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Brain structure
| The brain plays probably the most important role in the NS. One of its
main functions is to detect and transmit sensory signals.Dozens
of
different specialized receptor cells respond to changes in the
environment.They are mostly not neurons, but they are directly connected
to sensory neurons.These sensory neurons from all over the body
(except
the head) send their long,myelinated axon into the spinal cord via the
dorsal root of the spinal nerves.Neurons, which are transmitting
precisely localized information send axons to the top of the
spinal cord
, and there which are transmitting poorly localized information synapse
immediately with other neurons upon entering the spinal cord.
Sensory
neurons from the head send axons directly into the brain via cranial
nerves ( which are responsible specifically for something-e.g. optic
nerve). All signals from the environment are transmitted via
several"relay stations". At each one of them a signal will be
integrated
with other incoming signals from lower, higher or same-level processing
stages.A lot of the information processing takes place even before a
signal has reached the brain. Structures of
the brain
Roughly the brain consists of three main
parts- forebrain, midbrain and hindbrain.  The hind brain (little brain) is the place where the spinal cord enters the brain.It becomes visibly thicker.Medulla, pons and cerebellum are its main parts. The hind brain is not only a passageway, but also a vital structure containing several nuclei of the autonomic NS.Above it is the mid brain called "mesencephalon.Its functions include combination of information from different sense modalities and direction of attention.On the top of the brain stem is the diencephalon, a very important part of the fore brain.  A very important part in the diencephalon is the thalamus.It is a massive structure on the top of the mid brain,deep in the center of the brain.It is main relay station for all incoming sensory signals.The thalamus receives downward going in-put from higher areas,modulating the relay of sensory signals.Another important structure is the hypothalamus.It is situated below and in front of the thalamus.The hypothalamus is directly connected to the pituitary gland(controls the activity of all the other glands) and serves as a "gateway" to endocrine system, so this way the Nervous system can influence the endocrine system via the hypothalamus.  Telencephalon: Cerebral hemispheres: From the diencephalon, incoming signals go up to the cerebrum.The cerebral hemispheres are highly similar, but not identical .Each of them is covered in cerebral cortex( a thin layer of neurons), also contains several groups of sub-cortical nuclei(thigh cluster of neuron bodies). It contain white and gray matter , as the gray matter is the cortex and the sub-cortical nuclei and the white matter is the myelinated axons of neurons. Each of the hemispheres usually receives and sends out put from/ to the contra lateral side of the body (e.g. left hemisphere receives/sends output to the right side of the body). Basal Ganglia: It is a group of nuclei surrounding the thalamus, which is involved in motor control processes. It consists of globus pallidus , putamen, and caudate.Putamen and caudate are often referred as to corpus striatum. Amygdala is closely connected to this system, so it is sometimes described as a part of the basal ganglia, but usually it is described in the context of the limbic system. Limbic system: Consists of several interconnected cortical and subcortical areas and plays a crucial role in memory and emotion.Limbic system in the sub-cortical ares is almost a complete circle formed by fornix and hippocampus(very important role in forming new memories), ending in mammilary body and amygdala.The limbic system cortical area is the cingulate cortex (lowest bit of the cortex), which is directly above the corpus callosum( a part connecting the two hemispheres). the limbic system is connected to the hypothalamus and olfactory system. Cortex and Corpus Callosum The cerebral cortex is made of thin layers of neurons covering the whole hemisphere (not just the outside part, but the inner surface as well).The Corpus Callosum is thick bundle of axons connecting the two hemispheres .Virtually all signal transfer between the cortices of the two hemispheres is done via this structure. Cerebral Cortex It is highly folded, forming gyri( gyrus- outward folded areas) and sulci (sulcus-inward folded structures). The Longitudinal fissure is the largest sulcus , separating the two hemispheres.The smaller sulci are used to define boundaries of cerebral lobes: Occipital lobe: at the bback , responsible for visual perception Temporal lobe; at the sides, auditory perception Parietal lobe: at the top, somatosensory perception, inter-sensory and sensory motor integration Frontal lobe: at the front, planning and motor output  Some more things to do with the brain - Signal transmission and interpretation Sensory signals from the diencephalon are relayed to their appropriate primary sensory cortex i.e. visual signals- to visual cortex(occipital lobe) auditory signals to auditory cortex ( temporal lobe) signals from skin, muscles and joints to somato-sensory cortex (parietal lobe) All signals are identical - how a signal is interpreted depends entirely to the location in the brain where it arrives.Inside the specific sensory areas, signals arrive at positions corresponding to the position of a receptor cells(topographic representation): Somatotopic map- signals from the hand arrive in the hand areas of the somato sensory cortex, which is next to the arm area, which is next to the shoulder area. Retinotopic map- visual signals from neighboring positions arrive at neighboring positions in the primary visual cortex. Tonotopic map: auditory signals from adjacent areas of the cochlea arrive at adjacent areas in the primary auditory cortex !!! Multiple, differing maps exist for each sense modality - Direction of signal transmission Neurons transmit signals only in one direction (from the dendrites to the end of the axon), but receive signals from different sources. Someinput comes from earlier processing stages (bottom up or feed-forward), other comes from higher processing areas ( top down or feedback ), yet other input comes from neighboring neurons in the same area (lateral). Combination of feed-forward and feedback signal transmission means that signals are not just passively forwarded, but are also modified by everything else that is going on in the brain. -Motor output Cortical motor areas are located in the frontal cortex, at the boundary to the parietal cortex. Supplementary motor and premotor cortex are involved in planning, monitoring and sensory guidance of movements. Primary motor cortex is the final execution stage-its motor neurons send axons directly down the spinal cord . Cortical motor areas are interconnected with two sub-cortical structures , forming complex motor control circuits: Basal ganglia: modulate movements, particularly involved in selective inhibition of movements. Cerebellum: involved in maintaining posture and balance, timing of movements and motor learning. Both of them receive input from motor cortex, sensory cortex and from other sub-cortical structures.Motor signals are ultimately send down the spinal cord. In brief: The Central nervous system consists of brain and spinal cord.The spinal cord connects with the brain via the hindbrain. It consists of cerebellum (responsible for movement and posture), pons and medulla , which are continuation of the spinal cord and consist of autonomic nuclei.Over the hindbrain is the midbrain , wich consists of tectum ( responsible for perception and action) and tegmentum ( responsible for motor functions. Over the midbrain is the forebrain, which consists of two main structures- telencephalon and diencephalon. The telencephalon consists of the Cerebral cortex (cortical lobes, responsible for perception , action and cognition), Limbic system ( emotions) and basal ganglia ( motor control.The diencephalon consists of thalamus, which is general relay station and hypothal;amus , which is the gateway to endocrine system. | |
| Category: Brain and Behaviour | Added by: vessie (26/Mar/2010) | |
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