Fundamentals of the Nervous System and Nervous Tissue: Structured Study Notes

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Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master control and communication system of the body. It is responsible for monitoring changes, interpreting sensory input, and dictating motor output. These functions are essential for maintaining homeostasis and responding to the environment.

Sensory Input: Sensory receptors detect changes (stimuli) inside and outside the body and send this information to the nervous system.
Integration: The nervous system processes and interprets sensory input, deciding what action is needed.
Motor Output: The nervous system activates effector organs (muscles and glands) to cause a response.

Diagram showing sensory input, integration, and motor output

Divisions of the Nervous System

The nervous system is divided into two main anatomical and functional subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integrating and command center of the nervous system.
Peripheral Nervous System (PNS): Includes cranial nerves, spinal nerves, and ganglia. It connects the CNS to limbs and organs.

Diagram of CNS and PNS in the human body

Functional Organization of the PNS

The PNS is further divided based on the direction of signal transmission and the regions served:

Sensory (Afferent) Division: Conducts impulses from receptors to the CNS. Includes somatic and visceral sensory fibers.
Motor (Efferent) Division: Conducts impulses from the CNS to effectors (muscles and glands). Subdivided into:
- Somatic Nervous System: Voluntary control of skeletal muscles.
- Autonomic Nervous System (ANS): Involuntary control of cardiac muscle, smooth muscle, and glands. Includes sympathetic and parasympathetic divisions.

Functional organization of the nervous system

Subdivisions of Sensory and Motor Components

Somatic Sensory: General senses (touch, pain, pressure, vibration, temperature) and special senses (hearing, balance, vision).
Visceral Sensory: Senses from internal organs (stretch, pain, temperature, nausea, hunger), and special senses (taste, smell).
Somatic Motor: Voluntary control of skeletal muscles.
Visceral Motor (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is composed of two main cell types: neurons and neuroglia (supporting cells).

Neurons: Specialized cells that conduct electrical impulses (action potentials). They have longevity, do not divide (except neural stem cells), and have a high metabolic rate.
Neuroglia: Non-excitable cells that support, protect, and insulate neurons.

Structure of a Neuron

Neurons have a complex structure with distinct regions for receiving, integrating, and transmitting information.

Cell Body (Soma): Contains the nucleus and organelles. Chromatophilic bodies (Nissl bodies) are clusters of rough ER for protein synthesis.
Dendrites: Branching processes that receive signals and transmit them toward the cell body.
Axon: A single, long process that transmits impulses away from the cell body. Axons may branch (collaterals) and end in terminal boutons (axon terminals).

Labeled diagram of a neuron and its parts

Synapses

Synapses are specialized junctions where neurons communicate with other neurons, muscle cells, or glands. Most synapses use chemical messengers (neurotransmitters), but some are electrical (gap junctions).

Presynaptic Neuron: Conducts signal toward the synapse.
Postsynaptic Neuron: Receives the signal.
Types of Synapses:
- Axodendritic: Between axon terminals and dendrites (most common).
- Axosomatic: Between axon terminals and cell bodies.
- Axoaxonic: Between axon terminals of different neurons.

Types of synapses: axodendritic, axosomatic, axoaxonic Structure of a chemical synapse

Classification of Neurons

Neurons are classified structurally by the number of processes extending from the cell body and functionally by the direction of impulse conduction.

Structural Classification:
- Multipolar: Many dendrites, one axon (most common, e.g., motor neurons).
- Bipolar: One dendrite, one axon (rare, found in special senses).
- Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

Neuron Type	Processes	Location/Function
Multipolar	Many dendrites, one axon	Most CNS neurons, motor neurons
Bipolar	One dendrite, one axon	Special senses (retina, olfactory epithelium)
Unipolar	One process, splits into two	Sensory neurons in PNS

Comparison of structural classes of neurons Structural variations of neurons Functional classification of neurons

Neuroglia (Glial Cells)

Neuroglia are supporting cells that outnumber neurons and perform various functions in the CNS and PNS.

CNS Neuroglia:
- Astrocytes: Regulate neurotransmitter levels, blood flow, and ionic environment; support neuronal growth.
- Microglia: Phagocytic cells that remove debris and pathogens.
- Ependymal Cells: Line brain and spinal cord cavities; circulate cerebrospinal fluid.
- Oligodendrocytes: Form myelin sheaths around CNS axons.
PNS Neuroglia:
- Satellite Cells: Surround neuron cell bodies in ganglia.
- Schwann Cells: Form myelin sheaths around PNS axons.

Astrocyte in CNS Microglial cell in CNS Ependymal cells lining CNS cavities Oligodendrocytes forming myelin in CNS

Myelin Sheaths

Myelin sheaths are segmented, lipid-rich coverings that insulate axons and increase the speed of nerve impulse conduction.

In the PNS: Formed by Schwann cells wrapping around axons. Gaps between segments are called nodes of Ranvier.
In the CNS: Formed by oligodendrocytes, which can myelinate multiple axons.
Function: Myelination increases conduction velocity and prevents current leakage.

Formation of myelin sheath by Schwann cells Myelinated axon in PNS Myelin sheath gaps (nodes of Ranvier) Nonmyelinated axons in PNS Oligodendrocytes forming myelin in CNS

Gray and White Matter in the CNS

The CNS is organized into gray matter and white matter, each with distinct functions and locations.

Gray Matter: Contains neuron cell bodies, dendrites, and nonmyelinated axons. Site of synapses and integration.
White Matter: Composed of myelinated axons (tracts) that connect different CNS regions.

Gray matter in spinal cord Gray matter in CNS White matter in CNS

Gross Anatomy of Nerves

Nerves are cable-like organs in the PNS, consisting of bundles of axons (nerve fibers) wrapped in connective tissue layers.

Endoneurium: Surrounds individual axons.
Perineurium: Surrounds bundles of axons (fascicles).
Epineurium: Surrounds the entire nerve.

Structure of a peripheral nerve

Neuronal Integration and Reflex Arcs

Neuronal Integration

The CNS and PNS are functionally interconnected. Sensory (afferent) fibers bring information to the CNS, while motor (efferent) fibers carry commands to effectors. Interneurons in the CNS process and integrate information.

Integration of sensory and motor fibers in the spinal cord

Reflex Arcs

Reflex arcs are the basic structural and functional units of the nervous system, responsible for rapid, automatic responses to stimuli. They consist of five components:

Receptor: Detects the stimulus.
Sensory Neuron: Transmits afferent impulses to the CNS.
Integration Center: One or more synapses in the CNS.
Motor Neuron: Conducts efferent impulses to an effector.
Effector: Muscle or gland that responds to the impulse.

Reflex arc pathway Reflex arc with labeled components

Example: The knee-jerk reflex is a classic example of a simple reflex arc, involving a sensory neuron, an interneuron, and a motor neuron.

Additional info: The speed and efficiency of reflexes are critical for survival, allowing organisms to respond rapidly to potentially harmful stimuli without the need for conscious thought.