Nervous System Overview

Nervous System Overview

• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS consists of all the nerves that connect the CNS to the rest of the body.

Brain Structure

• The brain is divided into four major parts: the cerebrum, diencephalon, brainstem, and cerebellum.
• The cerebrum is the largest part of the brain and is responsible for higher-level functions like thinking, language, and memory.
• The diencephalon is located below the cerebrum and serves as a relay center for sensory information.
• The brainstem connects the cerebrum and cerebellum to the spinal cord.
• The cerebellum is responsible for coordinating movement and balance.

Neuron Structure

• Neurons are the basic functional units of the nervous system.
• The primary structure of a neuron includes the dendrites, initial segment, axon, and nerve endings.
• Dendrites receive signals from other neurons and transmit them to the cell body.
• The initial segment is where action potentials are generated.
• The axon transmits signals to other neurons or target cells where action potentials arrive at nerve endings.
• Nerve endings (presynaptic terminals) release neurotransmitters that communicate with other neurons.
• Many axons are covered by myelinated sheaths, formed by Schwann cells (PNS) or oligodendrocytes (CNS).

Neuron Excitation & Conduction

• Neurons generate two types of potentials: local (non-propagated) and action potentials.
• Local potentials are graded responses, whereas action potentials are all-or-none responses.
• Action potentials are generated by the opening and closing of voltage-gated ion channels, typically Na+ and K+ channels.
• Ion conductance depends on permeability and resistance of the membrane.

Action Potential Process

• Resting state: Neuron is at rest with no ion movement across the membrane.
• Depolarizing stimulus: Stimulus causes Na+ channels to open, allowing Na+ to enter the cell, leading to threshold potential.
• Rapid depolarization: More Na+ channels open, causing a rapid rise in membrane potential (upstroke).
• Na+ channels inactivate: Membrane potential moves towards +60 mV (Na+ equilibrium potential), but Na+ channels inactivate.
• Repolarization: Voltage-gated K+ channels open, allowing K+ to exit the cell, causing repolarization.
• After-hyperpolarization: Slow closure of K+ channels causes a brief period of hyperpolarization.
• Return to resting state: Membrane potential returns to its resting level after K+ channels close.

Action Potential Properties

• Threshold intensity: The minimum stimulus strength needed to trigger an action potential.
• Strength-duration relationship: Stronger stimuli have shorter durations to reach threshold, whereas weaker stimuli have longer durations.
• Refractory periods: Absolute refractory period (no response during depolarization) and relative refractory period (needs a stronger stimulus to elicit a response).
• All-or-none principle: An action potential will only occur if the threshold level is reached. If subthreshold, no action potential occurs. The size remains constant once threshold is reached.
• Conduction: Action potential propagates as positive charges flow into the region of depolarization, triggering local responses.

Myelinated Axons and Saltatory Conduction

• In myelinated axons, current flows only at the nodes of Ranvier, the gaps between myelin, due to the insulating effect of myelin.
• Depolarization jumps from one node to the next, called saltatory conduction, making signal transmission significantly faster.

Ion Concentration Effects on Excitability

• Decreased Na+ outside: Reduces AP strength but minimal impact on resting potential.
• Increased K+ outside: Lowers AP threshold, making neurons more excitable.
• Decreased K+ outside: Hyperpolarizes the membrane, reducing AP likelihood.
• Decreased Ca2+: Increases membrane excitability.
• Increased Ca2+: Decreases membrane excitability.

Glial Cells - Supporting Role

• Glial cells are non-neuronal cells in the CNS and PNS that support neuron function.
• Types of glial cells: oligodendrocytes, astrocytes, ependymal cells, microglia (CNS); Schwann cells and satellite cells (PNS).
• Functions: surround and hold neurons, supply nutrients and oxygen, insulate neurons from each other, destroy pathogens and remove dead neurons.

Blood-Brain Barrier (BBB)

• BBB is a specialized barrier between the blood and CNS, regulating what substances can enter the brain.
• it protects the brain from harmful substances in the blood.

Neurotrophins - Growth and Development

• Neurotrophins are a family of proteins that regulate the growth, survival, and function of neurons.
• They play essential roles in the development and maintenance of the nervous system.

Axonal Injury and Regeneration

• Peripheral nerve damage can often be repaired. The axon degenerates beyond the injury, but the distal stump survives.
• Axons regenerate from the proximal stump, guided by Schwann cells and neurotrophins from the distal stump.
• CNS axonal injury is more difficult to repair, as CNS neurons lack the growth-promoting chemicals and myelin inhibits axonal growth.
• CNS damage treatments focus on rehabilitation rather than repairing the damaged nerves.