Lecture 2: Neuroanatomy

The neuron

The neuron consists of dendrites, a soma (cell body) and an axon. Axons are long fibers connecting neurons to one another. An axon hillock is a place where an action potential should be at a certain threshold to cause some sort of signal. An action potential is a signal that travels along the axons, and it triggers an all-or-nothing response at the synapse. A synapse is a space between neurons. Action potentials lead to neurotransmitter release into the synaptic cleft. Both excitatory and inhibitory signals contribute to the summed signal at the axon hillock. Receptor cells in the postsynaptic membrane can adapt to under- or over-use of neurotransmitters. Some neurons can release more than one type of neurotransmitter, which depends on the type of stimulation (high or low).

Neurotransmitters: types

There are three main types of neurotransmitters. Small molecule neurotransmitters are neurotransmitters which act very quickly. Examples of these are acetylcholine, amines (dopamine, norepinephrine, adrenaline and serotonine), and amino acids (glutamate, GABA, glycine, histamine). Peptide transmitters are neurotransmitters which act very slowly. They consist of amino acid chains. Transmitter gases are neurotransmitters that very easily cross cell membranes and cause active metabolism. Examples of these are nitric oxide and carbon monoxide.

The effect of neurotransmitters can be excitatory, inhibitory or both. Each type of neurotransmitter pairs with multiple receptor types in the postsynaptic membrane.

Cortical cell layers

Different types of neurons are often organised in layers, which have differences in thickness, depending on the location in the brain. Sensory neurons (input), interneurons (relay) and motor neurons (output) are grouped in these layers. The layers are different in different cortical areas, depending on the primary function of the location. Brodmann’s functional map is a map which divides the brain in different areas based on cell structure (not on function!!).

Glia cells

There are different types of glia cells. Astrocytes are star shaped, symmetrical glia cells which provide structure and support, and have a contribution to the blood-brain barrier. Oligodendrytes are asymmetrical cells which produce myelin sheaths in the central nervous system. Myelin sheaths leave little, open spots so the action potential can jump from one gap (knots of Ranvier) to another as they speed up the process of the action potential. Microglial cells are small cells which fill up 10-15% of all cells in the brain. They fight infection and regulate waste disposal. Ependymal cells are small, ovoid cells, which cover the ventricular surface of the brain. They produce cerebrospinal fluid (CSF) in the brain. Schwann cells are not in the brain, nor spinal cord. They are part of the peripheral nervous system (all nerves outside the central nervous system) and produce myelin sheaths.

Major components of the CNS

The major components of the central nervous system are the forebrain (hemispheres, corpus callosum, telecephalon (deep cortical structure)), diencephalon (thalamus, hypothalamus, epithalamus), midbrain/mesencephalon (top of brainstem), hindbrain/metencephalon (cerebellum, pons), medulla oblongata and the spinal cord.

Grey matter are cell bodies. White matter are myelinated axons and connects neurons throughout both CNS and PNS. The majority of white matter consists of cortico-cortical connections (short fibers between adjacent areas and long fibers for more distant connections). Commisural fibers are fibers that cross from one hemisphere to the other. Homotopic is when they cross to the same place, heterotopic is when they cross to a different place. Projection fibers are fibers that connect to subcortical regions, the cerebellum or the spinal cord.

Afferent connections are connections coming to the brain and their main function is transporting sensory input. Efferent connections are connections coming from the brain and they transport motor output.

The brain consists of four lobes: the parietal lobe (memory, language, sensation), the occipital lobe (visual information), the frontal lobe (motor functions) and the temporal lobe (auditory information). The cerebellum has the main function of coordination and the brainstem regulates life functions like breathing.

The wrinkled surface of the cortex consists of gyri (bumps) and sulci (grooves, some are also called fissures). These gyri and sulci cause the brain to have a bigger surface, therefore can transport and process more information.

The ventricles are open spaces which contain cerebrospinal fluid, which circulates around. It gives protection to the brain.

Specific train aspects can lead to differences in the brain. An example: string players have one hand with highly developed motor skills, pianists have to hands with highly developed motor skills. Long term training can lead to brain change, which depends on aspects of training and on the sensitive developmental periods.

Subcortical structures

The basal ganglia are mainly for motor functions and consists of the caudate nucleus (spatial processing, specific moving, targeted actions), the putamen (regulation of movement), the globus pallidus (voluntarily movement), the substantia nigra (reward system) and the subthalamic nucleus (function not quite known).

The thalamus is the central station of the brain. All information goes through the thalamus, which determines where the information must go for further processing. It is a collection of smaller nuclei.

The limbic system’s main function is emotional regulation. It consists of the cingulate (processing of pain and emotions, learning), the hippocampus (memory), the hypothalamus (homeostasis) and the amygdala (emotions).

Directional planes

Functional pathways

Functions are often implemented as pathways or circuits, since functions involve multiple brain regions. Knowledge of structural connectivity can be informative in determining which regions work together. Sometimes functional connectivity cannot be easily identified from neural structure. For more increasingly complex behaviour, more and more of the brain will be involved.

The visual pathway is mainly based in the occipital lobe consists of a dorsal stream and a ventral stream. The main function of the dorsal stream is spatial visual information processing. Damage leads to impaired perception of movement and spatial neglect. The main function of the ventral stream is object recognition. Therefore damage leads to visual agnosia.

The auditory pathway is based in the temporal lobe. The signal spreads various cortical and subcortical regions for further processing. Tonotopic organization means that high tones are processed more medially and low tones are processed more laterally. Reduced white matter tracts between auditory and motor regions can lead to tone deafness.

Somatosensory pathways are based in the parietal lobe and regulate touch, vibration and position. Vibration crosses over in the lower medulla and terminates in the somatosensory cortex. Pain and temperature cross over in the cervical spinal cord, and terminate in the brainstem and cortical regions. Damage leads to deficits in haptic sensitivity (touch). The location and level/type of impairment how bad the damage is.

The motor pathway is based in the frontal lobe. Movement planning originates in frontal premotor and supplementary motor areas. You need different kinds of information for your movement. Movement execution involves the cerebellum, basal ganglia and the primary motor areas. Signals pass through the internal capsule to the muscles, crossing over in the medulla. Damage can lead to paralysis.

Primary and secondary regions

Primary regions are regions that are crucial to receiving input or sending output. Secondary regions are crucial to organizing that information, and to interpretation, planning and imagination. Associative regions are regions that are crucial to integration of information from multiple streams.

Brain damage

Atrophy (shrinking of the brain, gyri grow less thick and sulci grow bigger) is normal when growing older. Brain damage can be caused by different aspects, like vascular problems, trauma, tumors, developmental disorders (including neurodegenerative disorders), toxicity (alcohol, drugs, exposure to radiation) and infections.

The vascular system is a dense vascular network of arteries and veins. Arteries bring oxygen in and veins return de-oxygenated blood to the lungs. The middle cerebral artery is the largest vessel branching off from the internal carotid artery and it is the most common cerebral occlusion site. An ischemic stroke is caused by lack of oxygen, which leads to infarction. An artery gets blocked by a clot. The location of the occlusion determines where in the brain there won’t be enough blood due to poor perfusion. An haemorrhage Is a leak in the vascular system. It can be caused by hypertension or weakness in the blood vessel. Transient ischemic attacks (TIA) sometimes go unnoticed. Small white matter lesions can accumulate.

Gliomas are tumors that start in the glia cells of the brain or spine. Meningioma are slow growing tumors in the membranous layers of the brain. Brain metastases occur when cancer cells spread from their original site to the brain.

Reserve hypothesis

The reserve hypothesis is about the individual differences in the relationship between observed brain pathology and presented behaviour/symptomology. Brain reserve are differences brain structure. Cognitive reserve are differences in brain function. Both could explain differences in susceptibility to functional impairment in the presence of pathology. Better neural health predicts smaller impairments after damage, which are potentially driven by exercise and good nutrition. Better brain function predicts smaller impairment after damage as well, which are potentially driven by education level, cognitively and socially engaging activities.