Cortical and subcortical brain structures (Biological Aspects 2)

1) Central Nervous System (CNS): Brain and spinal cord

2) Peripheral Nervous System (PNS): see below

Somatic nervous system

It conveys sensory information (from skin, muscles, bones, joints, eyes and ears) to and from the CNS

It sends motor stimuli from CNS to muscles (voluntary movement)

Autonomic nervous system

It regulates involuntary movement activity

It controls functions such heart rate, blood pressure, kidney function and breathing

It is composed by the sympathetic and the parasympathetic nervous systems 

HINDBRAIN consists of:

the MEDULLA contains important nuclei responsible for regulation of breathing and cardiac functions

the PONS contains many ascending and descending fibre tracts and fibres linked to cerebellum

the CEREBELLUM coordinates movements 

MIDBRAIN includes:

tectum (colliculi)

tegmentum (red nuclei and substantia nigra)

cerebral aqueduct 

The FOREBRAIN consists of:

Telencephalo, which contains:

Neocortex

Basal ganglia

Limbic system

Olfactory bulb

Diencephalo, which contains:

Thalamus
Hypothalamus 

Fissure: it extends deeply into the brain v Sulcus: it is shallower Gyrus

Hemispheres: Left and right

Lobes: Frontal, Parietal, Occipital, Temporal 

Neocortex
It consists of 6 layers of cells, numbered from I to VI starting from the surface.

Two main cell types:

Pyramidal cells: large multipolar neurons – large dendrites and long axon – projection neurons

Stellate (or star-shaped) cells: smaller soma – shorter axon – local information processing

•Size and density of cell bodies varies across layers

For example, in Layer III large pyramidal cells – efferent to other cerebral area; IV layer rich of stellate cells – it receives afferent information from thalamus;

•The thickness of each layer varies across different cortical areas, according to the local function

9 Sensory areas  thick layer IV Motor area  thick layer V 

Neocortex

The variation of thickness and cell composition of each of the six layers is known as CYTOARCHITECTONICS.
On this basis Korbinian Brodmann developed Brodmann’s map.

Brodmann map consists of 52 areas (BA). 

(See diagram and learn main areas)

There is some correspondence between areas defined by cytoarchitectonic method and their function. 

Associative fibres, which connect areas within the same hemisphere:

Arcuate fasciculus
Uncinate fasciculus
Superior and inferior longitudinal fasciculi Cinguale fasciculus
Superior and inferior occipital frontal fasciculus

Commissural fibres, which connect same areas in the two hemispheres:

Corpus callosum

Anterior commissure

Posterior commissure

Fibres of projections, which send (efferent) receive (afferent) information between cortex and other structures of the brain

(e.g. mainly thalamus through internal capsula) 

(learn diagram)

Primary Sensory and Motor areas mediate elementary sensory or motor functions

Visual area
Auditory area
Primary somatosensory area

Motor area
Olfactory area

Associative areas Further process of information 

A few important issues about cortical and subcortical areas: (Schmahmann & Pandya, 2008)

- Lesions in basal ganglia and thalamus mimic deficits resulting from cortical lesions

Due to disruption of the interaction of domain-specific subcortical nuclei with the sensorimotor, association cortical areas.

- However there are qualitative differences between the manifestations of lesions in functionally related area of cortical and subcortical areas

Example:
Lesion of primary motor cortex hemiparesis

Lesion of putamen  slowness in movements

Lesion of thalamus (VL)  ataxia

Lesion of cerebellum lack of motor coordination 

Subcortcial areas are topographically arranged

Segregated loops 
Multiple parallel loops between cortical and subcortical areas with no cross-modal communication within subcortical nuclei

Complex behaviour required interaction between different functional domains Cortex in particular associative areas 

Basal ganglia are a group of subcortical structures.

The main structure are:

Globus Pallidus

Putamen

Caudate Nucleus

and:

Subthalamic nucleus

Substantia nigra

Basal ganglia receive direct input from cortical areas and limbic system however
They DO NOT project DIRECTLY to the cortex; Cortical projections travel trough the thalamus

(see diagram)

The function of basal ganglia is quite complex. They are strongly involved in movement and motor control.

They do NOT initiate movements but they are involved in a loop of modulation with the cortex that is constantly active.

Therefore they are actively involved in monitoring the force to be applied to a task and gross postural adjustments

Main neurotransmitters of Basal ganglia are

GABA, Dopamina and Glutamate

Malfunction or lesion of the basal ganglia:

Parkinson Disease (rhytmical tremor at rest, bradykinesia, muscles rigidity, difficulty in stopping walking once started; difficulty in motor coordination  falls) 

Parkinson Disease

Reduction of dopamina is mainly due to death of dopaminergic neurons in the substantia nigra. This causes a lack of input (through nigrostriatal path) to putamen 

then internal palllidus more activated 

then thalamus more hinibeted  less activation towards the cortexwhich turns in less movement. 

Huntington’s Chorea (jerks, muscle tone poor, possible change in personality, dementia) 

Huntington’s Chorea

Degeneration of striatum

Reduction of GABA neurons and increment of external pallidus activity

which strongly inhibits subthalamic n.then less inhibition of the thalamus  more motor activity - involuntary movements) 

Ballismus or hemiballismus

(severe involuntary movements of limbs)  Lesion of the subthalamic nucleus

Why involuntary movements?

In both Huntington’s Chorea and Ballismus syndromes, the involuntary movements are caused by a abnormal discharges of upper motor neurons that are not adequately inhibited (modulated) by basal ganglia. 

Cerebral cortical areas (motor, association, or limbic cortex) project in a topographically arranged manner to basal ganglia.

• Sensorimotor and parietal cortices  dorsal and mid-sectors of the putamen

• Association areas in prefrontal, posterior parietal and superior temporal cortices  caudate

• Orbital and medial prefrontal cortices and cingulate gyrus  ventral nuclei of putamen and caudate 

Behavioural-cognitive syndromes (Schmahmann & Pandya, 2008)

Lesions of the:
A) rostral head of caudate  impairment of working memory, strategy, formation, flexibility, neglect if right caudate or aphasia if left caudate

B) ventral striatum  Behavioural disorders (e.g. disinhibition, irritability, obsessive compulsive disorder) 

The thalamus has a key role in transforming information to and from the cortex.

It is a large two-lobed structure.
The m***a intermedia is a sort of bridge that connects the two lobes.

The internal medullary lamina divides the medial from the lateral group of nuclei.
External medullary lamina forms the lateral boundary of the thalamus medial to the internal capsule, which contains afferent and efferent nerve fibres p***ing to and from the thalamus to the cerebral cortex 

Thalamus

Thalamic nuclei are divided into:

Medial, Lateral and Anterior nuclei
plus the intralaminar nuclei 

Specific nuclei:

Each specific nucleus receives connection from the region of the cerebral cortex upon which it projects (usually primary motor and sensory areas)

Associative nuclei:

Receive input from several areas and each nucleus projects to one of the three ***ociation cortex (P-T-O ***. c.; PreF c.; Limbic c.)

Non-specific nuclei:

Widespread connections also towards other thalamic nuclei and some of these are inhibitory (e.g. reticular formation) 

Thalamo-cortical projections run through the internal capsule. In the internal capsule there are also the fibers from cortex to

medulla and spinal cord.

The internal capsule continues more laterally and superiorly with the corona radiata, which contains descending axons of the corticospinal tract. 

The Hypothalamus is a complex structure containing many nuclei and tracts.

 It controls the autonomic nervous system and the endocrine system with direct link to the pituitary gland.

Functions:

- Much of the endocrine system is controlled by hormones produced by cells in the hypothalamus. These hormones (realising – e.g. gonadotropin- releasing hormone) “instruct” the pituitary gland to secrete the specific hormone (e.g. gonadotropic hormone)

- Descending tracts to brain stem to regulate autonomic centres related to cardiovascular and breathing functions

- Integrating emotions and autonomic responses

- Link to centre for “set points” relating to appetite, body temperature -

Biological clock and circadian rhythms

- Link to limbic system 

Limbic lobe:

parahipppocampal gyrus

cingulate gyrus

subcallosal gyrus (anterior and inferior continuation of the cingulate gyrus)

Hippocampal formation (hippocampus, dentate gyrus and subiculum)

the hippocampus 

Lymbic system

Amygdala has two important projections to hypothalamus and dorso-medial thalamic nuclei and it receives an important afferent input from the olfactory tract

Emotional processing (e.g. fear) and social behaviour

Bilateral damage of amygdala results in Klüver-Bucy syndrome 

How the brain looks like?

Computed tomography (CT).

Various attenuation intensity of x-rays according to type of tissue. This is computerised into numbers.
Positive numbers (+1,000) are represented white (e.g. bone), negative numbers (-1,000) are represented black (e.g. air)

The brain and related structures in CT

Structure/fluid/space - grey scale

Bone/acute blood - very white

Grey matter - light grey

White matter - medium grey

Cerebrospinal fluid - dark grey to black

Subacute blood - light grey

Structure/fluid/space               T1              T2

Bone                              - very black - very black

Air                                 - very black - very black

Grey matter                  - dark grey - light grey

White matter                - light grey - dark grey 

Cerebrospinal fluid      - very black - very white

Edema                         - Dark grey - light grey to white

Advantages of CT scan:

- Clearly shows acute and subacute hemorrhages into the meningeal spaces and brain

-Better in showing bone and bone fractures

-Rapid (important for emergency) and cheap 

-Advantages of MRI scan:

-Excellent spatial resolution. It better differentiates white and grey matter than CT Scan.

-It clearly shows acute or subacute infarcts or ischemia or brain edema

-No radiation exposition and, at the present, not known harmful effects 

A sagittal MRI of a 23-year-old woman is located at, or immediately adjacent to, the midline. Which of the following spaces or structures would be in the image and would indicate a midline plane?

1. a)  Cerebral aqueduct

2. b)  Corpuscallosum

3. c)  Interventricular fossa

4. d)  Superiorcolliculus