Fundamentals of the Nervous System

Overview of the Nervous System

The nervous system is the master control and communication system of the body. It is responsible for monitoring changes, processing information, and dictating responses throughout the organism.

• Sensory receptors monitor changes inside and outside the body.

• Stimulus: Any change detected by sensory receptors.

• Sensory input: Information gathered by receptors about internal and external changes.

• Integration: The process of interpreting sensory input and making decisions about what should be done at each moment.

• Motor output: The response generated by activating effector organs (muscles or glands).

Example: When you touch a hot stove, sensory receptors in your skin detect the heat (stimulus), send sensory input to the brain, which integrates the information and sends a motor output to move your hand away.

Functions of the Nervous System

• Sensory Input: Detects changes (stimuli) in the environment.

• Integration: Processes and interprets sensory input.

• Motor Output: Activates effector organs to produce a response.

Divisions of the Nervous System

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

The nervous system is divided into two main anatomical divisions:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and command center.

• Peripheral Nervous System (PNS): Consists of cranial nerves, spinal nerves, and ganglia. It connects the CNS to the rest of the body.

Functional Divisions of the Nervous System

• Somatic Sensory: General senses (touch, pain, vibration, pressure, temperature) and proprioceptive senses (body position and movement).

• Special Somatic Senses: Hearing, balance, vision.

• Visceral Sensory: General senses (stretch, pain, temperature, nausea, hunger) from internal organs; special senses include taste and smell.

• Somatic Motor: Voluntary control of skeletal muscles.

• Visceral Motor: Involuntary control of smooth muscle, cardiac muscle, and glands (autonomic nervous system).

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is composed of two main types of cells:

• Neurons: Excitable cells that transmit electrical signals.

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

Neurons: Structure and Function

• Basic structural unit of the nervous system.

• Specialized for conducting electrical impulses (nerve impulses or action potentials).

• Longevity: Can live and function for a lifetime.

• Amitotic: Most do not divide after fetal development (except neural stem cells).

• High metabolic rate: Require abundant oxygen and glucose; neurons die after 5 minutes without oxygen.

Neuron Anatomy

• Cell body (soma): Contains the nucleus and organelles; site of most metabolic activity.

• Chromatophilic bodies (Nissl bodies): Clusters of rough ER and ribosomes for membrane renewal.

• Neurofibrils: Bundles of intermediate filaments providing structural support.

• Dendrites: Branching processes that receive signals and transmit them toward the cell body.

• Axon: Single, long process that transmits impulses away from the cell body; lacks Nissl bodies and does not synthesize proteins.

• Axon collaterals: Occasional branches along the axon.

• Terminal arborization (telodendria): Multiple branches at the end of the axon, ending in terminal boutons (axon terminals) where neurotransmitters are released.

Synapses

• Synapse: Specialized junction where a neuron communicates with another cell.

• Axodendritic synapse: Between axon terminals of one neuron and dendrites of another (most common).

• Axosomatic synapse: Between axon terminals and neuronal cell bodies.

• Presynaptic neuron: Sends the signal; contains synaptic vesicles with neurotransmitters.

• Postsynaptic neuron: Receives the signal.

• Synaptic cleft: Small gap separating the two neurons.

Classification of Neurons

Structural Classification

• Multipolar: More than two processes (many dendrites, one axon); most common type.

• Bipolar: Two processes (one dendrite, one axon); rare, found in special sensory organs.

• Unipolar (pseudounipolar): Single process that splits into two branches; primarily sensory neurons.

Functional Classification

• Sensory (afferent) neurons: Transmit impulses toward the CNS; mostly unipolar; cell bodies in ganglia outside CNS.

• Motor (efferent) neurons: Carry impulses away from the CNS to effectors; mostly multipolar; cell bodies in CNS.

• Interneurons (association neurons): Lie between sensory and motor neurons; confined to CNS; most are multipolar.

Neuroglia (Glial Cells)

Types and Functions

• Four types in the CNS: astrocytes, microglial cells, ependymal cells, oligodendrocytes.

• Two types in the PNS: satellite cells, Schwann cells.

• Support, protect, and insulate neurons; can divide throughout life; outnumber neurons 10:1.

Gray and White Matter in the CNS

• Gray matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; forms the butterfly-shaped region in the spinal cord.

• White matter: Composed of myelinated axons; surrounds gray matter in the spinal cord.

Gross Anatomy of the Nervous System

Nerves

• Nerves: Cable-like organs in the PNS, consisting of bundles of axons (nerve fibers) wrapped in connective tissue.

• Most nerves contain both myelinated and nonmyelinated sensory and motor axons.

• Connective tissue coverings:

  • Endoneurium: Surrounds each axon.

  • Perineurium: Surrounds bundles of axons (fascicles).

  • Epineurium: Surrounds the entire nerve.

Reflex Arcs

Components and Types

Reflex arcs are the basic functional units of the nervous system, responsible for rapid, automatic responses to stimuli.

• Components of a reflex arc:

  1. Receptor: Site where stimulus acts.

  2. Sensory neuron: Transmits afferent impulses to the CNS.

  3. Integration center: One or more synapses in the CNS.

  4. Motor neuron: Conducts efferent impulses from the integration center to an effector.

  5. Effector: Muscle or gland cell that responds to the efferent impulse.

• Monosynaptic reflex: Involves a single synapse (e.g., knee-jerk reflex); fastest type.

• Polysynaptic reflex: Involves multiple synapses and interneurons (e.g., withdrawal reflex).

Neuronal Circuits

• Diverging circuit: One presynaptic neuron synapses with several postsynaptic neurons (amplifies signal).

• Converging circuit: Many presynaptic neurons synapse on a single postsynaptic neuron (integration of information).

• Reverberating circuit: Circuit receives feedback via a collateral axon, producing a rhythmic output.

Additional info: Understanding the structure and function of the nervous system is essential for diagnosing and treating neurological disorders, as well as for comprehending how the body responds to internal and external stimuli.