Nerves and Spinal Cord

What are the 4 main ions involved in muscular contractions? Explain the relative amounts intercellular vs extracellular.

*intercellular

Na+ (5mM 1:28)

K+ (130mM 43:1) ← Only one that has more internal

Cl- (25mM 1:28)

Ca2+ (0.001mM 1:10000)

Explain the Sodium Potassium Pump

ATP is used to push 3Na+ out of the cell and 2K+ ions are then pumped into the cell. This creates a net positive extracellular environment (negative intercellular).

The potassium gradient is essential in secondary transport of other ions within the cell (like Cl-)

Polarized → Negative intracellular

Depolarized → Trying to make positive intracellular

EPSP

Excitatory Postsynaptic Potential

Generated by activation of neurotransmitters produced through the synapses of neurons that depolarize neurons → regulation

Promotes action potentials

Multiple EPSP causes action potentials

IPSP

Inhibitory Postsynaptic Potential

Generated by activation of neurotransmitters produced through the synapses of neurons that hyperpolarize neurons → regulation

Inhibits action potential

Depolarize membrane to reduce chance of action potential

Action Potential Curve (simple)

Dependent on the regulation of Sodium and Potassium gated channels

Sodium floods into cell to trigger action potential → rise in potential

Potassium exits cell to maintain membrane potential → decrease in voltage bc gradient returns back to normal

Polarization

1. Na+ channels are triggered, Na+ floods into cell → cell inside becomes more positive

2. -70mV → -55mV (must reach 55mV)

3. Voltage gated Na+ channels open in response to intermembrane charge reaching -55mV

4. Due to the voltage gated Na+ ions, Na+ ions flood into the cell rapidly → cell becomes very positive or depolarised (+40mV)

5. Action potential transfers down the neuron

6. YAY ACTION POTENTIAL

Depolarization

1. YAY ACTION POTENTIAL

2. Potassium ion channels open → potassium floods out of the cell to rebalance charges

3. Hyperpolarization occurs where too much potassium flows out so the intermembrane voltage dips to -75mV (more negative than resting) → relapse here where it cannot have another action potential

4. Gates close and the sodium potassium pumps work to redistribute the sodium and potassium ions back to the resting state

End Plate Potential

Triggered initially by neurotransmitters where sodium channels open (typically ACh to nAChRs)

Must reach a certain threshold to trigger the action potential and spread to the whole muscle for contraction

Local depolarization of muscle endplate at neuromuscular junction

Resting Membrane Potential

Slightly more negative inside than outside set by non-voltage gated channels

Voltage Amounts

Resting: -70mV

Max Voltage/Action Potential: 30mV

Hyperpolarization: -100mV

Return to Resting: -70mV

Types of Nerve Fibers

Type A

Type B

Type C

A-type Nerve Fibers

Large diameter and myelinated

Fastest of all nerve fibers

Rapid signals for motor and sensory control

B-type Nerve Fibers

Small diameter, thinly myelinated

Autonomic nervous system

Preganglionic fibers

C-type Nerve Fibers

Small diameter, unmyelinated

Slow conduction

Dull and diffusing pain, temperature

Postganglionic fibers

Associated for chronic and achey pain

Neuropathies most likely affect this

Ia: A-alpha fibers

Sensory Fiber

Proprioception

Muscle spindles + primary endings

Knee-jerk reflex

II: A-beta

Clinical test: Proprioception test, two point differentiation test

Touch, pressure, vibrations

Like feeling a textured surface

III: A-delta

Clinical test: Light touch, pin prick, cold temperature

Sharp pain, cold, light touches like tickle or stroke

Pain from a cut

IV: C type fibers

Clinical test: hot temperature

Dull pain, aches, warmth, itch, autonomic functions

Cauda Equina

The spinal cord goes from brainstem down to L1 or L2 where it “ends”. At that point, nerve roots split down and branch out, forming the cauda equina.

Grey Matter

Unmyelinated nerves synapse and conduct information. On the inside of vertebrae and surrounded by white matter.

White Matter

Myelinated and one way tracts that conduct information vertically up and down vertebrae. Ascending tracts are sensory, descending is motor.

Spinal Nerves

Mixed - contain both sensory and motor

They split off just before reaching the vertebrae where the sensory fibers enter on the dorsal side and the motor fibers enter on the ventral side

Sensory Pathway

First Order Neurons - Nerve enters dorsal root and synapse with 2nd order

Second Order Neurons - Ascend tract to brainstem

Third Order Neurons - Conduct information from brainstem to the sensory cortex

Motor Pathway

Upper Neurons - Start in motor cortex to brainstem/spinal cord

Lower Neurons - Starts from spinal cord vertebrae and travels to effector organs or muscles

Somatic Reflexes

Doesn’t usually require brain input (sometimes can)

• Reflex arc

  • Somatic Receptor

  • Sensory neuron

  • Interneuron

  • Motor Neuron

  • Effector muscle