Brain and Spinal Cord: Planes and Matter

• Identify the planes of reference when presented with pictures of sections of the brain and spinal cord

Planes of Reference

• Sagittal plane divides the brain into left and right halves
• Midsagittal is a sagittal section right in the middle
• Horizontal (Axial) divides the brain into superior (top) and inferior (bottom) parts
• Coronal divides the brain into anterior (front) and posterior (back) parts
• Transverse (Cross-section) is often used in spinal cord imaging, cutting perpendicular to its long axis

Gray and White Matter Definitions:

• Gray matter contains neuron cell bodies, dendrites, and synapses
• Gray matter is found in the cerebral cortex, basal ganglia, and spinal cord dorsal/ventral horns
• White matter contains myelinated axons that conduct signals
• White matter is found in the inner brain, corpus callosum, and outer spinal cord
• Areas of the CNS made up of mainly of neuron cell bodies are gray matter
• White matter are areas of the CNS made of myelinated axons

Nucleus vs Ganglion

• Nucleus is a collection of neuron cell bodies in the CNS (e.g., thalamic nuclei)
• Ganglion is a collection of neuron cell bodies in the PNS (e.g, dorsal root ganglia)

Afferent vs Efferent Pathways

• Afferent pathways carry sensory information to the CNS (ascending)
• Efferent pathways carry motor signals from the CNS to muscle/glans (descending)
• Sensory pathways travel out/distal to proximal
• Motor pathways travel in/proximal to distal
• Afferent pathways allow admittance to the CNS
• Efferent pathways allow exit to the CNS

Lesions

• Lesions are defined as any damaged area in the nervous system (stroke, tumor, injury)
• Identifying a lesions location helps diagnose neurological disorders based on functional loss
• Lesions are an area of damage or dysfunction.

Focal, Multifocal, and Diffuse Lesions

• Focal lesions are a single, localized area (e.g., stroke)
• Multifocal lesions are multiple distinct areas (e.g., multiple sclerosis)
• Diffuse lesions are widespread damage (e.g., traumatic brain injury)

Incidence vs Prevalence

• Incidence is the number of new cases over a period of time
• Prevalence is the number of cases at a specific time

Speed of Onset and Pattern of Progression

• Acute speed of onset is sudden, such as in a stroke
• Subacute speed of onset develops over days/weeks, such as in an infection
• Chronic speed of onset is a slow progression, such as in Alzheimer's
• A stable pattern of progression means symptoms remain constant
• A progressive pattern of progression means symptoms worsen over time
• A relapsing-remitting pattern of progression includes episodes of worsening followed by partial/full recovery

Neuroimaging Techniques:

• Computed Tomography (CT) detects bleeding, fractures, and tumors quickly
• Magnetic Resonance Imaging (MRI) shows high-detail images of soft tissue (brain, spinal cord, tumors)
• Diffusion Tensor Imaging (DTI) maps white matter tracts and is useful in brain injury and stroke recovery
• Positron Emission Tomography (PET) measures metabolic activity and is used for detecting cancer and dementia
• Functional Magnetic Resonance Imaging (fMRI) measures brain activity based on blood flow and is used in research

Computed Tomography (CT) Details:

• CT uses X-rays
• CT is used for acute hemorrhage, abnormalities, fractures of bone, calcified lesions, and sinus disease
• CT scans take approximately 5 minutes
• CT involves radiation exposure

Magnetic Resonance Imaging (MRI) Details:

• MRI uses magnetic fields and radio waves to detect hydrogen ions
• MRI is used for stroke, tumors, infection, and multiple sclerosis
• MRI scans take 30-120 minutes
• MRI does not involve radiation exposure

Diffusion Tension Imaging (DTI) Details:

• DTI uses magnetic fields and radio waves to detect water diffusion along axons
• DTI is used for detailed images of white matter tracts and surgical planning
• DTI scans take 30 minutes
• DTI does not involve radiation exposure

Positron Emission Tomography (PET) Details:

• PET detects radioactive isotopes as they travel through blood
• PET is used to measure blood flow, glucose metabolism, and oxygen consumption
• PET scans take 30 minutes
• PET involves radiation exposure

Functional Magnetic Resonance Imaging (fMRI) Details:

• fMRI measures changes in oxygenated blood flow
• fMRI is used to detect neural activity in the brain by evaluating changes in blood flow
• fMRI scans take 60 minutes
• fMRI does not involve radiation exposure

Orientations of Axial, Coronal, and Sagittal Images

• Axial (horizontal) images are viewed top-down
• Coronal (Frontal) images are viewed front-to-back
• Sagittal images are viewed from the side

Neuron Components

• Cell body or soma contains the nucleus and processes information
• Axons transmit electrical impulses
• Dendrites receive signals from other neurons
• Pre-synaptic terminal releases neurotransmitters
• Pre-synaptic neuron is the neuron sending the signal
• Synaptic vesicles store neurotransmitters
• Post-synaptic neuron is the neuron receiving the signal
• Neurotransmitter receptors on the cell membrane bind neurotransmitters on the post-synaptic membrane

Axoplasmic Transport

• Anterograde transport moves materials from the cell body to the axon (e.g., neurotransmitters)
• Retrograde transport moves materials back to the cell body for recycling
• Axoplasmic transport refers to the mechanism of transporting neurotransmitters and other substances from the soma to the presynaptic terminal
• Anterograde transport moves from the soma toward the presynaptic terminal
• Retrograde transport moves from the synapse back to the soma

Neuron Types based on Extensions:

• Unipolar neurons have one extension (e.g., sensory neurons)
• Bipolar neurons have two extensions (e.g., retina neurons)
• Pseudounipolar neurons have one extension that splits into two (e.g., dorsal root ganglia)
• Multipolar neurons have many extensions (e.g., motor neurons)

Glial Cell Types and Functions:

• Astrocytes maintain the blood-brain barrier and support neurons
• When stimulated, astrocytes propagate CA2+ waves
• Astrocytes signal to neurons through glutamate release
• Astrocytes transport nutrients from capillaries to neurons
• Astrocytes contribute to blood brain barrier
• Fibrous astrocytes provide supporting framework, are electrical insulators and limit the spread of neurotransmitters
• Protoplasmic astrocytes store glycogen, have a phagocytic function, take place of dead neurons, produce trophic substances and support metabolism
• Microglia act as the immune system of the CNS
• Are phagocytic -- Are inactive in normal CNS -- proliferate in disease and phagocytosis, joined by blood monocytes
• Oligodendrocytes provide myelin for CNS neurons
• Provide myelination of CNS axons
• Influences biochemistry of neurons
• Ependyma produce cerebrospinal fluid (CSF)
• Cuboidal or columnar ciliated epithelial cells
• Supports the CSF and lines surfaces

Myelin's Influence on Nerve Conduction:

Function: insulates axons, speeds up signal transmission via saltatory conduction at the Nodes of Ranvier.

• Mostly lipid sheath around the axon

• Insulates nerve fibers to increase speed of nerve impulse

• Not a continuous structure

• Small gaps, Nodes of Ranvier approximately every 1-2 millimeter

• Nodes of Ranvier are small gaps in myelin where action potentials “jump”, increasing conduction speed.

Diseases that affect myelin

• Multiple sclerosis (MS): Autoimmune attack on CNS myelin
• Guillain-Barre Syndrome: Autoimmune attack on PNS myelin

Ion Channels

• Modality-gated ion channels open due to stimuli such as touch
• Voltage-gated ion channels open due to electrical changes
• Ligand-gated ion channels open when a neurotransmitter binds

Events of the Action Potential

• At rest, a typical neuron has a resting membrane potential of -70mV, meaning the inside of the neuron is more negative compared to the outside. This is maintained by the sodium-potassium pump (Na+/K+ pump) and leak channels.
• Sodium ions (Na+) are more concentrated outside the neuron, while potassium ions (K+) are more concentrated inside the neuron.
• Action potential occurs in the three main phases:
• Depolarization (Neuron “fires”) -- A stimulus causes voltage-gated Na+ channels to open -- Na+ rushes into the neuron, making the inside more positive -- When the membrane reaches around +30mV, Na+ channels close.
• Repolarization (Neuron “Resets”) -- Voltage-gated K+ channels open, allowing K+ to leave the neuron. -- This restores the negative charge inside the neuron -- The membrane potential drops back toward resting levels (-70mV)
• Hyperpolarization (Extra Reset): -- Sometimes, too much K+ leaves, making the membrane even more negative than -70mV -- This is called hyperpolarization
• The sodium-potassium pump (Na+/K+ pump) helps return the neuron to its resting state.
• Too much K+ leaves results in neuron getting extra negative

Refractory Periods

• The neuron needs time to “reset” before firing again.
  • Absolute Refractory Period (Neuron cannot fire): No action potential can start
  • Because Na+ channels are inactivated and need time to rest
  • Neuron is busy and can't fire again
  • Relative Refractory Period (Neuron can fire, but needs a stronger stimulus): A new action potential can happen, but only with a stronger-than-normal stimulus
  • Because the neuron is still slightly hyperpolarized (extra negative)
  • Neuron is “recovering” – can fire if the stimulus is extra strong

All or None Property

• An action potential either happens completely or not at all. If the stimulus is strong enough to reach the threshold (-55mV), the neuron will always fire A stronger stimulus does not make the action potential stronger; it only makes neurons fire more frequently

Speed of Conduction

• The speed at which an action potential travels depends on two key factors:

  • Axon Diameter (Wider = Faster)

  • Larger diameter = less resistance = faster conduction

  • Smaller diameter = more resistance = slower conduction – Thicker wires send signals faster

  • Myelination (Insulation = Faster Signals)

  • Myelin insulates the axon, preventing signal loss

  • The action potential “jumps” from one Node of Ranvier to another – This process is called saltatory conduction, which is much faster than unmyelinated conduction

  • Myelinated = Super fast

  • Unmyelinated = Slow

• Nodes of Ranvier = Boost stations

Definitions of Terms

• EPSP: Increases the chance of an action potential
• IPSP: Decreases the chance of an action potential
• Temporal summation: Multiple signals over time increase the effect
• Spatial summation: Multiple signals from different neurons combine.

Embryonic Development

• The embryonic cell layer that gives rise to the nervous system is called the ectoderm
• The first three regions/enlargements of the developing embryonic brain are the forebrain, midbrain, and hindbrain
• The five regions that emerge from the first three regions are the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon

Neural Tube Defects

• Spina Bifida: Incomplete spinal cord formation
• Anencephaly: Missing parts of the brain.

Neuroplasticity

• Neuroplasticity: The brain's ability to reorganize itself
• Examples: Learning a new skill strengthens brain connection
• The ability of neurons to change their function, chemical profile (amount and types of neurotransmitters produced), and/or structure.

Types of Neuroplasticity

• Habituation
• Experience-dependent plasticity : learning and memory
• Cellular recovery after injury

Two major types related to learning and memory

• Long-Term Potential (LTP) Strengthening Connections – Definition: When a neuron is repeatedly activated, the connection between it and the receiving neuron becomes stronger.

• Mechanism: – More neurotransmitters are released – More receptors are added to the post synaptic neuron – Synapses become more efficient

• Example: Learning a new skill, like playing an instrument or studying for an exam, strengthens synaptic connections

• Use it = Strengthens it / More Practice = Stronger Neural Pathways

Long-Term Depression (LTD) Weakening Connections

• When a neuron is NOT used frequently, the connection weakens over time.

• Mechanism:

• Less neurotransmitters release

• Fewer receptors on the post-synaptic neuron – The synapse weakens or is removed

• Example: Forgetting a language you haven't spoken in years

• Use it or lose it; Neurons that aren't active weaken over time

Axonal Injury in the Peripheral Nervous System (PNS)

• When damaged can regenerate, but first, goes through degeneration Degenerative Events (After injury)
• Axon Damage Occurs: The neuron's axon is cut or crushed
• Wallerian Degeneration: The distal part breaks down and is cleared by Schwann cells and macrophages.
• Chromatolysis: The cell body swells, and the periphery moves as the neuron starts to repair processes.

Regenerative Events (Repair & Regrowth)

• Schwann Cells Proliferate: They form a generation tube to guide axonal growth.
• Axon Sprouting: small axon branches find the correct path.
• Successful Regeneration: If the axon finds the Schwann cell pathway, it can reconnect and regain function.

CNS vs. PNS Recovery

• CNS recovery includes the brain and spinal cord and can be limited
• PNS recovery includes peripheral nerves and is possible with Schwann cell help -CNS includes no growth-promoting factors; inhibitory environment -PNS: Schwann cells promote regeneration
• CNS include scar tissue forms, blocking regrowth after injury
• PNS include axons that can regrow if guided correctly
• CNS Spinal cord injury can lead to permanent paralysis
• PNS includes cut nerve hand leading to possible full recovery

Excitotoxicity

• Excitotoxicity causes excessive stimulation from neurotransmitters.
• A process where neurons are damaged or killed by excessive stimulation from neurotransmitters, especially glutamate.
• Excessive Glutamate Release over excites neurons
• Too much Calcium (Ca2+) Enters the Neuron
• Neuronal Damage or Death include conditions like stroke, traumatic brain injury and neurodegenerative diseases.
• Stroke: Lack of oxygen causes excess glutamate release; Results in Brain Damage
• Alzheimer's & Parkinson's: Ongoing excitotoxic damage contributes to cell death
• Traumatic Brain Injury (TBI): Neurons are overstimulated after injury, worsening brain damage.
• Too much excitement leads to the neuron to burnout with excess glutamate and can cause brain cell death

Learn to identify planes of reference (sagittal, horizontal, coronal, transverse) in brain and spinal cord sections. Understand the definitions and locations of gray matter (neuron cell bodies) and white matter (myelinated axons) within the central nervous system.