Nervous System Notes

Nervous System

Lecture 2: Nerve Impulse

• Message moves quickly at approximately $400 \text{ km/h}$.
• Normal nerve function requires an intact myelin sheath.

Neurone Types by Function

• Sensory Neurone (Afferent Neurone):
  • Includes sensory receptors.
  • Has axon terminals, axon, cell body, and dendrites.
• Interneurone:
  • Located in the spinal cord and relay neurones.
  • Has axon and dendrites.
• Motor Neurone (Efferent Neurone):
  • Connects to effector organs.
  • Has axon and dendrites.

Neuron Characteristics

• Resting Potential:
  • Neuron is at rest, with no stimulation.
• Active (Stimulated):
  • Action Potential: Depolarization and repolarization occur.
• Nerve Impulse:
  • Information passes along the axon, changing the potential difference across the membrane and generating an action potential.

Resting Potential

• Potential difference across the axon membrane when the neuron is not conducting an impulse.
• Caused by unequal distribution of charged ions inside and outside the neuron membrane.
  • Inside is more negatively charged relative to the outside, thus the axon is polarized.
• No stimulation means the axon is at rest and polarized.

Axon Polarization at Resting Potential

• Inner membrane: negative charge, low $[Na^+]$, high $[K^+]$, presence of anions like $Cl^-$, negatively charged proteins, and organic phosphates.
• Outer membrane: positive charge, high $[Na^+]$, low $[K^+]$, $Cl^-$ also present.
• These differences cause an electrical potential difference across the membrane, known as the resting potential (approximately $-70mV$).

Maintaining Resting Potential

• Three key ions:
  • Sodium $(Na^+)$
  • Potassium $(K^+)$
  • Large negatively charged organic molecules (amino acids and proteins)
• Involves two mechanisms:
  • $K^+/Na^+$ pump (active transport)
  • Non-voltage gated $K^+/Na^+$ channels (passive transport)

Factors Maintaining Ion Concentrations

• Active transport across the axon membrane
• Passive transport (diffusion)
• Internal anions
• Voltage-gated $Na^+$ & $K^+$ channels are both closed

Active Transport of Ions

• Against their electrochemical gradients via the $Na^+-K^+$ pump.
• Pumps are carrier proteins in the axon surface membrane.
• Actively transport $Na^+$ ions out of the axon and $K^+$ ions into the axon.
• Driven by energy from ATP.

Ion Movement

• For every 3 $Na^+$ ions pumped out, 2 $K^+$ ions are pumped in.
• Results in a greater concentration of $Na^+$ ions outside the axon.

Passive Transport of Ions (Diffusion)

• Active movement of ions by passive diffusion (down the concentration gradients).
• More non-voltage gated $K^+$ channels compared to $Na^+$ channels, so more $K^+$ diffuses out faster than $Na^+$ diffuses in.

Diffusion Rate

• Depends on the membrane's permeability to the ions.
• Membrane is more permeable to $K^+$ ions because there are more non-gated $K^+$ ion channels.

Charge Distribution

• $K^+$ ions diffuse out more compared to $Na^+$ ions moving in.
• $K^+$ ions that diffuse out increase the positive charge in the extracellular fluid outside the axon, making the inside of the axon negatively charged.
• Net charge: outer membrane is positive, inner membrane is negative.

Internal Anions

• Large organic molecules, like proteins, that cannot cross the membrane contribute to the negative charge within the axon.

Voltage-Gated Channels

• $N^+/K^+$ voltage-gated channels are both closed.
• The net result is that the outer membrane is positive compared to the inner membrane, establishing the resting potential.

Action Potential

• The change in the potential difference across an axon membrane during the passage of a nerve impulse.
• Phases:
  • Depolarization
  • Repolarization
  • Hyperpolarization
• Nerve impulse can only be transmitted as a series of electrical signals when the stimuli exceeds the threshold intensity (>-50 mV).

Action Potential Phases

• Occurs in 2-3 milliseconds.
  • Depolarization
  • Repolarization
  • Hyperpolarization

Ion Movement During Action Potential

• Axon at rest (polarized).
• Stimulus reaches a resting neuron, voltage-gated $Na^+$ channels open, and $Na^+$ starts to move in.

Depolarization

• More voltage-gated $Na^+$ channels open.
• More $Na^+$ moves in.
• Inner membrane becomes positively charged.

Repolarization

• Voltage-gated $K^+$ channels open.
• More $K^+$ moves out.
• Inner membrane becomes negatively charged.

Hyperpolarization

• Voltage-gated $K^+$ channels are slow to close.
• $K^+$ still moves out.
• Inner membrane becomes more negative.

Positive Feedback Cycle

• Opening of voltage-gated $Na^+$ channels follows a positive feedback cycle.
• More positivity in the cell leads to more voltage-gated $Na^+$ channels opening.

Depolarization (1 msec)

• Stimulus reaches a resting neuron, some voltage-gated $Na^+$ channels open.
• $Na^+$ diffuses into the axon.
• The inside of the neuron becomes more positive relative to the outside.
• The axon membrane depolarizes.
• More gates open, more $Na^+$ diffuses in, furthering depolarization.

Threshold Value

• When the membrane potential difference reaches a threshold value, many more gates open, leading to rapid diffusion of $Na^+$ and a sudden increase in the membrane potential difference (+35 mV).

Repolarization

• Reversal in polarity to +35 mV causes voltage-gated $Na^+$ channels to close.
• Voltage-gated $K^+$ channels open, and $K^+$ diffuses out of the axon.
• The outside of the neuron becomes more positive relative to the inside.
• The axon membrane is repolarized.
• Action potential alters from +35 mV to -70 mV.

Hyperpolarization

• Voltage-gated $K^+$ channels are slow to close; excess $K^+$ leaves the axon.
• Inner membrane becomes more negative, and the voltage falls slightly below -70 mV, resulting in hyperpolarization.
• Within a few milliseconds, voltage-gated $K^+$ channels close.
• Resting potential (-70 mV) is re-established.

Propagation of Nerve Impulse

• As a series of repolarization & depolarization along the axon.
• Information is transmitted along a neuron as a nerve impulse, which consists of a series of action potentials.
• When a neuron is stimulated, $Na^+$ flows into the neuron, causing depolarization of the inner membrane and generating an action potential.
• This part of the membrane is more positive relative to the adjacent part, which is still at resting potential.

Localized Electric Current (LEC)

• The difference in potential between the active and resting membrane parts creates a localized electric current (LEC).
• LEC stimulates the adjacent part of the membrane.
• $Na^+$ flows in, depolarizing and generating a second action potential.
• After the generation of the action potential in the second part, the first part of the membrane is repolarizing as $K^+$ flows out.

Impulse Propagation

• Impulse is propagated as a series of repolarization and depolarization along the axon.

Refractory Period

• Short period immediately after an action potential has passed, when the axon is unable to respond to another stimulus or transmit a new impulse (5-10 msec).
• The axon is insensitive to depolarization.

Absolute Refractory Period

• Period immediately after the repolarization phase of an action potential.
• The axon membrane is unable to respond to another stimulus.
• Lasts for 1 msec.

Relative Refractory Period

• After the absolute refractory period.
• The axon can transmit new impulses if the stimulus is more intense than normally required.
• Lasts for 5 msec.
• Resting potential is gradually restored by the $Na^+/K^+$ pump.

All or Nothing Law

• All action potentials are of the same amplitude.
• After the threshold is reached, the size of the action potential produced remains constant and is independent of the intensity of the stimulus.

Factors Affecting Impulse Transmission

Diameter of the Axon

• The larger the axon diameter, the faster the speed of impulse transmission.
• The smaller the diameter, the greater the resistance created by the axoplasm, lowering the speed of impulse transmission.

Myelinated Neurone

• An action potential can only be generated at nodes of Ranvier because $N^+$ & $K^+$ are able to move across the membrane.
• Hence, action potential jumps from one node of Ranvier to another along the axon, increasing the speed of impulse transmission. This is called saltatory conduction.