Bio Chapter 34- Neurons, Sense Organs, and Nervous Systems
Sympathetic vs. parasympathetic division
*The sympathetic and parasympathetic divisions always oppose each other -Sympathetic nervous system- all the nerves, neurons, axons that send information to a particular target that causes something to happen -Parasympathetic- does the same as the sympathetic, but opposite -Sympathetic nerve fibers cause a stimulation of heartbeat. Parasympathetic nerves slow it down -Sympathetic nervous system inhibits digestion. Parasympathetic activates it -Sympathetic relaxes airways. Parasympathetic constricts airways
Synaptic celft
-A narrow space, called the synaptic cleft, separates the presynaptic cell from the postsynaptic cell
Neuronal action potential- see slide 34
-Action potentials are transient changes in membrane potential measured at a defined location -These changes are simply caused by transient changes in membrane permeability -The temporary changes in membrane permeability involve just two ions: Na+ and K+ *Action potential = changes in permeability -Start at resting potential of -60. Since EK is negative and ENa is positive, the membrane potential has to be closer to EK than it is to ENa -Rapidly becomes more positive. Membrane permeability is changing. Membrane is becoming much more permeable to Na+ than to K+. ENa is positive, and the membrane potential is also positive -Becomes negative again and goes below the resting potential *This can all be understood based on changes in membrane permeability to K+ and Na+
Afferent pathways
-Afferent pathway- nerves carrying information to the CNS -Information coming into the CNS is often sensory in nature -Tempearture, sound, light
What do neurons share in common?
-Although neurons are highly variable in terms of structure and function, nearly all neurons share the following features: 1. Electrical excitability 2. Formation of synaptic connections with other cells 3. They contain specialized regions (extensions) of the cell body called axons and dendrites
Action potential
-An action potential is a rapid change in membrane potential that is caused by the opening and closing of ion channels in the plasma membrane of the neuron -The action potential can be divided into 5 phases: resting potential, threshold, rising phase, falling phase, recovery 1. Resting potential -The resting potential is the membrane potential of a neuron at rest -At this point, a small subset of K+ channels are opened, permitting K+ ions to enter and exit the cell based on electrochemical forces *There is no net movement of K+ ions. For each K+ ion that leaves the cell, another returns, maintaining the membrane potential at a constant value 2. Rise to threshold -As a depolarizing stimulus arrives at this segment of the membrane, Na+ channels open, permitting Na+ ions to enter the neuron -The increasing positive ions inside the cell depolarizes the membrane potential, making it less negative, and brings it closer to the threshold at which an action potential is generated 3. Rising phase -If the depolarization reaches the threshold potential, additional voltage-gated Na+ channels open -As positive Na+ ions rush into the cell, the voltage across the membrane rapidly reverses and reaches its most positive value -At the peak of the action potential, two processes occur simultaneously: --Many of the voltage-gated Na+ channels begin to close 4. Falling phase --Many more K+ channels open, allowing positive charges to leave the cell -This causes the membrane potential to begin to shift back towards the resting membrane potential -As the membrane potential approaches the resting potential, voltage-gated K+ channels are maximally activated and open -The membrane repolarizes beyond the resting membrane voltage -This undershoot occurs because more K+ channels are open at this point than during the membrane's resting state, allowing more positively charged K+ ions to leave the cell 5. Recovery -The return to steady state continues as the additional K+ channels that opened during the action potential now close -The membrane potential is now determined by the subset of K+ channels that are normally open during the membrane's resting state
Sound transduction in the human ear
-Base vibrates for high frequency -Basilar membrane vibrates for middle frequency -Apex vibrates for low frequency *When one part of the membrane starts to vibrate, hair cells in that region start to fire and send information to the brain
Overall ion concentrations do not change significantly
-Because there are so many ions inside and outside of cells, the number of ions that move across a membrane before net movement stops is only a very small fraction of the total ion concentration. -Thus, the bulk concentrations do not change significantly.
Central Nervous System (CNS)
-Brain -Spinal cord
How to determine the sign of a membrane potential
-By definition, the numerical sign of the membrane potential is always determined INSIDE with respect to outside of a cell -If there is an excess of negative charges INSIDE the cell relative positive countercharges, the membrane potential is negative -What are the signs of Vm? See slide 17 1. Negative 2. Zero
Predicting the membrane potential for a membrane permeable to more than one ion at a time (see slide 32)
-Consider a simplified cell with the following ion concentrations -If the membrane is permeable to both K+ and Cl- but much more permeable to chloride than to potassium or any other ion, what will be the sign of the membrane potential (Vm)?
Current
-Current is the movement of electric charges -In cells, current is based on movement of ions such as Na+, K+, Cl-, etc. -Current is the movement of electrons
Dendrites
-Dendrites receive electrical signals and send them to the cell body -Dendrites = "antennae" -Dendrites are extensions of the plasma membrane. They are highly branched -Dendrites are the portions of the nerve cells that receive signals from other neurons and send that information to the cell body, which will send information to the axon -There are many dendrites in every neuron
Deflection of the basilar membrane
-Different regions of the basilar membrane preferentially oscillate (deflect) in response to different sound frequencies
Diffusion of ions
-Diffusion of ions is controlled by both a concentration effect and an electrical effect -When they are equal, electrochemical equilibrium is reached
Efferent pathways
-Efferent pathways can be divided into the voluntary division and the involuntary/autonomic division -If you want to execute something, you need a neuron that sends information to a gland or organ- part of the efferent pathway *Efferent pathways Exit the CNS
Sensory cells
-First to encounter of information from the outside -Example- sensory cells in ear can sense sound. Sensory cells in your eyes can sense light. -Sensory cells receive stimuli and connect to a set of relay cells (generally neurons) -Information sent to specific areas of your brain that can make sense of the information
Glia
-Glia are also called glial cels -Glia continue to divide throughout adulthood -Glia- everything that is not a nerve cell -All sources of brain tumors come from dividing cells, which are glia
Glia
-Glia, or glial cells, provide support and maintain the extracellular environment of neurons -Glia outnumber neurons in the human brain, though they do not transmit electrical signals like neurons do -We will focused on two types of glial cells: Schwann cells and Oligodendrocytes
Stereocilia
-Hair cells have stereocilia ("hairs") that are physically displaced in response to sound and cause ion channels to open, generating a change in membrane potential (a receptor potential) -While they do NOT fire action potentials, they release neurotransmitters onto sensory neurons (relay cells), which then fire their own action potentials -Membrane potential is negative- nothing is happening. Then, they receive sound signal and depolarize and calcium enters the cell and release a neurotransmitter -Release neurotransmitter onto postsynaptic target
Diagram
-Has 3 axon termini
Hyperpolarized
-If the membrane potential becomes less positive than the resting potential, the membrane becomes hyperpolarized
Depolarized
-If the membrane potential becomes more positive than the resting potential (or whereever we start), the membrane becomes depolarized
Imbalance
-Imbalance implies that the two solutions on both sides of the membrane are not electrically neutral or balanced
Chemical synapse
-In a chemical synapse, neurotransmitters from a presynaptic cell bind to receptors in a postsynaptic cell
Membrane potentials
-In cells, we typically are interested in the transmembrane voltage, or membrane potential
Potassium leak channels *See slide 29
-In resting neurons, K+ ions diffuse OUT of the cell down their concentration gradient through potassium leak channels -These channels are K+ selective and are open all the time -Potassium ions move outside the neuron and leave an excess of negative charges inside the neuron
Concentrations of ions inside and outside animal cells
-Intracellular and extracellular ion concentrations are kept relatively constant and are established by ion pumps
Equilibrium potential vs. membrane potential
-Knowledge of an ion's equilibrium potential can help us to determine the sign of a cell's membrane potential -If the plasma membrane is selectively permeable to only one ion, the sign and magnitude of the membrane potential will equal the sign of that ion's equilibrium potential -If the plasma membrane is permeable to more than one ion at a time, then the sign (and magnitude) of the membrane potential will be a weighted average of each permeable ion's equilibrium potential
Membrane potentials - see slide 13
-Let's consider a simplified model where two fluid compartments (inside and outside) are separated by a plasma membrane -Only K+ and Cl- ions are present in solution -A K+ selective ion channel is present in the membrane, but is closed
Myelin
-Myelination increases the speed of action potential propagation down the axon -The thicker the myelin sheath, the faster the action potential propagates -Myelin electrically insulates axons so that charges don't leak out -Diseases such as multiple sclerosis that affect the amount or thickness of myelin impair action potential propagation -In MS, the myelin starts to break down and the speed of action potential propagation is really slow. Ions move out, and so depolarization is much less -Glial cells wrap the axon with a substance that electrically insulates the axon. That makes the movement of the action potential from the axon hillock to the axon terminal faster -It prevents charges from going out through the membrane -If you touch a hot plate, right after you touch it, it doesn't hurt. Small delay for action potential to propagate from the periphery to your spinal cord and into your brain. That's because those cells have less myelin than your other cells
Enteric division of the ANS
-Nerve cells internal to the gut wall -Regulates peristalsis and digestion
Afferent vs. efferent pathways
-Nerves can carry information towards the CNS (afferent) or away from the CNS (efferent)
Peripheral Nervous System (PNS)
-Nerves that connect the CNS with all tissues and sensors in the body *Sends messages to muscles, glands, etc.
Neurons
-Neurons are nerve cells -Most neurons are post-mitotic after birth -You are born with the vast majority of neurons in your brain and spinal cord at birth -Most neurons don't regenerate if you kill them (using drugs, etc.)
Electrical and chemical communication
-Neurons communicate with each other by means of electrical and chemical signals -Electrical signals pass down the axon and reach the axon terminal, where information is passed to other neurons either chemically or electrically at synapses -Electrical signals electrically activate the postsynaptic cell -A chemical can be secreted at the axon termini. That chemical will bind to the surface of the postsynaptic cell and cause it to activate electrically (example- glands and muscles)
What happens at the synapse?
-Neurons communicate with other neurons or target cells at synapses
Stroke
-Neurons in one part of the brain die -Even though sensory cells are sending information by relay cells to the brain, the brain doesn't know it's there -Same outcome as not having the sensory cells at all
ACh- see slide 44
-Once ACh has been released, it is quickly degraded by enzymes in the synaptic cleft
Electrical driving force
-Once you pass charges, you start to accumulate charge on one side vs. the other -See slide 20- as positive ions start to accumulate on the right, they will feel an electrical driving force in the opposite direction
Depolarization vs. Hyperpolarization
-Opening and closing ion channels allows neurons to change their membrane permeabilities, and thereby their membrane potentials, in response to a stimulus *Generally speaking, in a neuron, the starting point is the resting potential *See slide 33 -Depolarization, depolarization (less of a depolarizing stimulus), hyperpolarization
Parasympathetic neurons
-Parasympathetic neurons typically secrete acetylcholine
Peripheral nerves
-Peripheral nerves come off the brain and spinal cord -Nerves move down into the limbs and control movement. They send information to from the periphery to the CNS -Each nerve contains many axons, not just one -The spinal cord is continuous with the brain -In deuterostomes, the spinal cord is dorsal
Permeability
-Permeability is a measure of how readily ions can cross the membrane -The more channels are open, the higher the permeability -If a membrane is permeable to more than one ion at a time, but the relative permeability for one of the ions is much higher than for any other, the membrane potential (Vm) will be close to the equilibrium potential for the most permeable ion
Presynaptic and postsynaptic neurons
-Presynaptic and postsynaptic designations are relative to a particular synapse (or set of synapses) *Most often, a cell is both presynaptic and postsynaptic- it is all relative
Schwann cells
-Schwann cells are myelin-producing cells -Glial cells are responsible for myelination of axons -In the peripheral nervous system, they are called Schwann cells
Hair cells
-Sensory cells cells called hair cells in the inner ear convert mechanical pressure waves to electrical signals so that the nervous system can interpret the sound -Hair cells are specialized sensory cells
Receptor potentials- see slide 9
-Sensory cells convert sensory stimuli into electrical signals to generate receptor potentials -Unlike action potentials, the size and duration of receptor potentials varies with the strength of the signal/change in membrane potential -Sensory cells do NOT fire action potentials -Receptor potential- a change in membrane potential in a sensory cell in response to receiving a sensory signal -Stronger signal, then weaker signal on graph (less depolarization and time is shorter for second receptor potential)
Detecting sensory information
-Sensory signals such as sound, light, etc. are detected by sensory cells, which convert sensory signals into electrical signals -These are ultimately sent to specific areas of the brain (sensory cortex) that can interpret information -Sensory cells are often modified neurons that do not fire action potentials themselves, but they can cause postsynaptic target cells to fire action potentials -Sensory stimuli --> Sensory cells --> Relay cells (neurons) --> Specific areas in sensory cortex (brain)
Sympathetic neurons
-Sympathetic neurons typically secrete norepinephrine -Norepinephrine can accelerate heartbeat
Synapses
-Synapses are cell-to-cell contact points specialized for signal transmission -A signal initiated in one neuron will be passed to one or more target cells to activate that cell so that it secretes something (neurotransmitter, chemical, etc.) or activate a gland *See slide 8- many synapses occurring on one target cell
Na+/K+ ATPase pump
-The Na+/K+ ATPase pump establishes concentration gradients for Na+ and K+ -The pumps work all the time to maintain these gradients! -In mammals: Na+ in = lower, Na+ out = higher K+ in = higher, K+ out = lower
Nodes of Ranvier
-The Nodes of Ranvier are regularly spaced gaps where the axon is note covered by myelin, and where voltage-gated Na+ and K+ channels are clustered (allow permeability of Na+ and K+ to change)
Features of the action potential
-The action potential is an all or none event. Once the threshold is reached, the voltage-gated Na+ channels will open, and an action potential will occur -The shape is always the same- depolarizes, hyperpolarizes, and then comes back -Action potentials initiate at the axon hillock and then spread down the axon towards the axon terminal
Axon
-The axon generates and conducts electrical signals (action potentials) -The axon is usually the longest extension -There is only one axon in every neuron -Information that comes from the cell body will be transmitted down to the axon termini -The axon can branch and extend the number of target cells it can communicate with
Axon terminal
-The axon terminal is the region at the tip of the axon that forms synapses and releases neurotransmitters -Axon termini- where connections with other cells happens
Cell body
-The cell body contains the nucleus and many organelles -The cell body receives electrical signals from other cells -The cell body is also called the soma
Chemical driving force
-The chemical force is due to the concentration gradient -See slide 20- red and blue ions want to go from left to right because there is a higher concentration on the left than the right for both ions
Equilibrium potential
-The equilibrium potential (E) is the membrane potential at which the net movement of a particular ion stops -At the equilibrium potential, chemical and electrical gradients are equal and opposite -Each ion has its own equilibrium potential. For instance, EK is the potassium equilibrium potential and ENa is the sodium equilibrium potential *You should be able to predict the sign of an ion's equilibrium potential under a given set of conditions
Action potential frequency- see slide 41
-The frequency of firing action potentials seems to be a common method for encoding information -The shape and amplitude of an action potential waveform are the same in a given neuron -Pain intensity, pressure intensity, etc. might be encoded by different frequencies -High pain or pressure might have a higher frequency -Less intense signal- less pain, pressure, etc. has a lower frequency
Example of sensory detection: Sound detection
-The human ear funnels and amplifies sound waves that enter the auditory canal -Sound creates sound waves that displace the air -Displacements in the air are detected by the eardrum -The eardrum vibrates in response to the sound and sends vibrations onto the inner ear (looks like a snail) -Hair cells are sensory cells in the inner ear
Involuntary/autonomic division of efferent pathways
-The involuntary/autonomic division of efferent pathways controls physiological functions
Undershoot- see slide 36
-The membrane potential becomes more negative than it is at rest -PK is maximum at that time point
Transmembrane voltage/membrane potential (Vm)
-The membrane potential is the voltage that exists across a membrane on the inside vs. the outside -Anytime that charges cross the membrane and cause the solutions on both sides to have an imbalance of + and - charges, there will be a membrane potential -In order to get a potential difference, there must be two solutions on either side of the membrane. If one solution has an excess of one kind of charge, there is a net imbalance
Neuromuscular junction steps- see slide 43
-The neuromuscular junction is a chemical synapse between motor neurons and skeletal muscle cells 1. An action potential causes voltage-gated Ca+ channels to open in the presynaptic membrane, allowing Ca+ to flow in 2. The presynaptic neuro releases acetylcholine (ACh) from its axon terminals when vesicles fuse with the membrane 3. ACh diffuses across the cleft and binds to ACh receptors on the muscle cell -Channels selecting for calcium ions are embedded in the axon terminus -Ca2+ ions are voltage-gated. When the membrane depolarizes, they will open -Since the concentration of Ca2+ ions is higher outside the neuron than inside, ions flow into the neuron. This makes the inside of the plasma membrane of the axon terminus more positive -Calcium ions then bind to proteins inside of the axon terminus. This facilitates the fusion of synaptic vesicles with the plasma membrane of the axon terminus -This releases neurotransmitters into the synaptic cleft. They bind to targets on the postsynaptic membrane -The neurotransmitter is ACh (acetylcholine). ACh binds to ion channels that are embedded in the postsynaptic membrane, and that causes the postsynaptic membrane to depolarize
Neuromuscular junction
-The neuromuscular junction is the synapse between neurons and muscle fibers -Motor neurons tell the muscle fiber to fire their own action potentials to release calcium and cause myosin/actin to contract -The postsynaptic target is a muscle -When the action potential reaches the axon terminus, it will communicate with the post-synaptic cell in terms of a chemical signal. That chemical signal is stored in the axon terminus and is released when the action potential arises at the axon terminus. Then, the chemical message binds to the postsynaptic membrane
Ossicles
-The ossicles amplify the vibrations and transmit the pressure signal to the cochlea, which is filled with fluid -Eardrum amplifies movement by being connected to three bones called the ossicles -Amplify vibrations and send them to part of the inner ear called the cochlea -Inside the fluid-filled cochlea, hair cells cause the basilar membrane to oscillate in response to specific sound frequencies -You have hair cells that only respond to a particular frequency
Parasympathetic division of the ANS
-The parasympathetic division works in opposition to the sympathetic division
Postsynaptic neuron
-The postsynaptic neuron receives the message -The postsynaptic cell is the target cell
Presynaptic neuron
-The presynaptic neuron sends the message
Axon hillock
-The region just beyond the cell body, towards the end of the axon -The action potential initiates at the axon hillock -Moves down the axon to the axon terminal
Neuronal resting membrane potential
-The resting membrane potential (RMP) is the membrane potential of a "resting", or inactive, neuron -The resting membrane potential is typically between -60 and -70 millivolts (mV) -This means that the inside of the neuron has a slight excess of unbalanced negative charge at rest, and the outside has a slight excess of unbalanced positive charge -The neuronal resting potential is mostly due to the movement of K+ ions through potassium leak channels, which are always open
Sympathetic division of the ANS
-The sympathetic division prepares the body for emergencies (fight or flight)
Positive charges are balanced by negative charges
-The total number of positive and negative charges inside of a cell and outside of a cell is generally balanced -But, charges can be distributed unequally in different regions of the cell to give rise to voltages -Even though the overall charge of an entire cell and its surroundings is mostly balanced, there are areas where imbalances exist *Don't need an equal distribution of charges in each subcompartment of the cell, but the total charge is neutral
Tympanic membrane
-The tympanic membrane (eardrum) covers the end of the auditory canal and vibrates in response to sound (pressure) waves.
Electrical signals in the nervous system
-The unique electrical signals that move down the axon are called action potentials
Autonomic nervous system
-The vertebrate autonomic nervous system has three divisions: 1. Sympathetic division 2. Parasympathetic division 3. Enteric division
Voluntary division of efferent pathways
-The voluntary division of efferent pathways executes conscious movements
Relative permeability
-The weighting is determined by each ion's relative permeability -If one ions is more permeable than the rest, its sign is the sign of the membrane potential
Consider example 2 from the previous slide. Does a membrane potential exist after K+ channels are opened and time has passed? See slide 16
-There is a membrane potential -There is a net accumulation of charges on either side
Consider two different examples (slide 14). Does a membrane potential exist in either of the two examples shown?
-There is no membrane potential in either example -There are an equal number of positive and negative charges on both sides of each compartment
Now, let's open the K+ selective ion channels in the membrane. Will there be a net flow of K+ ion in either example? See slide 15
-There will be a net flow of K+ ions in the second example -K+ will go from high concentration to low concentration
Action potentials
-To understand how action potentials arise, we need to understand how ions are distributed across the neuronal plasma membrane -Action potential- the form of the electrical signal that is passed down the axon
Two forces regulate ion movement
-Two forces influence the movement of K+ ions across the membrane: 1. A chemical driving force due to the concentration gradient 2. An electrical driving force *At some point, net flow will stop. This occurs when the chemical driving force and the electrical driving force are equal and opposite each other.
Voltage
-Voltage, or electrical potential difference, exists if there is a net separation of positive and negative charges between two points/locations (one side of the membrane vs. the other) -Voltage exists ACROSS a membrane (NOT through or in a membrane)
What is required to trigger an action potential?
-Voltage-gated Na+ and voltage-gated K+ channels are responsible for action potentials - they cause sudden, large changes in membrane potential initiated by changes in membrane voltage. -Voltage-gated Na+ channels open in response to slight depolarization beyond a threshold value; then they deactivate. -Voltage-gated K+ channels also open in response to depolarization beyond the neuron's resting potential, just with a slight delay. They also deactivate soon thereafter.
Information is encoded in...
...action potential frequency
Communication can be...
...electrical and chemical
Nervous systems are composed of...
...neurons and glial cells
Neurons communicate at...
...synapses
Most neurons have four structural regions
1. Cell body 2. Dendrites 3. Axon 4. Axon terminal *See diagram on slide 6
Animals nervous systems have two main cell types:
1. Neurons 2. Glia *Both categories contain multiple cell types that differ in their structures and functions
Action potential problem- see slide 36
Compare and contrast the predicted relative membrane permeabilities to Na+ (PNa) and K+ (PK) at points 1, 3, and 4 -Time point 1- negative membrane potential. Therefore, PK is much higher than PNa. If PNa were higher, the membrane potential should approximate the equilibrium potential for Na+, which we know is positive
Relative concentrations for Na+ and K+ under standard conditions
Na+ -Intracellular = lower -Extracellular = higher K+ -Intracellular = higher -Extracellular = lower
Examples
See slide 25, 26
Consider the hypothetical below. If intracellular K+ and anions were increased from 100 to 150 mM, would EK be affected in any way? If so, how?
See slide 27
Vertebrate nervous system
The vertebrate nervous system is composed of: -Central nervous system (CNS) -Peripheral nervous system (PNS)
