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Functional Neuroanatomy

Next week’s reading…
STRUCTURE AND FUNCTIONS OF CELLS OF THE NERVOUS SYSTEM
*CARLSON, N. R.
XRX.57772 SCOTT
SCOTT-RESV (Available: 2HOUR)

Overview
A word about the funny names, and a few definitions
Anatomy versus Function: The Triune Brain Hypothesis
The Brainstem
The Limbic System
Cerebral Cortex

A word about the funny names…
Early anatomists named most brain structures (in Latin, for the most part) according to their similarity to commonplace objects:
amygdala = almond,
hippocampus = sea horse,
genu = knee,
cortex = bark
pons = bridge

A few definitions
Locations in the Brain
Described relative to neuraxis - an imaginary line drawn through the spinal cord up to the front of the brain
The front end is anterior
The back end is posterior
The terms rostral (toward the head) and caudal (toward the tail) are also used when referring to parts of the brain

A few definitions
Dorsal (back) refers to the top of the head and the back
The ventral (front) surface faces the ground.
These directions are somewhat more complicated in humans because our neuraxis bends, so that the top of the head is now perpendicular to the back.

The Neuraxis Anterior Posterior Dorsal Ventral

The Neuraxis Anterior Posterior Dorsal Dorsal Ventral

A few definitions
Lateral means to the side (away from the neuraxis)
Medial (or mesial ) means toward the middle (towards the neuraxis)
Proximal : Areas of the brain that are near to one another
Distal : Areas that are far from one another

A few definitions
Ipsilateral refers to structures on the same side of the body
E.g. the olfactory bulbs send ipsilateral connections to the brain - the right bulb connects to the right hemisphere, and the left bulb connects to the left hemisphere
Contralateral refers to structures on opposite sides of the body
E.g. the left motor strip connects to the right side of the body

A few definitions
Slices of the brain
Transversely , like a loaf of bread - also called frontal or cross sections
Parallel to the ground, giving us horizontal sections
Perpendicular to the ground and parallel to the neuraxis - sagittal section - midsagittal plane divides the brain in half along the longitudinal fissure
Because of our upright posture, cross sections of our spinal cord are actually parallel to the ground

Transverse/ Cross section Horizontal section Sagittal section

Gross Anatomy of the Brain

Functional divisions based on evolutionary criteria
Triune Brain Hypothesis (MacLean)
“ In its evolution, the forebrain of advanced mammals has expanded as a triune structure that anatomically and chemically reflects ancestral commonalities with reptiles, early mammals, and late mammals.” (1985)
Mammalian brain structure reflects its phylogeny

The three brains…
1. Reptiles (R-complex) = Brainstem
Basic regulatory and vegetative functions, “instincts”
2. Early Mammals = Limbic Cortex
Limbus = border or margin (Latin)
Explicit memories, emotion
Care and protection of offspring
Establishing territory
3. Late Mammals = Neocortex
Abstract reasoning, long-term planning and behaviour, higher sensory functioning, etc.

Is the Triune Brain Hypothesis Useful?
Intuitive, “textbook” way of dividing the structure of the brain based on its evolutionary usefulness and function
Most textbooks dealing with brain function use the triune division
Accounts how the mammalian brain evolved and became specialized, beyond basic survival functions common to all animals
Accounts for disproportionately large telencephalon

2. Emphasizes a distributed “systems” approach
As opposed to a modular approach
Processing of information proceeds through increasing levels of complexity
Multiple brain areas accomplish complex tasks
Diversity/redundancy of systems (I.e. memory)
Explains why partial functioning or recovery is possible

3. Competition between different brain areas
Conflict between different phylogenic areas of the brain--situations where different functions come into conflict. (I.e. smoking)
Antagonistic or inhibitory areas exist in the brain.
Mental disease may reflect an imbalance : over- or underactivity of a brain area
Role of drug or treatment is to restore balance

The Brainstem
Consists of all structures from the thalamus to the spinal cord
Regulatory functions: Eating, drinking, body temperature, sleep and waking, basic movement and learning
Generally speaking, these structures rule functions that are hard-wired, automatic, and not very plastic

The Brainstem

The Brainstem
Thalamus : A relay centre for sensory information (touch, vision, hearing); located near the middle of the cerebral hemispheres.
Fibres project to primary sensory areas in neocortex
There are separate nuclei for vision, touch, hearing
Not a “passive” structure because the majority (80%) of its connections are not from sensory neurons, but from the neocortex (including motor areas).

The Brainstem
Hypothalamus : Controls all aspects of motivated (pleasure and pain) and regulatory behaviour
Autonomic (vegetative) system
Superior to the pituitary gland, reciprocal connections with it
“ Master gland”: Closely involved in the regulation and secretion of hormones
0.3% of the brain’s weight

The Brainstem
Reticular Formation : Constellation of 90+ nuclei at the base of the brainstem
Bundles of fibres as well as projections that pass through to the forebrain from the spinal cord
A host of regulatory vegetative functions
connections with cerebral cortex and thalamus

The Brainstem
Cerebellum : conspicuous bulbous structure protruding from the posterior brain; “little brain”
Distinctive narrow folds (folia), similar to sulci in neocortex
Involved in aspects of learning and coordination of skilled or smooth movement
Posture, walking, equilibrium

The Brainstem
By and large, there is little that distinguishes mammals from reptiles in terms of brainstem structure and function
An evolutionary turning point occurred with the specialization of limbic cortex in mammals
Functions of brainstem aren’t replaced; rather, they are modified or enhanced by interacting with newer phylogenic structures.

The Limbic Cortex
Three-layer cortical structure covering the periphery of the brainstem, on the ventral surface of the lateral ventricles
Primarily known for its role in emotion, (emotional) learning and memory
Also plays a role in spatial learning and olfaction (memories of odour)

The Limbic Cortex

The Limbic Cortex
Hippocampus : L ocated next to the lateral ventricle in the temporal lobe.
Along with the fornix, mammillary bodies, and cingulate gyrus is involved in learning and memory.
Amygdala : located anterior to the hippocampus
only part of the limbic system responsible for emotional responses
Other areas involved with learning and memory of emotions (recognition of emotional events)

The Limbic Cortex
Fornix : a bundle of axons that connects hippocampus with other regions of the brain, including the mammillary bodies (containing some of the hypothalamic nuclei)

Cerebral Cortex
Six-layered structure
Includes most of the two symmetrical cerebral hemispheres.
The cerebral hemispheres contain the limbic cortex
The majority of the surface of the cerebral hemispheres is called the neocortex
Part of the cerebral cortex is buried in the frontal lobes (I.e. insula – taste, sensation, and memory)

Cerebral Cortex
In humans the cerebral cortex is very convoluted
About two-thirds of the brain surface is found in the sulci and fissures
The total surface area of the cortex is approximately 2360cm 3 , 3 mm thick
Sulci - small grooves
Fissures - large grooves
Gyri - bulges between sulci or fissures
Greatly enlarge the surface area of brain
Provides additional neurons for higher cognitive functions

Cerebral Cortex
The cortex is made up mostly of glia (support cells), and the cell bodies, dendrites, and interconnecting axons of neurons.
Neuron cell bodies are grayish brown- that is why the cortex is called gray matter
Beneath the cerebral cortex run millions of axons ensheathed in myelin--white matter--that connects the neurons of the cerebral cortex with those located elsewhere

Cerebral Cortex
Significant individual differences in the sulci and gyri. However, there are major landmarks common to everyone:
The longitudinal fissure divides most of the cortex into left and right sides.
The central sulcus provides an important dividing line between the anterior and posterior regions of the cerebral cortex
Anterior : planning and executing movements
Posterior : sensation, perception and learning

Cerebral Cortex
Although the two cerebral hemispheres cooperate with each other, they do not perform identical functions.
Some functions are lateralized – located primarily on one side of the brain
The left hemisphere is involved in analysis of information – extraction of elements that make up a whole
For instance the left hemisphere is good at recognizing serial events.

Cerebral Cortex
The right hemisphere is specialized for synthesis – putting isolated elements together to perceive a whole.
During normal functioning of the brain we are not aware of lateralization
Our perceptions and memories are unified by the corpus callosum – a band of axons that connects the two cerebral hemispheres
Connects topographically – i.e. Areas of the brain that are the same region on both sides
The corpus callosum is the largest commissure of the brain – cross-hemispheric connection

Cerebral Cortex
Basal ganglia: A collection of subcortical nuclei in the forebrain.
 Lie beneath the anterior portion of the lateral ventricles.
 The major parts: caudate nucleus, putamen, and the globus pallidus.
 These structures are involved in the control of movement (frontal lobe).
 Parkinson’s disease results from degeneration of the connections between the midbrain and the caudate nucleus and putamen.

Basal Ganglia

Cerebral Cortex
Its divided into 4 areas, or lobes, named for the bones of the skull that cover them
Frontal lobes - includes everything in front of the central fissure
Parietal lobe - is located on the side of the cerebral hemisphere, just behind the central sulcus and caudal to the frontal lobe
Temporal lobe - lateral and ventral to the frontal and parietal lobe
Occipital lobe - back of the brain - caudal to the parietal and temporal lobes

Cerebral Cortex
Three areas of the cerebral cortex receive contralateral information from the sensory organs.
Primary visual cortex - back of the brain on the inner surfaces of the cerebral hemispheres - upper and lower parts of the calcarine fissure
Primary auditory cortex is located on the upper surface of a deep fissure in the side of the brain - lateral fissure
Primary somatosensory cortex - vertical strip of cortex caudal to the central sulcus - receives information from the body senses primarily touch, pressure, pain

Cerebral Cortex

Cerebral Cortex
Primary motor cortex - just anterior to the central sulcus and the primary somatosensory cortex
most directly involved in the control of movement
neurons in different parts of the motor cortex are connected to different muscles in the body (homunculus)
connections to muscles are contralateral

Cerebral Cortex
The primary parts of the cortex take up a small proportion of the cortex
The rest of the cortex is made up of association areas.

Cerebral Cortex
Each primary sensory area of the cerebral cortex sends information to the adjacent regions - sensory association cortex
Usually posterior
Circuits of the neurons in these areas analyze the information received from the primary sensory cortex
Perception takes place there and memories are stored there

Cerebral Cortex
The regions of the sensory association cortex located closest to the primary cortices receive information from only one sensory system
E.g. region closest to primary visual cortex is involved in visual perception and stores visual memories
Regions further away from the primary cortices integrate information multiple sensory systems - involved in several kinds of perception and memories
Combining vision and hearing to recognize a face or to understand language

Cerebral Cortex
The frontal association cortex is involved in the planning and execution of movements
The motor association cortex is located directly rostral to the primary motor cortex
This region controls the primary motor cortex - direct control over behaviour
The rest of the frontal lobe, rostral to the motor association cortex is known as the prefrontal cortex - formulation of plans and strategies

Summary
Divisions of the brain based on structure and function may be different.
Normal brain functioning requires the interaction of many (inhibitory) systems.
Normal brain functioning requires a balance between all “three” brains.
If not, there is disease and dysfunction.

fornix

Basal ganglia

The Limbic Cortex Hippocampus

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by dominic54 on Oct 28, 2010

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