Nervous System and Nervous Tissue: Structure and Function

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Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into two major regions: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS): Composed of the brain and spinal cord. It serves as the main control center for processing information.
Peripheral Nervous System (PNS): Consists of all neural tissue outside the CNS, including nerves and ganglia. It connects the CNS to limbs and organs.
This division is helpful for understanding function but can be oversimplified in some contexts.

Nervous Tissue: Cellular Components

Both the CNS and PNS are composed of nervous tissue, which contains two basic types of cells: neurons and glial cells.

Neurons: The communicative cells of the nervous system, responsible for transmitting electrical signals.
Glial Cells (Neuroglia): Provide structural and metabolic support for neurons.

Anatomy of a Neuron

Neurons are specialized for the transmission of electrical signals. Their structure is adapted for this function.

Cell Body (Soma): Contains the nucleus and most major organelles.
Dendrites: Cell-membrane extensions that receive signals from other neurons via synapses.
Axon: Usually a single, long process that propagates nerve impulses to other cells. Axons can branch, allowing communication with multiple target cells.
Axon Terminal: The end of the axon, where synaptic end bulbs connect with target cells at synapses.
Axoplasm: The cytoplasm within the axon, which differs from that of the cell body.

Myelination and Nodes of Ranvier

Many neurons are wrapped in myelin, an insulating, lipid-rich substance produced by glial cells. Myelin increases the speed of electrical signal transmission.

Myelin Sheath: Produced by oligodendrocytes in the CNS and Schwann cells in the PNS.
Nodes of Ranvier: Gaps in the myelin sheath that are important for saltatory conduction (the jumping of action potentials from node to node).

Classification of Neurons by Structure

Type	Structure	Location/Function
Unipolar	One process (axon); found in invertebrates	Not found in humans
Pseudo-unipolar	One process splits close to cell body	Sensory neurons in humans
Bipolar	One axon, one dendrite	Olfactory epithelium, retina
Multipolar	One axon, two or more dendrites	Most common type in humans

Organization of Nervous Tissue

Nucleus: A localized collection of neuron cell bodies in the CNS.
Ganglion: A localized collection of neuron cell bodies in the PNS.
Tract: A bundle of axons (fibers) in the CNS.
Nerve: A bundle of axons (fibers) in the PNS.
Example: The optic nerve (PNS) becomes the optic tract (CNS) after the optic chiasma.

Basic Functions of the Nervous System

The nervous system has three main functions: sensation, integration, and response.

Sensation: Sensory functions detect changes (stimuli) within the body or environment. This information travels to the CNS via the sensory division of the PNS.
Integration: The CNS processes and interprets sensory input, integrating it with other stimuli, memories, and the current state of the body to determine an appropriate response.
Response: The nervous system produces a response based on integrated information. Responses can be voluntary (e.g., skeletal muscle contraction) or involuntary (e.g., smooth muscle contraction, gland activation).

Example: Withdrawing your hand from a hot stove is a rapid, involuntary response to a painful stimulus.

Divisions of the Peripheral Nervous System

Division	Main Function
Somatic Nervous System (SNS)	Voluntary motor responses (skeletal muscle contraction)
Autonomic Nervous System (ANS)	Involuntary control of smooth muscle, cardiac muscle, and glands; maintains homeostasis
Enteric Nervous System (ENS)	Controls smooth muscle and glands in the digestive system; sometimes considered part of the ANS

Glial Cells: Types and Functions

Glial cells (neuroglia) support neurons in various ways. They differ between the CNS and PNS.

Astrocytes (CNS): Regulate ion concentrations, help form the blood-brain barrier, and remove excess neurotransmitters.
Oligodendrocytes (CNS): Myelinate axons in the CNS.
Ependymal Cells (CNS): Produce and circulate cerebrospinal fluid (CSF).
Satellite Cells (PNS): Support neuron cell bodies in ganglia, similar to astrocytes in the CNS.
Schwann Cells (PNS): Myelinate axons in the PNS; each cell wraps around only one axon segment.

Ion Channels and Membrane Potential

The cell membrane separates the intracellular and extracellular environments. Charged particles (ions) cannot pass through the plasma membrane without help from ion channels (transmembrane proteins).

Ligand-gated Ion Channels: Open in response to signaling molecule (ligand) binding (e.g., acetylcholine).
Mechanically Gated Ion Channels: Open in response to physical distortion of the cell membrane (e.g., pressure on the skin).
Voltage-gated Ion Channels: Open in response to changes in membrane potential (e.g., voltage-gated sodium channels open at -55 mV).

Resting Membrane Potential

The resting membrane potential is the distribution of charge across the cell membrane when the neuron is not conducting an action potential.

Measured in millivolts (mV); typically around -70 mV (inside of the cell is negative relative to the outside).
Extracellular sodium ion concentration is about 10 times greater than intracellular; potassium ion concentration is greater inside the cell.
High concentration of anions (negatively charged ions and proteins) inside the cell.
Maintained by the sodium-potassium pump and ion leakage channels.

Action Potentials

An action potential is a temporary change in membrane potential that allows neurons to communicate.

At rest, the cell membrane is polarized.
A stimulus depolarizes the membrane to the threshold level (about -55 mV), opening sodium ion channels.
Sodium ions rush into the cell, causing rapid depolarization (up to +30 mV).
At +30 mV, sodium channels close and potassium channels open, allowing potassium to exit the cell (repolarization).
Temporary hyperpolarization may occur before returning to resting potential.
Action potentials are all-or-none events: if threshold is reached, the action potential always peaks at the same voltage.
Stronger stimuli result in more frequent action potentials, not larger ones.

Equation:

Resting membrane potential is maintained by the sodium-potassium pump:

Graded Potentials

Local changes in membrane potential are called graded potentials. They can be caused by the opening of ligand-gated or mechanically gated sodium ion channels. If large enough, a graded potential can trigger an action potential.

Summary Table: Key Terms in Nervous System Organization

Term	CNS	PNS
Cell body collection	Nucleus	Ganglion
Axon bundle	Tract	Nerve
Myelinating glial cell	Oligodendrocyte	Schwann cell

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