Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to coordinate rapid and specific responses to internal and external changes.

• Key Functions:

  • Sensory Input: Gathering information from sensory receptors about changes inside and outside the body.

  • Integration: Processing and interpreting sensory input to decide on a response.

  • Motor Output: Activating effectors (muscles and glands) to produce a response.

Divisions of the Nervous System

The nervous system is divided into two principal parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and control center, interpreting sensory input and dictating motor output.

• Peripheral Nervous System (PNS): Consists mainly of nerves that extend from the brain and spinal cord (cranial and spinal nerves) and the enteric nervous system in the gastrointestinal tract.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Transmits impulses from sensory receptors to the CNS. Includes:

  • Somatic sensory fibers (from skin, skeletal muscles, joints)

  • Visceral sensory fibers (from visceral organs)

• Motor (Efferent) Division: Transmits impulses from the CNS to effectors. Subdivided into:

  • Somatic Nervous System: Voluntary control of skeletal muscles.

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

    • Sympathetic Division (mobilizes body systems)

    • Parasympathetic Division (conserves energy, promotes housekeeping functions)

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue consists of two principal cell types:

• Neuroglia (Glial Cells): Small cells that support, protect, and insulate neurons.

• Neurons (Nerve Cells): Excitable cells that transmit electrical signals.

Neuroglia in the CNS

• Astrocytes: Most abundant, versatile, and highly branched. Support neurons, guide migration, control chemical environment, and influence neuronal functioning.

• Microglial Cells: Defensive cells that monitor neuron health and can phagocytize debris and microorganisms.

• Ependymal Cells: Line cerebrospinal fluid-filled CNS cavities; may be ciliated to help circulate fluid.

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers.

Neuroglia in the PNS

• Satellite Cells: Surround neuron cell bodies in the PNS.

• Schwann Cells: Form myelin sheaths around peripheral nerve fibers and are vital for regeneration of damaged fibers.

Neurons: Structure and Function

Neuron Structure

Neurons are large, highly specialized cells that conduct impulses. They have extreme longevity, are mostly amitotic, and have a high metabolic rate requiring continuous oxygen and glucose.

• Cell Body (Soma): Contains the nucleus and organelles; biosynthetic and metabolic center of the neuron.

• Dendrites: Short, branched processes that receive input and convey it toward the cell body as graded potentials.

• Axon: Single process that transmits impulses away from the cell body. May be myelinated or unmyelinated. Axon terminals are the secretory region, releasing neurotransmitters.

Myelin Sheath

The myelin sheath is a white, fatty covering that insulates axons and increases the speed of impulse transmission.

• In the PNS: Formed by Schwann cells wrapping around the axon in layers.

• In the CNS: Formed by oligodendrocyte processes; no neurilemma is present.

Classification of Neurons

• Structural Classification:

  • Multipolar: Many processes (1 axon, many dendrites); most common in CNS.

  • Bipolar: Two processes (1 axon, 1 dendrite); found in retina, ear, olfactory mucosa.

  • Unipolar (Pseudounipolar): One process that divides into peripheral and central branches; mainly sensory neurons in PNS ganglia.

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward CNS; mostly unipolar.

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

  • Interneurons: Lie between sensory and motor neurons; most abundant, mainly multipolar.

Membrane Potentials and Neural Signaling

Resting Membrane Potential

Neurons have a resting membrane potential (RMP) of approximately -70 mV, generated by differences in ionic composition and membrane permeability. The sodium-potassium pump maintains this potential by pumping 3 Na+ out and 2 K+ in.

• Depolarization: Membrane potential becomes less negative; increases probability of impulse generation.

• Hyperpolarization: Membrane potential becomes more negative; decreases probability of impulse generation.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential, essential for initiating action potentials. They decay with distance and are triggered by stimuli that open gated ion channels.

Action Potentials

An action potential (AP) is a brief reversal of membrane potential (~100 mV change) that does not decay with distance. It is the nerve impulse used for long-distance signaling in neurons and muscle cells.

• Steps of Action Potential:

  1. Resting state: All voltage-gated Na+ and K+ channels closed.

  2. Depolarization: Na+ channels open, Na+ influx.

  3. Repolarization: Na+ channels inactivate, K+ channels open, K+ efflux.

  4. Hyperpolarization: Some K+ channels remain open, Na+ channels reset.

• All-or-None Principle: An AP either happens completely or not at all.

• Propagation: AP is transmitted along the axon; only moves forward due to refractory periods.

Conduction Velocity

AP conduction velocity depends on axon diameter and degree of myelination:

• Larger diameter: Faster conduction.

• Myelinated axons: Saltatory conduction (jumps from node to node); faster than continuous conduction in unmyelinated axons.

Synapses and Neurotransmission

Synapses

Synapses are junctions that mediate information transfer between neurons or between a neuron and an effector cell. They can be electrical (rare, rapid) or chemical (most common, use neurotransmitters).

• Chemical Synapse Steps:

  1. AP arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

  3. Ca2+ triggers neurotransmitter release by exocytosis.

  4. Neurotransmitter diffuses across synaptic cleft, binds to receptors on postsynaptic membrane.

  5. Binding opens ion channels, creating graded potentials.

  6. Neurotransmitter effects are terminated (reuptake, degradation, or diffusion).

Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the likelihood of AP generation.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing the likelihood of AP generation.

• Summation: EPSPs and IPSPs can summate (temporal or spatial) to influence whether an AP is generated.

Comparison: Graded Potentials vs. Action Potentials

Neurotransmitters

Neurotransmitters are the chemical messengers of the nervous system. Most neurons produce two or more neurotransmitters, which may be released at different stimulation frequencies.

• Examples: Acetylcholine (ACh), dopamine, norepinephrine, epinephrine, amino acids (e.g., aspartate).

• Cholinergic Neurons: Release ACh; found in all preganglionic neurons, most parasympathetic postganglionic neurons, and somatic motor neurons. Functions include 'rest and digest' and muscle contraction.

• Adrenergic Neurons: Release norepinephrine (and sometimes epinephrine); found in most sympathetic postganglionic neurons. Functions include 'fight or flight' responses.

Grey Matter and White Matter

• Grey Matter: Contains neuronal cell bodies, dendrites, and unmyelinated axons; responsible for processing information, movement, memory, and sensory perception.

• White Matter: Composed mainly of myelinated axons; connects different brain regions and facilitates rapid communication.

Neuromuscular and Neuroglandular Communication

• Neuromuscular Junction: Connection between a neuron and a muscle cell, enabling movement and vital body functions.

• Neuroglandular Junction: Neuron communicates with a glandular cell, regulating secretion of substances such as sweat, saliva, or hormones.