Neuroanatomy: Structure and Function of Neurons and Nervous Systems

Objectives of Neuroanatomy Study

• Identify major neuron structures

• Explain neuron types and functions

• Describe action potential direction

• Explain myelin and conduction speed

• Distinguish neurons from glial cells

• Identify CNS vs PNS organization

• Compare somatic and autonomic systems

• Identify major brain regions and lobes

• Use neuroanatomical directions and planes

• Describe blood brain barrier and meninges

Neurons and the Neuron Doctrine

Basic Units of the Nervous System

• Neurons: 80-90 billion in adult humans; fundamental anatomical and functional units of the nervous system.

• Glial cells: Nearly as numerous as neurons; provide support and contribute to information processing.

• Neuron Doctrine: The brain is composed of metabolically, structurally, and functionally independent cells. Information is transmitted across synapses (gaps between neurons).

Key Figures

• Camillo Golgi: Developed the Golgi stain for visualizing neurons.

• Santiago Ramón y Cajal: Pioneered the neuron doctrine and mapped neural structures.

Neuronal Specialization for Information Processing

Functional Zones of Neurons

• Receive info: Dendrites and cell body collect and process signals.

• Integrate info: Integration zone (axon hillock) decides if a neural signal is made.

• Carry info: Axon conducts signals over long distances.

• Transfer info: Axon terminals release chemicals to communicate with other cells.

Types of Neurons

Type	Structure	Function
Multipolar	Multiple dendrites, single axon	Most common; integrates and transmits signals
Bipolar	Single dendrite, single axon	Specialized for sensory pathways (e.g., retina)
Unipolar	Single process splits into input and output zones	Common in sensory neurons

Functional Classification

• Sensory neurons: Carry info from peripheral tissue to brain.

• Interneurons: Complex dendrites, short axons; convey info within brain networks.

• Motor neurons: Long axons connect brain/spinal cord to muscles; control movement.

Axons & Myelin

Structure and Function

• Axon: Thin fiber transmitting impulses; varies in length, diameter, and myelination.

• Myelin: Fatty protein made by glial cells; insulates axons, increases speed and energy efficiency of action potentials.

• Nodes of Ranvier: Gaps in myelin; allow action potential to regenerate.

Conduction Velocity

• Determined by axon width and degree of myelination.

• Wider axon + more myelin = faster conduction.

Axonal Transport

Mechanisms

• Kinesin: Motor protein for anterograde transport (soma to terminal).

• Dynein: Motor protein for retrograde transport (terminal to soma).

• Transport occurs along microtubules within axons.

Axon Terminals and Synapses

Communication Points

• Axon terminals: End points where chemicals are released to communicate with other neurons.

• Axons may branch (collaterals) to innervate multiple cells.

• Synapse: Junction between presynaptic and postsynaptic cells.

Dendrites

• Dendritic spines: Increase surface area; facilitate neural plasticity.

• Surface lined with receptors for neurotransmitter binding.

Synaptic Structure and Function

Key Components

• Synaptic cleft: 20-40 nm gap between pre- and post-synaptic membranes.

• Synaptic vesicles: Contain neurotransmitters; fuse with membrane to release contents.

• Receptors: Proteins on postsynaptic membrane; initiate cellular responses.

Neuronal Communication Summary

• Neurotransmitters released from axon terminals bind to receptors on dendrites/soma of other neurons.

• Changes in postsynaptic neuron may trigger an action potential at the axon hillock.

• Action potential travels down axon, causing further neurotransmitter release.

• Process repeats for signal propagation.

Glial Cells

Types and Functions

Type	Main Function
Astrocytes	Support, regulate synapses, blood flow, pruning
Microglial cells	Remove debris, remodel neurons, pain perception
Oligodendrocytes	Form myelin in CNS
Schwann cells	Form myelin in PNS

• Glial cells can contribute to disease (e.g., multiple sclerosis, tumors).

• Unlike neurons, glial cells can continue dividing.

Central and Peripheral Nervous Systems

Organization

• Central nervous system (CNS): Brain and spinal cord.

• Peripheral nervous system (PNS): All nervous tissue outside CNS.

Somatic Nervous System

• Connects brain with muscles and sensory systems.

• Includes cranial nerves (12 pairs) and spinal nerves (31 pairs).

• Mostly voluntary and conscious control.

Autonomic Nervous System

• Controls viscera (organs and glands); not under voluntary control.

• Three divisions: Sympathetic (fight or flight), Parasympathetic (rest and digest), Enteric (gut function).

Key Equations and Concepts

• Action Potential Propagation: $V_{m} = V_{rest} + ext{input signals}$ (Membrane potential changes based on input signals at dendrites and soma)

• Conduction Velocity: $v ext{ (velocity)} ightarrow ext{increases with axon diameter and myelination}$

Example Table: Neuron Types and Functions

Neuron Type	Structure	Function
Multipolar	Many dendrites, one axon	Integration and transmission
Bipolar	One dendrite, one axon	Sensory pathways
Unipolar	Single process splits	Sensory neurons

Additional info:

• Neural plasticity refers to the ability of dendritic spines and synapses to remodel in response to experience.

• Diseases such as multiple sclerosis involve demyelination, affecting conduction velocity and neural communication.