Nerve, Muscle, and Synapse Flashcards
Stretch Reflex
the simplest stimulus - response paradigm that the human nervous system can generate
Muscle contraction in response to stretching within the muscle
Patellar-tendon stretch reflex
- tap the patellar tendon which attaches to the quads
- quad muscle stretches, making quads longer
- activation of nerve impulses in special receptors located in the quads
- nerve impulses are sent back to the spinal cord along the sensory neuron and activated another nerve cell which feeds back out onto the quads muscle, activating muscle and causing it to contract
- a jerk or swing of the foot outwards
Withdrawal Reflex
spinal reflex to protect the body from damaging stimuli
Components of Central Nervous System
Brain
Spinal Cord
Composed of the cerebral cortex, the cerebellum, the brain stem, and the spinal cord
Components of Peripheral Nervous System
Peripheral nerves
Receptors
2 Types of Cells in the Nervous System
Neurons
Glia
Afferent Neurons
Take information from the periphery to the CNS via the dorsal roots
Efferent Neurons
Take information from the CNS back out to the periphery via the ventral roots
Interneurons
Carry information between neurons
Glia
The glue of the nervous system
Oligodendrocytes
Makes myelin - important in action potential conduction in the CNS
Schwann Cells
Makes myelin for the PNS
Astrocytes
Physically and metabolically support neurons by buffering extracellular K+, removing the transmitter
Helps maintain the blood-brain barrier
Role in signalling and information processing
Microglia
serve an immune function in CNS
Reflex Loop
Circular in nature
Activation of receptor activates an afferent fibre which enters the dorsal root of the spinal cord -> activates an efferent fibre which leaves through the ventral root of the cod -> the efferent fibre will activate a muscle
Myelin
Coats the axons of neurons and allows them to transmit the nerve impulses more quickly
White Matter
Neurons with myelinated axons
Gray Matter
Neurons without myelinated axons
Interneurons
Dendrites
Receive information from the periphery or from other cells
Cell Body
Also called a soma - contains a nucleus
Axon Hillock
The very initial segment of the axon - processes information coming into the dendrites and generates a nerve impulse if there is enough input
Axon
Propagates nerve impulses from initial segment to axon terminals
Axon Terminals
Contains neurotransmitters in synaptic vesicles
Synapse
The junction between two neurons
Presynaptic Neuron
The neuron before a synapse
Postsynaptic Neuron
The neuron after a synapse
How does Information Flow in Neurons?
One direction only
dendrites + cell bodies down the axon towards axon terminals
Membrane Structure
Protein pumps and channels
- control movement of ions through the membrane
- pumps = active transport
- ion channels = passive or active transport
Resting Membrane Potential
The electrical potential difference between intracellular environment and extracellular environment
- -70 mV
- involves K+, Na+, Cl-
How is Net Negative Charge in the Membrane Set?
Na+/K+ pump is electrogenic, moving charge across the membrane
3 Na+ molecules move out of cell, 2 K+ molecules move into the cell
-net negative charge
How does Sodium Potassium Pump Create Gradients
Chemical Gradients -K+ wants to diffuse out of the cell -Na+ wants to diffuse into the cell Electrical Gradients -intracellular environment wants to become more positive
Leak Channels
- always open
- allow the passive flow of ions in and out of the neuron
- selective: each ion has its own leak channels through which only they can pass
K+ Leak Channels
The electrical force pushes K+ into the cell
The chemical force pushes K+ out of the cell
K+ tries to achieve a membrane potential closer to -90mV
Permeability is greater for K+, so it will be closer to that equilibrium than the equilibrium of Na+
Na+ Leak Channels
The electrical force and chemical force pushes Na+ into the cell
Na+ tries to achieve a membrane potential closer to +55mV
Action Potentials
Specific stimuli disrupt resting membrane potential by causing ion-selective channels to open
A large change in membrane potential from -70mV to +30mV and back to rest over a period of a few ms
Main types of ion channels
- voltage-gated ion channels
- ligand-gated ion channels
How Afferent Neurons are Activated
Sensory stimuli result in the increased opening of Na+ receptors, entry of Na+ into afferent fibre and depolarization of the afferent neuron
Threshold
If Na+ entry is sufficient to depolarize the neuron to its threshold (-50mV) the result is the opening of voltage-gated channels and an action potential
Voltage-Gated Ion Channels
- membrane depolarization opens voltage-gated Na+ channels
- depolarization removes the activation gate, allows Na+ to flow into the cell
- depolarization results in the opening of voltage-gated K+ channels and repolarization
Steps of Action Potential
- Resting membrane potential - voltage-gated Na+ channels are in the resting state and voltage-gated K+ channels are closed
- Stimulus causes depolarization to threshold
- Voltage-gated Na+ channel activation gates are open
- Voltage-gated K+ channels are open, Na+ channels are inactivating
- Voltage-gated K+ channels are still open, Na+ channels are in their resting state
- Resting membrane potential
Depolarizing Phase
Influx of Na+, membrane potential is positive
Repolarizing Phase
Influx of K+, membrane potential is negative
Refractory Period
When another action potential cannot be fired
Steps of Action Potential Transmission
- Axon is at resting membrane potential
- Activation results in the opening of voltage-gated ion channels
- Local depolarization of membrane causes adjacent voltage-gated na+ channels to activate
- New action potential is generated in the adjacent membrane
- Action potential only travels in one direction due to the refractory period
Electrotonic Conduction
Spread of a current inside of the axon
Myelination
- acts as an insulator and does not allow ions to move across axon where the myelin is
- faster transmission of action potential
Nodes of Ranvier
Myelination is discontinuous - nodes of Ranvier
Contains the voltage-gated Na+ channels as they aren’t found in myelinated regions of the axon
Saltatory Conduction
Electrotonic conduction at the node of Ranvier
Factors that Determine the Speed at which an Axon Potential Propagates Along Axon
- the size of the axon - the thicker the axon is in diameter, the faster it can propagate an action potential
- myelination - myelinated axons propagate action potentials faster than unmyelinated axons
Synaptic Transmission
The process whereby one neuron communicates with other neurons or effectors such as muscle cells at the synapse
Electrical Synapses
The physical connection between 2 cells which are very close together, allowing the passage of ions and small molecules
Connexin connects cells
Bidirectional
Fast communication between cells
Chemical Synapses
involves a presynaptic cell and a postsynaptic cell with no connection
not bidirectional
Neurotransmitters are stored in the presynaptic terminal where it is released and binds to receptors in the postsynaptic cell
Synaptic Cleft
The gap between the presynaptic and postsynaptic neurons
Directly Gated Chemical Synapses
- Transmitter binds
- Receptor channels open
- Ions pass through the channel
- Na+ passes through = excitatory = EPSP
- Cl- or K+ passes through = inhibitory = IPSP - Effects are fast in onset and short-lasting
- Receptor and effector are the same molecule
Indirectly Gated Chemical Synapses
- Transmitter binds
- Activates 2nd messenger system
- cAMP system activates protein kinases which phosphorylate a channel and cause it to open or close, causing changes in membrane permeability
- Ions flow, depolarization or hyperpolarization
- Slow onset, long-lasting, receptor and effect are different molecules
Chemical vs Electrical Synapses
Electrical synapses are inflexible
Chemical synapses are flexible
Inhibition is only possible with chemical synapses
Plasticity associated with indirectly gated synaptic transmission
Steps of Synaptic Transmission
- Action potential arrives in the presynaptic terminal
- presynaptic terminal depolarizes
- Voltage-gated Ca2+ channels open
- Ca2+ influx into the presynaptic terminal
- Increased Ca2+ causes synaptic vesicles to fuse with presynaptic membrane
- Transmitter released by exocytosis and diffuses across a synaptic clef, binds to, and open ligand-gated ion channels
- Ions flow across membrane as dictated by their concentration gradients and depolarize or hyperpolarize
- Transmitter removal and recycled or degraded, ion channel closes, PSP ends
Excitatory Transmitter
Glutamate
- binds to a receptor and opens ligand-gated Na+ channels
- Na+ enters the postsynaptic cell and results in small depolarization (EPSP)
- EPSPs are sub-threshold
Inhibitory Transmitter
GABA, glycine
- bind to receptor and opens ligand-gated Cl- channels
- Cl- enters postsynaptic cell and results in small hyperpolarization (IPSP) and prevents the generation of an action potential
Synaptic Potentials
Decay with distance
Can only travel short distances
Temporal Summation
When PSPs from a single axon overlap in time they add together.
EPSPs that are too small to initiate an action potential can sum together to bring the membrane to threshold
Spatial Summation
PSPs generated in different regions of the postsynaptic neuron are added together
Summation of EPSPs from different regions that are too small to form an AP can depolarize the membrane to threshold
PSPs vs APs
PSPs
- amplitude = graded
- duration = msec - sec
- location = on dendrites or somas
- conduction = passive (over short distance)
- function = change potential of postsynaptic neuron moving it closer or further from threshold
APs amplitude = all or none duration = msec location = initiated at the axon hillock - transmit to synaptic terminal conduction = active (long distances) function = Neuron will fire AP if the Axon Hillock decides to
Smooth Muscles
found in the walls of hollow organs of the body
its contraction reduces the size of structures
-arteries
-GI tract
-bladder
-lungs
not under voluntary control
Cardiac Muscle
Striated muscle in the walls of the heart
propels blood through the heart and through the blood vessels of the circulatory system
not under voluntary control
Skeletal Muscle
muscle attached to the skeleton
striated muscle
contraction of skeletal muscle under voluntary control
Endomysium
surrounds each muscle fibre in skeletal muscle and electrically isolates the muscle fibres from one another
Motor Neurons
Skeletal muscles contract when stimulated by the process of a motor neuron
Neuromuscular Junction
- One AP in motor neuron generates one AP in muscle cell
- Each muscle fibre is only innervated by one presynaptic axon
- No inhibitory transmitters released at the neuromuscular junction
Events at the Neuromuscular Junction
- Release of acetylcholine - the neurotransmitter ACh contained within the vesicles is liberated by exocytosis into the synaptic cleft + calcium ions are pumped out of the axon terminal
- Action potential propagation - the depolarization of the motor endplate initiates an action potential which propagates along the sarcolemma in all directions and down the t-tubules
- Calcium release from terminal cisternae - the action potential causes the release of calcium ions from the terminal cisternae into the cytosol
Sarcomere
the structural unit of a myofibril in a striated muscle
bound on either side by Z-lines
contractile elements are the myofilaments
-myosin = thick
-actin = thin
Myosin
forms thick filaments The head (cross-bridge) has the ability to move back and forth the flexing movement of the head provides the power stroke for muscle contraction two binding sites for cross-bridge -ATP -Actin low energy = myosin bent forward high energy = myosin head flat
Thin Filament
Actin
in unstimulated muscle, the position of the tropomyosin covers the binding sites on the actin subunits and prevent the myosin cross-bridges binding
To expose the binding sites for binding with myosin, the tropomyosin molecule must be moved aside
-facilitated by troponin
-calcium ions are released from the terminal cisternae and bind to troponin, causing a conformational change in the tropomyosin-troponin complex, dragging the tropomyosin off of the binding site
Steps of Cross-Bridge Cycling
- the influx of calcium, triggering the exposure of binding sites on actin
- the binding of myosin to actin
- the power stroke of the cross-bridge that causes the sliding of the thin filaments
- the binding of ATP to the cross-bridge which results in the cross-bridge disconnecting from actin
- the hydrolysis of ATP which leads to re-energizing and repositioning of the cross-bridge
- the transport of calcium ions back into the sarcoplasmic reticulum
Role of ATP in Cross-Bridge Cycle
- energizing the power stroke of the myosin cross-bridge
- disconnecting the myosin cross-bridge from the binding site on actin at the conclusion of a power stroke
- Pumping Ca+ back into the sarcoplasmic reticulum
Features of White Muscle Fibres
large in diameter reduced myoglobin surrounded by few capillaries relatively few mitochondria high glycogen content
Features of Red Muscle Fibres
about half the diameter of white muscle fibres large quantity of myoglobin surrounded by many capillaries numerous mitochondria low glycogen content
Metabolism in White Muscle Fibres
Muscles with many white muscle fibres are well suited for activities requiring power and speed for a short duration
many use glycolysis synthesizes ATP fast
rapid cross-bridge cycling - fast contractions
“fast twitch”
powerful due to large diameter
fatigue rapidly
Metabolism in Red Muscle Fibres
muscles with high number of red muscle fibres are suited for endurance activities
uses krebs cycle and oxidative phosphorylation for ATP synthesis
cross-bridge cycling occurs relatively slowly
“slow twitch”
fatigue resistance
