NERVOUS SYSTEM
• Central nervous system (CNS) – brain and spinal cord
• Peripheral nervous system (PNS) – nerves that communicate motor and sensory signals between CNS and rest of body
NEURON• Functional unit of nervous system• Relatively large cell body• Processes:
– Dendrites – convey signals from tips to cell body; often branched
– Axons – conduct signals away from body and toward tip; often single
• Myelin sheath – protective, insulating layer that covers many axons in vertebrates– Made by Schwann cells in the PNS– Made by oligodendrocytes in the CNS
• Axon ends at synaptic terminals– Synapse – site of contact between
synaptic terminal and target cell (neuron or effector cell – for example a muscle cell)
– Neurotransmitter – chemical messengers between neurons and other cells
ORGANIZATION OF NEURONS
• Sensory neurons – communicate sensory information from eyes and other senses and internal conditions– Senses, blood pressure, muscle tension,
CO2 levels)
• Interneurons – integrate sensory input and motor output; communicate only between neurons; make up vast majority of brain neurons
• Motor neurons – convey impulses from CNS to effector cells (muscles and glands)
MEMBRANE POTENTIAL
• Voltage measured across the membrane (like a battery)
• Inside of cell more negative• Typically –50 to –80 mV (resting
potential)• Sodium-potassium pump keeps ionic
gradient (3Na+ out, 2K+ in)
Charges Across Membranes
• Neurons have ability to generate changes in their membrane potential
• Resting potential – membrane potential of cell at rest (-60mV to -80mV)
• Gated ion channels control membrane potential – open to different stimuli– Hyperpolarization – increase in
electrical gradient•Open K+ channel (K+ moves out)
•Cell becomes more negative•No action potential because it makes it harder to depolarize
– Depolarization – decrease in electrical gradient•Open Na+ channel (Na+ moves in)•Cell becomes more positive•Action potential generated if threshold is reached (-50mV to -55mV)
–Massive change in voltage
•Threshold causes all-or-none event
–Action potential - massive change in membrane voltage that can spread along the membrane
ROLE OF GATED CHANNELS• Depolarizing – Na+ gates open rapidly so Na+
moves into cell• Repolarizing – K+ gates finally open and K+
moves out; Na+ gates close• Undershoot (Refractory Period) - K+ still open
(they are slower to close) and Na+ still closed so cell becomes even more negative than resting and cannot be depolarized
• Stronger stimuli result in greater frequency of action potentials and NOT from stronger action potentials
• Propagation– Action potentials move in one direction due to
refractory period
Propagation of the action potential
Na+ moves into cell starting action potential.
Depolarization spreads and K+ repolarizes initial area. Prevents action potential on that side.
SYNAPSES• Presynaptic cell – transmitting cell• Postsynaptic cell – receiving cell• Two types of synapses
– Electrical •Need gap junctions (channels between
neurons)•No delays
– Chemical•Narrow gap, synaptic cleft, between cells•More common than electrical in vertebrates
and most invertebrates•Require neurotransmitters (chemical
intercellular messengers)
•Depolarization of presynaptic membrane causes influx of Ca2+
•Increased Ca2+ in cell causes synaptic vesicles to fuse to cell membrane and release neurotransmitters via exocytosis
•Neurotransmitters diffuse to postsynaptic cell
•Postsynaptic membrane has gated channels that open when neurotransmitters bond to specific receptors
• A single neuron may receive many inputs simultaneously
• Neurotransmitters cause 2 different responses depending on the gates that are opened– Inhibitory
•(hyperpolarization)– Excitatory
•(depolarization)• Neurotransmitters are quickly degraded• Excitatory postsynaptic potential
(EPSP) – Na+ in and K+ out = depolarization• Inhibitory postsynaptic potential (IPSP)
- K+ out or CL- in = hyperpolarization
NEUROTRANSMITTERS
• Acetylcholine – one of the most common – can excite skeletal muscle and
inhibit cardiac muscle• Epinephrine and norepinephrine
– also function as hormones
• Dopamine – Usually excitatory– Excess dopamine can cause schizophrenia– Lack of dopamine can cause Parkinson’s
• Sertonin – Usually inhibitory
• Endorphins – Natural painkillers (morphine and opium
mimic endorphins shape)• Nitric Oxide (NO)
– Released during sexual arousal (increasing blood flow)
– Nitroglycerin used to treat chest pain
SKELETAL MUSCLE
•Attached to bones and responsible for their movement
•Consist of bundles of long fibers•Each fiber is a single cell with
many nuclei
•Each fiber made up of smaller myofibrils
•Myofibrils made of 2 kinds of myofilaments–Thin myofilaments
•2 strand of actin with a regulatory protein (tropomyosin)
–Thick myofilaments•Staggered arrays of myosin
•Striated muscle due to repeating light and dark bands
•Sarcomere – basic unit of muscle
•Contraction of sarcomeres results in muscle contraction.
•Actin and myosin slide pass each other to shorten the sarcomere.
• Sliding-filament model • Myosin head phosphorylated by ATP
making the head energized• Energized head attaches to actin
making cross-bridge• ADP and Pi released from head so it
goes back to relaxed state, sliding the thin filament toward center of sarcomere
• A new ATP binds to head releasing it from actin
• Creatine phosphate – stores phosphate in vertebrate muscles
How is skeletal muscle contraction regulated?
• An action potential begins in the brain and travels via nerve to muscle.
• The action potential causes neuron to release acetylcholine (neurotransmitter). This results in an excitatory response in muscle.
•Acetylcholine triggers action potential in T-tubules within muscle–T-tubules are infoldings of muscle cell’s cell membrane
•T-tubules touch sarcoplasmic reticulum and change is permeability to Ca2+ which means it releases Ca2+ –Sarcoplasmic reticulum – specialized ER that stores Ca2+
•Ca2+ binds to troponin which frees binding site for myosin head
What’s troponin and tropomyosin?
•Tropomyosin blocks myosin heads binding sites
•Troponin controls position of tropomyosin
•When Ca2+ binds to troponin, the shape of tropomyosin-troponin complex changes and frees binding site
•Summation and frequency of action potentials determine muscle tension
•One muscle cell only innervated by one motor neuron, but one motor neuron may innervated many muscle cells
•More cells activated = more tension
Big Picture – Making a muscle contract
• Action potential generated in brain and travels down nerve
• Action potential causes acetylcholine to diffuse across synapse to muscle
• Acetylcholine causes excitatory responses (action potential) that moves down T-tubules
• Change in membrane potential causes SR to release calcium
• Calcium binds to troponin, which then moves tropomyosin
• ATP used to bind myosin head to actin• Sarcomere contracts and then ATP used to
break bridge
Top Related