COGNITIVE SCIENCE 107A

Neuroanatomy: Basic Principles

Jaime A. Pineda, Ph.D.

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)

Dendrites

Cell body or perikaryon

Axon

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.

Cell Body

Nucleus (DNA) Golgi apparatus Endoplasmic reticulum Mitochondria Cytoskeleton

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

Synapses

Asymmetric: excitatory (Type1) Symmetric: inhibitory (Type2)

Pre- and post synapse

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

Oligodendrocytes/Schwann cells

Einstein’s Brain

Greater number of neuroglia

Larger inferior parietal cortex

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

Principles (cont.)

•  Brain contains an interconnected system of ventricles

Interventricular foramen

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

Functions of CSF

Brain contains a protective covering

Principles (cont.)

Hydrocephalus

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

Topography of CC

splenium genu

truncus

rostrum

isthmus

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.) •  Cortex is subdivided into areas of specialized function

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)