Nervous Tissue

Overview of the Nervous System
- Nervous tissue is composed of highly specialized cells, primarily neurons and glial cells, that form intricate communication networks throughout the body.
- Its primary role is to integrate and coordinate body functions, enabling responses to external and internal stimuli, and to maintain homeostasis.
- Key functions include the rapid control of body movement, processing of conscious thought, memory formation and storage, emotional regulation, and sophisticated communication between different parts of the body.
- Structurally, the nervous system is divided into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes all nerves outside the CNS.

Cells of Nervous Tissue

Neurons

Neurons are the fundamental functional units of the nervous system, specialized for detecting environmental stimuli, transmitting electrical signals (action potentials), and communicating with other neurons, muscles, or glands via chemical signals (neurotransmitters).
Structural Components of Neurons:
- Dendrites: Tree-like, branched extensions that receive incoming signals from other neurons and convey them towards the cell body. They are the primary receptive regions.
- Cell Body (Soma): The metabolic center of the neuron containing the nucleus, ribosomes, mitochondria, and other organelles necessary for synthesizing proteins and maintaining cell life.
- Axon Hillock (Trigger Zone): A specialized region at the base of the axon where graded potentials are integrated, and if the threshold is reached, action potentials are initiated.
- Axon: A single, long projection extending from the cell body that transmits electrical impulses (action potentials) away from the cell body towards target cells. Axons can be myelinated or unmyelinated.
- Myelin Sheath: A fatty insulating layer, formed by glial cells, that surrounds some axons, significantly increasing the speed of action potential propagation.
- Axon Terminal (Synaptic Terminal): The distal end of the axon, which forms synapses with other cells and is responsible for releasing neurotransmitters into the synaptic cleft.
Classification of Neurons:
- Neurons are generally classified based on their structure (number of projections) and function.
- Structural Classification (based on processes from the cell body):
  - Unipolar Neurons: Have a single process extending from the cell body that then divides into two, with one part extending to the periphery and the other towards the CNS. Primarily sensory neurons (afferent), detecting touch, pain, temperature, etc.
  - Bipolar Neurons: Have two processes extending from the cell body—one dendrite and one axon. Found in specialized sensory organs like the retina of the eye and the olfactory epithelium.
  - Multipolar Neurons: Possess multiple dendrites and a single axon extending from the cell body. They are the most common type in the CNS, including motor neurons (efferent) and interneurons.
- Functional Classification (based on signal direction):
  - Sensory (Afferent) Neurons: Transmit impulses from sensory receptors in the periphery towards the CNS.
  - Motor (Efferent) Neurons: Transmit impulses from the CNS to effector organs (muscles and glands).
  - Interneurons (Association Neurons): Lie entirely within the CNS and connect sensory and motor neurons, playing a crucial role in integration and complex neural pathways.

Glial Cells

Glial cells (neuroglia) are non-neuronal cells that provide essential physical, metabolic, and functional support to neurons, ensuring their optimal metabolic environment and facilitating efficient nerve impulse transmission. They do not transmit electrical impulses themselves.
Types of Glial Cells:
- Peripheral Nervous System (PNS) Glial Cells:
  1. Satellite Cells: Flattened cells that surround the cell bodies of unipolar neurons (ganglia) in the PNS. They provide structural support, regulate the external chemical environment, and act as a protective barrier, filtering blood toxins and supplying nutrients.
  2. Schwann Cells (Neurolemmocytes): Wrap tightly around peripheral axons, forming the myelin sheath (insulation) around many axons. A single Schwann cell can myelinate only one axon segment but can enclose multiple unmyelinated axons. Myelin dramatically increases the speed of nerve impulse conduction.
- Central Nervous System (CNS) Glial Cells:
  1. Astrocytes: Star-shaped cells that are the most abundant glial cells in the CNS. They have numerous functions including forming the structural framework of the CNS, regulating the chemical environment (e.g., K+ concentration, neurotransmitter uptake), guiding neuron migration, and forming a critical component of the blood-brain barrier (BBB) by inducing tight junctions between endothelial cells.
  2. Oligodendrocytes: Possess fewer branches than astrocytes and myelinate multiple axons within the CNS. A single oligodendrocyte can form myelin sheaths around segments of several different axons or multiple segments of the same axon, significantly improving conduction velocity.
  3. Microglial Cells: Small, mobile cells with thorny processes that act as the primary immune defense cells of the CNS. They monitor the health of neurons and remove cellular debris, pathogens, and damaged neurons through phagocytosis, akin to macrophages.
  4. Ependymal Cells: Cuboidal or columnar cells that line the ventricles of the brain and the central canal of the spinal cord. They possess cilia that assist in producing cerebrospinal fluid (CSF) and help circulate it throughout the CNS, providing cushioning, nutrient delivery, and waste removal.
The blood-brain barrier (BBB), primarily formed by tight junctions between endothelial cells of CNS capillaries and strengthened by astrocyte foot processes, is highly selective, restricting the passage of many substances from the bloodstream into the brain. While essential for protecting the brain, it significantly restricts drug delivery, though small, lipid-soluble (non-polar) molecules can often cross.

Action Potentials

Definition: Action potentials, also known as nerve impulses, are rapid, transient, and self-propagating electrical signals that represent a momentary reversal of the membrane potential, transmitting information along the neuron's axon without decrement.
Resting Membrane Potential (RMP): The potential difference across the plasma membrane of a neuron at rest, typically around $-70 ext{ mV}$ . This negative charge inside the cell relative to the outside is primarily maintained by:
- The differential distribution of ions (higher $K^+$ inside, higher $Na^+$ and $Cl^-$ outside).
- The selective permeability of the membrane to ions, predominantly via leak channels (more $K^+$ leak channels than $Na^+$ leak channels, allowing $K^+$ to diffuse out more readily).
- The activity of the sodium-potassium pump ( $Na^+/K^+-ATPase$ ), an active transport protein that expels three $Na^+$ ions for every two $K^+$ ions pumped into the cell, consuming ATP and contributing negatively to the RMP.
Formation of Action Potentials: Action potentials are generated through a sequence of rapid changes in membrane permeability to $Na^+$ and $K^+$ ions, mediated by voltage-gated ion channels.
- Ionic Movements: Ions move across the membrane primarily through ion channels, following their electrochemical gradients (both concentration and electrical gradients). Movement is from high to low concentration or towards the opposite charge.
- Generation: The $Na^+/K^+$ pump continuously works to maintain the steep concentration gradients of $Na^+$ and $K^+$ across the membrane, which are essential for action potential generation. Leak channels, especially for $K^+$ (potassium leak channels), contribute significantly to the RMP by allowing $K^+$ to diffuse out, making the inside of the cell negative.

Phases of an Action Potential

An action potential follows an "all-or-none" principle; once the threshold is reached, it fires with maximum intensity, regardless of the stimulus strength.
1. Depolarization: A stimulus causes voltage-gated $Na^+$ channels to open, leading to a rapid influx of $Na^+$ ions into the cell. This makes the inside of the membrane less negative and, once the threshold potential (typically around $-55 ext{ mV}$ ) is reached, a rapid and dramatic reversal of membrane potential occurs, reaching a peak of around $+30 ext{ mV}$ . This phase is excitatory.
2. Repolarization: Shortly after depolarization, voltage-gated $Na^+$ channels inactivate (close), and voltage-gated $K^+$ channels open, allowing $K^+$ ions to rapidly efflux out of the cell. This outflow of positive charge restores the negative charge inside the cell, bringing the membrane potential back towards the RMP.
3. Hyperpolarization (Undershoot): In many neurons, the voltage-gated $K^+$ channels remain open for a brief period after the membrane potential returns to the RMP, causing an excessive efflux of $K^+$ ions. This results in the membrane potential becoming even more negative than the RMP (e.g., $-90 ext{ mV}$ ) before the $K^+$ channels close and the RMP is re-established by the $Na^+/K^+$ pump and leak channels. This phase makes it harder for another action potential to be generated (inhibitory).

Triggering Action Potentials

Graded Potentials: Localized, small deviations from the RMP that are proportional to the strength of the stimulus. They can be either depolarizing (Excitatory Postsynaptic Potentials - EPSPs, often due to $Na^+$ influx) or hyperpolarizing (Inhibitory Postsynaptic Potentials - IPSPs, often due to $K^+$ efflux or $Cl^-$ influx). Graded potentials lose strength over distance, but if their summation (spatial or temporal) at the axon hillock reaches the threshold potential ( $-55 ext{ mV}$ ), an action potential is triggered.
Excitation/Inhibition: The balance between excitatory and inhibitory inputs determines whether a neuron will fire an action potential.
- Opening of $Na^+$ or $Ca^{2+}$ channels leads to depolarization, making the neuron more likely to fire an action potential (excitatory).
- Opening of $K^+$ or $Cl^-$ channels leads to hyperpolarization or stabilization of the membrane potential, making the neuron less likely to fire (inhibitory).
Propagation of Action Potentials: Once initiated at the axon hillock (due to a high density of voltage-gated $Na^+$ channels), action potentials are regenerated along the axon.
- The depolarization in one segment of the axon opens voltage-gated $Na^+$ channels in the adjacent segment, propagating the impulse unidirectionally away from the cell body.
Refractory Periods: Crucial for ensuring unidirectional propagation and limiting the frequency of action potentials.
- Absolute Refractory Period: During depolarization and the early repolarization phase, another action potential cannot be generated, regardless of stimulus strength, because voltage-gated $Na^+$ channels are either open or inactivated.
- Relative Refractory Period: During the later repolarization and hyperpolarization phases, a stronger than normal stimulus can generate another action potential because some $Na^+$ channels have reset, but the membrane is still hyperpolarized or recovering.

Myelination and Conduction Velocity

Myelin sheaths are crucial for accelerating action potential propagation.
Saltatory Conduction: In myelinated axons, the myelin acts as an electrical insulator, preventing ion flow across the membrane except at periodic gaps called Nodes of Ranvier. Voltage-gated $Na^+$ channels are highly concentrated at these nodes. The action potential