Cortex | Midbrain | Pons | Medulla | Cerebellum | Spinal Cord | Neurons | Peripheral Nervous System

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Cortex

The basic functional unit of the CNS is the neuron. Neurons are arranged in functional arrays called nuclei. Many of these nuclei have a distinctive histological appearance which is directly related to their function.

The Neocortex, often shortened to cortex, is that part of the nervous system that makes us distinctively human. Its cellular components are neurons, astocytes, oligodendroglia and microglia. The majority of the neocortex has a very similar appearance but some areas which are devoted to specific functions such as vision or movement have a characteristic appearance.

• The Motor cortex is located in the precentral gyrus also called Brodman areas 1,2 and 3. It contains the large pyramidal upper motor neurons of the corticospinal tract. Link to Motor
  
  
• The Visual Cortex, Brodman area 17, is located in the occipital lobe surrounding the calcarine fissure. Link to Visual

The white matter represents dense collections of myelinated axons. Myelin is a protein with a high lipid content that serves to insulate the axon. This insulation facilitates transmission of electrical impulses down the axon to the synapse.

Hippocampal Formation Link to Olfactory

• The hippocampal formation is a functional unit but for descriptive purposes can be divided into three parts. The Amygdala, the rostral hippocampus and the hippocampus proper.
  
  
• The Amygdala is actually a group of nuclei. The Entorhinal cortex covers the amygdala. The gross external structure is known as the uncus and it sits immediately superior to the tentorium. The rostral hippocampus is a convoluted structure immediately adjacent, inferior, medial and caudal to the amygdala. The structure is frequently resected in surgical interventions for the treatment of temporal lobe epilepsy.
  
  
• Later expansion for neurosurgical site
  
  
• The Hippocampus or seahorse is a distinctive structure. It is composed of two interlocking "C" shaped structures. One consists of large pyramidal neurons and merges into the subiculum and entorhinal cortex. The other is composed of small densely packed neurons which project to the large pyramidal neurons of Ammon's horn. This structure is called the fascia dentata and its neurons are called granule cells.
  
  
• The large pyramidal neurons make up the Cornu Ammonis. This is divided into four zones which have different functions and different susceptibilty to pathological processes. Zone CA4 is in the center. CA3 rounds the edge of the fascia dentata. CA2 is adjacent to the temporal horn of the lateral ventricle. The neurons here are more compactly arranged. CA1 is the largest and merges with the subiculum which in turn merges with the entorhinal cortex. CA1 is also known as the hypoxia sensitive zone. Neurons in this area are exquisitely sensitive to hypoxia and hypoperfusion injury. They are also damaged in mesial temporal lobe epilepsy and are one of the first group of neurons affected by Alzheimer neurofibrillary change.
  
  

Basal ganglia Link to extrapyramidal
Basal ganglia neurohistology animation 1
Basal ganglia neurohistology animation 2

• Neurons outside the neocortex are arranged in functional arrays known as subcortical nuclei. These subcortical nuclei are phylogenetically much older than neocortex, so that there are humbling similarities between these structures in animals such as cats and mice, and humans.
  
  
• The major subcortical nuclei are the basal ganglia which consists of caudate, putamen and globus pallidus. Each of these nuclei has distinctive features.
  
  

Thalamus Link to extrapyramidal and General Sensation
Coronal thalamus animation

• The thalamus is a subcortical structure but due to its complexity and functional importance, is generally not considered one of the subcortical nuclei. It is the major component of the diencepahalon. The thalamus, also known as the massa intermedia, is actually a complex of many nuclei. The dorsal medial nucleus plays an important role in memory and learning. The ventral posterior medial modulates many sensory functions. The subthalamic nucleus is an important part of the extrapyramidal motor system and when injured results in ballismus.

Geniculate Bodies Link to Visual and Auditory

• The Lateral and Medial Geniculate bodies are major relay nuclei for the visual and auditory systems, respectively. Some authors consider these structures as part of the thalamus since there are many functional connections with thalamic nuclei. Their distinctive appearance and anatomical location causes us to consider them separately from the thalamus.
  
  
• The lateral geniculate nucleus cups the temporal horn of the lateral ventricle at the level of the mid-hippocampus. The lateral geniculate has a distinctive laminated appearance which correlates geographically with retinal neuronal projections corresponding to visual fields. The medial geniculate nucleus is less well defined and slightly smaller but is located medial to the lateral geniculate.

Hypothalamus Link to autonomic

• The hypothalamus is below the thalamus. This is a tiny structure with tremendous functional importance. Nuclei located here control all basic biological functions such as hunger, sex drive and basal metabolic rate.

Epithalamus

• The pineal is one of two glands in the brain. It produces neuromelanin and modulates circadian rhytmns. It is located right in the middle of the bran. DeCartes called it the seat of the soul.
  
  
• The Habenula is immediately anterior to the pineal.

• The Midbrain is a small but anatomically and functionally very important stucture which is literally the "middle" of the brain.
  
  
• Tectum Link to Cranial nerves
  • The tectum is the upper or superior part of the midbrain surrounding the aqueduct of Sylvius. The tectum contains the superior and inferior colliculi which modulate visual and auditory reflexes and the occulomotor nucleus. In simplistic terms, the tectum helps focus your attention on the source of a loud noise or flash of light.
    
    
• Tegmentum Link to Motor systems
  • The tegmentum represents the lower part of the midbrain and is involved principally in motor function. The descending corticospinal and corticobulbar tracts (pyramidal tract) as well as ascending sensory tracts are contained in the cerebral peduncle. The substantia nigra and red nucleus are major components of the extrapyramidal motor system.

• Tectum Link to Cranial nerves
  
  • The tectum is the upper or superior part of the pons. It contains cranial nerve nuclei V, VI, VII.
    
    
• Tegmentum Link to Motor systems
  • The tegmentum represents the lower or inferior part of the pons and is involved principally in motor function. The descending corticospinal and corticobulbar tracts (pyramidal tract), ascending sensory tracts cerebellar crossing fibers and pontine nuclei are located here.
    
    

• The medulla forms the floor of the fourth ventricle and contains cranial nerve nuclei VIII, IX, X, XI and XII. The inferior medulla contains the pyramidal tract and the inferior olivary nucleus.
  

• The cerebellar cortex is a quite distinctive structure composed of folia rather than gyri. It consists of an outer molecular layer, a Purkinje neuron layer, granular neuron layer and the white matter.
  
  
• There are four nuclei, dentate, embolliform, fastigial and globose. An easy mnemonic, DEFG.
  

• Peripheral nerves are myelinated by Schwann cells. They carry axons of the Motor system to the skeletal muscle Link to pyramidal system. They also carry sensory input from the limbs and trunk to the Dorsal root ganglion. Link to general sensation.
  
  
• Finally they carry fibers of the sympathetic system to the sympathetic ganglia. Link to sympathetic. The Enteric ganglion located in the gut modulates basal metabolism and digestion.Link to parasympathetic

last update Jan 21, 2002