Neurons, Synapses, Muscles, and Movement Flashcards
Dendrites of a Motor Neuron
Receive chemical signals from sensory receptors/ other neurons and transform them into electrical signals which are sent to the cell body
Cell Body (Stoma) of a Motor Neuron
Contains the nucleus, cytoplasm, and organelles; key for metabolism and summation of input signals
Axons of a Motor Neuron
Carry signals away from the cell body to the end of the axon/axon terminal/synaptic terminal buttons (where neurotransmitters are released for communication with other neurons or effectors)
Myelin Sheath of a Motor Neuron
Made up of Schwann cells, forms an insulating layer on the axon that increases the speed of the signal along axon through saltatory conduction
Nodes of Ranvier of a Motor Neuron
Spaces in between the Schwann cells, contain membrane proteins: Na+/ K+ channels and pumps
What is membrane potential?
Difference in electrical charge across the plasma membrane
Neurons have a difference in _____ across their _____ due to the _______ ( ______ )
charge, membranes, distribution of positively charged ions (Na+ and K+)
How are electrical signals created?
By changing the membrane polarity
What is resting potential?
polarity of a neuron at rest (-70mV), difference in electrical charge of the plasma membrane of the neuron at REST
What is action potential?
Polarity of a firing neuron (+30mV)
What is the function and location of the Sodium-Potassium Pumps?
Located in the membrane of the axon, maintain resting potential
How do the Sodium-Potassium Pumps work?
Using ACTIVE transport (ATP), sodium-potassium pumps pump 3 Na+ ions OUT of the axon while pumping 2 K+ ions INTO the axon
What do the Sodium-Potassium Pumps create/ what is the result of their work?
OUTSIDE of neuron is more positive compared to inside of neuron (outside of axon and inside of neuron are POLARIZED) , creates resting potential of -70mV
The _____ of the cell is more _____ with respect to the _____. The internal and external environment are ______.
inside, negative, outside, polarized
Breakdown of Charges of Inside the Cell at Rest
-More negative
-Cations: LOTS of Potassium (K+) and few sodium (Na+)
-Anions: proteins, sulfate, phosphate (collectively A-) and few chloride (Cl-)
Break down of Charges Outside the Cell at Rest
-More positive
-Cations: LOTS of Sodium (Na+) and few potassium (K+)
-Anions: chloride (Cl-)
Action potential is an ________ event!
All-or-nothing
What must happen before an action potential can be generated and propagated down the axon?
Threshold potential must be reached (-55mV)
Step 1 of Action Potential Generation and Propagation: DEPOLARIZATION
Voltage-gated Na+ channels open (when threshold potential is reached) and Na+ rushes INTO axon (more Na+ outside of cell), causing more Na+ channels to open – domino effect (propagation) down the axon – membrane potential becomes more POSITIVE
Step 2 of Action Potential Generation and Propagation: REPOLARIZATION
K+ channels open (and Na+ channels close) and K+ rushes OUT of axon – domino effect down the axon – membrane potential becomes more NEGATIVE - becomes hyperpolarized)
Step 3 of Action Potential Generation and Propagation: RESTING POTENTIAL IS RESTORED
Restored (by sodium-potassium pumps: 3 Na+ OUT for every 2 K+ IN): This period called refractory period (another action potential CANNOT be fired until this period is complete - until the resting potential AND Na+/ K+ ion concentration gradients are restored)
What happens in myelinated neurons?
Action potentials travel FASTER down the axon because ion channels are ONLY positioned BETWEEN myelinated portions (at the Nodes of Ranvier) - called SALTATORY CONDUCTION (also require LESS ATP to return to resting potential)
What are nerve impulses?
action potentials propagated along axons of neurons
Neurotransmitters ______ enter postsynaptic cell
NEVER
What are the steps of synaptic transmission?
- An action potential arrives at END of the axon (axon terminal/ synaptic knob)
- Calcium channels open, calcium ions rush INTO the axon terminal/synaptic knob
- Calcium ions interact with vesicles (containing neurotransmitter) stored in the axon terminal, causing them to migrate to and fuse with the membrane of axon terminal/synaptic knob
- Neurotransmitter is released (by exocytosis) into the synaptic cleft (space between neurons/ neurons and effectors) and diffuses across the synaptic cleft
- Neurotransmitters bind to receptor proteins (ion channels) on the post-synaptic membrane
- Binding of neurotransmitter causes ion channels to open (changes their 3° structure) and:
* Na+ ions rush in the post-synaptic cell (causing depolarization: excitatory) OR
* Cl-ions rush into the post-synaptic cell (causing hyperpolarization: inhibitory) - Enzymes break down neurotransmitters into two or more fragments (ion channels close on postsynaptic membrane) and their pieces diffuse back into presynaptic neuron (reuptake) to be assembled in vesicles again
What is Acetylcholine (Ach)?
a neurotransmitter (made by combining choline and an acetyl group)
What is the function of Acetylcholine (Ach)?
Usually released by presynaptic neurons at neuromuscular junctions in order to trigger muscle contractions by binding to receptors (cholinergic/ nicotinic) in the membrane of postsynaptic muscle fibers (the motor end plate) to allow Na+ ions to diffuse into post-synaptic muscle fiber cells.
What is the function and location of the enzyme Acetylcholinesterase (AchE)?
Released by presynaptic cell or found in membrane of postsynaptic cell - continually breaks Ach down (back into choline and an acetyl group), as overstimulation of muscle fibers by Ach can lead to fatal convulsions and paralysis!
______ is taken back into the ______ through reuptake to be used to make _____ again
Choline, presynaptic cell, Ach
How do Neonicotinoid pesticides work and what are their effects?
-Neonicotinoid pesticides bind IRREVERSIBLY to Ach receptors (in postsynaptic muscle fiber cell membranes) in insects
-Block normal Ach binding (block/ prevent synaptic transmission)
-AchE is NOT able to break down neonicotinoids, so the effect is PERMANENT (paralysis/ no muscle contraction/ death)
What are the concerns of using Neonicotinoid pesticides?
-Composition of Ach receptors in insects is DIFFERENT than in mammals, so neonicotinoids bind to them much more readily/ strongly than ours
-Neonicotinoid use linked to reduced honeybee and bird populations
The ______ of a _____ _______ during _______ _______
length, sarcomere, shortens, muscle contraction
The ____ of _____ and _____ filaments do not change during muscle contraction, they simply _______ past one another
length, actin, myosin, slide
What are the steps of muscle contraction?
- Nerve impulse/ action potential arrives at the neuromuscular junction (between a motor neuron and a muscle cell) and acetylcholine (Ach) is released into the synaptic cleft
- Ach binds to protein receptors on the sarcolemma (muscle fibre cell membrane)
- Sodium channels open (on sarcolemma and in t-tubules) and sodium ions rush into the muscle cell, causing calcium channels (on sarcoplasmic reticulum) to open and release calcium ions
- Calcium ions bind to troponin (on actin filaments), causing troponin to change shape, moving tropomyosin and exposing the myosin-binding sites on the actin filaments
- ATP is hydrolyzed to ADP + Pi, providing energy for the myosin heads to bind to (form “cross bridges” with) the actin filaments
- Myosin heads move/ change shape/ bend, which pulls the actin filaments toward the center of the sarcomere (myosin filaments and actin filaments slide past each other) - this is called the “power stroke” - and the muscle contracts (shortens the sarcomere)
- ATP binds to myosin, releasing it from actin/ breaking “cross bridges”
Note: myosin can then hydrolyze ATP and bind to the next myosin binding site on actin to pull it in even farther in/ shorten the sarcomere even more
The muscle contraction cycle continues as long as….
ATP and calcium levels remain high in the sarcoplasm.
What are sarcomeres?
repeating units in the muscles
Sarcomeres are arranged from…. and ….
end to end, shorten during contraction
What are the thick filaments?
myosin
What are the thin filaments?
actin
What is the function of cartilage?
absorbs compression; reduces friction between bones
What is the function of synovial fluid?
provides nutrients to cartilage; reduces friction
What is the function of the joint capsule?
surrounds and seals joint cavity, limits range of motion, promotes stability
What is the function of tendons?
attach muscles (triceps and biceps) to bones
What is the function of ligaments?
connect radius, ulna and humerus (bone to bone)
What is the function of biceps?
muscles that contract to provide flexion (bending) of the arm
What is the function of triceps?
muscles that contract to provide extension (straightening) of the arm (note that biceps and triceps are antagonistic)
What is the function of the humerus?
upper arm bone that provides leverage and attachment for upper portions of muscles of the elbow
What is the function of the radius?
(forearm) is lower arm bone (smaller) that acts as a lever for the biceps (biceps attach to it)
What is the function of the ulna?
(forearm) is lower arm bone (larger) that acts as a lever for the triceps (triceps attach to it)
Flexion
decreases the angle between connecting bones (bending the joint)
Extension
increases the angle between connecting bones (straightening a joint)
Muscle Cell
a LONG fiber, (muscle fibre) formed from many cells that have fused together during development; as a result, each has many nuclei.
Sarcolemma
Muscle fibers (cells) are covered by a cell membrane called the sarcolemma. This is the same as the cell membrane in other cells.
T-Tubules
The sarcolemma has many tube-like invaginations into the muscle cell called T(transverse)-tubules
Sarcoplasm
Cytoplasm of muscle cells
Myofibrils
-parallel filaments within muscle cells that are made up of the contractile proteins actin and myosin (responsible for the banded look of striated muscles)
-myofibrils are made up of structures called sarcomeres
Sarcoplasmic reticulum
fluid filled membranous sacs that store calcium ions
Numerous mitochondria
positioned between myofibrils to provide ATP
Glycosomes
store glycogen (broken down to provide glucose for cellular respiration in mitochondria)
Myoglobin
oxygen-storing protein (red pigment)
List structures of a muscle from smallest to largest
Sarcomere, myofibril, sarcolemma, muscle fiber, muscular bundle/fascicle, muscle
