Introduction to Nervous Tissue
Chapter 11 Lecture 1: Introduction to Nervous Tissue
Overview of Nervous System Function
Central Nervous System (CNS)
Comprises the brain and spinal cord.
Responsible for information processing, integrating, processing, and coordinating sensory and motor commands.
Peripheral Nervous System (PNS)
Encompasses all nervous tissue outside the CNS, excluding the Enteric Nervous System (ENS).
Enteric Nervous System (ENS)
Consists of neural tissues within the walls of the gastrointestinal tract.
Aids in controlling digestive functions.
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Divisions of Peripheral Nervous System
Sensory (Afferent) Division:
Informs the CNS from receptors located in peripheral tissues and organs.
Sensory Receptors:
Detect changes such as position, touch, pressure, pain, and temperature.
Special Sensory Organs:
Include those for smell, taste, sight, balance, and hearing.
Motor (Efferent) Division:
Carries motor commands from the CNS.
Subdivisions:
Somatic Nervous System (SNS):
Also known as the voluntary nervous system, it allows for conscious control over skeletal muscles.
Autonomic Nervous System (ANS):
Known as the involuntary nervous system, it automatically regulates activities such as smooth muscle, cardiac muscle, glands, and adipose tissue.
Further branches into:
Parasympathetic Division
Sympathetic Division
General Functions of the Nervous System
Receptors:
Detect variations in internal or external environments.
Information Flow:
Information is relayed to the CNS by the sensory division of the PNS.
Information Processing:
Takes place within the CNS to integrate and distribute the received information.
Motor Commands:
Initiated by the motor division of the PNS, which then directs effectors to modify their activities.
Neurons: Anatomy and Function
Neurons
Comprised of three main regions:
Dendrites:
Responsible for receiving stimuli from both the environment and other neurons.
Characterized by being highly branched with dendritic spines.
CNS neurons primarily gather information at this location.
Cell Body:
Also known as the perikaryon, it contains the nucleus and organelles.
Composed of cytoplasm and features such as:
Neurofilaments:
Similar to intermediate filaments, providing structural support.
Neurofibrils:
Key part of the cytoskeleton.
Nissl bodies:
Clusters of rough endoplasmic reticulum (RER) and free ribosomes crucial for neurotransmitter synthesis.
Axon:
Transmits information towards other cells and has specific components:
Axon Hillock:
Where the axon originates from the cell body.
Initial Segment:
Site where action potential is initiated.
Axolemma:
Plasma membrane of the axon.
Axoplasm:
Cytoplasm of the axon, containing neurofibrils, neurotubules, vesicles, lysosomes, mitochondria, and enzymes.
Telodendria:
Fine extensions at the axon terminals where communication occurs at synapses.
Synapse and Communication
Synapse:
The juncture where a neuron (presynaptic cell) communicates with another cell (postsynaptic cell).
Typical Mechanism:
Neurotransmitters are released from the presynaptic membrane into a synaptic cleft and bind to receptors on the postsynaptic membrane.
Neurotransmitter Packaging:
Encased in synaptic vesicles found in axon terminals.
Collateral Branches:
Enable a single neuron to communicate with multiple cells simultaneously.
Types of Synapses
Neurotransmission Examples:
Neuron to neuron synapses.
Neuromuscular Junctions:
Connection between neurons and skeletal muscle fibers.
Neuroglandular Synapses:
Connections between neurons and gland cells.
Neuron Replacement & Classification
Neuron Replacement:
Most neurons in the CNS lack centrioles, limiting their ability to divide and regenerate after injury.
Exceptions to Regeneration:
Neural stem cells exist but are predominantly inactive.
Active examples found in:
Olfactory epithelium (smell)
Retina (vision)
Hippocampus (memory).
Neuronal Classification (by Structure):
Anatomical Classes of Neurons:
Anaxonic Neurons:
Small neurons with indistinguishable axons and dendrites, primarily in the brain and special sense organs; functions remain largely unknown.
Bipolar Neurons:
Featuring two distinct processes: a branching dendritic process and an axon.
Rare and located in special sense organs.
Unipolar Neurons:
Continuous dendrites and axons; cell body is offset.
Represents most sensory neurons in the PNS; axons can be extremely long, such as those transmit sensations from toes to spinal cord.
Multipolar Neurons:
Characterized by multiple dendrites and one axon; found predominantly in the CNS.
All muscle motor neurons are classified as multipolar and may also reach considerable lengths, conveying motor signals from the spinal cord to foot muscles.
Functional Classes of Neurons
Within the CNS and PNS:
Three primary functional classes:
Sensory Neurons:
Interneurons:
Primarily located in the CNS, they connect sensory and motor neurons and facilitate higher functions like memory and learning.
Motor Neurons:
Carry motor commands towards effectors.
Sensory Receptors and their Classification
Function:
Sensory receptors detect stimuli and are either processes of sensory neurons or cells monitored by sensory neurons.
Types of Sensory Receptors:
Interoceptors:
Monitor internal organs and systems; detect distension, deep pressure, and pain.
Proprioceptors:
Track the position and movement of skeletal muscles and joints.
Exteroceptors:
Monitor the external environment (e.g., for touch, temperature, pressure, and sensory inputs for special senses).
Efferent and Afferent Neural Pathways
Efferent Fibers:
Convey information from the CNS to effectors (soma: skeletal muscles; visceral: cardiac/smooth muscle and glands).
Afferent Fibers:
Transmit sensory information to the CNS.
Ganglion:
Collection of neuron cell bodies in the PNS.
Sensory Neurons:
Mostly unipolar with cell bodies in sensory ganglia, monitoring external environments and body position among other systems.
Somatic Motor Neurons:
Responsible for conscious control over skeletal muscles, possessing cell bodies in the CNS with axons extending within peripheral nerves.
Visceral Motor Neurons:
Part of the autonomic nervous system innervating non-skeletal effectors like smooth muscle, cardiac muscle, and glands; situated both in CNS and PNS with their cell bodies located in autonomic ganglia.
Neuroglia of CNS
Neuroglia (Glial Cells):
Support and protect neurons, constituting approximately half of the total volume of the nervous system.
Types of CNS Neuroglia:
Ependymal Cells:
Form a simple epithelium (ependyma) lining the central canal and brain ventricles; assists in producing and monitoring cerebrospinal fluid (CSF).
Microglia:
Phagocytic cells that eliminate debris, waste products, and pathogens, stemming from precursor monocytes/macrophages.
Astrocytes:
Generate structural support, maintain the blood-brain barrier, regulate ion, nutrient, and gas concentrations, absorb neurotransmitters, and form scar tissue post-injury.
Oligodendrocytes:
Stabilize axons and produce myelin, creating myelin sheaths that enhance nerve impulse transmission speed.
Myelination and Nervous Tissue Appearance
Myelinated Axons:
White appearance due to myelin lipid content; found primarily in CNS white matter, characterized by internodes and nodes of Ranvier.
Unmyelinated Axons:
Appearing gray due to cell bodies, dendrites, and unmyelinated axons, characteristic of the CNS gray matter.
Peripheral Nervous System (PNS) Neuroglia
Myelination in PNS:
Formed by Schwann cells, which creates the neurilemma on the outer surface; typically a single Schwann cell wraps a single internode.
Unmyelinated Axons in PNS:
A nonmyelinating Schwann cell can envelop segments of multiple axons, stabilizing their positions and isolating them from chemicals in the interstitial fluid; no nodes exist between adjacent nonmyelinating Schwann cells.
Axon Injury and Repair
Repair in the PNS:
Follows a four-step process upon damage.
Repair in the CNS:
Regeneration is constrained; numerous axons might be affected, and astrocytes produce scar tissue that obstructs regrowth, as well as secreting chemicals that inhibit axon regrowth.