Behavioral Neuroscience Lecture 2

Neuroanatomy

Introduction to Neuroanatomy

• Focus on the relationship between neurons and the nervous system.

• Includes numerous diagrams for visual comprehension.

• Discussion on the historical debate from the late 1800s regarding the continuity of neurons.

  • Golgi's Hypothesis: Proposed that neurons are continuous with each other.

Historical Perspectives on Neurons

• **Ramon y Cajal's Argument: **

  • Opposed Golgi’s view, positing that neurons are discrete (i.e., separate) entities.

  • Concept led to the Neuron Doctrine: Neurons do not physically connect but are very close, establishing synapses as the gaps between them.

Structure of Neurons

• Examination of neuron structure and function.

Major Parts of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Input zone where neurons collect information from the environment or other cells. Includes:

  • Dendritic spines for synaptic inputs.

• Axon:

  • Conduction Zone: Can electrically transmit signals over long distances.

  • Axon Terminals: Output zone for neuron signal transfer.

Organelles in Neurons

• Nucleus: Contains chromosomes.

• Mitochondrion: Generates energy for the cell utilizing fuel and oxygen.

• Ribosomes: Sites for protein synthesis.

• Endoplasmic Reticulum: Transports synthesized molecules to various locations within the cell; some ribosomes may be attached.

• Golgi Apparatus: Modifies and packages proteins for transport.

Diversity Among Neurons

• Although a “standard” neuron is discussed, there is notable diversity across species and types. Example neurons include:

  • Pigeon: Tectum ganglion cell

  • Monkey: Small pyramidal neuron in cortex

  • Mouse: Globus pallidus neuron

  • Locust: Motor neuron

  • Tree Shrew: Retinal horizontal cells

  • Turtle: Brainstem neuron

  • Human: Retinal ganglion cell

  • Rat: Thalamic neuron

  • Zebrafish: Neuron in reticular formation

Synaptic Communication

• Process by which neurons communicate using synapses.

Synapse Structure

• Presynaptic Neuron: Transmits the signal.

• Postsynaptic Neuron: Receives the signal.

• Synaptic Cleft: The gap between the presynaptic and postsynaptic neurons where neurotransmitters are released and received.

Types of Neurons

• Multipolar Neurons: Contain multiple extensions from the cell body.

• Bipolar Neurons: Have two extensions.

• Unipolar Neurons: One process extending from the cell body.

Distinctions Between Axons and Dendrites

Transport Mechanisms in Neurons

• Discusses how materials made in the cell body reach the axon terminus:

  • Anterograde Transport: Movement from the cell body to the axon terminals.

  • Retrograde Transport: Movement from axon terminals back to the cell body.

  • Utilizes motor proteins that “walk” along microtubules in axons for vesicle transport.

Glial Cells in the Nervous System

• Comprise about 50% of brain cells and 90% of brain mass.

Four Main Types of Glial Cells:

• Astrocytes:

  • Absorb and return chemicals from neurons.

  • Remove waste post-neuron death.

  • Regulate blood flow and nutrients to neurons.

• Microglia: Small cells removing waste and pathogens.

• Oligodendrocytes: Build myelin sheaths for CNS axons.

• Schwann Cells: Build myelin sheaths for PNS axons.

Myelin and Neuronal Communication

• Myelin Sheath: Insulates axons and is formed by oligodendrocytes (CNS) and Schwann cells (PNS).

• Nodes of Ranvier: Gaps between myelin sheaths crucial for action potential propagation.

• Multiple Sclerosis: A demyelinating condition illustrating the significance of myelin.

Neuroanatomical Techniques

1. Nissl Stain: Highlights cell bodies.

2. Golgi Stain: Fills entire neurons, allowing isolation of single neurons.

3. Immunocytochemistry (ICC): Uses antibodies for specific protein visualization.

4. Autoradiography: Specifies localization of radioactively labeled molecules in brain tissue.

5. In Situ Hybridization: Identifies mRNA expression in cells.

6. Tracing Pathways: Uses anterograde or retrograde tracers for understanding neuron connections.

   • Anterograde Tracers: Trace from cell bodies to axon terminals (afferents).

   • Retrograde Tracers: Trace from axon terminals back to cell bodies (efferents).

Anatomical and Directional Terms

• Rostral (anterior): Toward the front.

• Caudal (posterior): Toward the back.

• Dorsal: Toward the top.

• Ventral: Toward the bottom.

• Distinctions between gray matter (cell bodies, dendrites) and white matter (myelinated axons).

• Definitions:

  • Ganglion: Collection of cell bodies outside the CNS.

  • Nucleus: Collection of cell bodies within the CNS.

  • Nerve: Axon bundle outside the CNS.

  • Tract: Axon bundle within the CNS.

Peripheral and Central Nervous Systems

Peripheral Nervous System (PNS)

• Divided into:

  • Somatic nervous system: Controls voluntary movements.

  • Autonomic nervous system: Regulates involuntary functions.

    • Sympathetic Division: Prepares body for action.

    • Parasympathetic Division: Conservative functions - rest and digest.

    • Enteric Nervous System: Manages gastrointestinal system.

Central Nervous System (CNS)

• Comprises the brain and spinal cord.

• Major components of the brain include:

  • Forebrain: Includes the cerebral hemispheres, thalamus, and hypothalamus.

  • Midbrain: Involved in vision and hearing processing.

  • Hindbrain: Controls basic life functions.

  • Cerebellum: Coordination and balance.

Cranial Nerves and Their Functions

• Essential cranial nerves to know:

  • I Olfactory: Smell (sensory).

  • II Optic: Vision (sensory).

  • VIII Vestibulocochlear: Hearing and balance (sensory).

  • IX Glossopharyngeal: Taste and throat sensation (mixed).

  • X Vagus: Internal organ functions (mixed).

Gross Anatomy of the Brain

• Structures visible to the naked eye crucial for exams, including:

  • Gyri: Raised areas (e.g., precentral gyrus).

  • Sulci: Indented areas (e.g., postcentral gyrus).

  • Key areas: Thalamus, hypothalamus, and various lobes (frontal, temporal).

Potential Exam Questions

• Questions highlighting the composition and function of brain structures, e.g., Diencephalon includes

  • (C) The hypothalamus and the thalamus.

• Identify the glial cells responsible for myelin sheathing in the periphery:

  • (C) Schwann cells.

Introduction to Neuroanatomy

• Focus: Investigating the relationship between individual neurons and the architectural organization of the human nervous system.

• Methodology: Reliance on comparative biology and detailed histological diagrams to visualize complex neural circuits.

• The Reticular Theory vs. The Neuron Doctrine: A pivotal historical debate in the late 1800s regarding how the nervous system is built.

  • Camillo Golgi's Hypothesis: Proposed that the nervous system is a continuous network (syncytium) where neurons are physically fused together.

  • Santiago Ramón y Cajal's Discovery: Using the Golgi stain, Cajal demonstrated that neurons are individual, discrete cells that do not touch, foundational to the Neuron Doctrine.

Detailed Structure of Neurons

• Cell Body (Soma): The metabolic hub containing the nucleus, where major protein synthesis occurs.

• Dendrites (Input Zone): Specialized for receiving information.

  • Dendritic Spines: Small protrusions on dendrites that increase surface area for synapses; their plasticity is linked to learning and memory.

• Axon Hillock (Integration Zone): A specialized part of the cell body (or soma) that connects to the axon. It serves as the site where summation of incoming signals occurs to determine if an action potential will be triggered.

• Axon (Conduction Zone): A singular fiber that propagates electrical impulses. Axons can be extremely long, such as those traveling from the spinal cord to the foot.

• Axon Terminals (Output Zone): Specialized structures at the end of the axon that release neurotransmitters to signal the next cell.

Cellular Organelles and Proteomics

• Nucleus: Contains genomic DNA and is the site of transcription (DNA to mRNA).

• Mitochondrion: Produces energy in the form of $ATP$ ($Adenosine \space Triphosphate$) through cellular respiration.

• Rough Endoplasmic Reticulum (RER): Dotted with ribosomes, it is the primary site for the synthesis of membrane-bound and secreted proteins.

• Golgi Apparatus: Functions in the modification (e.g., glycosylation), sorting, and packaging of proteins into vesicles for transport to the synapse.

• Lysosomes: Contain enzymes for breaking down waste products and worn-out organelles.

Neuronal Diversity and Classification

• Morphological Types:

  • Multipolar Neurons: One axon and many dendritic trees (most motor neurons and interneurons).

  • Bipolar Neurons: One axon and one dendritic tree (found in specialized sensory systems like the retina).

  • Unipolar (Pseudounipolar) Neurons: A single process that divides into two branches; typically found in the somatosensory system.

• Inter-species Variation: Neuronal morphology varies significantly to accommodate specific ecological niches, such as the Tectum ganglion cell in pigeons or the Small pyramidal neuron in the monkey cortex.

Mechanisms of Synaptic Communication

• Synapse Components:

  • Presynaptic Membrane: The surface of the axon terminal where neurotransmitters are released.

  • Synaptic Cleft: A gap of approximately $20-40 \space nm$ where chemical diffusion occurs.

  • Postsynaptic Membrane: The surface of the receiving cell (usually a dendrite) containing specialized neurotransmitter receptors.

• Vesicle Cycling: Transmitters are stored in vesicles and released via exocytosis when an action potential arrives.

Axonal Transport Systems

• Microtubules: Provide the structural "tracks" for transporting materials between the soma and the terminal.

• Anterograde Transport: Movement from the soma to the terminal, powered by the motor protein Kinesin.

• Retrograde Transport: Movement from the terminal back to the soma (for recycling or carrying growth factors), powered by the motor protein Dynein.

Glial Cells and The Support Environment

• Astrocytes: Provide structural support, regulate the blood-brain barrier (BBB), and maintain the chemical environment (ion balance) for neurons.

• Microglia: The resident immune cells of the brain; they act as phagocytes to clear debris and pathogens.

• Myelinating Glia:

  • Oligodendrocytes: Responsible for myelination in the Central Nervous System (CNS); one cell can myelinate multiple axons.

  • Schwann Cells: Responsible for myelination in the Peripheral Nervous System (PNS); one cell wraps around a single axon segment.

Macro-Anatomy and Directional Terms

• Standard Planes:

  • Sagittal: Divides the brain into left and right hemispheres.

  • Coronal (Frontal): Divides the brain into front and back sections.

  • Horizontal (Axial): Divides the brain into top and bottom sections.

• Terminology:

  • Gray Matter: Dense with cell bodies and dendrites.

  • White Matter: High concentration of myelinated axons.

  • Nucleus vs. Ganglion: Both refer to clusters of cell bodies, inside (Nucleus) or outside (Ganglion) the CNS.

Functional Anatomy of the Brain Lobes

• Frontal Lobe: Responsible for higher-order cognitive functions, decision making, and voluntary motor control (Precentral Gyrus).

• Parietal Lobe: Processes somatosensory information like touch and pain (Postcentral Gyrus).

• Temporal Lobe: Essential for auditory processing, language comprehension, and memory formation.

• Occipital Lobe: Dedicated primarily to visual processing.

Nervous System Hierarchies

• Central Nervous System (CNS): Brain and spinal cord, protected by the three Meninges (Dura Mater, Arachnoid, and Pia Mater).

• Peripheral Nervous System (PNS):

  • Somatic: Relays sensory and motor info between the CNS and skin/muscles.

  • Autonomic: Manages involuntary internal environments.

    • Sympathetic: Mobilizes energy (fight or flight).

    • Parasympathetic: Conserves energy (rest and digest).