Questions and Answers
What is the external layer of gray matter in the brain called?
gyri
Which of the following types of neurons are found in the cerebrum?
The most external layer of gray matter in the cerebellum is called the ______ layer.
molecular
What are the large neurons in the Purkinje cell layer called?
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What is the internal layer adjacent to the white matter in the cerebellum called?
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White matter in the brain has more blood supply compared to gray matter.
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In which part of the central nervous system is white matter external to gray matter?
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What are the two major components of nerve tissue?
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Which of the following are functional categories of neurons? (Select all that apply)
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What structure in neurons is responsible for protein synthesis, visible in the light microscope?
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What is the typical resting membrane potential of a nerve cell?
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What are the two types of axonal transport?
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Dendrites are typically myelinated.
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Which of the following types of neurons has a single large process that branches?
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What type of synapses allow direct passage of ions from one cell to another?
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Match the following types of chemical synapses with their functions:
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What is the role of neurotransmitters at chemical synapses?
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What clinical conditions are associated with oligodendrocytes?
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What is the function of astrocytes in the CNS?
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How is the neurotransmitter reincorporated into vesicles?
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What are acetylcholine broken down into in the synaptic cleft?
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Inhibition of the enzyme that breaks down norepinephrine has detrimental effects in the treatment of depression.
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What type of synapse is formed between an axon and a dendrite?
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What is the specialized type of synapse called that represents the neuromuscular junction?
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The virus responsible for rabies is carried by wild ______.
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What do Schwann cells form around axons in the peripheral nerves?
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Match the cell types in the central nervous system (CNS) with their characteristics:
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Multiple sclerosis is caused by damage to the myelin sheath in the CNS.
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What is gliosis?
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What type of neuron do sensory craniospinal ganglia contain?
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What are the two major types of support cells in the peripheral nervous system (PNS)?
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What do free nerve endings mediate?
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Which sensory nerve ending is sensitive to vibrations and deep pressure?
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What is a primary function of the myelin sheath in neurons?
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Which of the following best describes axons?
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What role do lysosomes play in neurons?
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Which of the following statements about the cytoskeleton of neurons is true?
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Which type of cell is primarily responsible for producing myelin in the peripheral nervous system?
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What characterizes the processes of neurons?
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Which statement about neuroblasts in the adult brain is accurate?
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What is a key function of synapses in neurons?
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What is the primary function of inhibitory synapses in the postsynaptic cell?
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What size are synaptic vesicles typically found in the presynaptic knob?
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Which part of the neuron typically has receptor sites for neurotransmitters?
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What initiates local depolarization in the postsynaptic cell?
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How is excess plasma membrane removed after synaptic vesicle fusion?
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What type of synapse primarily occurs on the perikaryon of neurons?
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What is the primary role of neurotransmitters in a synapse?
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What occurs first when an action potential reaches the presynaptic terminal?
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What mechanism allows for the recapture of neurotransmitters in the synaptic cleft?
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What type of synapse is characterized by a connection between an axon and a perikaryon?
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Which neurotransmitter is primarily broken down by enzymes in the synaptic cleft?
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What is the main effect of inhibiting the enzymes that break down norepinephrine?
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Which of the following is NOT a morphotype of synapse?
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What is the primary role of the motor end-plate in the neuromuscular junction?
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In the context of synaptic transmission, what does endocytosis refer to?
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Which of the following components of the nervous system primarily supports nerve fibers in the peripheral nervous system?
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What is the primary function of dendrites in a neuron?
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Which statement accurately describes dendritic spines?
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Which characteristic distinguishes dendrites from axons?
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What type of neuron is primarily associated with pseudounipolar neurons?
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What structural feature allows dendrites to increase their synaptic connection potential?
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Which component is NOT typically found in dendrites?
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In terms of signal transmission, what is the role of the peripheral process of a pseudounipolar neuron?
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What distinguishes the surface of dendrites from axons?
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Which of the following describes the role of Schwann cells in the peripheral nervous system?
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Which type of cell in the central nervous system primarily functions in the repair of nervous tissue?
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Which statement about excitatory chemical synapses is correct?
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What is the main function of astrocytes in the central nervous system?
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Which structure forms the gap between the presynaptic and postsynaptic neurons in a chemical synapse?
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Which type of synapse is commonly associated with neurotransmitter release from axon terminals?
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What is the primary characteristic of retrograde axonal transport?
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Which type of neuron is commonly involved in sensory pathways and has one main process that splits into two branches?
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Study Notes
Characteristics of Nerve Tissue
- Provides rapid, specific communication between body organs.
- Comprises two major components: neurons (electrically excitable) and neuroglia (support cells).
- Average human body contains about 100 million neurons.
Functional Categories of Neurons
- Sensory Neurons: Gather information from receptors.
- Interneurons: Form communication networks between neurons.
- Motor Neurons: Convey impulses from the nervous system to effector cells.
Structure of Neurons
- Contain a large, rounded nucleus with a prominent nucleolus.
- Perikaryon (cell body) size ranges from 5 to 135 µm.
- Well-developed rough endoplasmic reticulum (RER) forms Nissl bodies visible in light microscopy.
- Well-developed Golgi complex for membrane and neurotransmitter production.
- High mitochondrial content for energy; may lack centrioles, limiting cellular division.
Cell Processes of Neurons
- Neurons have two types of processes: axons and dendrites.
- Synapses are formed for communication with other cells.
Axon Structure and Function
- Each neuron has one axon, designed to convey signals from the perikaryon.
- Axons can be very long (up to 1 meter), maintaining consistent diameter.
- Initiate from the axon hillock, which lacks Nissl bodies.
- Myelin sheath insulates many axons, facilitating rapid impulse transmission.
Axonal Transport
- Microtubules involved in transport within axons.
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Anterograde Flow: Materials move from perikaryon to axon terminal; consists of fast and slow axonal transport.
- Fast Transport: Up to 100-400 mm/day, for organelles and synaptic vesicles.
- Slow Transport: ~1-6 mm/day, for structural proteins.
- Retrograde Flow: Moves materials from axon terminal back to perikaryon, utilized by viruses and toxins.
Dendrites
- Typically several per neuron, designed to receive signals.
- Form extensive dendritic trees for increased synaptic contact area.
- Surface covered with dendritic spines where synapses form.
- Cytoplasmic composition similar to perikaryon, includes ribosomes and RER.
Major Types of Neurons
- Pseudounipolar Neurons: Sensory neurons with a single process that bifurcates into peripheral and central branches.
- Bipolar Neurons: Found in sensory organs, have one axon and one dendrite.
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Multipolar Neurons: Most common type; includes one axon and many dendrites, subclassified into:
- Golgi type I cells (long axons, motor neurons).
- Golgi type II cells (short axons, interneurons).
Electrophysiology of Nerve Tissue
- Nerve cells exhibit an electric membrane potential (~ -70 mV at rest).
- Action potentials are rapid positive changes in membrane potential, propagating at speeds up to 120 m/sec.
- Depolarization occurs with Na+ influx, hyperpolarization makes depolarization more difficult.
Types of Synapses
- Electrical Synapses: Represented by gap junctions, allowing direct ion transfer.
- Chemical Synapses: Predominant in mammals, convey signals via neurotransmitter release.
Chemical Synapse Structure
- Presynaptic Knob: Contains synaptic vesicles housing neurotransmitters.
- Synaptic Cleft: Narrow gap (~20 nm) between pre- and postsynaptic cells.
- Postsynaptic Membrane: Contains receptors for neurotransmitters, which trigger depolarization or hyperpolarization.
Morphotypes of Synapses
- Axodendritic: Connection between axon and dendrite.
- Axosomatic: Connection between axon and neuron body.
- Axoaxonic: Connection between two axons.
- Motor End-Plate: Specialized synapse between motor neurons and muscle cells, featuring junctional folds.
Clinical Comments
- Various toxins, such as curare and botulinum toxin, inhibit synaptic function and muscle depolarization, showcasing the importance of chemical synapses in bodily function.
Support Cells
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PNS Support Cells:
- Schwann Cells: Form myelin sheath; create Nodes of Ranvier.
- Satellite Cells: Support ganglia.
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CNS Support Cells:
- Astrocytes: Maintain blood-brain barrier, respond to CNS injuries (astrocytoma, glial scar).
- Oligodendrocytes: Form myelin sheath; involved in pathologies like oligodendrocytoma and multiple sclerosis.
- Microglial Cells: Act as immune cells in CNS.
- Ependymal Cells: Line ventricular cavities in the brain.
Peripheral Nervous System Structure
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Nerves:
- Endoneurium: Surrounds individual nerve fibers.
- Perineurium: Encloses bundles of fibers.
- Epineurium: Outer layer of nerve.
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Ganglia:
- Sensory craniospinal ganglia and autonomic ganglia.
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Special Ending Nerve Types:
- Meissner’s corpuscle (touch) and Pacinian corpuscle (pressure).
- Proprioceptors like muscle spindle.
Central Nervous System Structure
- Distinction between gray matter (neuronal cell bodies) and white matter (myelinated axons).### Curare Toxin
- Originally used by South American Indians for hunting.
- Binds to acetylcholine receptors as a muscle relaxant.
Botulinum Toxin
- Produced by the bacterium Clostridium botulinum, functioning as a neurotoxin.
- Prevents the release of acetylcholine from synaptic vesicles.
- Botox is a cosmetic application of botulinum toxin, used to relax facial muscles.
Autoimmune Diseases
- Diseases such as myasthenia gravis impact neuromuscular junctions, leading to extreme muscle weakness.
- Autoantibodies target acetylcholine receptor proteins, weakening muscle responses to nerve stimuli.
Rabies Virus Transmission
- Carried by wild mammals like skunks and raccoons; transmitted via bites.
- Virus replicates in muscle tissue for 1-2 weeks, during which vaccination can be effective.
- After replication, the virus reaches the motor neurons, spreading through retrograde axonal transport.
- Most of the CNS becomes affected, causing severe inflammation and seizures, also known as hydrophobia.
- Ultimately spreads to salivary glands for transmission between animals; no cure is available after symptoms appear.
Support Cells of the Nervous System
Peripheral Nervous System (PNS)
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Composed of Schwann cells and satellite cells.
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Schwann Cells: Form myelin sheath around axons, providing insulation for rapid impulse conduction.
- Nodes of Ranvier formed by gaps in myelin sheath facilitate saltatory conduction.
- Envelop unmyelinated axons; action potentials in these fibers propagate in a wave-like manner.
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Satellite Cells: Surround neuron cell bodies in ganglia, offering electrical insulation and supporting metabolic exchange.
Central Nervous System (CNS)
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Contains astrocytes, oligodendrocytes, microglial cells, and ependymal cells.
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Astrocytes: Largest neuroglial cells, offering structural support, forming the blood-brain barrier, and involved in metabolic substance transport.
- Two types: Protoplasmic (in gray matter) and Fibrous (in white matter).
- Astrocytomas are common tumors arising from astrocytes.
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Oligodendrocytes: Smaller neuroglial cells that myelinate axons in the CNS.
- Resemble Schwann cells but form multiple segments of the myelin sheath.
- Damage to myelin sheath is a hallmark of multiple sclerosis.
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Microglial Cells: Small phagocytic cells that increase in number with injury, believed to aid in debris removal.
- Associated with Alzheimer's and Parkinson's diseases due to their role in plaque formation and demyelination.
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Ependymal Cells: Line ventricles and spinal cord cavities, responsible for cerebrospinal fluid (CSF) production and absorption.
- Have microvilli and cilia to aid in CSF absorption.
Peripheral Nervous System Structure
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Comprises cranial, spinal, and peripheral nerves, ganglia, and special nerve endings.
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Nerves consist of myelinated and non-myelinated fibers, maintained by connective tissue layers:
- Endoneurium surrounds individual fibers.
- Perineurium surrounds nerve fascicles.
- Epineurium encloses entire nerves.
Ganglia Types
- Clusters of neuron cell bodies outside the CNS, encapsulated by connective tissue.
- Sensory Craniospinal Ganglia: Contain pseudounipolar neurons with processes extending to receptors and CNS.
- Motor Ganglia: Part of the autonomic nervous system, consisting of multipolar neurons.
Sensory and Motor Endings
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Motor End-plates: Specialized endings in skeletal muscles.
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Sensory Nerve Endings:
- Special Senses: Responsible for smell, sight, hearing, and equilibrium.
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Somesthetic Receptors: Distributed throughout the body; include free nerve endings for pain and encapsulated endings like Meissner’s and Pacinian corpuscles.
- Meissner’s Corpuscle: Provides touch sensation, located in skin of fingers and toes.
- Pacinian Corpuscle: Largest encapsulated sensory ending, detects vibrations and deep pressure.
Proprioceptors
- Collect information about joint angulations and muscle tension.
- Muscle Spindle: Specialized stretch receptor in skeletal muscle, containing intrafusal fibers surrounded by sensory and motor nerve fibers.
Central Nervous System Organization
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Composed of the spinal cord and brain, with clusters of neurons known as nuclei and organized nerve fibers into tracts.
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Gray Matter: Contains neuron bodies and unmyelinated fibers; vascularized by capillaries.
- In the spinal cord, gray matter is internal; organized into anterior (ventral) and posterior (dorsal) horns.
- In the brain, gray matter is external and features folds (gyri).
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White Matter: Composed of myelinated axons and fewer glial cells; denser with limited blood supply, and organized oppositely in the brain compared to the spinal cord.
Neurotransmitter Deactivation
- Released neurotransmitters are deactivated mainly through recapture and degradation.
- High-affinity reuptake allows up to 80% of catecholamines (e.g., dopamine, norepinephrine) to be recaptured into vesicles via endocytosis.
- Enzymes on the synaptic membrane break down remaining neurotransmitters; for instance, acetylcholine is converted into acetate and choline.
- Inhibiting norepinephrine breakdown or high-affinity reuptake is beneficial for depression treatment.
Morphotypes of Synapses
- Axodendritic synapses connect axons to dendrites.
- Axosomatic synapses link axons to neuron bodies (perikaryon).
- Axoaxonic synapses connect one axon to another axon.
- Motor end-plates are specialized synapses at neuromuscular junctions, containing presynaptic vesicles with acetylcholine.
- Type II inhibitory synapses hyperpolarize postsynaptic cells, reducing action potential likelihood.
Chemical Synapses
- Presynaptic knobs contain synaptic vesicles (40-60 nm), rich in neurotransmitters.
- Over 100 known neurotransmitters can induce depolarization or hyperpolarization, acting as either excitatory or inhibitory.
- The synaptic cleft is a narrow (~20 nm) gap between presynaptic and postsynaptic membranes.
- Action potential triggers calcium channel opening, facilitating neurotransmitter release into the synapse.
- Endocytosis removes excess plasma membrane after neurotransmitter release.
Dendrites
- Dendrites are numerous, short, tapered processes designed to convey signals toward the perikaryon.
- Dendritic trees can have up to 200,000 synapses, maximizing synaptic contact area.
- Dendritic spines, shaped like mushrooms, host most postsynaptic receptors.
- Dendrites lack myelin and contain ribosomes and rough endoplasmic reticulum but no Golgi apparatus.
Major Types of Neurons
- Neurons can be classified as pseudounipolar, bipolar, or multipolar based on shape and process number.
- Pseudounipolar neurons are primarily sensory, with one large process branching into peripheral and central processes that transmit sensory information to the central nervous system.
Axonal Transport
- Axonal transport occurs in two directions: anterograde (moving materials away from the cell body) and retrograde (moving materials toward the cell body).
- Anterograde transport includes both fast and slow transport mechanisms.
Neuron Structure
- Neurons feature distinct structures: one axon per cell, dendritic processes, and axon terminals.
- Axons vary in length (up to 1 meter), maintain a constant diameter, and lack Nissl bodies at the axon hillock.
- Myelinated axons enhance signal transmission speed, while myelin sheath formation abnormalities can lead to disorders.
Cytoskeleton of Neurons
- Neuronal cytoskeleton is well-developed, consisting of neurofilaments, microfilaments, and microtubules.
- Neurons generally lack centrioles, preventing cellular division, with limited neuroblast replacement in adults, notably in olfactory neurons.
Support Cells in Nervous System
- Peripheral nervous system support cells include Schwann cells (involved in myelination) and satellite cells.
- Central nervous system support cells encompass astrocytes (with multiple types), oligodendrocytes, microglial cells, and ependymal cells, each with distinct functions and clinical relevance.
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This quiz focuses on the characteristics and structure of nerve tissue, including neurons and their processes. You will be tested on your ability to identify nerve tissue in various specimens and understand its functional properties. Prepare to describe the intricacies of neurons, including dendrites and axons.