & K+

Compare the conduction of GP vs. AP

GP → decremental (not propagated) permit communication over short distances

AP → propagate & thus permit communication over long distances

Comapre the amplitude/size of GP vs. AP

GP → depending on strength of stimulus, varies from less than 1 mV to more than 50mV

AP → all or none; typically about 100mV

Compare the duration of GP vs. AP

GP → typically longer, ranging from several milliseconds to several minutes

AP → shorter, ranging from 0.5 to 2 msec

Compare polarity of GP vs. AP

GP → may be hyperpolarizing (inhibitory to generation of action potential) or depolaring (excitatory to generation of action)

AP → always consist of depolarizing phase followed by repolarizing phase & return to resting membrane potential

Compare the refractory periods of GP vs. AP

GP → NOT present; summation can occur

AP → present; summation cannot occur

Define propagation

action potentials must trabel from where they arise at the trigger zone to the axon terminals in order for communication to occur

• AP do not die out; they keep their strength as they spread across the membrane of a neuron

Differentiate between continuous vs. saltatory conduction

List the 3 factors that affect propogation speed

1. axon diameter → larger diamter axons propagate APs faster

2. amount of myelination → myelin increase speed of AP propagation

3. temperature → higher temperature inccreases speed of AP propagation

What is a synapse? What are the 2 types of synapse?

synapse → junction between neurons or between a neuron & an effector

1. electrical synapse → gap junctions connect cells & allow the transfer of info to synchronize the activity of a group of cells

2. chemical synapse → one-way transfer of info from a presynaptic neuron to a postsynaptic neuron

List the types of synapses between presynaptic & postsynaptic neurons

• axodendritic synapse

• axosomatic synapse

• axo-axonal synapse

Contrast the 2 types of postsynaptic potentials

• excitatory postsynaptic potentials

  • depolarizing postsynaptic potential

• inhibitory postsynaptic potentials

  • hyperpolarizing postsynaptic potential

True or false: A postsynapic neuron CANNOT receive many signals at once

FALSE

Neurotransmitters at chemical synapses cause either an ___ or ____ graded potential

excitatory (EPSP); inhibitory (IPSP)

List the 2 structures of neurotransmitters (receptors)

1. ionotrophic receptors → contains neurotransmitter binding site & ion channel

2. metabotropic receptors → contains a neurotransmitter binding site & is coupled at a separate ion channel by a G protein

Neurotransmitters can be removed from the synaptic cleft by:

1. Diffusion

2. Enzymatic degradation

3. Uptake into cells

What is summation? What does it cause?

• when several presynaptic end bulbs release their neurotransmitter at about the same time

• generates a nerve impulse

• may be spatial or temporal

Differentiate between spatial or temporal summation

spatial → AP at 40 msec

temporal → AP at around 60 msec

Describe the function of dendrites

dendrites receive stimuli through activation of ligand-gated or mechanically gated ion channels

• in sensory neurons, produce generator/receptor potentials

• in motor neurons & interneurons, produce excitatory & inhibitory postsynaptic potentials (EPSPs & IPSPs)

Describe the function of the cell body

cell body receives stimuli & produces EPSPs & IPSPs thru activation of ligand-gated ion channels

Describe the function of the “junction of axon hillock & initial segment of axon”

• trigger zone in many neurons

• integrates EPSPs & IPSPs

• if sum is depolarization that reaches threshold, initiates a nerve impulse

Describe the function of an axon

• propagates nerve impulses from initial segment (or from dendrites of sensory neurons) to axon terminals in self-regenerating manner

• impulse amplitude does NOT change as it propagates along axon

Describe the function of axon terminals & synaptic end bulbs (or varicosities)

• inflow of Ca+ caused by depolarizing phase of nerve impulse triggers exocytosis of neurotransmitter from synaptic vesicles

Where on the neuron are ligand-gated channels located?

plasma membrane of the dendrites

Where on the neuron are voltage gated Na+ & K+ channels located?

plasma membrane of the axon

Where on the neuron are voltage gated Ca2+ channels located

plasma membrane of the axon terminals & synaptic end bulbs

Are there any channels located on the cell body?

no

List the 6 major small molecule neurotransmitters

• acetylcholine

• amino acids

• biogenic amines

• ATP & other purines

• nitric oxide

• carbon monoxide

List & briefly describe the 4 major neuropeptides

1. SUBSTANCE P → found in sensory neurons, spinal cord pathways, & parts of brain associated w/ pain; enhances perception of pain

2. ENKECEPHALINS → inhibit pain impulses by suppressing release of substance P; may have role in memory & learning, control of body temp, sexual activity, & mental illness

3. ENDORPHINS → inhibit pain by blocking release of substance P; may have a role in memory & learning, sexual activity, & mental illness

4. DYNORPHINS → may be related to controlling pain & registering emotions

Define neural circuits

functional group of neurons that process specific types of info

List the 5 types of neural circuits

1. Simple series

2. Diverging

3. Converging

4. Reverberating

5. Parallel after-discharge

Define plasticity

the capability to change based on experience

define regenerate

the capability to replicate or repair

Although the nervous system exhibits ____, neurons have a limited ability to ____ themselves

plasticity; regenerate

In the CNS, there is little to no repair due to:

1. Inhibitory influences from neuroglia, particularly oligodendrocytes

2. Absence of growth-stimulating cues that were present during fetal development

3. Rapid formation of scar tissue

In the PNS, repair is possible if:

• the cell body is intact

• Schwann cells are functional

• scar tissue formation does not occur too rapidly

The steps involved in the repair process of neurons in the PNS are:

1. Chromatolysis

2. Wallerian degeneration

3. Formation of a regeneration tube