Nerve Signal Transmission

Nerve Signal TransmissionRaise your right hand.

Easy, right?You don’t even have to

think twice and your right arm is moving….

But what makes it happen???

How does your brain tell your body what to do?

The Nervous System

Central nervous system (CNS)– brain and spinal cordPeripheral nervous system (PNS)– sensory and motor

neuronsNerves – bundles of neurons

wrapped in connective tissue

Simple Nerve CircuitReflex: simple response-- sensory to motor neurons – Involuntary; not analyzed or

interpreted by brainGanglion (ganglia): cluster of nerve cell bodies in the PNSGlia: cell that provides support, insulation, and protection– astrocytes, radial glia,

oligodendrocytes, Schwann cells

Information Processing- External stimuli

detected- sight, sound, touch,

smell, taste, etc.- Information sent to the

CNS - analysis and

interpretation- Motor output is relayed

to the effector cells- muscle/ endocrine cells

Neuron StructureCell body~ nucelus and organellesDendrites~ receiving signalsAxons~ transmitting signals– Hillock~ connects

axon to cell body; generates signal

Synaptic terminals~ communicates w/ another cell (releases neurotransmitters)– Synapse~ neuron

junction (site of cell communication)

Neuron DiversitySensory neuron: convey information to spinal cordInterneurons: information integrationMotor neurons: convey signals to effector cell (muscle or gland)

Schwann Cells and Myelin

Schwann cells– PNS support cells– Wraps around axon,

creating layers of myelinMyelin sheath– supporting, insulating

layersNodes of Ranvier– Gaps between Schwann

cells

Membrane Potential- Intracellular/extracellular ionic concentration

difference - K+ diffuses out/Na+ in; large anions cannot follow

(selective permeability of the plasma membrane)- Voltage difference between -60 and -80 mV

It has potential….

Resting potential~ membrane potential of a neuron that is not transmittingEquilibrium potential~ magnitude of membrane voltage at equilibrium– Nernst equation- Eion = 62mV (log [ion]outside / [ion]inside

Gated Ion ChannelsOpen/ Close in response to stimuli…

Photoreceptors– Changes in light intensity

Vibrations in air – sound receptors

Chemical – neurotransmitters

Voltage – membrane potential changes

Graded PotentialsDepend on strength of stimulus– Threshold potential must be reached for reaction to occur

Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negativeDepolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative

Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)….The action potential is triggered….

1. Resting state – Both Na+ and K+ voltage-gated channels are

closed2. Threshold

– a stimulus opens some Na+ channels3. Depolarization

– action potential generated – Na+ channels open– cell becomes positive (K+ channels

closed)4. Repolarization

– Na+ channels close, K+ channels open; K+ leaves

– cell becomes negative5. Undershoot

– both gates close, but K+ channel is slow; resting state restored

Refractory period~ insensitive to depolarization due to closing of Na+ gates

Conduction of Action Potential

Movement of the action potential is self-propagating– Depolarization of one part

of axon triggers the action potential in the next part

Regeneration of “new” action potentials only after refractory period– This keeps signal moving

in the forward direction only

Action Potential Speed

Axon diameter (larger = faster; 100m/sec)Nodes of Ranvier (concentration of ion channels)– saltatory conduction- transmission “jumps” from one

node to the next– 150m/sec

Synaptic communicationDepolarization of the membrane (from a.p.) causes Ca+ influx (voltage gated channel)Ca+ causes vesicles to fuse with the presynaptic membrane and release neurotransmittersNeurotransmitters bind w/ ligand-gated ion channels on postsynaptic membraneNeurotransmitter releases from the receptor and the channels close

Postsynaptic Potential

EPSPs- excitatory postsynaptic potentialsIPSPs- inhibitory postsynaptic potentials

Indirect Transmission

Neurotransmitters bind with specific receptors instead of ion channelsMuch slower reaction time but last longerDifferent neurotransmitters produce diverse effectsPsychological drugs interact at the location of these receptors

Moving Muscles

Muscles contractMuscles move by shortening, or contracting… they cannot extend on their ownEach muscle has an antagonistic muscle that contracts to move in the opposite direction

What’s in a Muscle?Muscles are a bundle of muscle fibers.Muscle fibers: single cell w/ many nuclei composed of myofibrilsMyofibrils: longitudinal bundles composed of myofilamentsMyofilaments:– Thin~ 2 strands of actin protein

and a regulatory protein

– Thick~ myosin protein

Sarcomere: repeating unit of muscle tissue within a myofibril

Sarcomere StructureZ lines~ sarcomere borderI band~ only actinA band~ actin and myosin overlapH zone~ central sarcomere; only myosin

Sliding-filament modelTheory of muscle contractionSarcomere length reducedZ line length becomes shorterActin and myosin slide past each other (overlap increases)

Actin-myosin interaction1. Myosin head hydrolyzes

ATP to ADP and inorganic phosphate (Pi)-- “high energy configuration”

2. Myosin head binds to actin; forming a “cross bridge”

3. Releasing ADP and P, myosin relaxes sliding actin; “low energy configuration”

4. Binding of new ATP releases myosin head

Creatine phosphate~ supplier of phosphate to ADP

Contraction Regulation

Relaxation: – tropomyosin blocks

myosin binding sites on actin

Contraction: – calcium binds to

toponin complex– tropomyosin

changes shape, exposing myosin binding sites

ACTION!Acetylcholine released from synaptic terminal of motor neuron Acetylcholine binds w/ receptors and causes an action potential in the muscle fiberA.P travels down the T (transverse) tubules and triggers release of Ca+ from the sarcoplasmic reticulum– Modified endoplasmic

reticulumContraction begins when Ca+ reaches the sarcomere and binds to troponin

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