Chapter 11: Introduction to the Nervous System and Nervous Tissue

Chapter 11: Introduction to the Nervous System and Nervous Tissue

11.1 Overview of the Nervous System

The nervous system is responsible for controlling perception, voluntary movement, consciousness, personality, learning, memory, and maintaining homeostasis of physiological variables. It is divided into two main structural divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• CNS: Composed of the brain and spinal cord. The brain contains about 100 billion neurons, while the spinal cord contains about 100 million neurons. The spinal cord enables communication between the brain and most parts of the body below the head and neck.

• PNS: Consists of nerves (bundles of axons) and their branches, including 12 pairs of cranial nerves and 31 pairs of spinal nerves. Nerves are classified by their origin or destination.

The nervous system has three main functional divisions:

• Sensory (Afferent) Division: Gathers information from internal and external environments via sensory receptors. Includes somatic sensory (skeletal muscles, bones, joints, skin, special senses) and visceral sensory (organs such as heart, lungs, stomach).

• Integrative Functions: Performed by the CNS, analyzing and interpreting sensory stimuli to determine appropriate responses.

• Motor (Efferent) Division: Carries out actions in response to integration. Includes somatic motor (skeletal muscle, voluntary) and visceral motor (autonomic nervous system, involuntary control of glands, smooth and cardiac muscle).

Effectors are organs that carry out the effects of the nervous system.

11.2 Nervous Tissue

Nervous tissue is highly cellular, consisting of neurons and neuroglial cells. Neurons are excitable cells responsible for sending and receiving signals, while neuroglial cells provide support and protection.

• Neurons: Generally amitotic, long-lived, and consist of three parts: cell body (soma), dendrites, and axon.

• Cell Body (Soma): Metabolically active, contains organelles for protein synthesis (ribosomes, RER, Golgi apparatus, mitochondria). Nissl bodies are clusters of ribosomes and RER.

• Dendrites: Receive input and transmit electrical impulses toward the cell body. Do not generate action potentials.

• Axon: Single process that carries electrical signals away from the cell body. Arises from the axon hillock, may branch into axon collaterals and telodendria, ending in axon terminals (synaptic knobs).

• Axolemma: Plasma membrane of the axon; axoplasm is its cytoplasm.

• Axonal Transport: Movement of substances along the axon. Slow transport moves proteins away from the cell body; fast transport (using motor proteins and ATP) moves vesicles and organelles both retrograde (toward cell body) and anterograde (away from cell body).

Functional Regions of Neurons:

• Receptive region: Dendrites and cell body.

• Conducting region: Axon.

• Secretory region: Axon terminals.

Classification of Neurons:

• Structural:

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

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

  • Pseudounipolar: Single fused process that splits into two (sensory neurons for touch, pressure, pain).

• Functional:

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

  • Interneurons: Relay messages within CNS (multipolar).

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

Groups of Neuron Cell Bodies and Axons:

• CNS: Cell bodies in nuclei, axons in tracts.

• PNS: Cell bodies in ganglia, axons in nerves.

Neuroglia (Neuroglial Cells)

Supportive cells that maintain the environment, protect neurons, and assist in function. Six types: four in CNS, two in PNS.

• CNS Neuroglia:

  • Astrocytes: Anchor neurons and blood vessels, regulate extracellular environment, form blood-brain barrier, repair tissue.

  • Oligodendrocytes: Form myelin sheath in CNS.

  • Microglia: Phagocytic cells, clean up debris and pathogens.

  • Ependymal cells: Ciliated, circulate and help form cerebrospinal fluid.

• PNS Neuroglia:

  • Neurolemmocytes (Schwann cells): Form myelin sheath in PNS, aid in axon repair.

  • Satellite cells: Surround and support cell bodies in PNS, regulate environment.

The Myelin Sheath

Composed of repeating layers of plasma membrane, rich in phospholipids, cholesterol, and proteins. Myelin insulates axons, increasing the speed of action potential conduction (myelinated axons conduct 15-150 times faster than unmyelinated).

• Myelination: In PNS, Schwann cells wrap outward; in CNS, oligodendrocytes wrap inward.

• White matter: Myelinated axons; gray matter: cell bodies, dendrites, unmyelinated axons.

Regeneration of Nervous Tissue

Regeneration is limited. CNS axons rarely regenerate due to inhibitory factors and scar tissue. PNS axons can regenerate if the cell body is intact, following a multi-step process.

1. Distal axon and myelin sheath degenerate.

2. Growth processes form from proximal axon.

3. Schwann cells and basal lamina form regeneration tube.

4. Growth process enters tube.

5. Axon reconnects with target cell.

Regeneration is often incomplete.

11.3 Electrophysiology of Neurons

Neurons are excitable and conductive, generating electrical changes across their plasma membrane in response to stimuli. These changes are classified as local potentials (short distance) and action potentials (long distance).

• Resting membrane potential: $-70\,\text{mV}$

• Ion channels: Ligand-gated, voltage-gated, mechanically gated.

Local Potentials: Graded, decremental, reversible; can cause depolarization or hyperpolarization.

Action Potentials: Uniform, rapid depolarization and repolarization; all-or-none, irreversible, non-decremental.

• Phases:

  1. Depolarization to threshold.

  2. Voltage-gated sodium channels open, sodium enters, depolarization.

  3. Sodium channels inactivate, potassium channels open, repolarization.

  4. Sodium channels return to resting, repolarization continues.

  5. Hyperpolarization may occur before returning to resting potential.

• Refractory Period:

  • Absolute: No new AP possible.

  • Relative: Only strong stimulus can trigger AP.

Propagation: APs are self-propagating, moving in one direction due to refractory period. Conduction speed depends on axon diameter and myelination.

• Saltatory conduction: Myelinated axons, faster.

• Continuous conduction: Unmyelinated axons, slower.

Classification of Axons by Conduction Speed:

Type	Diameter	Myelination	Speed	Function
Type A	Largest	Myelinated	Fastest	Rapid CNS communication
Type B	Intermediate	Myelinated	Moderate	ANS efferent, sensory from organs
Type C	Smallest	Unmyelinated	Slowest	Pain, temperature, pressure

11.4 Neuronal Synapses

Synapses are junctions where neurons communicate with target cells. Two types: electrical and chemical.

• Electrical synapses: Cells are electrically coupled via gap junctions; transmission is bidirectional and nearly instantaneous.

• Chemical synapses: Use neurotransmitters; more common and efficient. Transmission involves synaptic vesicles, synaptic cleft, and neurotransmitter receptors.

Events at Chemical Synapse:

1. AP triggers calcium channels to open in presynaptic terminal.

2. Calcium influx causes vesicles to release neurotransmitters into synaptic cleft.

3. Neurotransmitters bind to postsynaptic receptors.

4. Ion channels open, leading to local potential and possibly AP.

Postsynaptic Potentials:

• EPSP (Excitatory): Brings membrane potential closer to threshold.

• IPSP (Inhibitory): Moves membrane potential farther from threshold.

Summation: Adding input from several postsynaptic potentials (temporal: rapid succession from one neuron; spatial: simultaneous from multiple neurons).

Termination of Synaptic Transmission: Occurs via diffusion/absorption, degradation, or reuptake.

11.5 Neurotransmitters

Neurotransmitters bind to receptors to induce postsynaptic potentials. Two main receptor types:

• Ionotropic: Part of ligand-gated ion channels; rapid, short-lived effects.

• Metabotropic: Connected to ion channels via G-proteins; slower, longer-lasting effects. Second messengers (e.g., cAMP) are involved.

Major Neurotransmitter Groups:

Group	Examples	Effects	Notes
Acetylcholine (ACh)	ACh	Excitatory/inhibitory	NMJ, CNS, ANS; degraded by AChE
Biogenic amines	Norepinephrine, Epinephrine, Dopamine, Serotonin, Histamine	Varied	Homeostasis, cognition, mood; catecholamines from tyrosine
Amino acids	Glutamate, Glycine, GABA	Excitatory/inhibitory	Glutamate (excitatory), Glycine & GABA (inhibitory)
Neuropeptides	Substance P, Opioids, Neuropeptide Y	Varied	Pain relief, nervous system depression

Neuromodulation: Neuromodulators alter synaptic transmission without directly exciting or inhibiting postsynaptic neurons.

Psychiatric Disorders:

• Schizophrenia: Excess dopamine; treated by blocking dopamine receptors.

• Depression: Deficiency in serotonin, norepinephrine, dopamine; treated with SSRIs.

• Anxiety: Abnormalities in norepinephrine, serotonin, GABA.

• Bipolar disorder: Treated by blocking sodium channels to reduce AP generation.

11.6 Functional Groups of Neurons

Neurons form networks called neuronal pools and neural circuits to perform common functions.

• Neuronal pools: Groups of interneurons in CNS, defined by synaptic connections.

• Input neurons: Initiate signals in pools.

• Neural circuits: Patterns of synaptic connections; two main types:

  • Diverging circuit: One neuron branches to multiple postsynaptic neurons; allows amplification and widespread communication.

  • Converging circuit: Multiple neurons converge on a single postsynaptic neuron; important for skeletal muscle control.

• Stabilization mechanisms: Inhibitory circuits and synaptic fatigue.

• Epileptic seizures: Result from abnormal, disorganized electrical activity; usually end due to synaptic fatigue.

Additional info: Academic context was added to clarify structural and functional classifications, synaptic mechanisms, and clinical relevance. Tables were inferred and expanded for clarity.