Upload
phungduong
View
217
Download
3
Embed Size (px)
Citation preview
Neurons
“The mysterious butterflies of the soul, the beating of whose wings may some day – who knows? clarify the secret of mental life.”
S. Ramon y Cajal
Neurons • Functional units of
communication • About 1011 (100 billion)
cells • Independent units (Neuron
Doctrine) • Bioelectrically driven
(Functional polarity) • Categorized in terms of
Function (sensory, motor); Location (cortical, spinal); NT (cholinergic);
Shape (pyramidal, stellate)
Variety of Multipolar Neurons Differ in genes expressed, chemicals, shape, arborization, connectivity patterns…
Structure function
104 connections per neuron
1015 total interconnections
INPUT INTEGRATION
OUTPUT
Cajal formulated Law of Dynamic Polarization Dendrites generally receive synaptic input (i.e. are postsynaptic) and axons generally send synaptic output (i.e., are presynaptic)
Dynamic polarization (processes of input, integration, output) may be considered “computation.”
However, DP is NOT independent of the neuroanatomy and can occur in both directions.
Spines Dendrites may be spiny or aspiny
30,000 – 40,000 spines on the largest pyramidal neurons Increase receptive surface They receive most of the excitatory input Undergo morphological changes; regulate synaptic transmission
Cytoskeleton
• Formed by 3 types of proteins – Microtubules (tubulins) – Microfilaments (actins) – Intermediate filaments
• Microtubules (> 10% of total brain protein): intracellular transport, determine cell morphology
Protein conformation governed by quantum effects – Roger Penrose and Stuart Hameroff
Quantum states believed to be too sensitive and fragile to disruption by thermal energy (environmental decoherence) to affect the macroscopic nature of proteins and other macromolecular structures.
Microtubule lattice – computational quantum machine? (Orchestrated Objective Reduction) noncomputability
Myelination
• Insulates axon • Speeds up conduction
without increasing diameter of axon
• Saves energy
Organization of pyramidal cells in cortex
Layers of distinct cells; radially organized. Layers allow for separation of inputs and outputs. Unique apical dendrite allows for distribution of information to/from other layers.
Neuroglial Cells • Physical and metabolic support • 90% of cells in brain • Four types in CNS
– Astrocytes (maintenance/support) – Oligodendrocytes (myelin)* – Microglia (macrophages) – Ependymal (line ventricles)
*Schwann cell is the major glial cell in PNS
Glial Functions • Astrocytes
• Constitute 20-50% of the volume in most brain areas • Originate from radial glial cells – migration/guidance • Source for CAMs (N-CAM, laminin, fibronectin), growth
factors, and cytokines (signaling proteins involved in immune function)
• Regulate neurotransmitter uptake/inactivation (contain ion channels)
• Detoxification of CNS • Astrogliosis – response to infection/disorders
• Microglia • 5-20% of total cells in the mouse brain • Mediators of immune response (macrophages) • Secrete cytokines and growth factors • Constantly moving and analyzing the CNS for damaged
neurons, plaques, and infectious agents
Principles of Brain Organization • Subdivided into 3 primary
and 5 secondary regions + spinal cord – Forebrain
• Telencephalon (hemispheres) • Diencephalon (thal/hypothal)
– Midbrain • Mesencephalon (sup.colliculi)
– Hindbrain • Metencephalon (cerebellum) • Myelencephalon (medulla)
Neural plate > neural tube
Choroid Plexus • Each ventricle contains
choroid plexus (ependymal cells)
• 60-80% of CSF comes from CP; rest from extrachoroidal sources
• Total CSF=130-150 ml • Rate = 20 ml/hr
Principles (cont.)
• Brain is composed of two hemispheres connected by a set of fibers (corpus callosum)
• 200-250 million fibers • Monotremes and
marsupials do not have a corpus callosum
• Agenesis and split brain patients
Principles (cont.)
• Cortex is subdivided into four major external lobes (plus the internal limbic lobe)
Limbic lobe
Found only in the medial view of the brain. Its major responsibilities include olfaction and emotions
Principles (cont.) • Some areas of cortex (primarily sensory and motor areas) are topographically mapped.
V1 (retinotopic) A1 (tonotopic) M1 (mototopic) S1 (somatotopic)
Principles of Neuronal Circuitry • Long hierarchical neuronal connections (macrocircuits)
– Sensory/motor pathways – Point-to-point connections – Long axons (myelinated)
• Local circuit (microcircuits) – Interneurons (unmyelinated) – Short axons
• Single source divergent – Modulatory systems – Global reach (unmyelinated)