Organization and Cellular Structure of the Nervous System

8.1 Organization of the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body functions and enabling communication between different body parts. It is composed of specialized cells called neurons, which transmit electrical and chemical signals.

• Neurons with processes secrete neurotransmitters: Neurons communicate by releasing chemical messengers called neurotransmitters at synapses.

• Emergent properties: The nervous system exhibits properties and functions that arise from the interactions of its many components, such as consciousness and learning.

Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and coordinates activity throughout the body.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. It connects the CNS to limbs and organs.

  • Sensory (afferent) neurons: Carry information from sensory receptors to the CNS.

  • Efferent neurons: Transmit signals from the CNS to effectors (muscles and glands). Subdivided into:

    • Somatic motor division: Controls voluntary movements by innervating skeletal muscles.

    • Autonomic division: Regulates involuntary functions (e.g., heart rate, digestion). Further divided into:

      • Sympathetic branch: Prepares the body for 'fight or flight' responses.

      • Parasympathetic branch: Promotes 'rest and digest' activities.

• Enteric Nervous System: A network of neurons in the walls of the digestive tract. It is regulated by the autonomic nervous system but can function independently to control digestive processes.

Table 8.1 Synonyms in Neuroscience

This table provides alternative terms commonly used in neuroscience for key structures and concepts.

Term Used in This Book	Synonym(s)
Action potential	AP, spike, nerve impulse, conduction signal
Autonomic nervous system	Visceral nervous system
Axon	Nerve fiber
Axonal transport	Axoplasmic flow
Axon terminal	Synaptic knob, synaptic bouton, presynaptic terminal
Axoplasm	Cytoplasm of an axon
Cell body	Cell soma, nerve cell body
Cell membrane of an axon	Axolemma
Glial cells	Neuroglia, glia
Interneuron	Association neuron
Rough endoplasmic reticulum	Nissl substance, Nissl body
Sensory neuron	Afferent neuron, afferent

8.2 Cells of the Nervous System

Neurons: Structure and Function

Neurons are the primary signaling cells of the nervous system, specialized for the transmission of electrical and chemical signals.

• Carry electrical signals: Neurons generate and propagate action potentials, enabling rapid communication.

• Functional unit: Neurons are the basic structural and functional units of the nervous system.

Neuron Classification

• By structure (number of processes):

  • Multipolar neurons: Have multiple dendrites and a single axon; most common in the CNS.

  • Pseudounipolar neurons: Have a single process that splits into two branches; typical of sensory neurons.

  • Bipolar neurons: Have one dendrite and one axon; found in sensory organs like the retina.

  • Anaxonic neurons: Lack a distinct axon; found in the brain and retina.

• By function:

  • Sensory (afferent) neurons: Transmit sensory information to the CNS.

  • Efferent neurons: Carry commands from the CNS to effectors.

  • Interneurons: Connect neurons within the CNS and integrate information.

Neuron Structure

• Cell body (soma): Contains the nucleus and organelles; metabolic center of the neuron.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body.

• Collaterals: Branches of the axon that allow communication with multiple targets.

• Axon terminals: Specialized endings where neurotransmitters are released.

• Varicosities: Swellings along the axon where neurotransmitters can also be released.

Examples and Applications

• Multipolar efferent neuron: A typical motor neuron that controls skeletal muscle contraction.

• Bipolar neuron: Found in the retina, involved in visual processing.

Axonal Transport

Classification by Speed

Axonal transport is the process by which materials are moved along the axon, essential for neuron function and survival.

• Fast axonal transport:

  • Moves organelles and vesicles at rates up to 400 mm/day.

  • Anterograde transport: From cell body to axon terminal.

  • Retrograde transport: From axon terminal to cell body.

• Slow axonal transport:

  • Moves cytoplasmic components by axoplasmic flow at 0.2–2.5 mm/day.

Example: Fast Axonal Transport

• Proteins and organelles synthesized in the soma are transported rapidly to the axon terminal for synaptic function.

• Damaged organelles and signaling molecules are returned to the soma for recycling or degradation.

Summary Table: Types of Axonal Transport

Type	Direction	Speed	Materials Moved
Fast (Anterograde)	Cell body to axon terminal	Up to 400 mm/day	Organelles, vesicles, proteins
Fast (Retrograde)	Axon terminal to cell body	Up to 400 mm/day	Recycled materials, growth factors
Slow	Cell body to axon terminal	0.2–2.5 mm/day	Cytoskeletal elements, soluble proteins

Additional info: Axonal transport relies on molecular motors (kinesin and dynein) that move cargo along microtubules within the axon.