Neuron Structure and Function

• Neurons are the fundamental units of the brain and nervous system, responsible for receiving sensory input from the external world, for sending motor commands to our muscles, and for transforming and relaying the electrical signals at every step in between.

Neuron Structure

• Neurons are diverse in size, shape, and electrochemical properties.
• Most neurons have the same fundamental structures: a cell body, dendrites, and an axon.
• The cell body (soma) contains the nucleus and other organelles and integrates signals from the dendrites.
• The size of the soma varies and ranges from 4 to 100 μm in diameter depending on neuron type.
• Dendrites are branching extensions emanating from the cell body that receive signals from other neurons.
• Dendrites can have specialized structures called dendritic spines, which increase the surface area for receiving signals.
• The axon is a long, slender projection that transmits signals to other neurons, muscles, or glands.
• Axons can range from a few millimeters to over a meter in length.
• The axon hillock is the region where the axon arises from the cell body, and where action potentials are initiated.
• Axons can branch, forming axon collaterals, allowing a single neuron to communicate with multiple target cells.
• The axon terminal is the end of the axon that forms a synapse with another neuron or target cell.
• Synapses are specialized junctions where neurons communicate with each other through chemical or electrical signals.
• Neurons can be classified based on the number of processes extending from the cell body:
• Unipolar neurons have a single process.
• Bipolar neurons have two processes.
• Multipolar neurons have multiple processes.

Neuron Types and Functions

• Sensory neurons:
• Respond to stimuli such as light, touch, or sound.
• Transmit signals from sensory receptors to the central nervous system.
• Motor neurons:
• Send signals from the central nervous system to muscles or glands.
• Control movement and other bodily functions.
• Interneurons:
• Connect sensory and motor neurons within the central nervous system.
• Involved in higher-level processing and complex functions.
• Projection neurons:
• Transmit signals over long distances to different brain regions.
• Local circuit neurons:
• Communicate with nearby neurons within a specific brain region.
• Glial cells support neurons
• Astrocytes:
• Maintain the chemical environment.
• Provide nutrients.
• Repair injury.
• Form the blood-brain barrier.
• Oligodendrocytes:
• Form myelin sheath in the central nervous system.
• Schwann cells:
• Form myelin sheath in the peripheral nervous system.
• Microglia:
• Immune cells of the brain.
• Remove debris and fight infection
• Ependymal cells:
• Line ventricles and spinal cord.
• Help produce cerebrospinal fluid.

Microscopic Neuroanatomy

• Microscopic neuroanatomy, or neurohistology, involves the study of the nervous system at the cellular level using microscopy techniques.
• Key techniques include:
• Tissue preparation:
• Fixation to preserve tissue structure.
• Sectioning to obtain thin slices.
• Staining:
• Highlights specific cellular components.
• Common stains include Golgi stain (for visualizing entire neurons) and Nissl stain (for visualizing cell bodies).
• Microscopy:
• Light microscopy, electron microscopy, and fluorescence microscopy are used to visualize different aspects of neurons and brain tissue.

Cellular Components

• Nucleus:
• Contains the cell's DNA.
• Essential for gene expression and regulation.
• Endoplasmic reticulum (ER):
• Involved in protein synthesis and folding.
• Smooth ER is involved in lipid synthesis and calcium storage.
• Golgi apparatus:
• Processes and packages proteins.
• Forms vesicles for transport.
• Mitochondria:
• Produce energy (ATP) through cellular respiration.
• Abundant in neurons due to their high energy demands.
• Cytoskeleton:
• Provides structural support and facilitates transport within the neuron.
• Includes microtubules, neurofilaments, and microfilaments.
• Myelin Sheath:
• Insulating layer around axons formed by oligodendrocytes (in the central nervous system) and Schwann cells (in the peripheral nervous system).
• Increases the speed of action potential conduction

Synapses

• Synapses are the points of communication between neurons and other cells.
• Types of synapses:
• Chemical synapses:
• Use neurotransmitters to transmit signals.
• The presynaptic neuron releases neurotransmitters into the synaptic cleft, which bind to receptors on the postsynaptic neuron.
• Electrical synapses:
• Use gap junctions to directly transmit electrical signals between neurons.
• Faster than chemical synapses but less flexible.
• Synaptic transmission:
• Action potential reaches the axon terminal of the presynaptic neuron.
• Voltage-gated calcium channels open, allowing calcium ions to enter the axon terminal.
• Calcium influx triggers the release of neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic neuron, causing a change in its membrane potential.
• Neurotransmitters are cleared from the synaptic cleft by reuptake, enzymatic degradation, or diffusion.

Neural Circuits

• Neurons are organized into circuits that perform specific functions.
• Simple circuits:
• Reflex arc:
• Involves a sensory neuron, an interneuron (in some cases), and a motor neuron.
• Produces a rapid, involuntary response to a stimulus.
• Complex circuits:
• Neural networks:
• Involve many interconnected neurons.
• Responsible for higher-level processing, such as learning, memory, and decision-making.
• Neural pathways:
• Specific routes through the nervous system that transmit signals between different brain regions.
• Neuroplasticity:
• The ability of neural circuits to change and reorganize over time in response to experience or injury.
• Neurogenesis:
• The formation of new neurons, which occurs in certain brain regions, such as the hippocampus and olfactory bulb.

Microscopic Features

• Axonal transport
• Fast axonal transport
• Transports membrane-bound organelles
• Anterograde: kinesin
• Retrograde: dynein
• Slow axonal transport
• Transports cytosolic proteins and cytoskeletal proteins
• Anterograde only
• Dendritic spines
• Increase surface area for synapses
• Morphology is activity-dependent
• Nodes of Ranvier
• Gaps in the myelin sheath
• High concentration of voltage-gated sodium channels
• Allows for saltatory conduction