nervous system

Nervous tissue composition

Made of two main cell types: neurons (which send signals) and glial cells (which support and protect neurons).

Function of neurons

Communication — sending electrical and chemical signals throughout the body.

Types of neurons

Sensory neurons (detect inputs), motor neurons (cause actions/movement), and interneurons (process signals in between).

Location of neurons

Located throughout both the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Main function of glial cells

Supporting, protecting, and maintaining the environment for neurons ('glue' of the nervous system).

Major types of glial cells

Oligodendrocytes, Schwann cells, microglia, ependymal cells, astrocytes, and satellite cells.

Location of glial cells

Found in both the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Myelin sheath

A fatty coating wrapped around axons that acts like plastic insulation on an electrical wire to speed up signals.

Myelin coverage of axons

Formed in separate individual sections with tiny unmyelinated gaps in between, rather than one continuous layer.

Unmyelinated gaps between myelin sections

Nodes of Ranvier — bare spots along the axon where the signal gets recharged and jumps forward.

Myelin producer in CNS

Oligodendrocytes (Easy memory: CO = CNS uses Oligodendrocytes).

Myelin producer in PNS

Schwann cells (Easy memory: PS = PNS uses Schwann cells).

Main function of myelin

Insulates the axon so electrical action potentials can travel much faster down the neuron.

Effect of increasing myelination on action potential speed

Speed increases significantly (thicker myelin = faster signal).

Saltatory conduction

The fast way an action potential travels down a myelinated axon by leaping from node to node ('saltar' = to jump).

Action potential propagation during saltatory conduction

The electrical signal leaps across bare gaps (Nodes of Ranvier) instead of crawling through every millimeter.

Types of signals for neuronal communication

Chemical signals (neurotransmitters across gaps) and electrical signals (action potentials down axons).

Chemical signals used by neurons

Neurotransmitters (chemical messengers sent across the synapse).

Electrical signals used by neurons

Action potentials (voltage spikes traveling down the axon).

General communication sequence in the nervous system

Chemical signal $\rightarrow$ electrical signal $\rightarrow$ chemical signal.

Path of chemical signals between neurons

Axon terminal of sending neuron $\rightarrow$ synaptic cleft (gap) $\rightarrow$ dendrites/cell body of receiving neuron.

Path of electrical signal on a neuron

Axon hillock (start point) $\rightarrow$ axon body $\rightarrow$ axon terminal (end point).

Effect of excitatory neurotransmitters

They excite the neuron by pushing its voltage up toward threshold to spark an electrical signal (Green light).

Examples of excitatory neurotransmitters

Acetylcholine, dopamine, and glutamate.

Effect of inhibitory neurotransmitters

They quiet the neuron down by pulling its voltage away from threshold to stop an electrical signal (Red light).

Examples of inhibitory neurotransmitters

GABA, serotonin, and glycine.

Classes of neurotransmitters based on their effect

Excitatory (signals 'GO') and inhibitory (signals 'STOP').

Definition of action potential

A rapid, temporary electrical impulse that travels down an axon to deliver a nerve message.

Membrane potential changes during action potential

The voltage rapidly shoots up (becomes positive) and then quickly drops back down (becomes negative).

Location of action potential occurrence

Along the axonal membrane of a neuron.

Trigger for an action potential

A graded potential that depolarizes the cell up to the threshold level.

Result of an action potential

Triggers the exocytosis and release of neurotransmitters at the axon terminal.

Resting membrane potential of a neuron

$-70\,mV$ (the normal idle baseline voltage inside a quiet neuron).

Resting membrane potential definition

The electric potential across the neuron's membrane when it is resting and not firing signals.

Threshold definition

The essential voltage point of no return that must be hit to trigger an action potential.

Threshold value

$-50\,mV$ (moving up from $-70\,mV$ to $-50\,mV$ triggers the fire).

Consequence of reaching threshold

Voltage-gated $Na^+$ (sodium) channels suddenly burst open, initiating depolarization.

Transition from -70 mV to -50 mV

Driven by incoming depolarizing graded potentials summing up.

Purpose of graded potential

To shift the resting voltage close enough to $-50\,mV$ threshold to spark an action potential.

Definition of graded potential

A small, localized change in membrane voltage caused by the combination of incoming signals.

Outcome when threshold is reached

An action potential fires completely (All-or-None principle).

Outcome when threshold is not reached

No action potential fires (the signal simply fades away).

Types of graded potentials

Depolarizing (pushes voltage UP toward firing) and hyperpolarizing (pulls voltage DOWN away from firing).

Name for depolarizing graded potential

EPSP (Excitatory Postsynaptic Potential).

Effect of depolarization on membrane potential

Makes the inside of the cell more positive, shifting voltage closer to threshold.

Cause of an EPSP

Excitatory neurotransmitters opening $Na^+$ channels to let positive charge inside.

Name for hyperpolarizing graded potential

IPSP (Inhibitory Postsynaptic Potential).

Effect of hyperpolarization on membrane potential

Makes the inside of the cell more negative, pulling voltage further away from threshold.

Cause of an IPSP

Inhibitory neurotransmitters opening $Cl^-$ channels to let negative charge inside.

Ion movement during EPSP

Ligand-gated $Na^+$ channels open, letting positive $Na^+$ ions rush INTO the cell.

Ion entering during depolarizing graded potential

$Na^+$ (sodium).

Effect of Na+ entry on cell

The cell interior becomes more positive because positive ions enter.

Effect of Na+ entry on membrane potential

Shifts voltage upward toward the $-50\,mV$ threshold.

Example of excitatory neurotransmitter

Acetylcholine (ACh).

Ion movement during IPSP

Ligand-gated $Cl^-$ channels open, letting negative $Cl^-$ ions rush INTO the cell.

Ion entering during hyperpolarizing graded potential

$Cl^-$ (chloride).

Effect of Cl- entry on the cell

The cell interior becomes more negative because negative ions enter.

Effect of Cl- entry on membrane potential

Shifts voltage downward, further away from threshold.

Example of inhibitory neurotransmitter

GABA.

Three phases of action potential

1. Depolarization (voltage spikes up); 2. Repolarization (voltage drops down); 3. Hyperpolarization (voltage dips below baseline).

First phase of action potential

Depolarization (the rising voltage phase).

Voltage change during depolarization

$$-50\,mV \rightarrow +30\,mV$$

Channels opening during depolarization

Voltage-gated $Na^+$ (sodium) channels.

Ion movement during depolarization

$Na^+$ rushes INTO the cell (influx).

Na+ movement during depolarization

Influx (moving into the cell interior).

Event at +30 mV

Voltage-gated $Na^+$ channels snap shut and voltage-gated $K^+$ channels open.

Easy memory for depolarization

DEP = $Na^+$ Dives IN (Sodium enters, making voltage positive).

Second phase of action potential

Repolarization (the falling voltage phase).

Voltage change during repolarization

$$+30\,mV \rightarrow -70\,mV$$

Channels opening during repolarization

Voltage-gated $K^+$ (potassium) channels.

Ion movement during repolarization

$K^+$ rushes OUT of the cell (efflux).

K+ movement during repolarization

Efflux (moving out of the cell interior).

Reason membrane becomes negative again in repolarization

Positive potassium ions ($K^+$) leave the neuron, taking positive charge away.

Easy memory for repolarization

REP = $K^+$ Runs OUT (Potassium exits, restoring negative voltage).

Third phase of action potential

Hyperpolarization (the temporary undershoot phase).

Voltage change during hyperpolarization

$$-70\,mV \rightarrow -90\,mV$$

Cause of hyperpolarization

Voltage-gated $K^+$ channels close slowly, letting too much positive $K^+$ escape.

Ion leaving during hyperpolarization

$K^+$ (potassium).

Voltage reached during hyperpolarization

$$-90\,mV$$

Easy memory for hyperpolarization

$K^+$ stayed open too long, causing the cell to overshoot and become extra negative ($-90\,mV$).

Memorized resting membrane potential

$$-70\,mV$$

Memorized threshold

$$-50\,mV$$

Peak of depolarization value

$$+30\,mV$$

Lowest value during hyperpolarization

$$-90\,mV$$

Depolarization voltage range

$$-50\,mV \rightarrow +30\,mV$$

Repolarization voltage range

$$+30\,mV \rightarrow -70\,mV$$

Hyperpolarization voltage range

$$-70\,mV \rightarrow -90\,mV$$

Memory sequence of membrane potentials

$-70\,mV \rightarrow -50\,mV \rightarrow +30\,mV \rightarrow -70\,mV \rightarrow -90\,mV \rightarrow -70\,mV$ (Rest $\rightarrow$ Threshold $\rightarrow$ Peak $\rightarrow$ Rest $\rightarrow$ Dip $\rightarrow$ Rest).

Definition of "all-or-none" action potential

Once threshold is reached, the action potential fires completely with identical height and speed every single time.

Requirement for action potential to occur

The membrane voltage must reach the threshold value of $-50\,mV$.

Conduction without decrement meaning

The action potential stays at 100% full strength all the way down the axon without weakening.

Self-propagating definition

Once started at the axon hillock, the signal automatically triggers the next membrane section all the way to the end.

Representation of stimulus intensity

The frequency of action potentials (firing more times per second = stronger feeling/signal).

Name for action potential number and intensity relationship

Recruitment (more intense stimuli cause higher firing frequency and recruit more neurons).

Factors increasing action potential speed

1. Wider axon diameter; 2. Presence of a fatty myelin sheath.

Effect of increased axon diameter on speed

Speed increases because wider pathways offer less electrical resistance.

Effect of increased myelination on speed

Speed increases dramatically because the signal leaps between nodes.

Types of refractory periods

Absolute refractory period (impossible to fire again) and Relative refractory period (possible to fire again, but requires a stronger stimulus).

Absolute refractory period effect

A second action potential CANNOT fire, no matter how strong the incoming signal is.

Reason no AP occurs during absolute refractory period

$Na^+$ channels are inactive and locked shut while recovering from the previous spike.