Introduction of nervous systems /part-2/

Understanding Synapses in Neurons

Types of Synapses

• The connection site between two or more neurons is called a synapse, which can be categorized into two main types: electrical and chemical synapses.

• Electrical synapses involve gap junctions that allow for rapid ion transfer between cells through protein channels known as connexons, formed by connexin proteins.

Chemical Synapses

• Chemical synapses are more abundant and consist of a presynaptic part (axon terminal) and a postsynaptic part (dendrite, soma, or axon).

• Different types of chemical synapses include axodendritic (between an axon and dendrite), axosomatic (between an axon and cell body), and axoaxonic (between two axons).

Structure of Presynaptic and Postsynaptic Membranes

• The presynaptic membrane contains numerous vesicles filled with neurotransmitters such as dopamine, acetylcholine, serotonin, etc. Each vesicle holds only one type of neurotransmitter.

• The postsynaptic membrane has many receptors that bind to released neurotransmitters. This area also features a network of actin filaments known as the subsynaptic web.

Functionality of Synapses

• Axodendritic and axosomatic synapses can be either excitatory or inhibitory; however, axoaxonic synapses are exclusively inhibitory.

• The initial segment of the neuron is crucial for signal transmission; it lies between the first myelin sheath and the axon hillock.

Inhibitory Mechanisms in Synapse Types

• Axoaxonic synapses inhibit signals by connecting an axon to another neuron's initial segment or terminal.

• Other rare types include dendrodendritic and somatosomatic synapses, which are also inhibitory.

Developmental Aspects

• During embryonic development, neurons form connections that will later become functional chemical synapses.

Visual Representation

• Animations illustrate how neurotransmitters are released at chemical synapses, showing both excitatory (e.g., sodium ions entering the postsynaptic area) and inhibitory mechanisms where different ions may enter instead.

Role of Glial Cells

• Astrocytes support neurons by providing nourishment; they come in two forms: protoplasmic astrocytes related to gray matter with short branched processes.

Astrocytes and Oligodendrocytes in the CNS

Overview of Astrocytes

• Protoplasmic astrocytes are star-shaped cells characterized by a potato-like nucleus and numerous short, branched processes.

• The branches of astrocytes that attach to capillaries are referred to as vascular feet, playing a crucial role in nutrient exchange.

• There are two types of astrocytes: protoplasmic (found in gray matter) with short processes, and fibrous (in white matter) with longer branches.

Functions of Astrocytes

• Main functions include protection, support, nourishment, insulation, and proliferation within nervous tissue.

• Astrocytic proliferation can lead to gliosis, which is a significant limitation in the regeneration of the nervous system as it inhibits axon growth.

Oligodendrocytes: Structure and Function

• Oligodendrocytes are starlike cells with dense nuclei that wrap around axons to form myelin sheaths in the CNS.

• Unlike Schwann cells that create one myelin sheath per cell in the PNS, oligodendrocytes can form multiple myelin sheaths around several axons.

Microglia and Ependymal Cells

Microglia Characteristics

• Microglia are small cells with many branches; their primary function is phagocytosis within the nervous system.

• Inactive microglia have numerous processes while active microglia appear rounder like macrophages.

Ependymal Cells Structure

• Ependymal cells are cuboidal or columnar epithelial-like cells lining brain ventricles and spinal cord central canal; they possess cilia for movement.

• A key difference from typical epithelial cells is that ependymal cells lack a basal lamina composed of collagen type IV and glycoproteins.

Schwann Cells vs. Oligodendrocytes

Schwann Cell Functionality

• Schwann cells (or neurolemmocytes), unlike oligodendrocytes, wrap entirely around individual axons to form myelin sheaths.

Differences in Myelination

• Each Schwann cell forms one myelin sheath around one axon while an oligodendrocyte can create multiple sheaths for different axons simultaneously.

Cell Membrane Composition Comparison

Lipid vs. Protein Ratios

• Hepatocyte membranes consist of 50% proteins and 50% lipids; however, Schwann cell membranes contain approximately 80% lipids and only 20% proteins.

• This higher lipid content contributes to the white appearance of myelin sheets formed by Schwann cells when they encircle axons.

Embryonic Development Insights

Embryo Size Reference

• The size reference for a six-week embryo is approximately 1 cm or 10 mm based on crown-rump length measurements.

Embryology and Nervous System Overview

Embryo Development and Arterial Branches

• The size of an embryo at the 8th week is approximately 3 cm, indicating early developmental stages.

• Discussion on branches of the external carotid artery, emphasizing its connection to the internal carotid artery and various branches known as "k SMS."

• Key branches of the ophthalmic artery are identified, including medial talasia celia and central retina.

• Additional branches mentioned include lacrimal, anterior ethmoidal, posterior ciliary, and others related to ocular anatomy.

• Importance of memorizing these arterial branches for understanding vascular supply in embryonic development.

Nerve Fiber Types

• Introduction to three main types of nerve fibers: A, B, and C; with A type further divided into four subtypes (alpha, beta, gamma, delta).

• Characteristics of A alpha fibers include a diameter of 20 micrometers and a velocity of 100 m/s; A beta fibers have smaller diameters and slower velocities.

• C type fibers are described as unmyelinated with low diameter (1 micrometer) and slow velocity (1 m/s), associated with dull pain sensations.

• Distinction between fiber functions: A alpha for motor control in skeletal muscles; A delta for sharp pain; B type for pre-ganglionic autonomic fibers.

• Explanation of how touching activates faster sensory pathways that can inhibit pain signals through gate theory.

Pain Perception Mechanisms

• The interaction between different types of nerve fibers during pain perception is discussed. Touching an area can activate faster C type fibers which help close the "gate" on pain signals.

• Gate theory is introduced as a mechanism explaining why massage or touch can alleviate pain by activating faster sensory pathways over slower pain pathways.