Nervous Tissue
Nervous Tissue Study Notes
Organization of the Nervous System
Nervous System Categories:
Structural: Divides the nervous system anatomically into the Central Nervous System (CNS) and Peripheral Nervous System (PNS). The CNS acts as the control center, while the PNS serves as the communication lines.
Functional: Categorizes based on the roles and activities performed. These include:
Sensory Input: Detecting internal and external changes.
Integration: Processing and interpreting sensory input.
Motor Output: Initiating responses to integrated information.
Structural Organization: CNS and PNS
Central Nervous System (CNS):
Comprises the brain and spinal cord. These are the primary centers for information processing, memory, and thought. They are highly protected by bones (skull and vertebral column) and three layers of connective tissue membranes called meninges.
Peripheral Nervous System (PNS):
Consists of all nervous tissue outside the CNS. This includes cranial nerves (12 pairs originating from the brain), spinal nerves (31 pairs originating from the spinal cord), and ganglia (clusters of neuronal cell bodies located outside the CNS, forming relay stations).
Functional Organization: Sensory and Motor Nervous Systems
Functions of the Nervous System:
Collecting Information:
Receptors are specialized structures throughout the body that detect specific changes (stimuli) in both the internal (e.g., blood pressure, pH levels, stretch in organs) and external (e.g., touch, temperature, light, sound) environments. This sensory information is then relayed by nerves to the CNS.
Processing and Evaluating Information:
The CNS receives the sensory input, interprets its meaning, and determines what response, if any, is required. This involves complex processes such as memory, learning, decision-making, and emotional responses.
Responding to Information:
Once a response is determined, the CNS initiates nerve impulses that are transmitted to effectors (muscles or glands). These effectors then carry out the appropriate actions, such as muscle contraction (e.g., pulling your hand away from a hot stove) or hormone secretion (e.g., adrenal glands releasing adrenaline).
Functional Divisions of the Nervous System
Sensory Nervous System:
Also known as the afferent nervous system, meaning it transmits information to the CNS.
Function: Responsible for receiving sensory information from receptors throughout the PNS and transmitting these nerve impulses to the CNS for interpretation.
Components:
Somatic Sensory (General Somatic Senses): Detects consciously perceived general senses from the skin, skeletal muscles, and joints. These include touch, pain, pressure, vibration, and proprioception (the sense of body position and movement).
Visceral Sensory: Conveys impulses from internal organs (viscera) such as the stomach, heart, urinary bladder, and intestines. These sensations are often not consciously perceived (e.g., blood pressure) or are interpreted as general discomfort or organ stretch.
Motor Nervous System:
Also known as the efferent nervous system, meaning it transmits information away from the CNS.
Function: Responsible for transmitting nerve impulses from the CNS to muscles and glands in the PNS, causing them to respond (e.g., muscle contraction or gland secretion).
Components:
Somatic Motor (Voluntary Nervous System): Transmits impulses from the CNS to skeletal muscles, leading to voluntary muscle contractions. These are movements you consciously control, like walking or typing.
Autonomic Motor (Involuntary Nervous System): Regulates cardiac muscle, smooth muscle (found in internal organs and blood vessels), and glands. Its actions are involuntary and largely outside conscious control, managing vital functions like heart rate, digestion, respiration, and body temperature. This system is further divided into the sympathetic (often for 'fight-or-flight' responses) and parasympathetic (for 'rest-and-digest' functions) divisions.
Neurons
Definition: The fundamental structural and functional units of the nervous system, specialized for rapidly conducting nerve impulses (action potentials) over long distances.
Characteristics:
High metabolic rate: Neurons require a constant and abundant supply of oxygen and glucose to fuel their intense electrical and chemical activity.
Extreme longevity: Most neurons formed during fetal development are designed to function throughout an individual's entire lifetime, making them some of the longest-lived cells in the body.
Nonmitotic (amitotic): Most mature neurons have lost the ability to divide and cannot be easily replaced if destroyed. Exceptions include specialized neurons in certain regions like the olfactory epithelium (sense of smell) and parts of the hippocampus (memory formation).
Structure of Neurons
Neurons are composed of three main structural regions, which facilitate their role in transmitting signals:
Cell Body (Soma): The neuron's control center, containing the nucleus, cytoplasm, and all typical organelles. It is particularly rich in Nissl bodies (dense clusters of rough endoplasmic reticulum), which are highly active in protein synthesis to produce neurotransmitters and other vital proteins.
Dendrites: Typically short, highly branched processes extending from the cell body. They are the primary receptive regions, designed to receive nerve impulses from other neurons and conduct these impulses towards the cell body. Their extensive branching allows a single neuron to receive signals from many other neurons.
Axon: A single, typically long, slender projection that extends from a specialized conical region of the cell body called the axon hillock. The axon's primary function is to transmit nerve impulses away from the cell body towards other neurons, muscles, or glands.
Key Structures Related to Axons:
Axon Collaterals: Side branches that extend from the main axon, allowing a single neuron to transmit signals to multiple target cells simultaneously.
Telodendria (Axon Terminals): Fine terminal branches found at the distal end of the axon and its collaterals.
Synaptic Knobs (Terminal Boutons): Expanded, knob-like regions at the very tips of the telodendria. These contain numerous synaptic vesicles filled with neurotransmitters, which are chemical messengers released into the synaptic cleft to communicate with target cells.
Neuron Classification by Structure:
Unipolar (Pseudounipolar): Have a single, short process that emerges from the cell body and immediately divides into two processes, forming a T-shape. One process extends to the periphery (functioning as a dendrite by receiving sensory input), and the other projects into the CNS (acting as an axon). Most sensory neurons are unipolar.
Bipolar: Possess two processes—one dendrite and one axon—extending from opposite sides of the cell body. They are relatively rare and found in specialized sensory organs like the retina of the eye and the olfactory epithelium for the sense of smell.
Multipolar: Characterized by multiple dendrites and a single axon. This is the most common type of neuron in the CNS, including all motor neurons and most interneurons, allowing for complex integration of information.
Neuron Function
Classification by Function:
Sensory Neurons (Afferent): Specialized to detect stimuli (e.g., touch, light, pain) and transmit nerve impulses from sensory receptors in the PNS to the CNS for processing and interpretation.
Motor Neurons (Efferent): Transmit nerve impulses from the CNS to effectors (muscles or glands) in the PNS, initiating a response such as muscle contraction or glandular secretion.
Interneurons (Association Neurons): Located entirely within the CNS. They receive impulses from sensory neurons, integrate and process this information (acting as the 'middlemen' of the nervous system), and then communicate with motor neurons or other interneurons. They form the vast majority of neurons in the body (approximately ).
Glial Cells (Neuroglia)
Definition: Non-excitable cells in the nervous system that outnumber neurons (by about to ratio) and primarily protect, nourish, and support neurons. Unlike most mature neurons, glial cells are capable of mitosis (cell division).
Types of Glial Cells in CNS
Astrocytes:
The most abundant (comprising about of CNS glial cells) and versatile glial cells, often star-shaped. They have numerous crucial functions:
Blood-brain barrier (BBB) formation: Their perivascular feet wrap around capillaries in the CNS, helping to regulate the movement of substances from the blood into the brain tissue, thus protecting neurons from harmful substances.
Regulation of fluid composition: Control the chemical environment around neurons, including ion concentrations (especially ) and neurotransmitter levels, by absorbing excess ions and neurotransmitters.
Structural support: Provide a framework for CNS neurons, guiding their migration during development and occupying space vacated by degenerating neurons.
Replacement of damaged neurons: Can proliferate to form scar tissue (gliosis) in damaged brain areas, helping to reduce the spread of inflammation but also hindering axonal regeneration.
Assistance in neuronal development: Guide the migration and connection of developing neurons during embryogenesis.
Ependymal Cells:
Squamous-to-columnar shaped cells that line the internal cavities (ventricles) of the brain and the central canal of the spinal cord. Many are ciliated, and their beating cilia help circulate cerebrospinal fluid (CSF), primarily produced by the choroid plexus (a specialized vascular structure covered by ependymal cells).
Microglial Cells:
Small, motile cells with immune-like functions. They act as the resident macrophages of the CNS, engulfing infectious agents, cellular debris, and dead neurons through phagocytosis. They are activated during injury or infection and migrate to the site of damage.
Oligodendrocytes:
Cells with cytoplasmic extensions that provide insulation to CNS axons by forming myelin sheaths. Unlike Schwann cells in the PNS, a single oligodendrocyte can myelinate segments of multiple axons simultaneously, increasing the speed of nerve impulse conduction.
Types of Glial Cells in PNS
Satellite Cells:
Flattened cells that closely surround the cell bodies of neurons in PNS ganglia (e.g., dorsal root ganglia). They regulate the exchange of nutrients and wastes between the neurons and their environment, providing protection and support similar to CNS astrocytes.
Neurolemmocytes (Schwann Cells):
Wrap around PNS axons, producing myelin for insulation. Unlike oligodendrocytes, each neurolemmocyte typically myelinates only a single segment of one axon. They are crucial for the regeneration of damaged PNS axons, forming a regeneration tube.
Myelination of Axons
Purpose: Myelination significantly increases the speed of nerve impulse conduction. The myelin sheath, composed of multiple layers of lipid and protein (derived from the plasma membrane of glial cells), acts as an electrical insulator, preventing ion leakage across the axon membrane.
Mechanism: Impulses 'jump' from one exposed region of the axon (called Nodes of Ranvier, which are unmyelinated gaps) to the next. This rapid propagation is called saltatory conduction (from Latin saltare, to leap), which is far faster and more energy-efficient than continuous conduction in unmyelinated axons because it requires less ATP to pump ions.
Myelination occurs when neurolemmocytes in the PNS and oligodendrocytes in the CNS wrap tightly around axons, forming concentric layers of their plasma membrane until the cytoplasm is largely squeezed out, leaving behind a compact, fatty sheath.
Axon Regeneration
Ability: Damaged axons in the PNS have a limited but significant capacity for regeneration, provided the neuronal cell body remains intact and a critical amount of the neurolemma (the outer part of the Schwann cell) survives. Regeneration in the CNS is generally very limited or nonexistent due to inhibitory factors released by oligodendrocytes and astrocytes, and the lack of a robust regeneration tube.
Process: After an injury to a PNS axon, the distal (farther from the cell body) portion of the axon and its myelin sheath undergo Wallerian degeneration (fragmentation and disintegration). However, the surviving neurolemmocytes (Schwann cells) and the endoneurium form a regeneration tube. Neurolemmocytes within this tube secrete growth factors, guiding the sprouting axon to regrow towards its original target. This process can be slow (about mm per day) and success is not guaranteed.
Protective Coverings of Nerves
Nerves are macroscopic bundles of parallel axons (nerve fibers) that are surrounded and protected by three sequential layers of connective tissue, providing structural integrity, support, and protection against physical trauma:
Endoneurium: A delicate layer of areolar connective tissue that surrounds and electrically insulates each individual axon. It also contains capillaries that supply nutrients to the axon and Schwann cells.
Perineurium: A thicker layer of dense irregular connective tissue that bundles groups of axons into structures called fascicles. This layer protects the fascicles, helps maintain the internal microenvironment of the nerve, and forms a blood-nerve barrier.
Epineurium: The outermost, tough, and fibrous layer of dense irregular connective tissue that encloses the entire nerve. It provides strong protection against stretching and injury and supports the major blood vessels and fat that supply the nerve.
Synapses
Definition: Specialized junctions where one neuron communicates with another neuron or with an effector cell (such as a muscle cell or gland cell). This is the crucial point for information transmission throughout the nervous system.
Components of a Synapse:
Presynaptic Neuron: The neuron that transmits the signal towards the synapse. Its axon terminal (synaptic knob) contains numerous synaptic vesicles filled with neurotransmitters.
Postsynaptic Neuron: The neuron or effector cell that receives the signal away from the synapse. Its membrane, called the postsynaptic membrane, contains specific receptors for neurotransmitters.
Synaptic Cleft: The narrow, fluid-filled space (typically to nm wide) that separates the presynaptic and postsynaptic membranes.
Types of Synapses:
Electrical Synapses: Involve direct physical contact between neurons via gap junctions. These junctions allow ions to flow directly from one cell to another, enabling very rapid and synchronized transmission of electrical signals. They are less common in the mature human nervous system but are found in specific areas like the cardiac muscle and some brain regions for rapid communication.
Chemical Synapses: The most common type of synapse. They utilize neurotransmitters (chemical messengers) to transmit signals across the synaptic cleft. When a nerve impulse arrives at the presynaptic terminal, neurotransmitters are released, diffuse across the cleft, and bind to receptors on the postsynaptic membrane, causing an electrical change (excitation or inhibition) in the postsynaptic cell. This process involves a synaptic delay (about to ms) due to the time required for neurotransmitter release and binding.
Neural Integration and Neuronal Pools
Neuronal Pools: Functional groups of interconnected neurons within the CNS that process specific types of information. These pools, sometimes called neural circuits, exhibit distinct patterns of connection that determine their functional properties:
Converging Circuits: Involve multiple presynaptic neurons synapsing with a single postsynaptic neuron or a smaller number of postsynaptic neurons. This allows for spatial summation and the integration of diverse signals from many sources into a single, focused output (e.g., sensory input from various body parts converging on a single motor neuron for a balance response).
Diverging Circuits: A single presynaptic neuron branches to synapse with multiple postsynaptic neurons, which then may synapse with even more neurons. This allows for the widespread dissemination of a signal to several parts of the nervous system (e.g., a single thought or motor command from the brain spreading to activate numerous skeletal muscles for coordinated movement).
Reverberating Circuits: Neurons in the circuit send collateral branches back to excite previous neurons in the pathway, creating a positive feedback loop. This allows for continuous, repetitive activity for a period of time, leading to rhythmic activities (e.g., controlling breathing, walking, short-term memory, or maintaining consciousness). The activity stops when a neuron becomes fatigued or an inhibitory signal interrupts the cycle.
Parallel-after-discharge Circuits: The input neuron diverges to stimulate several chains of neurons, each of which has a different number of synapses (and thus different synaptic delays). These chains then converge on a single output neuron. This arrangement allows for prolonged, complex processing and ensures that the output neuron fires for a longer duration or at different times, which is useful for tasks requiring precise, complex patterns of output like mathematical calculations or intense mental activity.
Development of the Nervous System
Timeline: The development of the nervous system is one of the first systems to begin formation, starting in the third week of embryonic development.
Origins: The nervous system is derived from the ectoderm, the outermost germ layer. A specialized plate of ectoderm, the neural plate, forms along the dorsal midline of the embryo. This neural plate gives rise to the neuroectoderm, the specific tissue of origin for nervous system structures.
Neural Tube Formation: The neural plate folds inward, forming a neural groove. The edges of this groove, called neural folds, elevate and eventually fuse around the fourth week of development to form the neural tube. This tube then gives rise to the entire central nervous system (CNS), specifically the brain (from the cranial end) and the spinal cord (from the caudal end). Cells that break off the neural folds during fusion form the neural crest, which gives rise to many components of the PNS including sensory neurons, postganglionic autonomic neurons, Schwann cells, and also non-neural structures like melanocytes and some facial bones.