Nervous System Flashcards
Explain the organization of the nervous system (& diagram/drawing)
The nervous system is how the body communicates in a fast and transient (brief) way through reflex responses. The nervous system consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord and acts as the integrating center of the nervous system. The PNS consists of an afferent (sensory) division and an efferent division. The afferent (sensory) neurons take stimulus information to the CNS and the efferent neurons take information from the CNS to target cells/tissues. The Efferent division has two subdivisions: the motor (somatic) division and the autonomic division. The motor (somatic) division of the efferent division of the PNS are voluntary neurons that take information to skeletal muscles. The autonomic division are involuntary neurons that take information to smooth muscle, cardiac muscle, exocrine glands/cells, some endocrine glands/cells, and some adipose tissue. The Autonomic division has two subparts: the sympathetic and parasympathetic. The sympathetic is involved with acute stress (flight or fight). The parasympathetic is involved with day to day (rest and digest). The sympathetic and parasympathetic systems work antagonistically, meaning when one is working the other is not.
Explain glial cells and the different types of glial cells in both the CNS and PNS.
CNS
Glial cells in the CNS are the “glue” cells of the nervous system. Originally glial cells were thought to be simply the “scaffold” upon which the neurons were structure (glia = “glue”), but now glial cells are appreciated as a diverse group of nervous system cells which is as functionally important as neurons. Without glial cells there is no neuron function. There are three types of glial cells: oligodendrocytes, microglia, and astrocytes. The oligodendrocytes (a type of glial cell) myelinate neurons, meaning they put a fatty sheath around the axon. The microglia (a second type of glial cell) perform immune functions and have a newly-appreciated function in potential infectious diseases that cause neurodegenerative diseases (they are the brain-equivalent of the immune system). The astrocytes (the third type of glial cell) promote brain sub architecture and are involved in forming the BBB (blood-brain barrier).
PNS
The PNS has two types of glial cells→ Schwann cells and satellite cells. The Schwann cells are responsible for myelination and the satellite cells are found at PNS neuron synapses and act as support cell bodies.
Explain how astrocytes act as “neuronal support”
Astrocytes are very important, in fact you cannot get a functioning brain without astrocytes. There are different astrocyte types in the brain (cerebral cortex vs subcortex), and they have different functions. The different parts of the brain are different in part because of the astrocytes that are there establishing function. Astrocytes are also involved in the formation of the blood-brain barrier (BBB), because they induce tight junctions in endothelial cells. They (astrocytes) do this by telling the endothelial cells in blood vessels to form a tight junction so that nothing can get in or out and they promote region-specific blood flow. Astrocytes also act as “neuronal support” in the following ways: metabolic help, regulation of K+ homeostasis for a proper signaling environment, and regulating neurotransmitter homeostasis using the Tripartite Synapse Model, and glial cell transmission through the Ca2+ wave.
Astrocytes act as metabolic help by providing glucose and lactate. Astrocytes take up glucose from the blood vessels and distributing it throughout the brain.
Astrocytes also act as neuronal support by regulating K+ homeostasis for a proper signaling environment. Astrocytes “clean up” the K+ leaked out when an action potential in a neuron occurs.
Astrocytes also regulate neurotransmitter homeostasis using the Tripartite Synapse Model. The ends of an astrocyte physically envelope neuron synapses and allows monitoring & regulation of neurotransmitter activity. One example is through the uptake of neurotransmitters: First tripartite gets activated → then astrocytes induce a calcium wave (“Ca2+ wave”) → and then Ca2+ flows from astrocyte to astrocyte using gap junctions→ and the Ca2+ wave plays a role in signaling.
Explain how the AP is then transmitted to the next neuron (with Diagram)
Transmission of electrical info (impulse to the next cell)
- Presynaptic cell sends signals to postsynaptic cell
- Signaling by release of neurotransmitters from synaptic vesicles into synaptic cleft
- A protein mediated process
- Signaling by release of neurotransmitters from synaptic vesicles into synaptic cleft
- At the synapse (where the pre- and post-synaptic cell meet)
- AP causes voltage-gated Ca2+ channels to open
- Ca2+ influx causes neurotransmitter release by fusing the vesicle to the membrane
- Ca2+ binds to synaptotagmin protein present on the surface of vesicles
- Binding activates SNARE proteins on the vesicle and cell plasma membrane to associate
- Lipid bilayers of the cell membrane and vesicle membrane fuse together
- Neurotransmitters diffuses into the synaptic cleft
- Neurotransmitters bind to a receptor on the stimulus-gated Na+ channel
- Stimulus-gated Na+ channel opens and allows Na+ to diffuse into the cell
- Influx of Na+ causes a graded potential
What are the consequences of Refractory Periods (of Voltage-gated Na+ channels)
Consequences:
- Propagation of Action Potential: The refractory periods of voltage-gated Na+ channels make it so action potential can only travel in one direction, because Na+ diffuse in all directions as a result of chain reaction of Na+ channels opening down the axon, but b/c of refractory peiords channels that are “behind” the AP are in absolute refractory (since they have just been active and none have reset) and therefore the charge disperses before they reset and by the time they do the charge is no longer sufficient to activate them again. This allows the adjacent voltage-gated Na+ channels to be activated due to the influx and local diffusion of Na+ to adjacent channels without reactivating previously activated channels.
- The Neuron translates the strength of the original stimulus into the amount of neurotransmitters to be released : Relative refractory periods of the voltage-gated Na+ channels allows the few voltage-gated Na+ channels that have reset to respond if there is a stronger graded stimulus. A stronger stimulus (in the form of more local Na+ ions) will be needed to ensure that the few available channels can open and be able to send another AP. An increase in stimulus → temporarily increases the number of APs and → therefore more neurotransmitters are released. A stronger stimulus can occur due to temporal or spatial summation. Temporal summation is the accumulation of multiple signals/graded potentials. Spatial summation is when more neurons are “talking” to the target neuron.
- Speed of the Action Potential can be Increased : Saltatory conduction is when because of the myelination of axon results in localized spaces (nodes of Ranvier = spaces between myelination containing voltage-gated Na+ and K+ channels), faster propagation occurs due to LESS K+ ion leakage. There is less K+ ion leakage because there are fewer leaky K+ channels in the membrane where parts of the neuron are wrapped by Schwann cell myelination, which avoids loss of positive charge inside the cell, making it easier to move sodium ions down the channel. An increased diameter of axon can also result in increased rate of the impulse because a larger diameter allows molecules to be able to move faster because it decreases the friction.
Explain the process of action potential.
(1) Resting Membrane Potential (RMP) : -70 mV
- At RMP, voltage-gated Na+ channel is not active
- activation gate is closed
- Inactivation gate is open
(2) Depolarizing Stimulus
- Graded Potential
- -initial change in membrane potential occurs via simple diffusion of Na+ ions into postsynaptic cell
- -stimulus-gated channels (in dendrites & cell body) open upon receiving stimulus (usually when a neurotransmitter binds) → Na+ travels through channel via simple diffusion → dendrites receive an influx of Na+
- -a graded potential must be strong enough, aka meaning enough Na+ ion flow into neuron for neuron to fire an action potential
- -positive charge from the graded potential is “summed” at the trigger zone (beginning of axon)
(3) Membrane depolarizes → to threshold voltage (-55 mV). Voltage-gated Na+ channel and Voltage-gated K+ channels begin to open
- Activation gate (of voltage-gated Na+ channel) opens at threshold voltage (-55 mV)
- Occurs as a response to change in charge by activity of voltage-sensitive helices in membrane
- Positive charge causes conformational change in the voltage-sensitive 𝛼-helix
- At -55 mV there is enough Na+ to create a positive charge that will cause the 𝛼-helix to change conformation
- Inactivation gate at this point remains open
- Inactivation gate closes ~1ms later
- Na+ influx stops
- Inactivation gate prevents too much Na+ coming into cell
- ACTION POTENTIAL!!!
- -AP sends signals (neurotransmitters) to next neuron
- -results in graded potential in next neuron
- -once a graded potential becomes strong enough, then an AP occurs → process repeats
- -an AP is governed by the activity of the voltage-gated Na+ channels, only lets Na+ ion in when the voltage-gated channel is activated
(4) Rapid Na+ entry depolarizes cell
- Gating of voltage-gated Na+ channels controls AP movement and character
- 1st → Voltage-gated Na+ channels open
- Local Na+ ion influx
- Membrane potential rises to +30 mV due to many channels opening in the local vicinity
(5) Na+ channels close, & slower K+ channels open
- Inactivation gate closes ~1ms later
- Na+ influx stops
- Inactivation gate prevents too much Na+ coming into cell
- Gates will resent after signal passes
- All gating changes based solely on conformational change in protein
- There are at least 9 isoforms of the V.G. Na+ channels
- Different neurons use different isoforms of Na+ channels
- Na+ channels close ~ 1 ms latter
- Relative Refractory (occurs after some time has passed)
- Some channels in local area begin to reset and become sensitive to charge
- Excitability of membrane starts to increase back toward normal
- Na+ channels will reset in somewhat random order until they are all completely reset
- Membrane slowly increases the ability to respond to charge
- Voltage-gated K+ channels open
- Also gated by charge, but open more slowly
- Called “delayed rectifying” channels
(6) K+ moves from cell → to ECF (extracellular fluid)
- Causes K+ to leave the cell (efflux of K+)
(7) K+ channels remain open & additional K+ leaves cell, hyperpolarizing it
(8) Voltage-gated K+ channels close, less K+ leaks out of cell
(9) Cell returns to resting ion permeability and resting membrane potential (RMP): -70 mV
- Membrane returns to RMP by efflux of K+
- Absolute Refractory (occurs right after a neuron has undergone AP)
- The activation gate opens and the inactivation gate closes
- Soon after the inactivation gate closes, all the Na+ channels in the
Explain Resting Membrane Potential and the factors contributing to it.
The resting membrane potential of a neuron is -70 mV. Two factors influence membrane potential:
- The uneven distribution of ions across the cell membrane. Normally, sodium (Na+), chloride (Cl-), and calcium (Ca2+) are more concentrated in the extracellular fluid (ECF) than in the cytosol. Potassium (K+) is more concentrated in the cytosol than in the extracellular fluid (ECF)
- Differing membrane permeability to those ions. The resting cell membrane is much more permeable to K+ than to Na+ or Ca2+. This makes K+ the major ion contributing to the resting membrane potential
Gated Channels Control the Ion Permeability of the Neuron:
- Mechanically gated ion channels are found in sensory neurons and open in response to physical forces such as pressure or stretch
- Chemically gated ion channels in most neurons respond to a variety of ligands, such as extracellular neurotransmitters and neuromodulators or intracellular signal molecules
- Voltage gated ion channels respond to changes in the cell’s membrane potential. Voltage gated Na+ and K+ channels play an important role in the initiation and conduction of electrical signals along the axon.
What is a graded potential?
Graded potentials are the initial change in membrane potential that occurs via simple diffusion of Na+ ions (that at rest are on the outside of the cell) into the postsynaptic cell.
Explain Electrical Signaling in the Nervous System
Signaling takes advantage of the resting membrane potential (RMP) -70 mV. Charge separation across the membrane will change rapidly yet transiently. Relatively few ions need to cross the membrane to get a large change in charge. This change in charge happens locally right on either side of the membrane. When the change in charge (the graded potential) is great enough, meaning it reaches the threshold voltage -55 mV the neuron will fire an action potential.
Neurotransmitters
Neurotransmitters (NT)
- Many different types are made from amino acids
- Provides easy and economical way of forming many different types of nt
- Can be excitatory or inhibitory on the postsynaptic cell
- GABA→ main inhibitory NT of the CNS
- Glutamate→ main excitatory NT of the CNS
- Neurotransmitters are never a “one size fits all”
Central Nervous System:
BRAIN
Brain
- highly vascularized
- Makes big demands on body resources
- Brain is only 2% of our body weight, but 15% to 25% of our cardiac output (depending on activity)
- Cerebrospinal fluid (CSF) circulates around brain/spinal cord
- The brain “floats” in CSF
- Cushions/protects the brain
- Helps to reduce blood pressure
Central Nervous System:
SPINAL CORD
Spinal cord architecture
- Signals come and go through roots
- Ganglia are collection of sensory neuron cell bodies in the roots
- Ex: dorsal root ganglion
- Ganglia are collection of sensory neuron cell bodies in the roots
- Information travels up and down the spinal cord through interneurons
- Interneurons: a neuron in the CNS that synapses with another neuron
- How information travels in the spinal cord:
- Information enters through dorsal roots via sensory neurons
- Sensory neurons synapse with interneurons in dorsal horn
- Interneurons in ascending tracts take information up the spinal cord and to the brain
- Processing and decisions are made in the brain
- “Decision” information from the brain travels down the spinal cord through interneurons in the descending tracts
- Interneurons synapse with efferent neurons in ventral horn
- Information exits ventral roots via efferent neurons
- Information is taken to the designated part of the body
- Information enters through dorsal roots via sensory neurons
SPINAL CORD DIAGRAM
CNS
BRAIN FUNCTION
- The brain functions by regions (location, location, location)
- The brain is assembled in a hierarchical fashion
- Brainstem
- Involved in rudimentary, life-supporting functions
- “Keep the person alive!”
- Ex: breathing, heart rate, blood pressure
- Midbrain is the “relay station”
- Midbrain contains the thalamus, hypothalamus, and pituitary gland
- Thalamus: receives information and routes it to the correct part of the brain→ the “relay station”
- Hypothalamus: homeostasis command center→ basic response to stimuli
- Pituitary: controls the body’s hormones→ basic response to stimuli
- Midbrain contains the thalamus, hypothalamus, and pituitary gland
- Cerebrum is the rest of the brain
- Contains the majority of the brain’s volume
- The cerebral cortex is the outer shell (~2 mm) of the cerebrum
- Densely packed with neurons
- Cerebral cortex is involved in higher cognition
- All the advanced function in humans is due to our cerebral cortex
- Contains the primary motor cortex
- Part of the cerebral cortex located on the frontal lobe
- Controls motor function
- The vast majority of the cerebrum is involved in cortex intercommunication
- Brainstem
- The brain is assembled in a hierarchical fashion
CNS
Brain Function:
REGION SPECIFIC ORGANIZATION
- Region specific organization on the cerebral cortex
- Ex: primary motor cortex
- Plans and executes body movement
- Has “sub-regions” devoted to specific muscles at specific body parts
- Ex: primary motor cortex
- Region-specific organization by hemisphere
- Left hemisphere:
- Involved in logic, reasoning, etc.
- Basic logic/interpretation of things
- Mathematical reasoning
- Involved in logic, reasoning, etc.
- Defects (lesions) in the left hemisphere:
- Aphasia→ defect in language ability
- Broca’s area (“speech” forming area of the brain):
- Causes defects in ability to form/speak words
- Wernicke’s area:
- Causes defects in language comprehension
- Lose the ability to interpret speech
- Cannot be lied to because they don’t understand the words
- Broca’s area (“speech” forming area of the brain):
- The Oliver Sacks example:
- Aphasics thought Reagan’s speech was humorous
- They were Wernicke’s area aphasics→ they were-hypersensitive to his mannerisms
- All they saw was his mannerisms, movements, and inflections
- They couldn’t process the meaning of his words
- Aphasics thought Reagan’s speech was humorous
- Aphasia→ defect in language ability
- Right hemisphere:
- Involved in feeling, emotion, abstraction, etc.
- More creative
- Big picture/ imagination
- Defects (lesions) in the right hemisphere:
- Agnosia→ defect in emotional content or meaning
- Can only understand word meaning
- Can’t understand slang, body language and tone
- The Oliver Sacks example:
- Agnosic thought the speech by Reagan was unintelligible
- His words weren’t “precise” enough→ his speech seemed misleading or he seemed unstable in his delivery
- The agnosic couldn’t process the emotional meaning behind the words→ was only left with the exactness of the actual words
- Agnosic thought the speech by Reagan was unintelligible
- Agnosia→ defect in emotional content or meaning
- Involved in feeling, emotion, abstraction, etc.
- Left hemisphere:
Organizaiton of Nervous System
