nervous system 2

Neurons (Structure & Functions)

• Basic functional unit of nervous system

• Conduct electrical signal, integrate info

• Various shapes, sizes ; share basic characteristics

Neuron Anatomy

• Cell body – contains organelles

• 2 types of cytoplasmic extensions:

  • 1. Dendrites:

    • receive info, many short dendrites, highly branched

  • 2. Axon:

    • Sends signal, one long axon per neuron

    • Axon hillock:

      • Base of axon where signals are generated

    • Synaptic terminals:

      • Often branched at end of axon

    • Nerve:

      • Axons of many neurons held together with connective tissue

Types of Neurons

• Sensory: Afferent Neurons

  • Sensory receptor → nervous system

• Integration: Interneurons

  • Only interact with other neurons

  • 90% of neurons

• Motor: Efferent Neurons

  • Nervous system → effector (muscle, etc.)

Membrane Potential

• Selectively permeable membrane

• Polarized:

  • Difference in charge on either side (inside - , outside +)

• MP is potential energy

• Measured in voltage – how different are the two sides of membrane

• Excitable cells:

  • Cells that can rapidly change MP (neurons & muscle cells)

Resting Potential

• MP of cell at rest: -70mV

• Due to Na/K pump & Leaky K channels

• Sodium Potassium Pump:

• Transmembrane transport protein

• Found throughout neuron

• 1 cycle – 3 Na+ out, 2 K+ in

• Active transport

• Leaky K+ Channels:

• Always open

• K+ pumped in, diffuses out

 Changes to Membrane Potentia

• Hyperpolarization – negativity inside cell inc

• Depolarization – positivity inside cell inc 

Threshold Potential

• MP required to trigger action potential

• -55mV for most neurons

Action Potential (general process)

• Electrical signal within neurons

• Depolarization crosses threshold

  • → opens voltage-gated channels

  • → large change in MP

• Membrane proteins allow passage of specific ions via facilitated diffusion

  • Open at specific MP

• Voltage-gated K+, Na+ channels involved in action potentials

action potential (sequence of events)

• Neurons at resting state (MP = -70mV)

  • Voltage-gated channels closed

• Stimulus

  • → Na+ channels open (NOT voltage gated)

  • → Na+ enters → depolarization

• Possible outcomes:

  • Small stimulus:

    • Few channels open – does not reach threshold, no AP

  • Strong stimulus:

    • Many channels open – strong depolarization, if reaches threshold → AP

• Rising Phase:

  • Voltage-gated Na+ channels open

    • rapid depolarization

    • MP reaches +35mV

• Falling Phase:

  • Na+ channels close

  • Membrane impermeable to Na+

Refractory Period

• Voltage-gated K+ channels open at +30 mV

• Bc we don’t want multiple APs at same time

• → K+ diffuses out

• → MP rapidly falls

Undershoot:

• Na+ closed, K+ open

  • → hyperpolarization

• K+ channels then close

  • → MP returns to resting potential

Conduction of AP

• AP is all or nothing (happens or doesn’t)

• Always the same when it does

• Intensity of sensation depends on…

  • # neurons stimulated

  • Freq. of stimulus

  • NOT strength of AP

Intro (conduction of AP)

• Signal propagates as series of APs along axon

• Voltage shift in one region → triggers Na+ channels further down

• Area behind in refractory period – Unidirectional signal

 Continuous Conduction

• Occurs in unmyelinated axons (gray matter)

• Smooth wave of depolarization

• Every spot depolarizes & repolarizes

Saltatory Conduction

• Occurs in myelinated axons (white matter)

• Requires myelin sheath, Internodes, Nodes of Ranvier

myelin sheath

• Fatty insulation for AP, rich in myelin

• ( Made by oligodendrocytes (CNS) & Schwann cells (PNS) )

Internodes

• Regions covered by myelin - no depolarization

Nodes of Ranvier:

• No myelin - lots of Na+, K+ channels

  • → depolarization only at nodes

• Signal jumps from node to node

• >50x faster than unmyelinated, more E efficient