Introduction to the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is a complex network responsible for controlling perception, voluntary movement, consciousness, learning, memory, and homeostasis. It works closely with the endocrine system to regulate vital functions such as respiratory rate, blood pressure, body temperature, sleep/wake cycles, and blood pH.

• Voluntary system: Directs conscious actions and experiences.

• Homeostasis: Maintains internal stability in conjunction with the endocrine system.

Structural Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain (about 100 billion neurons) and spinal cord (about 100 million neurons). The spinal cord merges with the brain at the foramen magnum and extends to the first or second lumbar vertebra.

• Peripheral Nervous System (PNS): Composed of cranial nerves (12 pairs) and spinal nerves (31 pairs) and their branches. Nerves are bundles of axons surrounded by connective tissue and blood vessels.

Functional Divisions of the Nervous System

• Sensory (Afferent) Division: Gathers information from internal and external environments via sensory receptors.

• Integrative Functions: CNS analyzes and interprets sensory input, determining appropriate responses.

• Motor (Efferent) Division: Executes responses by transmitting signals to effectors (muscles and glands).

PNS Sensory Division

• Somatic Sensory: Signals from skin, muscles, bones, joints, and special senses (vision, hearing, taste, smell, balance).

• Visceral Sensory: Signals from internal organs (heart, lungs, stomach, intestines, kidneys, bladder).

PNS Motor Division

• Somatic Motor: Voluntary control of skeletal muscles.

• Visceral Motor (Autonomic Nervous System): Involuntary control of smooth muscle, cardiac muscle, and glands.

Nervous Tissue

Nervous tissue is composed of about 80% cells and 20% extracellular matrix. The two main cell types are neurons and neuroglial cells.

• Neurons: Excitable cells that send and receive signals (action potentials). Responsible for sensory, integrative, and motor functions. Generally amitotic and long-lived.

• Neuroglial Cells: Supportive cells, more numerous than neurons, capable of mitosis.

Structure of a Neuron

• Cell Body (Soma): Contains nucleus, Nissl bodies (clusters of rough ER and ribosomes), Golgi apparatus, nucleoli, and mitochondria. Maintains cell metabolism.

• Dendrites: Short, branched processes that receive input and transmit it toward the cell body.

• Axon: Long process that conducts action potentials away from the cell body. Features include axon hillock, axon collaterals, telodendria, axon terminals (synaptic knobs), axolemma (plasma membrane), and axoplasm (cytoplasm).

Axonal Transport

• Slow Axonal Transport: Moves cytoskeletal proteins away from the cell body (1–3 mm/day).

• Fast Axonal Transport: Moves vesicles and organelles using ATP and motor proteins (up to 400 mm/day anterograde, 200 mm/day retrograde).

Functional Regions of Neurons

• Receptive Region: Dendrites and cell body.

• Conducting Region: Axon.

• Secretory Region: Axon terminals.

Structural Classification of Neurons

• Multipolar: One axon, multiple dendrites (most common).

• Bipolar: One axon, one dendrite (retina, olfactory epithelium).

• Pseudounipolar: Single axon with two branches (sensory neurons for touch, pressure, pain).

Functional Classification of Neurons

• Sensory (Afferent): Carry signals toward CNS; usually pseudounipolar or bipolar.

• Interneurons: Relay messages within CNS; multipolar; most numerous.

• Motor (Efferent): Carry signals from CNS to effectors; multipolar.

Organization of Neuron Cell Bodies and Axons

Neuroglia (Neuroglial Cells)

Neuroglia support, protect, and maintain the environment for neurons. They can divide and fill in gaps when neurons die. There are six types: four in the CNS and two in the PNS.

Neuroglia in the CNS

• Astrocytes: Star-shaped; anchor neurons and blood vessels, regulate extracellular environment, form blood-brain barrier, repair damaged tissue.

• Oligodendrocytes: Form myelin sheaths around CNS axons.

• Microglia: Act as phagocytes, ingesting debris and pathogens.

• Ependymal Cells: Ciliated; circulate and help form cerebrospinal fluid.

Neuroglia in the PNS

• Neurolemmocytes (Schwann Cells): Myelinate PNS axons; assist in axon repair.

• Satellite Cells: Surround and support PNS neuron cell bodies; regulate extracellular environment.

The Myelin Sheath

The myelin sheath is a multilayered covering formed by oligodendrocytes (CNS) or Schwann cells (PNS) that insulates axons and increases conduction speed.

• Composition: Phospholipids, cholesterol, proteins, and unique lipids.

• Myelinated axons: Conduct impulses 15–150 times faster than unmyelinated axons.

• Nodes of Ranvier: Gaps between myelinated segments where action potentials are regenerated.

• White Matter: Myelinated regions in CNS; Gray Matter: Unmyelinated regions.

Myelination Differences

• PNS: Schwann cells wrap outward, forming neurolemma; each cell myelinates one axon segment.

• CNS: Oligodendrocyte processes wrap inward; each cell can myelinate multiple axons.

Regeneration of Nervous Tissue

• CNS: Limited regeneration due to inhibitory factors, lack of growth factors, and scar formation by astrocytes.

• PNS: Possible if cell body is intact; involves Wallerian degeneration, formation of a regeneration tube by Schwann cells, and reconnection with the target cell.

Electrophysiology of Neurons

Neurons are excitable and can generate electrical changes across their membranes in response to stimuli. These changes are essential for nerve impulse transmission.

Resting Membrane Potential

• Definition: The voltage difference across the membrane at rest (about -70 mV).

• Maintained by: Leak channels, gated channels, and the sodium-potassium pump.

• Sodium-Potassium Pump Equation:

• Polarization: Negative inside relative to outside.

Ion Channels

• Leak Channels: Always open.

• Ligand-Gated Channels: Open in response to chemical signals.

• Voltage-Gated Channels: Open/close in response to voltage changes.

• Mechanically Gated Channels: Open/close in response to mechanical stimuli.

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative (e.g., Na+ influx).

• Repolarization: Return to resting potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Local and Action Potentials

• Local (Graded) Potentials: Small, reversible, decremental changes in membrane potential; can be depolarizing or hyperpolarizing.

• Action Potentials: Rapid, uniform depolarization and repolarization; all-or-none; nondecremental; generated at the trigger zone and propagated along the axon.

States of Voltage-Gated Channels

• Potassium Channels: Resting (closed) or activated (open).

• Sodium Channels: Resting (activation gate closed), activated (both gates open), inactivated (inactivation gate closed).

Events of an Action Potential

1. Local potential depolarizes axolemma to threshold (−55 mV).

2. Voltage-gated Na+ channels open; Na+ enters (depolarization to +30 mV).

3. Na+ channels inactivate; voltage-gated K+ channels open (repolarization).

4. K+ channels return to resting state; membrane may hyperpolarize.

5. Resting membrane potential restored by leak channels and Na+/K+ pump.

Refractory Periods

• Absolute Refractory Period: No new action potential possible.

• Relative Refractory Period: Stronger stimulus required for new action potential.

Propagation of Action Potentials

• Self-propagating, unidirectional (due to refractory period).

• Depolarization at one segment triggers the next; previous segment repolarizes.

Conduction Speed

• Diameter: Larger axons conduct faster.

• Myelination: Myelinated axons use saltatory conduction (jumping between nodes of Ranvier); unmyelinated use continuous conduction.

Neuronal Synapses

A synapse is the junction where a neuron communicates with another cell. Synapses can be electrical or chemical.

Types of Synapses

• Electrical Synapses: Direct electrical connection via gap junctions; bidirectional, nearly instantaneous; synchronize activity (e.g., in cardiac muscle).

• Chemical Synapses: Use neurotransmitters; unidirectional; allow for varied and modifiable responses.

Events at a Chemical Synapse

1. Action potential arrives at axon terminal; voltage-gated Ca2+ channels open.

2. Ca2+ influx triggers neurotransmitter release into synaptic cleft.

3. Neurotransmitter binds to postsynaptic receptors (ionotropic or metabotropic).

4. Ion channels open, generating a local potential (EPSP or IPSP).

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarization, brings membrane closer to threshold.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization, moves membrane further from threshold.

Summation and Neural Integration

• Temporal Summation: Rapid, repeated input from one presynaptic neuron.

• Spatial Summation: Simultaneous input from multiple presynaptic neurons.

• EPSPs and IPSPs can combine to determine if threshold is reached at the trigger zone.

Termination of Synaptic Transmission

• Diffusion and Absorption: Neurotransmitter diffuses away and is absorbed.

• Degradation: Enzymatic breakdown in the synaptic cleft.

• Reuptake: Neurotransmitter is taken back into the presynaptic neuron.

Neurotransmitter Receptors

• Ionotropic Receptors: Ligand-gated ion channels; fast, short-lived effects.

• Metabotropic Receptors: G-protein coupled; slower, longer-lasting, and more varied effects via second messengers (e.g., cAMP).

Major Neurotransmitters

• Acetylcholine (ACh): Excitatory at neuromuscular junctions; broken down by acetylcholinesterase.

• Biogenic Amines (Monoamines): Include catecholamines (norepinephrine, epinephrine, dopamine), serotonin, and histamine. Involved in mood, attention, and homeostasis.

• Amino Acid Neurotransmitters: Glutamate (excitatory), GABA and glycine (inhibitory).

• Neuropeptides: Substance P (pain), opioids (endorphins, enkephalins), neuropeptide Y (feeding behavior).

Neuromodulation

• Neuromodulators: Neurotransmitters that modulate synaptic transmission by altering neurotransmitter release, receptor sensitivity, or gene expression.

• Neurohormones: Hormones that modulate neuronal activity.

Psychiatric Disorders and Treatments

• Schizophrenia: Linked to excess dopamine; treated by dopamine receptor blockers.

• Depressive Disorders: Linked to deficiencies in serotonin, norepinephrine, and/or dopamine; treated with SSRIs and related drugs.

• Anxiety Disorders: Involve norepinephrine, serotonin, and GABA; treated with antidepressants and GABA enhancers.

• Bipolar Disorder: Treated by drugs that block sodium channels to reduce action potentials.

Neuronal Pools and Circuits

• Neuronal Pools: Groups of interneurons with a common function; input neurons branch to multiple postsynaptic neurons.

• Diverging Circuits: One neuron branches to many; amplifies signals.

• Converging Circuits: Multiple neurons converge on one; allows spatial summation.

• Inhibitory Feedback: Prevents overexcitation; mediated by neuromodulators.

• Synaptic Fatigue: Decreased sensitivity due to prolonged neurotransmitter presence.

Epileptic Seizures

• Epilepsy: Recurrent abnormal electrical activity (seizures) due to loss of inhibitory control in neuronal pools.

• Symptoms: Sensory disturbances, loss of consciousness, jerking movements.

• Treatment: Medications to prevent seizures and restore inhibitory function.

Summary Table: Key Terms and Definitions

Example: When you touch a hot surface, sensory neurons (pseudounipolar) carry the signal to the CNS, interneurons process the information, and motor neurons (multipolar) send a signal to your muscles to withdraw your hand.