The meninges are three connective tissue layers that surround the brain:
The pia and arachnoid together are called the leptomeninges. The dura is called the pachymeninx.
The pia mater is tightly attached to the brain and follows the gyri, grooves, and sulci. Beneath the arachnoid mater is the subarachnoid space, which contains cerebrospinal fluid (CSF).
Meningeal nociceptors are located in the dura mater. These pain-sensitive fibers detect pain in headaches and in meningeal irritation.
Cerebrospinal fluid (CSF) is produced mainly by the choroid plexuses and partly by ependymal cells. It circulates through the ventricles and the subarachnoid space.
About 150 mL of CSF is present in the ventricles at any time. It is continuously generated and reabsorbed. Reabsorption occurs at the arachnoid granulations.
The cells of the choroid plexus are joined by tight junctions, forming the blood-CSF barrier. CSF is essentially an ultrafiltrate of plasma:
CSF functions include:
The brain has 4 ventricles:
The lateral ventricles communicate with the third ventricle through the interventricular foramina. The third ventricle is located in the diencephalon.
The cerebral aqueduct of Sylvius is located in the midbrain and connects the third ventricle to the fourth ventricle.
The fourth ventricle is diamond-shaped. It is located posterior to the pons and upper medulla and anterior to the cerebellum. It connects to the subarachnoid space via:
CSF flow follows this path:
The basal nuclei (basal ganglia) include:
The putamen and caudate nucleus together are called the striatum (or neostriatum). The putamen and globus pallidus together form the lentiform (or lenticular) nuclei.
The main function of the basal nuclei is motor control, with additional effects on cognition and eye movements.
The caudate nucleus is a C-shaped structure closely associated with the lateral wall of the lateral ventricle. The anterior limb of the internal capsule separates it from the putamen.
The globus pallidus is divided into:
The subthalamic nucleus is located under the thalamus.
The substantia nigra is a midbrain structure with two distinct parts:
The pars compacta is the source of a clinically important dopaminergic pathway to the striatum. Loss of neurons in this area causes Parkinson’s disease. A functionally analogous area is the ventral tegmental area, located nearby, which makes a dopaminergic projection to the nucleus accumbens. The pars reticulata contains GABA neurons.
The basal nuclei receive excitatory afferents from the cerebral cortex and thalamus:
The limbic system includes:
It is involved in memory and emotions.
The hippocampus is also called Ammon’s horn because it is shaped like two horns of a ram. It is located in the medial temporal lobe and is important for:
Long-term potentiation is vital for memory formation in the hippocampus. The hippocampus is one of the earliest structures involved in Alzheimer’s disease. Hippocampal atrophy is seen in schizophrenia, PTSD, depression, and chronic stress. Pyramidal cells of the hippocampus have been implicated in temporal lobe epilepsy.
The amygdala is an almond-shaped group of neurons located deep in the medial temporal lobe. It is involved in motivation, mediating the emotional aspects of memory, and processing emotions such as fear.
The mammillary bodies are involved in spatial and episodic memory consolidation and storage (as part of the Papez circuit), recollective memory, emotion and reward behaviors, and goal-directed behaviors. Damage to the mammillary bodies from alcoholism and thiamine deficiency causes Wernicke- Korsakoff syndrome, presenting with anterograde amnesia and confabulation.
The Papez circuit is the most important relay in the limbic system. It begins and ends at the hippocampus.
From the hippocampus, signals are relayed:
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