Nervous System Flashcards
What are the three functions of the nervous system?
- detect changes in internal, external environments
- integrates info, make unconscious and conscious decisions
- stimulates muscles and glands to respond
What are the two divisions within the nervous system?
- central nervous system (CNS) - brain & spinal cord, analyze & coordinates
- peripheral nervous system (PNS) - sensory/afferent division & motor/efferent division
What is the job of the sensory division?
-receives input from special senses (eyes, ears, nose, mouth), from internal organs (visceral sensory neurons), and from joints and skeletal muscle (somatic sensory neurons)
What is the job of the motor division?
- sends commands
- 2 parts: autonomic nervous system, somatic motor neurons
What is the autonomic nervous system?
- mostly involuntary
- sympathetic (fight or flight) to cardiac muscle, smooth muscle, glands
- parasympathetic (rest or digest) to skeletal muscle
What is the role of somatic motor neurons?
-mostly voluntary
What are the special characteristics of neurons?
- long-lived - entire lifetime
- amniotic - can’t divide
- high metabolic rate: needs lots of “food”: oxygen, and glucose
What are neurons made of?
-dendrites, cell body, dendritic spines, axons
What is the S&F of dendrites?
-dendrites: highly branched processes (stick out) that receive information
What is the S&F of dendritic spines?
-increase surface area to receive information
What is the S&F of the cell body?
-cell body: soma: large to produce neurotransmitters, clusters of cells in CNS are called nuclei, clusters of cells in PNS are called ganglia
What is the S&F of axons?
- length varies
- may be myelinated to increase the speed of impulse transmission
- may have collateral branches = side branches
- synaptic knobs at the end hold vesicles with NT
- axons wrapped in connective tissue
What are the types of connective tissue that cover the axons?
- endoneurium: covers single axon; has capillaries
- perineurium: covers bundles of axons (nerve fascicles) has arteries and veins
- epineurium: covers bundles of nerve fascicles
What are bundles of axons called in the CNS and PNS?
CNS: tract
PNS: nerve
What are the types of neurons?
- multipolar
- bipolar
- unipolar
- anaxonic
What are the characteristics of a multipolar neuron?
- many dendrites, 1 long axon
- mostly common in CNS, all motor neurons
What are the characteristics of a bipolar neuron?
- 1 dendrite, 1 axon
- rare, special sense organs
What are the characteristics of a unipolar neuron?
- dendrites continuous with the axon
- sensory neurons in the PNS
- cell body in dorsal root ganglion
What are the characteristics of a anaxonic axon?
-can’t distinguish dendrites and axons
What is the classification by function for sensory neurons?
- mostly unipolar
- carry info from sensory organs/receptors to CNS
- eg. exteroceptors: info from outside: touch, vision, sight
- eg. interoceptors: monitor internal organs
- eg. proprioceptors: monster muscle and joint position
What is the classification by function for interneurons?
- mostly multipolar and found in CNS but some anaxonic
- between sensory and motor neurons
- integrate info
What is the classification by function for motor neurons?
- ALL multipolar but the cell bodies are in the CNS
- carry info to muscles and glands
What is a characteristic of neuroglia?
-smaller than the neurons but also outnumber them 10:1
What are the 4 types of neuroglia in the CNS?
-astrocytes, microglia, oligodendrocytes, ependymal cells
What is the S&F of astrocytes?
- most common, star-shaped
- surround to maintain blood-brain barrier and control transport of the material into the interstitial fluid
- create supportive network for neuron
- recycle NT
- guides neuronal migration in the embryo (growth)
What is the S&F of microglia?
- small cells with “thorny” processes
- defend and remove debris (no WBC in CNS)
What is the S&F of oligodendrocytes?
- processes wrap around portions of multiple CNS axons
- to myelinated/insulate axons to increase the speed of action potential
What is the S&F of ependymal cells?
- look like epithelial
- ciliated cells joined to tight junctions
- lines ventricles of the brain, central canal of spinal cord
- produce, monitor, circulate, cerebral spinal fluid (CSF)
What are the two types of neuroglia in the PNS?
- schwann cells
- satellite cells
What is the S&F of Schwann cells?
- whole cells wrapped around part of one axon = myeline axon –> AP travels 150x faster
- many Schwann cells needed to myeline 1 axon
- adjacent Schwann cells don’t touch (gap= node of Ranvier
- AP is propagated at nodes of Ranvier
- demyelination results in less sensation and control = MS
- Schwann cells also guide axon growth during neuron repair
What is the S&F of satellite cells?
- surround cell bodies in ganglia
- help regulate the environment around the neurons
What is the resting potential?
-difference in voltage (+,-) across the membrane when the cell is at rest
What generates the resting membrane potential?
- most Na+ in body lies outside cells, most K+ is inside cells - a bit motor negative on the inside of the membrane (-70 mV)
- cells have proteins (large molecules stay inside the cells) with negatively charged amino acids
- cells have leaky K+ channels
Why does K+ leak out of cell?
-leaky K+ channels
-K+ diffuses down the electrochemical gradient out of cells
-some K+ is pumped back in via Na+/K+ pump
= overall negative charge left inside the cell (70mV)
Where is the location of the graded potential?
-dendrite –> soma –> axon hillock
Where is the location that an action potential happens?
-axon hillock –> axon –> synaptic knob
What is the location of neurotransmitters?
-synaptic knobs –> synaptic cleft –> dendrites (of another neuron)
What are the three types of channels?
1) GP- ligand gated (molecule biding)/mechanically gated (Na/Cl channels)
2) AP- voltage gated Na+/K+ channels
3) NT- voltage gated Ca2+ channels
What are the stimulus/triggers of the channels?
1) GP- forced movement (ligand or mechanically gated
2) AP- ion flow, depolarization (increase and charge)
3) NT- depolarization
What are the distances that can be travelled?
1) GP-short distance travelled within soma through cytosol
2) AP-long distance travelled (cm) regenerated along the axon
3) NT-short distance across the synaptic cleft
What are the events at the dendrites?
- stimulus opens ligand-gated or mechanically-gated ion channels
- ion flow generates graded potentials
What stimuli open ligand-gated channels?
- NT
- food (smell molecules)
- chemicals in body (eg. glucose, CO2)
- chemicals released by injured cells
- light (triggers reaction that produce molecules)
What stimuli open mechanically-gated channels?
- touch, pressure, vibration
- stretch
- sound
When do positive ion flows generate graded potentials?
- if positive ions enter dendrites- increases positive charge in cells = cell depolarizing - makes cell more likely to fire
- if positive ions leaves dendrites -makes cell more negative = hyperpolarization and less likely to fire)
- negative ions enter dendrites
What factors change the strength of the graded potential?
- week stimulus: few channels open, few ions flow
- strong stimulus: many channels open, many ions flow
Why can a GP be excitatory?
-Na+ or K+ influx = depolarization = excitatory postsynaptic potential (EPSP)
Why can a GP be inhibitory?
-K+ efflux/Cl- influx = hyperpolarization = inhibitory postsynaptic potential (IPSP)
What determines if a threshold is reached?
-summation of all the EPSPs and IPSPs
What happens if there is a weak stimulus?
-threshold is not reached = no signal passed on
What happened in a strong stimulus?
-spacial summation or temporal summation
What is spacial summation?
- ions for many synapses add up because many terminals release Its simultaneously
- few AP
What is temporal summation?
- high frequency of firing
- ions from one synapse add up over time because the new NT released before initial amounts degraded
What is the axon hillock?
- voltage-gated channels present at axon hillock
- if axon hillock reaches threshold potential, voltage-gated channels open –> AP is generated
- if axon hillock does not reach threshold potential, voltage-gated channels stay closed –> NO AP
What does the amount of depolarization have to be in order for an AP to take place?
- -55mV
- if its -30mV perhaps multiple will fire
What are the characteristics of an action potential?
- brief reversal in membrane potential =-70mV
- always the same strength
- only axons and muscle cells have excitable membrane - they have voltage-gated channels therefore can generate an AP
What is the first step of an action potential?
- resting membrane potential = -70mV
- voltage gated Na+, K+ channels @ axon hillock and axon are closed, capable of opening
What is the second stop in an AP?
- stimulus triggers graded potential in soma
- depolarization spreads through soma to voltage-gated Na+ channels at axon hillock
What is the third step of an AP?
- when depolarization reaches -55mV, threshold potential reached
- voltage Na+ channels at axon hillock open
- outer gates open fast, inner gate close slowly
What is the fourth step of an AP?
- fast depolarization to +30mV
- Na+ rushes down electrochemical gradient
- depolarization opens next set of voltage-gated Na+ channels, AP propagated along axon
What is the turning point in an AP?
- step 5
- slow inner gates closed, Na+ channels closed
- no more Na+ enters
- voltage K+ channels open
What is repolarization?
- step 6 in AP
- K+ rushes down the electrochemical gradient
What is hyperpolarization?
-step 7 of AP
-K+ gates slow to close
-excess K+ leaves the cell
Na+/K+ will eventually restore ion distribution
How is the AP ‘“all or none”?
- weak stimulus: subthreshold no AP
- strong stimulus: threshold reached AP
- stronger stimulus: threshold reached more often, increased AP but all AP are the same strength
What is the refractory period in an AP?
-ensures unidirectional propagation of APq
What is the absolute refractory period?
- Na+ gates open or inactivated
- region can’t respond to another stimulus
- only Na+ channels further along axon can open
- ensures AP
What is the relative refractory period?
- gates have been reset, are ready to open but cell is hyperpolarized
- threshold stimulus won’t trigger another AP
- stronger stimulus will reopen Na+ channels, trigger AP
What is AP propagation?
1) AP along unmyelinated axons travel by continuous propagation
- voltage gated Na+ channels open along an axon
2) AP along myelinated axons travels by salutary conducting 150x faster
- myeline insulate fibres
- voltage-gated Na+ channels at nodes of Ranvier
- AP generated only at nodes
What is the synaptic knob?
- synapse is junction between 2 neurons or between neuron and effector
- converts electrical impulse to chemical signal and back again
What are the events at a synapse?
1) voltage-gated Ca2+ channels open
- Ca2+ floods in
2) Ca+ triggers synaptic vesicles to fuse with axonal membrane
- NT released by exocytosis
- Ca2+ removed by mitochondria
3) NT binds to receptors, open ion channels - open Na+ channels
- depolarization = EPSP
- opens K+ or Cl- channels - hyperpolarization =IPSP
4) NT removed from postsynaptic receptor by:
- degration (break down) of enzymes or removed by a transporter
- uptake by neuroglia cell =astrocytes or neuron that released it (presynaptic neuron) for storage (sometimes keeps parts to make something new)
- diffusion
What are neurotransmitters (NT)?
- may bind to different types of receptors on different postsynaptic cells
- receptor type determines effect of NT on cell
- NT may be excitatory, inhibitory or both depending on type/location on receptor
What is Achetylcholine?
- NT
- excites skeletal muscle, inhibits cardiac muscle
eg. snake venom & curare inhibit binding of Ach to receptor –> flaccid paralysis (NT can’t bind to receptor and channels don’t open so no AP)
eg. nerve gas prevents Ach removal from synapse –> muscle spasms leading to death (diaphragm is a muscle) (NT builds up channels stay open producing too many AP)
What are Monoamines?
NT
- norepinephrine
- dopamine
What is norepinephrine?
- excitatory/inhibitory depending on location
- feeling good
- tricyclin antidepressants block NE removal from synapse –> enhance good feelings
What is dopamine?
- excitatory/inhibitory
- feeling good skeletal muscle control
- coccaine binds competitively to dopamine
- reuptake transported
- given to Parkinson’s patients to control complex movement
- fewer NT, channels stay closed - no AP
- Add NT substitute –> channel opens
What is serotonin?
- NT
- inhibitory
- regulates mood, sleep, appetite, nausea
- feeling good
- prozac blocks seretonin uptake –> enhances positive feelings to reduce depression
- LSD blocks seretonin activity –> inhibitory effect reduce excess AP –> hallucinations
What affect does prozac have on AP?
- NT builds up and opens more channels
- cells stays hyperpolarized
- decreased firing fear and anxiety centres
What affect does LSD have on AP?
- seretonin can’t bind therefore channel stays closed
- K+ can’t leave but continues to enter through Na+/K+ pump
- (+) charge builds up and cell depolarizes
- excessive EPSP excess AP
What are amino acids NTs?
eg. gabba
- inhibitory, opens Cl- channels
- principle inhibitory NT in brain
- alcohol and value –> augment effects
What are neuropeptides?
eg. endorphins
- inhibitory
- widely distributed in brain, inhibits pain
- morphine, heroine are structurally similar (bind to receptors and mimic effect)
What are neuromodulators?
eg. nitric oxide
- excitatory
- brain, spinal cord induces muscle relaxation
- nitroglycerin released NO –> relaxes smooth muscle of the blood vessels and increases blood flow in the heat, reduces angina
