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NEU_LEC_4

CELLS OF THE BRAIN LECTURE 4 CHAPTER 1, SECTIONS 1.1, 1.2

INTRODUCTION TO NERVOUS SYSTEM

Key Topics:

  • Neurons: Fundamental units responsible for transmitting information throughout the body.

  • Glial Cells: Support cells that play various roles in maintaining homeostasis, forming myelin, and supporting neurons.

  • Divisions of the Nervous System: Categorization into the Central and Peripheral Nervous Systems.

  • Terminology used in the study of the Nervous System: Defined terms essential for understanding the anatomy and physiology of the nervous system.

CELLS OF THE NERVOUS SYSTEM

The nervous system is a highly organized network comprised of two primary cell types: Neurons and Glia.

Neurons:

  • The adult human brain contains approximately 100 billion neurons, specialized for processing and transmitting information through electrical and chemical signals.

  • Neurons play crucial roles in processes such as reflex actions, sensory perception, and information processing in the brain.

  • Neurons consist of major parts: dendrites (receive signals), soma or cell body (processes signals), and axon (transmits signals).

Glial Cells (Glia):

  • Glial cells significantly outnumber neurons, estimated at about 360 billion in the nervous system.

  • They provide essential support for neurons, including protection, nutrition, and maintenance.

  • Functions include:

    • Oligodendrocytes: Form the myelin sheath in the CNS.

    • Schwann cells: Myelinate axons in the PNS.

    • Microglia: Act as immune cells in the brain, cleaning up debris.

    • Astrocytes: Maintain the blood-brain barrier and regulate blood flow to neurons.

STAINED SECTION OF BRAIN

Key cell types observed in stained sections of the brain include:

  • Neuron

  • Glial Cell

  • Epithelial Cell

Scale:

Approximately 50 µm in height, allowing for detailed observation of cellular organization.

GLIAL CELLS

Functions of Glial Cells:

  • Oligodendrocytes and Schwann cells: Essential for myelination, improving signal conduction speed.

  • Microglia: Monitor the brain's environment, responding to injury by migrating to damaged areas and undergoing proliferation.

  • Astrocytes: Provide biochemical support to endothelial cells that form the blood-brain barrier and help regulate neurotransmitter levels.

TYPES OF GLIAL CELLS

  • Astrocytes: Regulate blood flow and nutrient delivery based on metabolic needs of neurons.

  • Oligodendrocytes: Involved in the formation of the myelin and nutrient delivery to neurons.

  • Microglia: Function as the immune system of the CNS, responding to threats by producing inflammatory cytokines and removing cellular debris.

NEURONS

Functions of Neurons:

  • Neurons receive and transmit signals to other neurons and muscles through synapses.

  • They form complex networks known as circuits essential for voluntary and involuntary actions, such as reflexes and sensory processing.

NEURONAL CONNECTIONS

  • Neurons establish circuits that enable coordinated functions throughout the nervous system, involving direct communication with muscle fibers to facilitate movements.

PARTS OF A NEURON

  • Nucleus: Contains genetic material necessary for the functioning of the neuron.

  • Dendrites: Specialized for receiving incoming signals through synaptic connections.

  • Soma/Cell Body: Key site for metabolic activity, containing organelles for energy and protein synthesis.

  • Axon: Transmits action potentials (electrical impulses) away from the cell body; its length allows for long-distance signal propagation.

  • Axon Terminals: Site of neurotransmitter release, playing a crucial role in synaptic communication.

FUNCTIONS OF NEURON PARTS

  • Dendrites: Act as input units to gather information; connected to input zones for integration.

  • Axon Hillock: Acts as a threshold point, determining if the integrated signal is strong enough to generate an action potential.

  • Cell Body: Concentrates resources for the entire neuron and supports overall cellular health.

  • Axon Terminals: Release neurotransmitters; essential for communication across synapses.

FUNCTIONAL ZONES OF NEURONS

  • Input Zone: Composed of dendrites, integrates incoming signals.

  • Integration Zone: Where dendrites converge; action potentials are generated here if the input surpasses a certain threshold.

  • Conduction Zone: The axon propagates electrical signals towards target neurons.

  • Output Zone: Axon terminals transmit signals to other neurons or target tissues through neurotransmitter release.

PATH OF INFORMATION FLOW

  • Information flows from dendrites, to axon hillock which generates an action potential, travelling down the axon, and finally reaching the axon terminals for communication with other cells.

MYELIN SHEATH

  • Myelin: A lipid-rich layer providing insulation to axons, significantly speeding up electrical signal transmission through saltatory conduction at the Nodes of Ranvier.

MULTIPLE SCLEROSIS (MS)

  • Multiple Sclerosis: An autoimmune disease affecting myelin in the CNS. Symptoms may include:

    • Slowed movement and coordination

    • Visual disturbances such as blurred vision or loss of sight

    • Auditory impairments and sensory deficits

NEURONS: DIVERSITY AND SPECIALIZATION

  • Neurons can be classified into:

    • Sensory Neurons (Afferent): Carry signals from sensory receptors to the brain, facilitating sensation.

    • Motor Neurons (Efferent): Transmit signals from the brain to muscles, responsible for movements.

    • Interneurons: Most populous, involved in local processing and reflex actions.

TYPES OF NEURONS BY SHAPE

  • Multipolar Neurons: Many dendrites for complex information processing.

  • Bipolar Neurons: One dendrite and one axon, typical in sensory pathways like the retina.

  • Unipolar Neurons: Single extension acts as both axon and dendrite, predominantly found in sensory nerves.

NEURONS COMMUNICATE AT SYNAPSES

  • Communication occurs at synapses, specialized junctions for signal transmission.

COMPONENTS OF A SYNAPSE

  • Presynaptic Neuron: Sends signals via neurotransmission.

  • Postsynaptic Neuron: Receives signals through receptors located on dendritic spines.

  • Synaptic Cleft: Gap where neurotransmitter diffusion occurs.

NEUROTRANSMITTER ACTION

  • Neurotransmitters bind to receptors on the postsynaptic neuron, triggering cellular responses such as an influx of ions leading to potential action in the postsynaptic neuron.

TERMINOLOGY IN NERVOUS SYSTEM STUDY

  • Lamina: Layer of neuronal bodies organized in specific patterns.

  • Column: Neuronal structures exhibiting similar properties in vertical alignment.

  • Nerve: Bundles of axons in the peripheral covering sensory and motor pathways.

  • Ganglion: Clusters of neuron bodies outside the CNS.

  • Gyrus & Sulcus: Convolution and indentation patterns on the brain’s surface, increasing functional area.

  • Tract: Pathway of axons within the CNS; can reflect projections from one area to another.

  • Nucleus: Cluster of neuron cell bodies in the CNS, often involved in specific functions.

DIVISIONS OF THE NERVOUS SYSTEM

The nervous system is categorized into two main divisions:

  • Central Nervous System (CNS): Comprising the brain and spinal cord, central processing unit for information and control.

  • Peripheral Nervous System (PNS): Includes all neural structures outside the CNS, connecting it to the rest of the body.

AUTONOMIC AND SOMATIC DIVISIONS OF PNS

  • Somatic Nervous System: Links CNS to skeletal muscles for voluntary actions:

    • Includes cranial and spinal nerves for voluntary and reflex actions.

    • Functions primarily in facilitating conscious movements.

  • Autonomic Nervous System: Manages involuntary processes:

    • Sympathetic Nervous System: Activates body in stress response (fight-or-flight).

    • Parasympathetic Nervous System: Conserves energy during restful states (rest-and-digest).

CRANIAL NERVES IN SOMATIC NERVOUS SYSTEM

The somatic system comprises 12 cranial nerves:

  • Sensory Only: Olfactory (I), Optic (II), Vestibulocochlear (VIII)

  • Motor Only: Oculomotor (III), Trochlear (IV), Abducens (VI), Spinal Accessory (XI), Hypoglossal (XII)

  • Mixed Functions: Trigeminal (V), Facial (VII), Glossopharyngeal (IX), Vagus (X)

SPINAL NERVES OF THE SOMATIC NERVOUS SYSTEM

Consists of 31 pairs of spinal nerves:

  • Categorized by spinal segments: cervical, thoracic, lumbar, sacral, coccygeal.

  • Essential for transmitting sensory information to the brain and motor commands back to muscles, facilitating bodily responses to stimuli.