Lesson 18 - Nervous System

CNS (central nervous system)

Brain and spinal cord; processes information, receives sensory input, and creates responses

PNS (peripheral nervous system)

Everything outside the CNS; transmits sensory information to the CNS and motor commands to organs and tissues

Somatic nervous system

Controls voluntary movement of skeletal muscles

Autonomic nervous system (ANS)

Regulates involuntary functions of organs, smooth muscle, cardiac muscle, and glands

Enteric nervous system

Neural network of the gut that communicates with the brain through the gut

Electrical signals

Neuronal signals produced by changes in membrane voltage

Ion channels

Membrane proteins that allow specific ions to cross the membrane down their electrochemical gradients

Lidocaine

Drug that blocks voltage

Leak channels

Ion channels that are typically open and allow specific ions to move passively without ATP

Mechanically gated channels

Channels that respond to mechanical stimuli, such as pressure, stretch, or vibration

Example of mechanically gated channels

Cochlear hair

Resting membrane potential

Voltage across a resting neuron's membrane, typically around −70 mV

Why is the inside of a resting neuron negative?

Unequal ion distribution and selective membrane permeability create a net negative charge inside

Major extracellular ions

Sodium (Na⁺) and chloride (Cl⁻) are more concentrated outside the cell

Major intracellular ions

Potassium (K⁺), proteins, and phosphate are more concentrated inside the cell

Why does potassium leave a resting neuron?

More K⁺ leak channels than Na⁺ leak channels allow greater net K⁺ movement out of the cell

Why do many negatively charged proteins remain inside the cell?

They are too large or cannot pass through the membrane's ion channels

Graded potentials

Local changes in membrane potential that make the cell more positive or more negative

Why are graded potentials called graded?

Their amplitude varies with stimulus strength

Effect of a stronger stimulus on a graded potential

Can open more ion channels, producing a larger change in membrane voltage

Summation

Combining multiple graded potentials to produce a larger overall change in membrane potential

Where do graded potentials mainly occur?

Dendrites and the cell body

Action potential

Rapid, all

Depolarization

Membrane potential becomes more positive, primarily as Na⁺ enters through voltage

Threshold potential

Membrane voltage that triggers an action potential, typically around −55 mV

What happens when threshold is reached?

Voltage

Repolarization

Membrane potential becomes more negative as Na⁺ channels inactivate and K⁺ exits through voltage

Hyperpolarization

Membrane potential briefly becomes more negative than resting potential because K⁺ channels close slowly

Absolute refractory period

Period during which another action potential cannot be initiated, regardless of stimulus strength

Relative refractory period

Period during which another action potential is possible but requires a stronger

Effect of a stronger stimulus on an action potential

Increases action potential frequency, not its amplitude

Ion movement during an action potential

Na⁺ enters during depolarization, K⁺ exits during repolarization, and ion pumps maintain ion concentration gradients

How is resting potential restored after an action potential?

K⁺ channels close, and the membrane returns toward resting potential; ion pumps maintain concentration gradients

Continuous conduction

Action potentials propagate along successive sections of an unmyelinated axon

Saltatory conduction

Action potentials are regenerated at nodes of Ranvier, appearing to jump between myelinated sections

Which type of conduction is faster?

Saltatory conduction, because myelin allows rapid propagation between nodes of Ranvier

Effect of myelination on conduction speed

Greater myelination generally increases conduction speed

Effect of axon diameter on conduction speed

Larger axon diameter generally increases conduction speed by reducing internal resistance

Effect of temperature on conduction speed

Within physiological limits, higher temperature generally increases conduction speed

Nerve impulses

Action potentials initiated at trigger zones and propagated along axons

Trigger zone

Region near the axon hillock and initial segment where action potentials are typically initiated