This includes their length, cell body (soma) position and the comparative distribution of dendrites and axon terminals If you're seeing this message, it means we're having trouble loading external resources on our website.
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Within the division of the brain are cavities called vesicles, which later grow to form expansions or ventricles.
- The lateral ventricles occupy the cerebral hemispheres.
- The third ventricle is in the diencephalon, and is connected to the lateral ventricles by the foramen of Munro.
- The fourth ventricle lies within the metencephalon and myelencephalon and is connected to the third ventricle by the aqueduct of Sylvius.
- Cerebrospinal fluid is contained within the ventricles and acts as both a drainage system as well as a source of nutrition for the brain.
Although most brains have nearly straight axes, the brain of embryos of birds and mammals acquires three flexures- The cephalic flexure is in the mesencephalon and is concave ventrally
- The pontine flexure is in the metencephalon and is concave dorsally
- The cervical flexure is within the posterior part of the myelencephalon and is concave ventrally.
For the adult brain of vertebrates we use three primary subdivisions: the cerebrum, the cerebellum and the brainstem.The brainstem is the first region to form in development, is the least variable, and receives all the cranial nerves (except for the terminal and olfactory nerves). Part of the adult metencephalon and all of the diencephalon , mesencephalon and myelencephalon are included in the brainstem. The brainstem controls most of the vegetative functions of the body, and is thus vital for life.
The cerebellum and pons (the ventral part of the metencephalon of birds and mammals that has a band of transverse fibers) are the principal adult derivatives of the metencephalon. The cerebellum and pons contribute to coordination of motor function.
The cerebrum is the adult derivative of the telencephalon, and dominates the brain in both size and control.
Surrounding the adult brain are layers of mesodermally-derived connective tissue called meninges (singular: meninx). Whereas cyclostomes and fishes only have a single envelope called the primitive meninx, amphibians have two layers, consisting of an outer dura mater which is extremely dense and protective, and a pia-arachnoid or secondary meninx which is more delicate and vascular. Mammals have three meninges: pia mater (which follows all the convolutions of the brain and is the most interior), the arachnoid layer (which is delicate and sends strands to the pia mater), and the dura mater (the outer, more protective meninx). The area between the dura mater and the arachnoid layer is called the subdural space; the area between the arachnoid layer is called the subarachnoid space. An additional layer of tissue lies between the two hemispheres of the cerebrum and is called the falx cerebri.
Posterior brainstem: medulla through midbrain
The posterior brainstem is the site for connection of cranial nerves into the central nervous system. Each cranial nerve has a nucleus in the posterior brainstem for each type of fiber it carries (e.g., somatic sensory, visceral sensory, somatic motor, visceral motor).
The reticular formation is found in all vertebrates, and is a network of short interneurons in the brainstem that forms a primitive integrating system. It projects into the cerebrum, cerebellum, cranial nuclei, and the spinal cord, and is essential for consciousness as well as control of the cardiovascular and respiratory systems.
The ruber nucleus and substantia nigra are two other important parts of the brain that are located in the posterior brainstem. The ruber nucleus plays a role in the coordination of motor functions. The substantia nigra is involved in the memory of learned tasks, and death of its cells is associated with Parkinson�s disease.
The roof of the midbrain is called the tectum. The tectum of non-mammalian vertebrates is the site for the optic lobes, which are the primary center for the perception of vision. In mammals, vision is perceived in the cerebrum. However, while the mammalian cerebrum tells the animal what an object is, the tectum tells the mammal where in space a visual object is. In the tectum the optic lobes are called anterior colliculi. Behind them are the posterior colliculi which may be important in coordination of auditory reflexes. Together the colliculi form four bumps called the corpora quadrigemina.
Other features of the posterior brainstem include are the pyramidal tracts, which are tracts of motor fibers that run from the cerebral cortex to the spinal cord without interruption. The cerebra peduncles are also important, as they are the sites where the cerebellum joins the brainstem.
Anterior brainstem: diencephalon
The anterior brainstem differs from the posterior part in having no nuclei for cranial nerves and no reticular formation, and for relating to functions that are more highly evolved.
The dorsal part of the diencephalon is the epithalamus, most of which is nonnervous in function. It includes two evaginations: the parietal organ and the pineal body, which function as an endocrine gland and a sense organ.
The lateral parts of the diencephalon are called the thalamus. The thalamus is a relay center to the cerebrum, and functions in awareness, as well as in the perception of pain and pleasure.
The ventral part of the diencephalon is the hypothalamus. It controls most of the autonomic functions of the body, including water balance, temperature regulation, appetite and digestion, blood pressure, sleep and waking, sexual behavior, and emotions. On the ventral surface of the hypothalamus is the optic chiasma, where the optic nerves converge and cross. The hypophysis (pituitary gland) also lies on the ventral side, and functions as an endocrine gland
Cerebellum and pons
The cerebellum develops from the dorsal part of the metencephalon
� highly convoluted in mammals and birds into convex folds or gyri (singular: gyrus) and concave grooves or sulci (singular: sulcus)
� in cross section, the cerebellum is composed of mostly white matter on the cortex and branching white matter in the interior, giving a tree-like appearance called the arbor vitae
� functions to control motor coordination and to maintain equilibrationCerebrum
The cerebrum is the largest part of the brain, and develops from the telencephalon.
Each half of the cerebrum termed a cerebral hemisphere� the olfactory bulb is at the anterior end of each hemisphere
� the corpus striatum - a group of basal nuclei in the base of the cerebrum through which white fibers pass and functions in the movement of muscle masses, and some visual perception
� the cortex - forms the roof and side walls of the cerebrum; the hippocampus, which is important in spatial memory, may be damaged in individuals with Alzheimer�s disease.Definitions:Astrocytes - star-shaped nutritive and supportive glia cells of the central nervous system Central pattern generators - groups of neurons in the spinal cord and in the brain whose activity is responsible for innate cyclical movements of body partsGanglion - group of neuron cell bodies that lie peripheral to the central nervous system in vertebrates
Neuroglia - cells in the central nervous system that help to support, protect and maintain the neurons
Microglia - small neuroglial cells of mesodermal origin, some of which are phagocytic
Node of Ranvier - regions of the axon that lie between the Schwann cells, where the plasma membrane of a myelinated axon is close to the extracellular fluid
Oligodendrocytes - neuroglial cells of ectodermal origin that myelinate axons in the central nervous system, and forms the white matter of the central nervous system (unmyelinated axons are grey matter)
Plexus - networks of nerves or blood vessels formed before nerves are distributed to the muscles
Schwann cells - also called neurilemma. Cells of neural crest origin that form a thin sheath that surrounds an unmyelinated axon, or, after having myelinated an axon, lies on the surface of the myelin sheath
How are motor neurons transmitted?
The upper and lower motor neurons form a two-neuron circuit. The upper motor neurons originate in the cerebral cortex and travel down to the brain stem or spinal cord, while the lower motor neurons begin in the spinal cord and go on to innervate muscles and glands throughout the body.
How does a neuron transmit impulses?
The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized—that is, there is a difference in electrical charge between the outside and inside of the membrane.
What part of neurons transmit impulses?
The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the diameter of the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator.
What transmits impulse from sensory to motor neuron?
A neuron that transmits the impulse from the sensory neuron to a motor neuron is a relay neuron. Relay neuron allows sensory and motor neurons to communicate with each other.
