Summary of "Types of Tissue Part 4: Nervous Tissue"

Main Ideas and Concepts:

Introduction to the Nervous System and Nervous Tissue:
- The nervous system controls voluntary and involuntary actions by transmitting signals from the brain to the body.
- Nervous Tissue is the fourth type of tissue studied after epithelial, connective, and muscle tissue.
- The primary cell of Nervous Tissue is the Neuron, specialized for electrical signal transmission.
Neuron Structure and Function:
- Neurons have a unique structure with extensive branching cytoplasmic extensions.
- Key parts of a Neuron:
  - Cell body (soma): Contains the nucleus and organelles.
  - Dendrites: Receive incoming signals, increasing surface area.
  - Axon: A long projection that transmits electrical impulses from the axon hillock to axon terminals.
  - Axon terminals: Release neurotransmitters to communicate with other neurons or muscles.
- Neurons are amitotic (do not divide) and can live for many decades.
- Supporting cells called neuroglia (glial cells) protect and support neurons.
Myelin Sheath and Schwann Cells:
- Many axons are wrapped in a Myelin Sheath to insulate and speed up signal transmission.
- Schwann Cells form the Myelin Sheath in the Peripheral Nervous System by wrapping around axons multiple times.
- Nodes of Ranvier are gaps between Schwann Cells that allow faster signal conduction and branching.
Types of Neurons:
- Classified by structure:
  - Unipolar: Single axon that splits into two branches (sensory function).
  - Bipolar: One axon and one dendrite (found in specialized sensory organs).
  - Multipolar: One axon and multiple dendrites (most common, ~99% of neurons).
- Classified by function:
  - Sensory (afferent) neurons: Carry signals from sensory receptors to CNS (usually unipolar).
  - Motor (efferent) neurons: Carry signals from CNS to muscles/glands (multipolar).
  - Interneurons: Connect sensory and motor neurons within CNS (multipolar, most abundant).
Basic Features of Neuronal Signal Transmission:
- Regions of a Neuron:
  - Receptive region: Typically dendrites, where stimuli are received.
  - Trigger zone: Initiates electrical impulses.
  - Conducting region: Axon transmits the electrical impulse.
  - Secretory region: Axon terminals release neurotransmitters.
- Electrical impulses arise from membrane potential differences caused by ion concentration gradients.
- Ion channels (non-gated and gated: chemically, voltage, mechanically gated) regulate ion flow, generating electrical currents.
- Resting membrane potential is maintained by sodium-potassium pumps.
- Signals can be:
  - Graded potentials: Short distance, variable strength.
  - Action potentials: Long distance, all-or-none impulses.
- Action potential phases:
  - Depolarization (sodium influx)
  - Repolarization (potassium efflux)
  - Hyperpolarization and return to resting state
- Synapses are junctions where neurons communicate, categorized by connection points (axodendritic, axosomatic, axoaxonal).
Organization of the Nervous System:
- Divided into:
  - Central Nervous System (CNS): Brain and spinal cord; control center for processing and integration.
  - Peripheral Nervous System (PNS): Nerves outside CNS; communication network.
- PNS further divided into:
  - Sensory (afferent) division: Carries signals to CNS.
  - Motor (efferent) division: Carries signals from CNS to effectors.
- Motor division subdivided into:
  - Somatic nervous system: Controls voluntary skeletal muscles.
  - Autonomic nervous system: Controls involuntary smooth and cardiac muscles.
- Autonomic nervous system further split into:
  - Sympathetic division: Prepares body for "fight or flight."
  - Parasympathetic division: Promotes "rest and digest" functions.

Detailed Bullet Points of Methodology / Instructions:

Neuron Structure:
- Identify cell body with nucleus and organelles.
- Note dendrites for signal reception.
- Trace axon from axon hillock to terminals.
- Recognize Myelin Sheath and Nodes of Ranvier.
- Understand axon terminals release neurotransmitters.
Neuron Classification:
- Structural:
  - Unipolar: Single axon with two branches.
  - Bipolar: One axon, one dendrite.
  - Multipolar: One axon, multiple dendrites.
- Functional:
  - Sensory neurons: toward CNS.