C2.2: Neural Signaling

Nervous System

consists of Central Nervous System and Peripheral Nervous System

Central Nervous System

CNS, brain and spinal cord

Peripheral Nervous System

PNS, connects CNS w/ rest of body

Neurons

Nerve cells that relay messages via electrical impulses, converts sensory information into electrical signals

Neuron Structure

Consists of Soma, Dendrite, and Axon

Soma

Cell body of neuron containing nucleus, organelles, and cytoplasm, carries out metabolic processes

Dendrite

Multiple, short fibers that receive and convert chemical information from other neurons to electrical impulses

Axon

single, elongated fiber that passes electrical impulses to synapses to relay info to other neurons/effectors

Factors that affect nerve impulse speeds

Diameter of axon and myelination

Diameter of Axon

Increase axon diameter, decrease resistance so impulses travel more quickly, often used where speed is vital.

ex. squid have axon w/ diameter 500 microns, travels faster than human axon w/ diameter 1 micron

Myelination

Myelin sheath wraps around axons of some neurons, composed of schwann cells made of proteins and phospholipids

Gaps between schwann cells called Nodes of Ranvier (basically relays) so impulses "jump" from one node to the next bc no Na+/K+ channels inside myelin sheaths

This greatly increases speed of transmission (1m/s to 100 m/s for humans)

Synapse

Junction between 2 neurons and other neurons/effector cells (narrow, about 20 nm)

Direction of Signal Transmission

Signal travels in 1 direction as either action potential (electrical) or as neurotransmitters (chemical) between neurons

*no action potentials btwn synapses bc no membranes*

Where Are Synapses Located?

Between:

Sensory receptors and neurons

Two neurons (CNS)

Neurons and muscle fibers/glands (effectors)

Oscilloscopes

Measure membrane potential, consists of two electrodes placed inside and outside of neuron membrane

data displayed on graph, x-axis=time (ms), y-axis=membrane potential (mV)

Resting Potential

-70 mV, it is the voltage difference across membrane due to imbalance of ions, consists of charge across membrane when neuron NOT firing

*more negative inside than outside*

Factors of Resting Potential (3)

1. Na+/K+ Pump, the two ions are transported unequally, 3 Na+ out for every 2 K+ in

2. Na+ and K+ diffuse back across the membrane via simple diffusion (through membrane), membrane more permeable to K+ so diffuses faster, resulting in a steeper concentration gradient for Na+ so even more positive outside than inside

3. More negatively charged proteins inside neuron

Action Potential

Neuron "firing," change in membrane potential which produces electrical impulses, involves movement of positively charged ions.

Depolarization

Neuron becomes more positive, going from -55 mV to +30 mV, [Na+] inside cell increases toward equilibrium as Na+ voltage gated channels open at -55 mV (threshold). Na+ rushes into axon, known as influx

Repolarization

Restoration of membrane potential, begins when K+ voltage gated ion channels open at +30 mV, resulting in K+ rushing out of axon (efflux) due to greater concentration of K+ inside

*neuron becomes more negative*

Hyperpolarization

Exiting K+ surpasses (becomes lower than) resting potential, becomes more negative

Refractory Period

Sate of recovery required for neuron to fire again, prevents action potential from traveling backward. Ionic distribution is reversed (more Na+ in and more K+ out), Na+/K+ pump works to restore resting potential

Threshold Potential

Relies on the all or none principle, stimulus must be strong enough to break threshold (-55 mV) to open voltage gated ion channels.

Any signal less than -55mV will not trigger an action potential, and action potential is always same magnitude once initiated

Positive Feedback

Occurs in action potentials, when neuron becomes more positive, threshold is reached, then more Na+ diffuses in.

Cell then becomes more positive, opening more Na+ channels, causing process to repeat

Action Potential Propagation

Movement of an action potential along an axon:

Depolarization creates concentration gradient btwn adjacent areas of axon, leading to diffusion occurring across these regions.

Inside axon, Na+ moves from depolarized (+) to polarized (-) regions, outside axon, Na+ moves from polarized to depolarized regions

Overall process increases adjacent membrane potential to -55mV, opening up Na+ channels and allowing action potential to move through next region of axon.

Saltatory Conduction

the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.

Unmyelinated Axons

every Na+ and K+ channel must open for action potential to spread, slower

Myelinated Axons

Areas wrapped w/ 20 or more phospholipid bilayers (made of Schwann cells) to prevent ion movement (no channels).

Channels are clustered in Nodes of Ranvier, or exposed areas btwn myelin sheaths, local currents force action potential to "jump" from one node to the next

Synaptic Transmission Process

1. Action Potential arrives at presynaptic axon terminal

2. Opens voltage-gated calcium channels, causing Ca2+ influx into axon terminal

3. vesicles containing neurotransmitters fuse with cell membrane and neurotransmitters undergo exocytosis and cross synapse

4. Excess NTs are recycled (reuptake) or hydrolyzed (destroyed)

5. NTs bind to specific receptors on postsynaptic neuron, opening ligand-gated channels, causing Na+ influx, initiating action potential

Excitatory Postsynaptic Potentials

EPSP, when signal causes depolarization in postsynaptic neurons, opens ligand-gated Na+ or Ca2+ channels resulting in action potential

Acetylcholine

Released in neuromuscular junctions to initiate muscle contractions, made from choline (diet) and acetyl group from aerobic respiration

Initiates action potential, only binds for postsynaptic receptor for short time to prevent fatal convulsions and paralysis (must be continually removed)

Acetylcholinesterase

Enzyme that brakes down acetylcholine into its two parts, choline is absorbed in endocytosis back into presynaptic neuron and paired with another acetyl group

Exogenous Chemicals

Chemicals that enter body from outside source

Neonicotinoids

Synthetic chemicals that bind to nicotinic acetylcholine receptors that cannot be degraded by acetylcholinesterase.

Insects have greater proportion of these synapses and neonicotinoids lead to overstimulation of receptors in insects and paralysis/death (used in insecticides)

Cocaine

Excitatory Psychoactive drug (stimulant), binds to and blocks dopamine reuptake transporters so dopamine builds up in synapses and postsynaptic neuron continuously fires (leads to sustained euphoria)

Inhibitory Postsynaptic Potentials

IPSP, when signal causes hyperpolarization in postsynaptic neurons (more negative so harder to start action potential)

ex. GABA (g-aminobutyric acid) , calms down nervous system to process input in organized way, opens Cl- channels

Summation Effect

More than one neuron forms synapses with postsynaptic neurons, one EPSP not enough to break threshold potential so either one must fire repeatedly or several must fire simultaneously

Summation is when all neurotransmitters combine to fire action potential, if depolarization is greater than hyperpolarization, then threshold is reached.

Perception of Pain

Pain Receptors makeup endings of sensory neurons, detect high temps, acids, and chemical like capsaicin chili

Contains channels for positively charged ions, once threshold is met, fired actions potentials travel to spinal cord and brain

-prefrontal cortex allows awareness of situation

-effectors (muscles) reduce exposure to stimulus

Emergent Properties

Consiousness, state of complex awareness that emerges from interaction of neurons in brain, property of the system as a whole rather than any one component

*idea that the system is greater than the sum of its parts*