Introduction to Neurobiology Lecture 2: “Structure of the Nervous System-Basic concept in neuroanatomy-”

Is that any better?

The central nervous system

White and grey matter

Grey matter - neurons

Grey matter - glia AstrocytesBergmann glia

Neurons and glia

White matter - Myelin

Laminar organization – cerebellar cortex

Laminar organization – cerebral cortex

Nuclear organization – Thalamus and basal ganglia

The central nervous system

The ventricular system

The autonomic nervous system

The cerebral hemisphere

The Thalamus

The basal ganglia

The limbic system

The basal ganglia and limbic system

The central nervous system

The spinal cord

Terminology: Fissure Sulcus (pl. Sulci) Gyrus (pl. Gyri) Laminar organization (pl. Cortices) Nuclear organization (Nucleus pl. Nuclei) Afferent pathway Efferent pathway White matter Gray matter

The basal gangalia Anatomy: The basal ganglia form a set of interconnected nuclei in the forebrain. Overall the basal ganglia receive a large amount of input from cerebral cortex, and after processing, send it back to cerebral cortex via thalamus. This major pathway led to the creation of the popular concept of cortico-basal ganglia-cortical loops. Inside the basal ganglia there are too many connections and pathways to cover in this paragraph. Just briefly: The cortex sends excitatory input to the striatum. The principle neuron of the striatum is the famous medium spiny neuron, which sends its inhibitory output on to the globus pallidus. The globus pallidus can also be excited by cortical activity, namely by a pathway that travels through the subthalamic nucleus first. The globus pallidus is really divided into two segments, only one of which sends output (yet again inhibitory!) to the thalamus and on to cortex, thus completing the loop. The larger segment of globus pallidus (GPe) just inhibits the subthalamic nucleus and itself. The functional significance of this connection is still quite mysterious! Similar to the cerebellum the basal ganglia are also implicated in learning, and the system that is thought to be important here is the dopaminergic input received from the Substantia nigra pars compacta. Probably the best known fact regarding the basal ganglia is that a lesion of this dopaminergic pathway causes Parkinson’s disease. Physiology: Numerous research projects have recorded electrical activity in the basal ganglia. Unfortunately for the experimentalists seeking clear answers, the recorded activity in behaving animals can be related just about to any component of sensory input, motor preparation, and movement execution. One thing is sure however: The medium spiny neurons are active only at a very slow rate, and furthermore the connection to the GP takes more time than most pathways in the brain. In contrast to cerebellum this system seems unsuitable for the fast feedback control of ongoing movement. Neurons in GP in contrast are active at a very high rate. This could be very useful, if both decreases and increases in activity need to be communicated accurately to the thalamus. Since GP neurons are inhibitory in thalamus, a decrease in activity actually would disinhibit the thalamus, and thus activate cortex. Single cell properties of various cell types in the basal ganglia are also quite unique and interesting, and intracellular recordings in brain slices and anesthetized animals have showed how specific features of single neuron properties could be important in the ongoing function of the basal ganglia. Function: As is true for the cerebellum, the ultimate answers about the exact function of the basal ganglia in the control of behavior have yet to be established. One very good candidate is called “Action Selection Hypothesis”. In this model the basal ganglia would be the arbiter of which of the potential actions that cortex might be planning actually gets executed. This fits together well with the idea that dopamine is a system mediating learning based on reward. This could train the basal ganglia to chose behaviors that have been rewarding in the past. The overall lack of action found in Parkinsons’ disease is also easily reconciled with the idea of action selection. The other major symptom, namely movement tremor, however, is not. The presence of movement tremor and other specific motor problems, have led some people to believe that the basal ganglia may play a role in the planning and coordination of specific movement sequences. Thus, the temporal sequencing of movements is another intriguing function of the basal ganglia.

The limbic system Essentially the limbic system is the set of brain structures that forms the inner border of the cortex. In an abstract topological sense, each cortical hemisphere can be thought of as a sphere of gray matter, with a hole punched through it in the area where nerve fibers connect it to the subcortical structures of the basal forebrain. The hole is surrounded by a ring of cortical and noncortical areas that combine to make up the limbic system. The cortical components generally have fewer layers than the classical 6-layered neocortex and are often classified as archicortex. The limbic system includes many structures in the cerebral cortex and subcortex of the brain. The term has been used within psychiatry and neurology, although its exact role and definition have been revised considerably since the term was introduced. The following structures are, or have been considered to be, part of the limbic system: Amigdala Involved in signalling the cortex of motivationally significant stimuli such as those related to reward and fear in addition to social functions such as mating. Hippocampus Required for the formation of long term memory and implicated in maintenance of cognitive maps for navigation. Parahippocampl gyrus Plays a role in the formation of spatial memory Cingulate gyrus Autonomic functions regulating heart rate, blood pressure and cognitive and attentional processing Fornix carries signals from the hippocampus to the mammillary bodies and septal nuclei.. Hypothalamus Regulates the autonomic nervous system via hormone production and release. Affects and regulatesblood pressure, heart rate, hunger, thirst, sexual arousal, and the sleep/wake cycle. Nucleus accombens: Involved in reward, pleasure, and addiction. Function The limbic system operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, the brain's pleasure canter, which plays a role in sexual arousal and the "high" derived from certain recreational drugs. These responses are heavily modulated by dopaminergic projections from the limbic system. The limbic system is also tightly connected to the prefrontal cortex that might be related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery, called lobotomy. Patients who underwent this procedure often became passive and lacked all motivation.