Nervous Tissues and Neurons

Nervous Tissues

• Nervous system (NS) coordinates and controls all body functions.
• NS has two major divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS).
• CNS includes the brain and spinal cord.
• PNS includes cranial, spinal, and peripheral nerves.
• Nervous tissue is specialized tissue within the NS.
• Nervous tissue consists of two types of cells: Neurons and Glial cells.

Neurons

• Neurons are the structural and functional units of the nervous system.
• Neurons respond to environmental changes (stimuli).
• Neurons rapidly change electrical potential responding to stimuli, forming excitable cells.
• Neurons react quickly to stimuli by reversing ionic gradients (membrane depolarization).
• Depolarization starts from the stimulus point and travels across the neuron's plasma membrane.
• This propagation (action potential/nerve impulse) travels along the neuron.

Structure of the Neuron

• Neurons have three main parts: Cell body (perikaryon), dendrites, and axon.
• Cell body (perikaryon): contains the nucleus, Nissl bodies (rough ER), neurofibrils, mitochondria, and Golgi apparatus.
• Cell body is the main part of the neuron where cytoplasmic processes originate.
• Nucleus: one or two prominent nucleoli.
• Neurons lack centrosomes, therefore cannot multiply.
• Neurofilaments: strands of proteins within the neuron's cytoplasm, notably in axons' distal parts.

Neurofilaments

• Neurofilaments are protein strands appearing as neurofibrils.
• They are present in the neuron's cytoplasm and extend into dendrites and axons.
• More observable in the distal part of the axon.

Neurotubules

• Neurotubules are proteins measuring 20-30 nm.
• Varying lengths, they extend into different neuron parts.
• More abundant in dendrites than axons.
• Assist with transporting materials throughout neuron cytoplasm and processes.

Nissl Bodies

• Nissl granules are small, basophilic granules in neuron cytoplasm.
• They contain attached ribosomes, found in neuron somata and dendrites.
• More prominent in active neurons (motor than sensory).
• Responsible for protein synthesis in neurons.

Mitochondria

• Mitochondria are filamentous structures in neurons.
• Occupy neuron somata and cytoplasmic processes.
• More prominent at synapses and end-plates.
• Cluster at regenerating dendrite tips.
• Are considered the neuron's powerhouses, producing ATP.

Lipofuscin

• Lipofuscin is a yellow pigment in neuron cell bodies.
• Represents the product of degenerative activity (wear and tear) in neurons.
• Increases with age in quantity.

Centrioles

• Present in nerve centrosomes.
• Not involved in mitotic spindle formation (most neurons don't divide in adults).
• Possibly involved in neurotubule generation and maintenance.

Dendrites

• Short, branched processes attached to the neuron's cell body.
• Contain ribosomes, granular endoplasmic reticulum (Nissl bodies), mitochondria, microfilaments, and microtubules.
• Transmit impulses towards the nerve cell body.

Axon

• The longer process of a nerve cell.
• Originates from the neuron's somata, referred to as axon hillocks.
• Lacks ribosomes and Nissl substance but contains other organelles.
• Transmits impulses away from the nerve cell body to other cells (nerve, muscle, and gland).

Classifications of Neuron

• Classified by number of processes (multipolar, bipolar, unipolar, anaxonic), function (sensory or motor), or size (Golgi type I or II).
• Multipolar neurons: one axon and two or more dendrites (most common).
• Bipolar neurons: one axon and one dendrite (sensory function, found in retina, olfactory mucosa, and inner ear).
• Unipolar neurons: one fused axon emerging like a T from the cell body with peripheral and central processes (most functionally sensory; found in spinal and cranial ganglia).
• Anaxonic neurons: many dendrites, but no true axon and do not produce action potentials (regulate electrical changes).

Classifications Based on Function

• Sensory neurons: afferent, receive stimuli from receptors throughout the body.
  • May be unipolar or bipolar.
• Motor neurons: efferent, send impulses from the CNS to effector organs (muscles and glands).
  • Motor neurons are multipolar.
• Golgi Type I: large somata and long axons (e.g., pyramidal cells of the motor cortex, Purkinje cells of the cerebellum).
• Golgi Type II: small somata and short axons; often inhibitory (e.g., amacrine cells of the retina).

Glial Cells (Neuroglia)

Are non-excitable cells in the CNS that support neuronal survival and activities, more abundant than neurons. Examples include astrocytes, oligodendrocytes, microglia, and ependymal cells.

Astrocytes

• Most abundant glia, highly branched cells.
• Star-shaped with a rounded nucleus.
• Contain mitochondria, lysosomes, Golgi apparatus, microfilaments, and glycogen.
• Exist as fibrous (white matter) and protoplasmic (grey matter).
• Provide support to neurons, transfer nutrients, and maintain the blood-brain barrier.

Oligodendrocytes

• Smaller, rounded cells with the ability to proliferate.
• Produce myelin sheaths around axons, providing electrical insulation in the CNS.
• May be involved in metabolic activities.

Microglia

• Smallest glial cells in the CNS.
• Migrate through neuropil, scanning for damaged cells & invading microorganisms.
• Normally quiescent, becoming active during injury or inflammation.

Ependymal Cells

• Columnar or cuboidal cells lining brain ventricles and the central canal of the spinal cord.
• Possess cilia and microvilli. Essential for cerebrospinal fluid (CSF) production & modification, and facilitating CSF movement and absorption.

Schwann Cells

• Specialized neuroglial cells in the peripheral nervous system.
• Form myelin around segments of axons.
• Have trophic interactions with axons, allowing myelination.

Blood-Brain Barrier (BBB)

• Functional barrier controlling the passage of substances from blood to the CNS tissue.
• Composed primarily of capillary endothelium with tight junctions.
• Astrocytic feet form the limiting layer, completely surrounding the basal lamina.
• Regulates molecule/ion passage between blood and brain.
• Protects neurons and glia from toxins, infectious agents, maintaining stable interstitial fluid composition for normal function.

Clinical Correlate

• Chromatolysis: Nissl substance degeneration in the cell body.
• Multiple sclerosis: myelin sheath damage.
• Astrocytosis (gliosis): astrocyte proliferation.
• Demyelination: severe oligodendrocyte damage.

This quiz evaluates your understanding of nervous tissues, focusing on their structure and function in the nervous system. You'll learn about the central and peripheral nervous systems, as well as the role of neurons and glial cells in transmitting signals. Dive deep into neuronal responses and action potentials to test your knowledge!