Week 5: Nerves Continued

what is the basis of animal integration

nerves

what are the 2 types of nervous systems?

central nervous system and peripheral nervous system

what are the 2 main parts of the central nervous system?

what does the CNS serve as?

brain and spinal cord

serves as the major center for processing and integrating info

what are the 2 main nervous systems (ns) of the peripheral nervous system?

autonomic ns = regulates involuntary processes like heart rate, digestion

somatic ns = deals w/ seonsory system and voluntary control of skeletal muscles

what are the 2 main ns in the autonomic ns?

sympathetic ns and parasympathetic ns

what are the 2 categories never cells fall into?

neurons and neuroglia cells

define neurons

excitable cells that are capable of transmitting/generating electric signals

define neuroglia cells

non-excitable nerve cells that carry out functions specialized nerve cells cannot

in jellyfish, are neuroglia cells present, which type organism are they ALWAYS in?

the neuroglia cells are rare or absent in jellyfish but ALL 5 types are found in verts

list and define the 5 types of neuroglia cells

1. astrocytes - provide support and nourishment for neurons

2. schwann cells - from myelin sheaths around single PNS axons

3. oligodendrocytes - CNS myelin forms sheaths around several axons

4. microglia cells - carry out immunity duties w/in the cerebral spinal fluid

5. ependymal cells - line third ventricle of the brain and spinal canal; produce cerebral-spinal fluid

which neuroglia cells are part of the central ns and peripheral ns?

Central ns: oligodendrocytes, astrocytes, microglia, ependymal

Peripheral ns: astrocytes, Schwann cells

which neuroglia cell is this?

Schwann cells

which neuroglia cell is this?

oligodendricytes

which neuroglia cell is this?

ependymal cells

which neuroglia cell is this?

astrocytes

which neuroglia cell is this?

microglia

what are the 8 factors which make up the basic structure of excitable neurons

1. cell body or soma

2. cell processes

3. dendrites

4. axon

5. collateral axons

6. Schwann cells

7. Nodes of Ranvier

8. Axon terminals, synaptic batons, or presynaptic terminal

what does the cell body or soma contain?

contains cellular organelles

what are the cell processes part of?

extensions arising from the soma

what do dendrites receive?

impulses from other neurons or sensory cells

what is the purpose of axons?

to convey signals away from the soma toward other cells

where are collateral axons located?

branches off the main axon

what do Schwann cells make up?

make up the myelin sheath (if present)

what do the Nodes of Ranvier make up?

gaps in the myelin sheath (if present)

what do axon terminals, synaptic batons, or presynaptic terminals produce/secrete?

produces and/or secretes neurotransmitter substances

label the excitable neuron

1. dendrite

2. nucleus

3. cell body

4. axon

5. myelin sheath

6. node of ranvier

7. direction of impulse

8. axon terminal

9. synapse

what is the pathway of electrical signals running through the neuron?

dendrites receive the info => the soma integrates it => the axon carries action potential => the axon terminals pass the info on

list and define the 2 basic types of excitable neuron conduction types to move from one end of the neuron to the other

be sure to describe AP signals

1. electronic conduction - AP signals are propagated along entire axon in myelinated neurons

2. salutatory conduction - AP signal “jumps” b/w nodes of Ranvier, skipping myelinated sections of the axon in unmyelinated neurons

describe the 5 step process of the synapse, in which transmission w/out signal loss occurs

1. when an AP reaches the axonal baton, transmitter vesicles mobilize along the pre-synaptic membrane in response to Ca++ release

2. pre-synaptic vesicles empty transmitter substances into the synaptic cleft (aka: synaptic junction or nexus)

3. postsynaptic membrane ligand gated receptor channels receive transmitter substances and initiate an AP along the post synaptic neuron

4. transmitter substances are removed by diffusion, absorption, or enzymatic breakdown

5. the signal is rectified (i.e., the signal can only move in one direction)

define neurotransmitter substances

chemical messengers that cross the synaptic cleft to depolarize a postsynaptic neuron or effector target

what are the 4 major neurotransmitter classes and their functions

• monoamines: dopamine, histamine, adrenaline, serotonin

  • wide range of functions like behavior, arousal, and other physiological responses

• peptides: oxytocin, vasopressin, opiods, endorphins

  • larger molecules made from chains of amino acids and often produce longer-lasting effects, some can also function as hormones

• amino acids: glutamine, gamma, aminobutyric acid

  • important in CNS to increase/decrease neuronal activity

• other:

  • acetylcholine = communication b/w neurons and muscles

  • adenosine

  • nitric oxide = gas so doesnt follow same typical vessicle release pathway

what are the 7 “small molecule” neurotransmitters that do most of the work in mammals and what do they transmit?

1. acetylcholine - neuromuscular transmitter

2. gamma-aminobutyric acid - primary CNS inhibitory neurotransmitter

3. norepinephrine - catecholamine CNS and PNS neurotransmitter

4. dopamine - central nervous system pain/pleasure processing

5. glutamate - CNS neurotransmitter key to learning and memory

6. histamine - stimulates post-synaptic CNS receptors

7. serotonin - CNS neurotransmitter regulating mood

how many subtypes can neurotransmitters have?

provide example

neurotransmitters can have several subtypes

ex = serotonin has 15 subtypes

how does acetylcholine work differently in arthropods compared to other verts and advanced inverts?

acetylcholine is a neuromuscular transmitted in all vert and advanced inverts except for arthropods in which its ineffective at motor nerve endings

how does epinephrine work differently b/w non-human animals and fish/amphibians?

epinephrine is a transmitter substance in the para-sympathetic nervous system of most non-human animals, but is the sympathetic nervous system transmitter substance in fish and amphibians

what is the norepinephrine?

aka nonadrenaline, is the major transmitter substance in human sympathetic nervous system

what is gamma-aminobutyric acid (GABA)?

inhibitory neuromuscular transmitter substance for most crustaceans, insects, and other inverts as well as higher verts

what are the 4 morphological classifications of neurons?

pyramidal, bipolar interneuron, unipolar sensory neuron, multipolar motor neuron

define the pyramidal neuron

specialized multipolar CNS neurons and the primary excitation unit of the mammalian prefrontal cortex (the thinky-thinky part of your brain)

define the bipolar interneuron

a PNS neuron transmitting impulses b/w other neurons, usually associated w/ sensory/motor neurons and reflex arcs

define unipolar sensory neuron

sensory neurons w/ cell bodies housed in the spinal or cranial vault as nerve ganglia

define multipolar motor neuron

majority of neurons found in the CNS that function as motor neurons and interneurons

describe how action potential voltage and speed of conduction vary across animal groups.

action potential voltage (-120 mV) is relatively constant across animal groups, while the speed of conduction varies tremendously

what is the trend of the speed of conduction comparing inverts to verts?

inverts have slower conduction rates than verts

what are the 3 factors determining speed of nerve conduction?

synaptic connection type, neuronal diameter, axonal modifications

what is the result of highly specialized gap junctions across inverts and verts?

highly specialized gap junctions increase speed in many inverts and in limited areas of vertebrate CNS

how does current flow w/ a membrane separation from 2nm to non-existent gap junctions?

w/ membrane separation from 2nm to non-existent gap junctions allows current to flow directly b/w nerve cells through openings b/w adjacent cells

what transmission is faster than neurotransmitter systems?

ephaptic (electrical) or septal synapse transmission

what is the flow that ephaptic junctions provide?

two-way flow but the signal is unrectified

• what is the invert solution to transmission speed?

• provide 4 invert examples

• giant neurons bc they ALWAYS coordinate large muscle groups for quick escape or withdrawal

• cockroach jump-forward response, crayfish tail-flip response, earthworm withdrawal response, squid jet propulsive mantle contraction

how has the giant neuron originally found in inverts evolved to other animal groups?

present in most fish and amphibians, the neuron innervated tail muscles used to avoid aerial predators

what 2 animal groups are giant neurons absent in?

absent in tailless amphibians and bottom dwelling species

what are the regions of giant neurons determined by?

transmitter dominance NOT morphological boundaries

how much larger are giant neurons than an animal’s regular motor neurons?

fast responding giant neurons may be 50-100x the diameter of the animal’s regulator motor neurons

what is the major problem of giant neurons?

the major problem w/ giant neurons is that 100-fold increase in velocity requires a 1000-fold increase in diameter and 10,000 fold increase in volume => sooooo great for roaches but not whales

what is the vert solution to conduction speed?

myelinated neurons

describe the rate of nerve conduction in verts?

verts have one of the highest rates of nerve conduction in the animal kingdom: 120m/sec (270 mph) in mammals vs. 30m/sec (70mph) in fast earthworm fibers

what are myelinated neurons covered in?

myelinated neurons have axons covered by a thin sheath of fat-like myelin

what encircles the axon?

Schwann cells and oligodendrocytes have annular Nodes of Ranvier (i.e., they encircle the axon)

what occurs when depolarization of the exposed cell membrane occurs? how does this affect the travel of AP?

this is salutatory (to leap or dance) and generates a current strong enough to “trip” the next mode, as a result AP does NOT travel down the axon but rather “jumps” b/w nodes

describe the 4 steps of saltatory conduction action in a myelinated axon?

1) initial segment stimulation yields a local disturbance => 2) the disturbance produces an AP at the Ranvier node Nodal depolarization trips the next node in line => 3) after depolarizing nodes are hyperpolarized, preventing the signal from moving backward => 4) the process is faster than continuous conduction, where the impulse must travel the entire unmyelinated neuron

what is the invert solution to “myelinated” neurons

inverts have functionally equivalent “myelin-like” sheathes that have multiple loosely wrapped membranes w/ extracellular spaces forming ion reservoirs that change the cable properties

how do neuron nodes differ in inverts compared to verts

unlike verts, invert neuron nodes occur in larger numbers, are irregularly spaces, and may be either annular or fenestrated (i.e., are restricted to “spots”)

what is the best studied invert regarding myelin?

earthworm myelin consists of 20-200 layers that are not always compact

describe the layers of non-compact regions of myelin

non-compact regions typically have thin layers of cytoplasm b/w glial cell membranes

what is notable about invert phylogeny and the patter of distribution of myelin-like sheaths?

even tho many invert groups have nerve sheaths, strikingly similar to myelin but the pattern of distribution is not consistent w/ invert phylogeny

describe the ability of ions to compromise the invert sheath

the ability of ions to compromise the invert sheath insulation is limited

what is the advantage of myelinated fibers vs non-myelinated fibers in earthworms?

while conduction speed of earthworm myelinated fibers is high compared to that for non-myelinated fibers of the same diameter, the advantage is only a few fold

describe the presence of myelin sheaths in lampreys and hagfish and other chordates

Lampreys and hagfish lack myelin sheaths, whereas all other chordates have well-developed sheaths

describe the presence of myelin sheaths in decapods

Among decapods, shrimp have myelin sheaths, but crabs do not

describe the presence of myelin sheaths in earthworms and leeches, what does this suggest for all inverts

earthworms have myelinated nerves, but leeches do not: suggesting that myelin like sheaths evolved independently among invertebrate groups

what organism has the fastest recorded conduction speed in myelin-like axons?

Kuruma shrimp exhibits conduction rates ranging between 90 and 200 m/s, (450 mph) compared to 100–120m/s for the fastest myelinated vertebrate axon while unsheathed earthworm neurons have speeds of only 0.5 to 2.0 m/sec (1.1 to 4.5 mph)