Chapter 12: Nervous system

nervous system vs endocrine

endocrine uses hormones to communicate while nervous system uses electrical and chemical means

central nervous system

brain and spinal cord

peripheral nervous system

nerves and ganglia

nerve

bundle of nerve fibers (axons) wrapped in fibrous connective tissue

ganglion

location of neuron cell bodies in PNS

peripheral nervous system is divided into BLANK and BLANK, each with BLANK and BLANK subdivisions

Peripheral nervous system is functionally divided into
sensory and motor divisions, each with somatic and
visceral subdivisions

sensory (afferent) division

carries signals from receptors (sense organs) to CNS

somatic sensory division

carries signals from receptors in the skin, muscles, bones, and joints

Visceral sensory division

carries signals from the viscera (heart, lungs, stomach, and urinary bladder)

motor (efferent) division

carries signals from CNS to effectors

somatic motor division

carries signals to skeletal muscles, voluntary

visceral motor division

carries signals to glands, cardiac and smooth muscle, involuntary. further divides into sympathetic, parasympathetic and enteric plexus

sympathetic division of ANS

stimulates and prepares for action

parasympathetic division of ANS

calming

enteric plexus

in digestive tract wall, coordination and communication

3 properties of neurons

excitability, conductivity, and secretion

sensory (afferent) neurons

detect stimuli and transmit information toward the CNS

interneurons

receive signals from other neurons, process it, and make decision. found in CNS

motor (efferent) neurons

send signals out to muscles and gland cells

cell body (neurosoma, soma, or perikaryon)

contains nucleus and organelles. Mitochondria, lysosomes, inclusions, and rough ER. no centrioles

neurites

extensions of the neuron

dendrites

primary site for receiving signals from other neurons

axon (nerve) fiber

rapid conduction of nerve signals

axon hillock

mound on one side of cell body

axoplasm

cytoplasm of axon

axolemma

membrane of axon

axon terminal

forms synapse with next cell

multipolar neuron

one axon and multiple dendrites, most common in body

bipolar neuron

one axon and one dendrite. olfactory cells

unipolar

single process leading away from cell body. the single branch acts as both the axon and the dendrite

anaxonic neuron

many dendrites but no axon. brain, retina, and adrenal gland

axonal transport

passage of materials along an axon

anterograde transport

movement away from cell body, via kinesin

retrograde transport

movement up the axon toward the cell body, via dynein

fast anterograde transport

organelles, enzymes, synaptic vesicles, small molecules

fast retrograde transport

recycled materials or some pathogens

slow axonal transport

always anterograde. transport of enzymes, cytoskeletal components, new axoplasm

neuroglia

supportive cells of nervous system

oligodendrocytes

form myelin sheaths in CNS

ependymal cells

line internal cavities of brain and secrete CSF (CNS)

microglia

macrophages in CNS

astrocytes

create BBB in CNS, diverse functions

Schwann cells

envelop axons of PNS and form myelin sheath, also help with regeneration of nerve fibers

satellite cells

surround nerve cell bodies in ganglia in PNS and provide insulation, also regulate chemical environment

gliomas

tumors of glial cells

can mature neurons perform mitosis? relate this to tumors

no, so they rarely form tumors. brain tumors usually arise from meninges and metastasis

myelin sheath

layers of insulation around an axon. mostly lipid, 20% protein

How does myelination happen in PNS?

• Schwann cell spirals around a small section of single axon

• lays down layers of its membrane which form the myelin sheath

• neurilemma is the outermost coil

endoneurium

thin layer of fibrous connective tissue external to the neurilemma

what is external to the neurilemma?

the basal laminate and endoneurium

neurilemma

thick outermost coil of myelin in the PNS. contains the cell nucleus and most of the cytoplasm of the Schwann cell

How does myelination occur in the CNS?

• oligodendrocyte can wrap around small portions of many axons

• myelination spirals inward toward axon

• no neurilemma

node of ranvier

gaps between myelin

internal segments

myelin covered segments

initial segment

bare section of axon between the axon hillock and first glial cell

trigger zone

axon hillock and initial segment

multiple sclerosis

oligodendrocytes and myelin sheets in CNS deteriorate.

tay-sachs disease

abnormal accumulation of glycolipid GM2 in the myelin sheath

What affects speed of a neuron?

diameter (large= faster)

myelin (myelinated = faster)

nerve regeneration

Can occur in PNS if cell body remains intact

-macrophages clean up tissue debris

-cell body swells

-axon creates growth processes

-Schwann cell forms regeneration tube

Very slow

Electrical potential

Difference in concentration between charged particle from one point to another

Current

Flow of a charged particle

Resting membrane potential

Charge across membrane. -70mV

Is the inside of the membrane negative or positive?

the ICF is MORE NEGATIVE

Potassium influence of RMP

K+ is more concentrated in the ICF. The membrane is more permeable to K+ so it can leak out and membrane becomes more negative. The electrical charge pulls the positive K back inside

Sodium influence of RMP

Na+ is more concentrated in the ECF and the membrane is less permeable to Na+. Na+ diffuses into the cell down concentration gradient but only cancels a small amount of negative charge

Sodium-potassium pump

Helps establish the RMP and ensure there are more Na+ outside and more K+ inside. Uses one ATP to bring 3 Na+ out of cell and 2 K+ into cell.

Local potentials

Local, temporary change in membrane potential. Occurs at dendrites and soma. Purpose is to process sensory info and determine whether neuron should trigger an action potential.

What are the characteristics of local potentials?

-graded: Varys in magnitude depending on stimulus strength (ex: strong stimuli can open more Na+ channels)

-decremental: weaker as they get further away

-reversible: if stimulus ceases, membrane potential returns to RMP

-can be excitatory: depolarize = more likely to have AP

-can be inhibitory: hyper-polarize = less likely to have AP

Action potential

Rapid up and down change in voltage created by opening and closing of voltage-gated channels. Begins in the trigger zone, where there are enough voltage-gated ion channels

Describe the steps of an action potential

-local potential spreads to axon hillock

-threshold must be met (-55 mV)

-Na+ channels open quickly and K+ voltage gated channels open slowly

-depolarization: Na+ influx into cell and membrane potential reaches +35 mV

-Na+ channels close and inactivate

-Polarization: the K+ channels are now fully open and K+ leaves the cell. Membrane becomes more negative

-hyperpolarization: membrane becomes more negative than RMP

-membrane returns to RMP

Explain depolarization

Influx of Na+ makes the membrane more positive and it peaks at +35 mV.

Explain repolarization

Efflux of K+ out of the cell. Makes the membrane negative again

Do the concentrations of Na+ and K+ change significantly during an AP?

No. Movement of only a few ions has a big effect on the membrane potential

What are the characteristics of an AP?

• All or none law: neuron fires go to maximum voltage if threshold meets.

• Non decremental: does not get weaker with distance

• Irreversible: travels all the way down the axon and cannot be stopped

All or none law

Threshold must be met in order for action potential to proceed. If threshold is met, the action potential proceeds to maximum voltage.

Absolute refractory period

No stimulus of any strength will trigger an action potential. Caused by closing of Na+ voltage-gated channels, which prevents another depolarization

Relative refractory

An unusually strong stimulus can trigger an AP. During hyperpolarization, a larger depolarization (local potential) can trigger an AP

Continuous conduction

Occurs on unmyelinated axons because they have voltage-gated channels down the length of their axon. Action potential begins at the trigger zone and depolarization travels down. voltage-gated channels are opened immediately next to new action potential

Saltatory conduction

Action potentials “jump” from node to node. Electrical signal passes passively between nodes but is strong enough to depolarize membrane to threshold at nodes of ranvier and Na+ channels open, so a full strength action potential occurs

How does myelin increase speed of action potential?

-insulates and prevents ions from leaking

-creates more distance between the intracellular fluid and extracellular fluid so fewer ions are needed to depolarize the membrane

Synapse

Point where an axon terminal meets the next cell

Presynaptic neuron

Nerve cell that sends the signal across the synapse

Postsynaptic neuron

Neuron that receives signals from the presynaptic neuron at the synapse

synaptic cleft

Gap between neurons

Chemical synapse

Release of neurotransmitters

Electrical synapse

gap junctions join cells and electrical signals spread from cell to cell. Very fast but cannot integrate into

Synaptic vesicles

Found in axon terminal of presynaptic neuron and contain the neurotransmitter

Acetylcholine

Neurotransmitter formed from acetic acid and choline

Amino acids

Neurotransmitters glycine, glutamate, aspartate, and GABA

Monoamines

Class of neurotransmitters. Remove the -COOH group from amino acids but amine group remains

purines

Adenosine and ATP

Gases

Nitric acid and carbon monoxide

Neuropeptide

Chains of amino acids that function as hormones or neuromodulators

How do synapses vary in their modes of action?

Depends on type of receptor on post synaptic neuron. Can be excitatory or inhibitory

Excitatory cholinergic synapse

• Cholinergic synapse—acetylcholine (ACh) is theneurotransmitter
• Steps in synaptic transmission:
• Action potential depolarizes the axon terminal, opens voltage-gated Ca channels
• Ca enters, triggers exocytosis of ACh
• ACh diffuses across cleft, binds to postsynaptic receptors
• ACh receptors are ligand-gated ion channels that open and allow Na and K across the membrane
• Entry of Na causes depolarizing postsynaptic potential
• If strong enough, depolarization spreads to the trigger zone, raises the membrane potential up to threshold to trigger an action potential

GABA-nergic synapse

y-aminobutyric acid (GABA) is the neurotransmitter. This is an inhibitory synapse.

1. AP triggers the release of GABA into the synaptic cleft.

2. GABA receptors are chloride channels

3. Cl- entry hypolerpolarizes the postsynaptic membrane

4. Postsynaptic neuron is inhibited (less likely to fire an action potential)

adrenergic synapse

norepinephrine is the neurotransmitter. slower than cholingeric and GABA-ergic synapses. advantage is enzyme amplification

1. monoamines and neuropeptides bind to G-protein coupled receptors on postsynaptic membrane

2. activation of second messenger systems such as cAMP

How does the signal cease at the synapse?

• presynaptic cell stops releasing neurotransmitters

• neurotransmitter already in the synapse is cleared

Explain clearance of a neurotransmitter

1. neurotransmitter degradation: enzyme in synaptic cleft breaks down neurotransmitter

2. reuptake: neurotransmitter or its breakdown products reabsorbed into axon terminal

3. Diffusion: neurotransmitter or its breakdown products diffuse away from synapse into the ECF