|
|
|
THE CENTRAL NERVOUS SYSTEM (THE BRAIN AND SPINAL CORD) I) The Meninges: Both organs are surrounded by the meninges which consists of three membranes: 1) Dura Mater- Around the brain, the dura mater consists of two layers. The outermost layer is fused to the periosteum of the skull and is separated from the inner layer of the dura mater by the dural sinus (a large flat vein). Around the spinal cord the pia mater consists of only one layer which is separated from the inner walls of the vertebrate by an epidural space containing adipose tissue. 2) Arachnoid - This is the middle layer of the meninges and around the brain it is separated from the inner layer of the dura mater by the subdural space. The subdural space contains lymphatic fluid which acts to reduce friction between the dura mater and arachnoid layer. Around the spinal cord, the arachnoid is attached to the single layer of dura mater 3) Pia Mater - inner layer-contains blood vessels serving the brain. The pia mater is separated from the arachnoid layer by the subarachnoid space in both the brain and spinal cord, consisting of a web of collagen fibers and filled with cerebrospinal fluid (CSF). II) The Blood Brain Barrier: 1) The capillaries of the brain are impermeable to water soluble compounds without the assistance of specific carriers (only lipid soluble substances can enter without carriers. 2) There is a separate transport system for glucose, large amino acids, and glycine (the smallest amino acid). This transport system involves facilitated diffusion. 3) All other tissues outside the nervous system are exposed to hormones, all amino acids, from the blood. Some hormones and amino acids are neurotransmitters. If these were allowed to enter into the nervous system the neurons would fire uncontrollably. 4) The bulbous feet of the astrocytes wrapped around the capillaries control all of this. III) The Spinal Cord
IV) The Brain Consists of six major regions: 1) cerebrum 2) dinencephalon (thalamus and hypothalamus) 3) midbrain (mesencephalon) 4) pons 5) medulla oblongata 6) cerebellum **brainstem consists of the midbrain, pons, and medulla oblongata (page 443) General Anatomy and functions:
The Ventricles of the Brain a) Four ventricles : ----two lateral ventricles (1st and 2nd ventricles), in each cerebral hemisphere----- ----the 3rd ventricle (in the diencephalon)---- ---the 4th ventricle in the pons and medulla (below the cerebellum b) Ventricular communication: ----3rd ventricle attached to the lateral ventricles by the interventricular foramen--- ----the 3rd ventricle is attached to the 4th ventricle by way of the cerebral aquaduct (aquaduct of sylvius)--- ----the 4th ventricle attaches to the central canal of the spinal cord. Cerebrospinal Fluid (CSF) a) Produced by the choroid plexus (one in each ventricle) from circulatory system by ependymal cells (one of the supporting cells of the central nervous system) b) Cerebrospinal fluid circulates out of the ventricles into the central canal and "bathes" the outside of the brain and spinal cord within the subarachnoid space. c) It supports and provides cushioning forthe brain. The brain "floats" in CSF (the brain weighs 1400 grams in the air and only 50 grams in CSF) d) Approximately 500 ml of CSF is produced per day by the choroid plexis. The total volume of the CSF is always maintained at 150 ml, so the CSF is replaced every 8 hours. e) CSF transports nutrients, chemical messengers, and waste products. f) CSF leaves the subarachnoid space and diffuses across arachnoid granules (extensions of the arachnoid into the dura mater). Granules penetrate a large cerebral vein called the superior saggital sinus returning the CSF to venous circulation. g) CSF is similar to blood plasma. CSF contains less protein, and has a different ion concentration. CSF has more NaCl, H+, and less Ca2+ and K+) The Cerebral Cortex (divided into four lobes: frontal, parietal, temporal and occipital lobes) (page 435 figure 12.8) Divided into motor and sensory areas by the central sulcus (fissure of Rolando). Anterior to the fissure of Rolando is the motor cortex and posterior to the fissure of Rolando is the sensory cortex. a) The motor cortex (the frontal lobe) ----Precentral gyri: This is also called the primary motor cortex where impulses of voluntary movement to skeletal muscles originates.---- -----Premotor cortex: This is also called the motor association area and coordinates learned movements---- -----Prefrontal cortex (conscious intellect) Found in both hemispheres it coordinates information from all association areas of the cortex. Functions in predicting the future consequences of events, contains feelings of frustrations, tension, and anxiety. If one severs the connections to the prefrontal cortex, anxieties, frustrations are removed. Prefrontal lobotomies were performed to "cure" mental illnesses associated with violent behavior. ----Broca's area (controls speech) found in left hemisphere only, it regulates patterns of breathing and vocalization for speech. If damage occurs one has difficulty in speaking or constantly talk but use the wrong words. ****association areas interpret incoming data or coordinate a motor response (memory is stored within respective association areas) b) The sensory cortex (parietal, occipital and the temporal lobes) ----Postcentral gyri: This is also called the primary sensory cortex and receives sensory information from touch, pressure, pains and temperature receptors (found in the parietal lobe)----- -----Primary visual cortex: Receives visual information (found in the occipital lobe--- ---Visual association cortex (occipital lobe)--- ---Gustatory cortex receives taste sensations (parietal lobe and insula)--- ---Primary auditory cortex receives hearing sensations (found in temporal lobe)--- ----Auditory association area (temporal and parietal lobes)--- ----Olfactory cortex receives sense of smell (temporal lobe)--- Cortical Connections a) Each area on a single hemispheric cortex is interconnected by white matter (axons) beneath the cerebral cortex b) Each hemisphere is interconnected by the corpus callosum. c) Some white matter also interconnects the cortex with the diencephalon, brains stem, cerebellum, and spinal cord. Cerebral Processing Centers a) control complex motor activities, and performs analytical functions. b) Right hemisphere is called the Representational Hemisphere and is responsible for spatial visualization and analysis. It also is responsible for analysis by touch (many musicians and artists are left handed) c) Left hemisphere is called the Categorical Hemisphere and contains the General Interpretative center involving language and mathematical calculations, speech center (Broca's area), and writing. Memory a) can be fact or skill memories b) can be short term or long term memories c) memories are stored within their representational association cortex (familiar sounds in the auditory cortex, familiar faces in the the olfactory cortex, familiar tastes in the gustatory cortex, familiar smells in the olfactory cortex) Basal a) Consists of the caudate, putamen and globus pallidus (figure 12.10 page 441). b) The basal nuclei do not start movements but provide pattern and rhythm such as would be used in running or walking. c) They also inhibit excessive movement by involuntary control of skeletal muscle tone, posture, coordination of learned movement patterns. The Diencephalon a) includes the thalamus and the hypothalamus which includes the 3rd ventricle with the CSF, choroid plexus in the roof of the diencephalon, and pineal body, an endocrine gland which secretes melatonin controlling day night cycles. b) The thalamus receives all sensory information (except for olfaction), and acts as a filter in that it only allows a small amount of the sensations into the primary cortex while the rest is relayed to the cerebral nuclei and centers in the brain stem. The thalamus also is responsible for the coordination of voluntary and involuntary motor commands. c) The hypothalamus 1) contains centers for rage, pleasure, pain, thirst, hunger, and sexual arousal. 2) It also adjusts and coordinated the activities of the autonomic centers in the pons and medulla 3) coordinates neural and endocrine activities (via the pituitary gland) 4) produces hormones: ADH and oxytocin 5) coordinates voluntary and autonomic functions 6) maintains normal body temperature The midbrain (superior and inferior colliculi) a) part of the brain stem b) includes two pairs of sensory nuclei: the superior and inferior colliculi (colliculi-small hills) that direct involuntary motor reflexes to sudden visual and auditory stimuli. c) includes motor nuclei for two cranial nerves for eye movements (III and IV: oculomotor and trochlear). d) a center for the reticular formation it contains the RAC (reticular activating system). When unstimulated we become inactive; when stimulated we become attentive or more wakeful e) It contains the substantia nigra which inhibits the cerebral nuclei by releasing dopamine (a neurotransmitter). If substantia nigra is damaged then less dopamine is available resulting in a gradual increase in muscle tone resulting in Parkinson's disease (Substantia nigra maintains muscle tone and posture). The Pons a) links the cerebellum with the midbrain, diencephalon, cerebrum, and spinal cord. b) contains sensory and motor neurons for cranial nerves V-VIII c) contains nuclei for the involuntary control of pace and depth of respiration The Cerebellum a) makes rapid adjustments in muscle tone and position for balancing and equilibrium (modifies the activity of the nuclei in the brains stem) b) programs and fine tunes voluntary and involuntary movements in the cerebrum and brain stem via the cerebral peduncles. c) The cerebellum can be permanently or temporarily affected by drugs and alcohol by a condition called ataxia (a disturbance in balance). The Medulla Oblongata a) connecting and forming communication between the brain with the spinal cord b) contains sensory and motor neurons for cranial nerves VIII-XIII c) contains centers for regulating vital autonomic functions: The cardiovascular center (cardiac center, vasomoter centers), and respiratory centers are examples. The Limbic System a) Consists of a group of structures located on the medial aspect of each cerebral hemisphere and diencephalon. Its cerebral structures encircle (limbus=ring) the upper part of the brains stem (Figure 12.8), and include parts of the rhinencephalon (the septal nuclei, cingulate gyrus, parahippocampal gyrus, dentate gyrus, and C-shaped hippocampus), and part of the amygdala. In the diencephalon, the main limbic structures are the hypothalamus and the anterior nucleus of the thalamus. The fornix (arch) and other fiber tracts link these limbic system regions together. b) establishes emotional states and related behavioral drives c) links conscious intellectual functions of the cerebral cortex with the unconscious autonomic functions of the brain stem d) contains long term memory storage and retrieval ***damage to hippocampus from Alzheimer's disease results in memory loss e) hypothalamic centers control emotional states such as rage, fear, sexual arousal f) reflex movements that can be consciously activated (ie mamillary bodies--little breasts-- or mammilla of the hypothalamus that control reflex movements such as chewing, licking, and swallowing. |
|
|
