Nervous Tissue: Structure, Function, and Physiology

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Nervous Tissue

An Introduction to the Nervous System

The nervous system is a complex network responsible for communication, coordination, and control throughout the body. It includes the brain, spinal cord, sensory receptors, and nerves that connect to other organ systems.

Neurons: Specialized cells for intercellular communication.
Neuroglia (glial cells): Support, protect, and maintain the environment for neurons.

Overview of the nervous system divisions

Divisions of the Nervous System

Anatomical Divisions

Central Nervous System (CNS): Brain and spinal cord; processes and coordinates sensory data, motor commands, and higher functions (intelligence, memory, learning, emotion).
Peripheral Nervous System (PNS): All nervous tissue outside the CNS; delivers sensory information to the CNS and carries motor commands to peripheral tissues.

Functional Divisions of the PNS

Afferent Division: Carries sensory information from receptors to the CNS.
Efferent Division: Carries motor commands from the CNS to effectors (muscles, glands, adipose tissue).
Somatic Nervous System (SNS): Controls voluntary and involuntary (reflex) skeletal muscle contractions.
Autonomic Nervous System (ANS): Controls subconscious actions (smooth/cardiac muscle, glands); includes sympathetic (stimulating) and parasympathetic (relaxing) divisions.
Enteric Nervous System (ENS): Network in the digestive tract, capable of local reflexes independent of the CNS but influenced by the ANS.

Neurons

Structure and Function

Neurons are the basic functional units of the nervous system, specialized for communication, information processing, and control.

Cell Body (Soma): Contains the nucleus, nucleolus, mitochondria, rough endoplasmic reticulum (RER), and ribosomes.
Dendrites: Short, branched processes that receive signals from other neurons.
Axon: Long process that propagates action potentials to other cells.
Axon Hillock: Region where the axon attaches to the cell body and where action potentials are initiated.
Telodendria: Fine extensions at the end of the axon, ending in axon terminals (synaptic terminals).

Anatomy of a typical neuron Detailed neuron structure and action potential direction

Axonal Transport

Materials move between the cell body and axon terminals via neurotubules, powered by proteins such as kinesin and dynein.

Structural Classification of Neurons

Anaxonic: Small, indistinguishable axons and dendrites; found in brain and sense organs.
Bipolar: One dendrite, one axon; rare, found in special sense organs.
Unipolar: Fused axon and dendrite; most sensory neurons in PNS.
Multipolar: One axon, multiple dendrites; most common in CNS and all motor neurons to skeletal muscle.

Functional Classification of Neurons

Sensory Neurons (Afferent): Transmit sensory information to the CNS.
Motor Neurons (Efferent): Carry instructions from the CNS to effectors.
Interneurons: Connect sensory and motor neurons; involved in integration, memory, and learning.

Neuroglia

Types of Neuroglia in the CNS

Astrocytes: Maintain the blood-brain barrier, provide structural support, repair tissue, guide development, and regulate the interstitial environment.
Ependymal Cells: Line ventricles and central canal; produce and circulate cerebrospinal fluid (CSF).
Oligodendrocytes: Myelinate CNS axons, increasing action potential speed; provide structural framework.
Microglia: Phagocytic cells that remove debris, waste, and pathogens.

Types of neuroglia in the CNS Oligodendrocytes and astrocytes in CNS tissue Ependymal cells and microglia in CNS tissue

Types of Neuroglia in the PNS

Satellite Cells: Surround neuron cell bodies in ganglia; regulate the environment around neurons.
Schwann Cells: Myelinate peripheral axons; assist in repair after injury.

Types of neuroglia in the PNS Myelinated axon with Schwann cell Schwann cells enclosing unmyelinated axons TEM of unmyelinated axons with Schwann cell Stages of myelin sheath formation by Schwann cell

Neural Responses to Injury

Wallerian Degeneration: Axon distal to injury degenerates; Schwann cells form a path for regrowth.
Regeneration in CNS: Limited by astrocyte activity (scar tissue, inhibitory chemicals).

Fragmentation of axon and myelin after injury Schwann cells form cord for axon regrowth Axon buds grow along Schwann cell cord Axon regrowth and enclosure by Schwann cells

Membrane Potential

Resting Membrane Potential

All cells have a membrane potential due to ion movement. In neurons, this is crucial for signal transmission.

Resting Membrane Potential: About –70 mV in neurons; inside is negative relative to outside.
Key Ions: Na+ (outside), K+ (inside), Cl–, and negatively charged proteins (inside).
Sodium-Potassium Pump: Maintains gradients by pumping 3 Na+ out and 2 K+ in, using ATP.

Ion gradients and sodium-potassium pump Electrical gradients and resting membrane potential

Ion Channels

Passive (Leak) Channels: Always open; permeability varies.
Active (Gated) Channels: Open/close in response to stimuli.
Chemically Gated: Open when specific chemicals bind (e.g., ACh).
Voltage-Gated: Open/close with changes in membrane potential.
Mechanically Gated: Open in response to physical distortion.

Chemically gated ion channel Voltage-gated ion channel Mechanically gated ion channel

Graded Potentials

Local changes in membrane potential; decrease with distance from stimulus.
Can be depolarizing (Na+ influx) or hyperpolarizing (K+ efflux).
Stronger stimuli produce larger changes.

Resting state of neuron membrane Stimulus applied, sodium channels open Spread of graded potential Depolarization, repolarization, and hyperpolarization

Action Potentials

Generation and Propagation

Action potentials are rapid, propagated changes in membrane potential that travel along axons.

All-or-None Principle: If threshold is reached, an action potential occurs; otherwise, it does not.
Phases:
1. Depolarization to threshold
2. Activation of voltage-gated Na+ channels (Na+ influx)
3. Inactivation of Na+ channels, activation of K+ channels (K+ efflux, repolarization)
4. Return to resting potential (brief hyperpolarization)
Refractory Periods: Absolute (no new AP possible), Relative (strong stimulus needed).

Action potential generation, part 1 Depolarization to threshold Activation of sodium channels and rapid depolarization Inactivation of sodium channels, activation of potassium channels Temporary hyperpolarization Return to resting membrane potential $Absolute and relative refractory periods$

Propagation Types

Continuous Propagation: In unmyelinated axons; AP moves segment by segment.
Saltatory Propagation: In myelinated axons; AP jumps from node to node, increasing speed and efficiency.

Continuous propagation, step 1 Continuous propagation, step 2 Continuous propagation, step 3 Continuous propagation, step 4 Continuous propagation, cycle repeats Saltatory propagation, step 1 Saltatory propagation, step 2 Saltatory propagation, step 3 Saltatory propagation, step 4

Axon Types and Conduction Speed

Type A fibers: Large, myelinated, fast (120 m/s); carry critical sensory and motor information.
Type B fibers: Medium, myelinated, intermediate speed (18 m/s).
Type C fibers: Small, unmyelinated, slow (1 m/s); carry less urgent information.

Synapses

Types of Synapses

Electrical Synapses: Direct connection via gap junctions; rapid transmission.
Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; most common type.

Structure of a chemical synapse

Events at a Cholinergic Synapse

Action potential arrives at axon terminal, depolarizing the membrane.
Voltage-gated Ca2+ channels open; Ca2+ triggers exocytosis of ACh.
ACh binds to receptors on postsynaptic membrane, causing depolarization.
ACh is broken down by acetylcholinesterase (AChE); choline is reabsorbed and recycled.

AP arrives at axon terminal Ca2+ influx and ACh release ACh binds to postsynaptic receptors ACh breakdown and recycling

Synaptic Delay and Fatigue

Synaptic Delay: Time required for neurotransmitter release and binding (0.2–0.5 ms).
Synaptic Fatigue: Occurs when neurotransmitter supply cannot keep up with demand during intense stimulation.

Neurotransmitters and Neuromodulators

Classes and Effects

Excitatory: Cause depolarization (e.g., glutamate, ACh at neuromuscular junctions).
Inhibitory: Cause hyperpolarization (e.g., GABA).
Biogenic Amines: Norepinephrine, dopamine, serotonin.
Dissolved Gases: Nitric oxide (NO), carbon monoxide (CO).
Neuropeptides: Opioids (endorphins, enkephalins, dynorphins).

Mechanisms of Action

Direct: Open/close ion channels directly (e.g., ACh, glutamate).
Indirect via G Proteins: Activate second messengers (e.g., cAMP) to alter cell activity (e.g., norepinephrine, dopamine).
Indirect via Intracellular Enzymes: Lipid-soluble gases diffuse and activate enzymes inside the cell (e.g., NO, CO).

Direct effects of neurotransmitters Indirect effects via G proteins Indirect effects via intracellular enzymes

Information Processing

Postsynaptic Potentials

Excitatory Postsynaptic Potential (EPSP): Graded depolarization; increases likelihood of action potential.
Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization; decreases likelihood of action potential.
Summation: EPSPs and IPSPs combine temporally (rapid, repeated stimuli at one synapse) or spatially (simultaneous stimuli at multiple synapses) to determine if threshold is reached.

Presynaptic Modulation

Presynaptic Inhibition: Reduces neurotransmitter release (e.g., GABA).
Presynaptic Facilitation: Increases neurotransmitter release (e.g., serotonin).

Summary

Neural information is relayed as action potentials.
Neurotransmitters and neuromodulators can excite, inhibit, or modulate postsynaptic responses.
Integration of signals at the postsynaptic neuron determines the final response.