The Nervous System and Neuronal Excitability Lecture Review

Foundational Concepts: This chapter applies foundational concepts from previous chapters ($1-6$) to the nervous system, demonstrating how basic principles underpin complex body systems.

Integration:
- Processes sensory information.
- Controls all body responses and activities.
- Stores information.
Sensory Input:
- Detects and monitors internal and external stimuli.
- Examples: fullness of stomach, bee sting.
Motor Output:
- Causes a response in effectors, such as muscles and glands.

Central Nervous System (CNS):
- Comprises the brain and spinal cord.
Peripheral Nervous System (PNS):
- Includes cranial and spinal nerves.
- Contains sensory receptors.

The functional unit of the nervous system.
Incapable of mitosis once formed, though some neurons have a limited ability for repair.
Excitable cells:
- Respond to physical and chemical stimuli.
- Produce and conduct electrical signals.
- Release chemicals for regulation and communication, primarily neurotransmitters (NT).

Cell Body: Contains a single nucleus.
- Axon Hillock: A thick base of the cell body where the axon originates.
Processes:
- Dendrites: Carry signals TO the cell body.
  - Specialized for contact with other neurons.
  - Often branched and contain dendritic spines to increase surface area for synaptic contact.
- Axon: Carry impulses AWAY from the cell body.
  - Typically a single, long process starting at the axon hillock.
  - Branches at the end to form axon terminals.
  - Axon terminals contain vesicles filled with neurotransmitters.

SENSORY Neurons: Transmit information from the PNS to the CNS.
INTERNEURONS: Interpret sensory information and may elicit a response; found entirely within the CNS.
MOTOR Neurons: Transmit information from the CNS to the PNS (to effectors).

Cluster of cell bodies:
- In the PNS: ganglion/ganglia.
- In the CNS: nucleus/nuclei.
Bundle of axons:
- In the PNS: nerve.
- In the CNS: tract.

Astrocytes:
- Most common glial cell, star-shaped.
- Form the blood-brain barrier by covering capillaries in the brain.
- Provide structural support.
- Direct/guide neurons during development and new neural connection formation.
Oligodendrocytes:
- Fairly common.
- Form the myelin sheath around multiple axons in the CNS (myelination), similar to Schwann cells in the PNS.
Microglia:
- Found near blood vessels.
- Phagocytic cells that engulf and clear away dead or damaged cells, debris, and pathogens.
Ependymal Cells:
- Form an epithelial layer lining chambers and canals in the CNS.
- Produce cerebrospinal fluid (CSF).

Satellite Cells:
- Flat cells that surround cell bodies in ganglia.
- Provide support and protection for neurons within ganglia.
Schwann Cells:
- Wrap around axons of PNS neurons.
- Produce a myelin sheath (myelination).

Most axons in the PNS and some in the CNS are myelinated.
The myelin sheath is formed by glial cells wrapping tightly around axons.
Function: Acts as an electrical insulator and significantly speeds up the conduction of nerve impulses.
Nodes of Ranvier: Unmyelinated gaps along an axon between myelin-forming cells.
- These nodes are in direct contact with the extracellular fluid (ECF).
Loss of Myelination (Demyelination):
- CNS neurons: Example is Multiple Sclerosis (MS).
- PNS neurons: Example is Guillain-Barre syndrome.
- These are autoimmune diseases where the immune system attacks and destroys myelin, impairing neural function (both sensory and motor).

Excitable cells (neurons, muscle cells) can change their membrane potential when adequately stimulated.
This change creates an electrical signal that spreads along the cell membrane.

Charged particles: Ions ($ ext{Na}^+, ext{K}^+, ext{Cl}^-$, etc.), some proteins, $ ext{PO}_4^{3-}$ are in solution inside and outside cells.
Electrical Gradient: Opposites attract, like repels. If these charges are separated by a boundary (the cell membrane), a difference in electrical charge (a potential) is created across the boundary.
Potential Energy: This potential contains potential energy.
Voltage: This electrical potential can be measured as a voltage (unit: millivolts, ext{mV}).
Membrane Potential ( ext{V}_{ ext{m}}): The difference in electrical charge inside vs. outside the cell, across the cell membrane.

The membrane potential of a cell at rest.
Range: Approximately -50 ext{ mV} to -100 ext{ mV} (cell type dependent).
- The negative sign indicates that the inside of the cell is negatively charged relative to the outside.
Excitable cells can radically alter their RMP when stimulated.
RMP in neurons is typically -70 ext{ mV}.

Different Ionic Makeup:
- ext{Na}^+ concentration is high outside the cell.
- ext{K}^+ concentration is high inside the cell.
Differential Membrane Permeability (Leakage Channels):
- The membrane is barely permeable to ext{Na}^+ (a small amount trickles in).
- The membrane is quite permeable to ext{K}^+ (more K^+ flows out).
- This imbalance, with more positive ions leaving than entering, makes the inside of the cell more negative.
Role of ext{Na}^+/ ext{K}^+ Pumps:
- These active transport pumps constantly work to counteract the leakage of ions and maintain the concentration gradients.
- They pump ext{Na}^+ out and ext{K}^+ in, returning ions to their original positions.
- Each pump cycle moves 3 ext{ Na}^+ ions out of the cell and 2 ext{ K}^+ ions into the cell, which also contributes to maintaining the negative RMP inside the cell (as more positive charge leaves than enters).