Test 3

• Sensory input
• neural integration
• motor output

transmitting impulses/signals from sense organs(receptors) to CNS for interpretation, function of PNS

decision making, only happens in CNS

transmitting impulses (commands) from the CNS to effectors so that they can carry it on, function of PNS

The NS communicates electrically (with action potentials) and chemically (with neurotransmitters), to send messages quickly from the CNS (the brain and spinal cord) to the periphery of the body (skeletal muscles) and from the periphery of the body (receptors) to CNS

CNS: Brain (enclosed and protected by the cranium) and spinal cord (enclosed and protected by the vertebral column). Have somas of interneurons.

PNS: Nerves=a collection of nerve processes/axons (ex. spinal and cranial nerves) and ganglia=a collection of somas of neurons (like dorsal root ganglion). Has sensory (afferent) and motor (efferent division); somatic and visceral (autonomic) division. Have axons of sensory and motor neurons.

The Central Nervous System and The Peripheral Nervous System

• Somatic
• Autonomic (visceral)
• Sensory
• Motor (skeletal muscle, smooth muscle, cardiac muscle, glands)

this division mainly controls skeletal muscles of the body. It is voluntary.

(visceral motor division) It carries signals to internal organs (viscera) like glands, cardiac muscles, and smooth muscles. It is involuntary.

• Sensory receptors
• afferent pathway
• control center (in CNS)
• efferent pathway
• effectors

• specialized structures that detect changes, aka stimuli or signals=sensory input (temperature, pressure, touch, etc.). 
• Examples: the eyes, Merkel/tactile cells, ears, temperature and pain receptors (nociceptors), Pacinian, Meissner, hair receptor

nerve pathway through which sensory input is transmitted to CNS from receptors. Formed by axons of sensory neurons.

(In CNS) interneurons that analyze the sensory input and make a decision to respond to the change. Interneurons are also called association neurons.

nerve pathway through which motor output is sent away from CNS to effectors. Carried by axons of motor neurons.

Motor Output- carries out the decision. Examples: skeletal muscles contract; smooth muscle relaxes blood vessels; glands produce sweat.

Neurons and Glial cells (neuroglia)

• Neurons are structural units of the nervous system, large cells with dendrites and axons.
• Neuroglia are 10:1 to neurons. Help neurons do their jobs, highly mitotic.

excitability, conductivity and secretion.

aka responsiveness, ability to respond to a signal/stimulus

the ability to conduct an electrical signal (think axons)

ability to produce and eliminate chemicals called neurotransmitters (think vesicles with neurotransmitters (NT) in axons

sensory, interneuron/association neuron and motor neuron

• Sensory neurons conduct afferent/sensory information to CNS from receptors. Responsible for carrying sensory input.
• Interneurons make decisions. Are responsible for neural integration. Always in CNS.
• Motor neurons conduct efferent/motor information from CNS to effectors. Responsible for carrying motor output.

this neuron has ONE extension coming off the soma that divides into a dendrite and an axon

Has ONE dendrite and ONE axon. Rare.

Has MULTIple dendrites and ONE axon. Most common neuron.

Unipolar, bipolar, and multipolar

Dendrites. Usually short and branching. Local potentials are usually created on dendrites and then travel to axon hillock. A neuron usually has multiple dendrites

Soma –this part of the neuron contains the nucleus and other organelles

Axons. A neuron has one axon. The axon can have a myelin sheath(myelinated axons/fibers) or lack a myelin sheath (unmyelinated axons/fibers). Action Potential starts here

Seen in sensory neurons of dorsal root ganglion (outside CNS)

Found in sensory organs (smell and vision)

Found mostly in CNS

• Oligodendrocytes
• Ependymal cells 
• Microglia 
• Astrocytes

form the myelin sheath in the brain and the spinal cord

line the cavities of the brain (ventricles) and spinal cord (central canal)

phagocytize and destroy microorganisms, foreign matter, and dead nervous tissue(macrophages).

participate in forming the Blood-Brain Barrier

• Schwann cells
• Satellite cells

these glial cells form the neurilemma and the myelin sheath around all PNS nerve fibers. (has to do with the exterior of PNS nerve fibers)

provide support to neurons: electrical insulation and regulate chemical environment of neurons. (has to do with the interior of PNS nerve fibers)

Across the plasma membrane of neurons and muscle cells, the voltmeter measures a voltage (from -40mV to -90mV). This potential difference in a resting cell is called a resting membrane potential (Vm) and the membrane is said to be polarized. This resting potential exists only across the plasma membrane

measure of potential energy generated by separated charge (V or mV). It is the potential between two points. The greater the difference in charge, the higher the voltage

flow of electrical charge from one point to another. Depends on voltage and resistance

the ability to transport/allow solutes through a membrane. Ex: membrane is permeable to K and Na, but impermeable to negatively charged ions.

Solutes that cannot diffuse directly through the membrane will require channels to cross the membrane. When ion channels are open, these ions can pass through easily and the permeability is increasing. When ion channels are closed, solutes will not be able to pass through and the permeability will decrease.

Ex: in depolarization wave of AP, permeability to Na increases due to opening of additional voltage-gated Na channels, while during repolarization wave of AP, permeability to Na decreases, since the voltage gated Na channels are inactivated/closed

• Anions- high ICF (stuck inside)
• Sodium- high ECF, low ICF
• Potassium- high ICF, low ECF
• Chloride- high ECF, low ICF

• concentration gradient: concentration across plasma membrane due to selective permeability. (normally permeable to Na and K)
• electrical potential: voltage difference across membrane that depends on concentration gradient and selective permeability

• Constantly open:  are always open and allow ions to move in and out of the cell as long as the solute fits through the channel opening (reason why plasma membrane is normally selectively permeable to Na and K)
• Closed: these channels ONLY open in response to signals

• ligand-gated
• voltage-gated
• mechanically-gated

open in response to chemicals attaching to them (ex: ligand-gated Na channels open in response to ACh)

open in response to voltage change (ex: voltage gated Na channels on the axon hillock open at threshold and generate an AP)

open in response to physical signals (like stretch)

Resting membrane potential: a difference in charge across the membrane

a)the concentration gradient of ions: more Na in ECF and less in ICF; more K in ICF and less in ECF; high concentration of anions in ICF and b) permeability of membrane to ions: permeable to Na and K and impermeable to anions

Constantly open ion channels allow ions (K and Na) to pass through the plasma membrane due to their concentration gradient: Na diffuses into the cell and K diffuses out of the cell. Vm is maintained by the Na+/K+ pump that works continuously to actively move Na out of the cell and K into the cell. 

• it maintains concentration gradient where theres more Na in ECF (positive charge on the outside of membrane) and more K in ICF (negative on the inside of the membrane due to large concentration of anions in ICF). 
• Vm has a negative sign, because the inside of the membrane is negatively charged due to anions (stuck in the cell due to impermeability of membrane).

• voltage-gated Na (fast) channels open first and cause depolarization wave of AP (Na rushes in and brings + charge)
• voltage-gated K (slower) channels open at peak of AP (when Na channels close). K moves out and takes away + charge causing repolarization. as well as a short hyperpolarization at the end of AP.

Minimum electrical change required for voltage gated channels to open and generate an AP. Usually around -55mV. If threshold is not reached, voltage gated channels cannot open and an AP cannot be generated.

Vm decrease. Inside of plasma membrane becomes less negative (closer to 0)

Vm increase. The inside of the plasma membrane becomes more negative (moves further from 0)

return to a resting membrane potential (moves closer to original value)

During action potential, voltage-gated channels (Na and K) will not open and these drastic changes in voltage (de-, hyper-, re-polariztion) repolarization not possible.

• Chemicals attach to ligand-gated channels on dendrites and cause local potential
• local potential travels to axon hillcock (trigger zone)
• If local potential is strong enough to reach threshold at trigger zone, voltage-gated Na channels open
• Na ions rush into neuron and start depolariztion wave of AP
• Once Na channels are inactivated, voltage-gated K channels open fully and K exits neuron---> repolarization
• action potential (nerve signal) is conducted along axon. Unmyelated- slower conduction. Myelated- faster

a nerve signal

slower

The Na+/K+ pump allows for future action potentials by working continuously to maintain and restore the concentration gradient of a resting cell; more Na in ECF and more K in ICF. Which then allows for depolarization and repolarization of the next AP.

period of absolute resistance to stimulation = no stimulus will trigger an action potential.

period of relative resistance to stimulation = only a strong signal can trigger an AP

• Absolute: when Na+ gates are inactivated and no movement of Na is possible. If Na cannot move across the membrane and enter the cell, no depolarization is possible, Cell cannot reach the threshold, which means no AP can be generated
• Relative: any time that K+ gates are still open and the cell is hyperpolarized. K is moving out of the cell and depolarization is possible, but only in response to a very strong signal.

AP is only able to travel one way, from the axon hillock to the synaptic knob, allowing the passage of signals from sensory to interneurons to motor neurons and not the other way.

• In an unmyelinated axon, there are voltage-gated Na+ channels all along the length of the axon.
• The continuous wave of de- and re-polarization travels from one region of the membrane to the next (self-propagating AP)
• Called continuous conduction

• This is a type of conduction seen with myelinated fibers/axons.
• saltatory = to jump, the signal moves very fast through the myelin sheath to the next node of Ranvier.

• AP is only formed in the axon hillock and propagated by the voltage gated channels found in the Nodes of Ranvier
• the signal moves very fast through the myelin sheath to the next node of Ranvier.

a larger diameter will make an action potential travel faster along the axon because of less peripheral resistance.

an mylenated axon will conduct an AP (nerve signal) faster than an unmyelinated axon, since the signal can jump over the areas of the myelin sheath, called internodes

• Synaptic knob/axonal terminal
• Synaptic cleft
• Synaptic vesicles
• Presynaptic cells
• Postsynaptic cells
• Receptors

knob-like area that stores the neurotransmitters in synaptic vesicles and has voltage-gated Calcium channel that open in response to an AP to allow calcium ions to enter the axon terminal/synaptic knob

space between the pre- and post-synaptic neurons

hold the neurotransmitters before they are released

cell before synapse

cell after synapse

found on postsynaptic cell of synapse, bind the neurotransmitters, are ligand gated channels.

• Pre- cells that release neurotransmitters into synaptic cleft and send signal across synapse. 
• Post- cells have receptors for neurotransmitters and process signal

• 1. A nerve signal (AP) arrives at the synaptic knob
• 2. Voltage-gated Ca2+ channels in the synaptic knob open
• 3. Calcium ions enter the synaptic knob (bc concentration in ECF is higher)
• 4. Ca2+ triggers exocytosis of the NT (Ach, Epi, substance P or other) from the synaptic vesicles into synaptic cleft
• 5. The neurotransmitter diffuses across the synaptic cleft
• 6. The neurotransmitter binds to ligand-gated channels on post-synaptic cell
• 7. These channels open and allow ions like Na+ or Cl- to enter the post-synaptic cell
• 8. As ions enter the post-synaptic cell, they depolarize or hyperpolarize the postsynaptic cell (creating a postsynaptic potential-IPSP or EPSP)

• Acetylcholine (Ach)
• Catecholamines (Epinephrine and Norepinephrine)
• Neuropeptides (substance P)
• Amino acids (GABA)

Receptors are ligand-gated channels. These channels will only open when the NT binds to the receptor. Each receptor is specifically built to only bind a a specific neurotransmitter.

Ligand-gated channels open. They can be Na channels or Cl channels. Once open, channels allow ions to enter the postsynaptic cell and depolarize= excite (if Na+) or hyperpolarize=inhibit (if Cl-) the postsynaptic cell.

synapse in which acetylcholine (Ach) is the neurotransmitter

• Present in motor neurons responsible for activating skeletal muscles
• Ach binds to ligand-gated Na+ channels on postsynaptic cell to allow Na+ to enter the cell and depolarize it (EPSP)
• post-synaptic neuron is excited and is more likely to reach the threshold and “fire” (generate an AP to send along to next neuron)

this synapse uses amino acid (GABA) as neurotransmitter

• Present in motor neurons responsible for activating skeletal muscles
• GABA binds to ligand-gated Cl- channels on the post-synaptic cell to allow Cl- to enter the postsynaptic cell and hyperpolarize it (IPSP)
• post-synaptic neuron is inhibited and is less likely to reach the threshold and “fire” (generate an AP to send along to next neuron)

Excitatory postsynaptic potential (EPSP) – graded depolarization will bring a normal Vm closer to the threshold

Inhibitory postsynaptic potential (IPSP) – graded hyperpolarization will bring a normal Vm farther away from the threshold

• Sodium is main role. If it can make it into membrane-->depolarization (closer to threshold). 
• K ions leaving cell (take + charge)--> Repolarization
• Cl- enter thru ligand-gated channels--> negative charge--> hyperpolarization (away from threshold)
• Opening of K+ channels allows K to leave making inside more negative--> hyperpolarization. Until Na/K pump reestablishes concentration gradient

ONE presynaptic neuron stimulates ONE postsynaptic neuron multiple times within a brief period of time. All EPSPs in the postsynaptic neuron add up (summation) and ARE more likely to reach the threshold. The postsynaptic neuron is more likely to fire.

MULTIPLE presynaptic neurons stimulate ONE postsynaptic neuron. All EPSPs add up (summation) and ARE more likely to reach the threshold. The postsynaptic neuron is more likely to fire.

• (graded potentials) Localized changes in membrane potential.Need chemical neurotransmitters and depend on ligand-gated channels
• graded-magnitude varies in size, reversible, travel short distances, decrease in size/strength as they travel, excitatory (EPSP) or inhibitory (IPSP)

• more pronounced change in membrane potential with a reversal and total amplitude of about 100 mV.
• If local potential is strong at trigger zone, threshold (-55 mV) is reached, voltage-gated channels open and neuron “fires” (generates an AP)
• all-or-nothing law-if threshold is reached there is a full AP; if threshold is not reached, there is no AP, irreversible, travel a long distance, don’t decrease in size/strength as they travel, which means they have the same voltage at end of axons as they had in the beginning of axon

Ach and epinephrine (Epi)

amino acid - (GABA)