Neurons are specialized nerve cells essential for intercellular communication.
Learning Outcomes
Sketch and label a typical neuron's structure.
Describe the functions of each component of the neuron.
Classify neurons based on their structure and function.
Understanding Neuron Function
Knowledge of neuron function necessitates familiarity with its structural components.
Neurons serve as the basic functional unit of the nervous system.
Characteristics of Neurons
Neurons are long-lived and possess a high metabolic rate, similar to skeletal muscle cells.
Have an excitable plasma membrane capable of generating and propagating action potentials, which requires significant energy resources.
Energy expenditure is critical for synthesizing and secreting chemical compounds necessary for neurotransmission.
Mitochondria in neurons produce energy to meet these demands.
Most neurons lack centrioles, which prevents them from dividing; hence, they cannot be replaced if lost to injury or disease.
Exemptions include neural stem cells in the adult nervous system, like those in the nose and the hippocampus, which are involved in olfactory receptor regeneration and memory storage, respectively.
Research is ongoing to understand triggers for neural stem cell activity to prevent or reverse neuron loss.
Structure of Neurons
Neurons display various shapes and typically consist of four main regions:
Cell Body (Soma)
Dendrites (branching extensions)
Axon (a long projection)
Teledendria (terminal branches of the axon)
Axon Hillock: The region between the cell body and the axon.
Components of the Cell Body (Soma)
Contains a large nucleus with a prominent nucleolus.
The cytoplasm is referred to as the Perikaryon, which contains:
Neurofilaments and neurotubules for cytoskeleton support.
Organelles for energy production and neurotransmitter synthesis.
Mitochondria generate ATP for the neuron's activity.
Rough Endoplasmic Reticulum (RER) with ribosomes produces proteins and forms dark-stained regions known as Nissl bodies (named after Franz Nissl).
Nissl bodies contribute to the gray matter seen in the brain and spinal cord.
Dendrites, Axons, and Teledendria
Dendrites:
Highly branched, extending from the cell body.
Participate in intercellular communication with key functions in receiving information.
Dendritic spines (0.5 - 1 µm projections) engage in synapses and account for 80-90% of a neuron's surface area.
Axons:
Long cytoplasmic processes, potentially up to 1 meter in length, propagate action potentials.
Axoplasm contains neurofibrillas, neurotubules, vesicles, lysosomes, mitochondria, and enzymes.
Axolemma: The plasma membrane around the axoplasm.
Axons may branch into collaterals, allowing a single neuron to communicate with multiple other cells.
Teledendria:
Terminal branches of the axon that end in axon terminals or synaptic terminals involved in cell communication.
Axonal Transport
Movement of materials between the cell body and axon terminals is termed Axonal Axoplasmic Transport.
Transport types:
Slow Stream: A few millimeters per day (e.g., substances like proteins).
Fast Stream: Can reach speeds of up to 1,000 mm/day, average around 300 mm/day.
Anterograde Flow: Movement from the cell body to the axon terminal (via kinesin).
Retrograde Flow: Movement from axon terminal to cell body (via dynein).
Clinical Note: Rabies utilizes retrograde flow to enter the CNS after a bite from a rabid animal, showcasing how certain pathogens could bypass neural defenses through axonal transport.
Classification of Neurons
Neurons can be classified structurally or functionally.
Structural Classification
Neurons may be anaxonic, bipolar, unipolar, or multipolar based on dendrite and axon relationships:
Anaxonic Neurons: Small; numerous dendrites without visible axons; found in the brain and special sense organs; functions are poorly understood.
Bipolar Neurons: Have one dendritic and one axonal process; rare, occurring in special sense organs (like sight and smell); cell body is situated between the two processes; maximum length under 30 µm.
Unipolar Neurons: One continuous process that merges dendrites and axon with the cell body off to the side; primarily sensory neurons in the PNS; can be very long.
Multipolar Neurons: Most common in the CNS; feature multiple dendrites and one axon; all motor neurons controlling skeletal muscles are multipolar, transmitting motor commands from spinal cord to muscles.
Functional Classification
Neurons can also be classified into three main functional categories:
Sensory Neurons (Afferent Neurons): Part of the PNS; their cell bodies are in peripheral sensory ganglia; deliver information from sensory receptors to the CNS; classified as unipolar neurons with processes known as afferent fibers.
Proprioceptors: Monitor muscle/joint position and movement.
Motor Neurons (Efferent Neurons): Carry instructions from CNS to effector organs and tissues; axons known as efferent fibers; controlled systems include:
Somatic Nervous System (SNS): Consciously controlled motor neurons to skeletal muscles.
Autonomic Nervous System (ANS): Involuntarily innervates smooth muscle, cardiac muscle, and glands; includes preganglionic and postganglionic fibers.
Interneurons (Association Neurons): Located between sensory and motor neurons; outnumber all other neuron types; mainly in the brain and spinal cord; responsible for integration; facilitate coordination of motor activity, memory, planning, and higher-level cognitive functions.