Brain Notes PDF

# The Central Nervous System: The Brain ## I. Development of the nervous system ### A. Embryologically, the brain is generated from an ectoderm tube that develops from three divisions: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon) 1. The midbrai...

# The Central Nervous System: The Brain ## I. Development of the nervous system ### A. Embryologically, the brain is generated from an ectoderm tube that develops from three divisions: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon) 1. The midbrain and hindbrain - together form the parts of the brain stem (pons, medulla oblongata) and cerebellum. - are the most primitve portions of the brain (reptilian brain) - contain the basic neural machinery for survival that is regulated automatically - control of heartbeat, blood vessels, respiration, etc. - will also help to regulate muscles for movement, balance, and posture 2. The forebrain - The posterior part of the forebrain will develop into the diencephalon - The mammalian brain - Concerned with emotions and memories - Develops into the thalamus and hypothalamus - The most anterior part of the forebrain will develop into the telencephalon, which will overgrow the more primitve parts of the brain and become the two cerebral hemispheres (cerebrum) - Located on its surface (the cortex) are all of the primary sensory and motor regions - This is where our most advanced centers for "thinking" are located (neocortex) - The most recently evolved area of the brain. ### B. The adult brain is a three-pound mass of gelatinous tissue composed of about 100 billion neurons and neuroglia ## II. The Cerebrum (telencephalon/neocortex) ### A. is the largest part of the brain, consisting of two cerebral hemispheres separated by the deep longitudinal fissure ### B. Each hemisphere can be subdivided into regions known as lobes (frontal, parietal, occipital and temporal) that are defined by the grooves and ridges of the brain. 1. Fissures, deep grooves - The longitudinal fissure separates the two hemispheres - The transverse fissure separates the hemispheres from the cerebellum 2. Sulci: shallow grooves - lateral sulcus: separates the temporal lobe from rest of the brain - central sulcus: separates the frontal lobe from the parietal lobes - parietal-occipital sulcus separates the parietal lobe from the occipital lobe 3. Gyri are the ridges on each side of a sulcus or fissure, i.e., the precentral and post-central gyri (gyrus is the singular form) ### C. Each hemisphere can also be divided into a thin surface layer known as the cortex, with the inner area known as the medulla. 1. Surface anatomy - The Cerebral cortex - is the 2 mm thick (6 cells deep) outer layer of the cerebral hemispheres - consists of gray matter, which is the dendrites and cell bodies of neurons - 75% of all cell bodies in the nervous system are found in the cortex ### D. Major functional areas of the cerebral cortex. The cortex was divided into multiple areas based upon their function (sensory, motor or association). 1. The frontal lobe functional areas - The prefrontal cortex (anterior association area) - is the site of higher intellectual processes, i.e., reasoning, working and long term memory, deliberation, motor and cognitive anticipation, worry, concentration, planning, problem solving, knowledge of consequences - Is the last area of the brain to fully develop (usually not fully developed until early 20's). - Damage to this area (i.e., a prefrontal lobotomy) results in personality reversals, lack of caring and the inability to carry out the above intellectual functions - The primary motor cortex (located on the precentral gyrus) - spatially represents (meaning your body is mapped out on the gyrus) and controls the voluntary skeletal muscles on the opposite (contralateral) side of the body - Contains the cell bodies of neurons and is the source of impulses going to your voluntary skeletal muscles to make them move (plastic, specific areas can expand at the expense of neighboring areas) - conscious control resulting in precise and skilled movement of voluntary skeletal muscles - Damage to this area results in muscle weakness or paralysis (spastic); reflexes are still present - The premotor cortex - programs semi-voluntary motor activities - initiates impulses for posture and gross movements as well as learned movements of a complex and sequential nature, i.e. walking, piano playing, bike riding, swimming (stores motor programming) - inhibits and modifies impulses generated by the primary motor cortex - frontal eye field - initiates impulses for movement of the extrinsic eye muscles. - Broca's area - is a motor programming area located just anterior to the primary motor cortex that represents muscles found in the face, tongue, lips, palate and vocal cords. - is usually the most highly developed on the left side of the brain - programs muscles involved in symbolic communication such as those used for speech, writing and signing - sends impulses to the primary motor cortex indicating which muscles, in which order should contract in order for speech, writing and signing to occur. - Damage to this area results in motor (Broca's) aphasia - the inability to communicate via speech, writing or signing - speech is slow and only occurs with great effort - sentences are incomplete - individuals may know what they want to say but can't properly form the words - Also inhibits and modifies impulses from the primary motor cortex 2. The temporal lobe - is where the primary auditory cortex is located - at the base of the post central gyrus (both hemispheres) - where impulses, generated in the ear for hearing, terminate and generation of the sound occurs (hear the bell or dog barking) - it is surrounded by the auditory association area – interprets and identifies the sounds received by the primary cortex (identifies what is making the sound) - Wernicke's area - is located posterior to the auditory cortex and is best developed on the left side of the brain - Has 3 primary functions: - is responsible for comprehension of the written and spoken word especially in children and when adults are reading unfamiliar words (often bypassed in people with advanced reading skills) - puts what we want to say into a pattern of impulses (formulates verbal abstractions) that is sent to Broca's area to be verbalized - is connected to the primary visual cortex via the angular gyrus so we can "see what we hear + hear what we see", e.g. when reading you create a mental picture of what you read (i.e.g., a word is heard (auditory cortex), interpreted in Wernicke's area and finally sent via the angular gyrus to the primary visual cortex where visual images are made) - If Wernike's area is damaged - the individual can't understand either the written or spoken word - speech, writing and reading are impaired - Also inhibits and modifies impulses from the primary motor cortex 3. Parietal lobe - The postcentral gyrus is the location of the primary somatosensory (somesthetic) cortex - This area receives impulses representing the general senses (hot, cold, pain, stretch, etc.) from the skin, muscles, joints and internal organs from the opposite (contralateral) side of the body and interprets them to generate the sensation (allows you to feel the sensation) - The body is spatially represented (mapped out) on this gyrus (note: the greater the number of receptors in an area of the body, the greater the representation of that part of the body in the postcentral gyrus). - The role of the postcentral gyrus in perception - this area specifically locates the source of a sensation - determines the specific kind of sensation (hot, cold etc.) - determines the intensity of a sensation via the frequency of impulses arriving per second - mentally makes you think you felt something from the source of impulse - Somatosensory association area - Located posterior to the primary somatosensory area. - Interprets what you are feeling (size and texture) so help you identify something by how it feels (can identify a key in your pocket without seeing it.) - primary gustatory cortex - is where impulses generated by the tongue, that represent taste, terminate - is located at the base of the insula, deep to the temporal lobe - The angular gyrus - connects Wernicke's area with the primary visual cortex - transforms visual patterns of impulses into auditory patterns and the reverse 4. The occipital lobe - The primary visual cortex - Impulses generated by the retina of the eye terminate in this area and a visual image is created - Damage to this area results in some degree of blindness - The visual association areas - are where recognition and interpretation of visual stimuli occur - where movement, color, shape (and texture) are processed - stores visual memories 5. The association areas - surround each primary sensory cortex and interpret sensory data - Posterior association area/Gnostic area - receives input from all of the sensory association area - integrates and coordinates sensory data from all the regions and generates a single "thought" - sends the "thought" to the prefrontal area where decisions etc. are made (i.e., does the body need to act on the thought) - Also important for the special awareness of one's own body. ## E. Medulla of the cerebral hemispheres 1. is composed of white matter consisting primarily of myelinated axons - commissural fibers - axons that link the two hemispheres allowing sharing of information - Form the corpus callosum, the largest commissure that links the two cerebral hemispheres - Association fibers are axons that connect areas within a given hemisphere - Projection fibers are ascending sensory and descending motor axons that link the cortex with lower centers of the brain, i.e. the brain stem, and the spinal cord 2. The basal nucleus (ganglia) - are collections of nuclei (clusters of cell bodies in the CNS) imbedded in the medulla of each cerebral hemisphere that's divided into 3 parts: - caudate nucleus - the putamen - globus pallidus - Functions of of the basal nuclei includes: - Receives sensory input from the cortex - Sends impulses to the premotor cortex to influence skeletal muscle movement - Important in starting and stopping voluntary movements - Prevents spontaneous or inappropriate muscle contractions - Monitors intensity of movements so they are smooth and orderly - Plays a role in behavior and cognition - It is functionally associated with the substantia nigra in the midbrain and subthalamic nucleus in the diencephalon - The substantia nigra releases the neurotransmitter dopamine to the basal nuclei to help control movement (dopamine is primarily an excitatory neurotransmitter, but has an inhibitory effect on the basal ganglia) - Death of neuron in the substantia nigra results in Parkinson's disease - Parkinson's disease - is a progressive paralytic disorder - Signs include rhythmic movements, a slowing of voluntary movements, shaking, a shuffling gait, difficulty swallowing and speaking, rigidity of arms and legs, and loss of facial expression - Loss of cells (up to 70%) in the substantia nigra that produce dopamine results in excessive stimulation of the cortex via the thalamus - Loss of dopamine may create an imbalance between dopamine and acetylcholine - Suspected causes include encephalitis and slow metallic poisons from the environment (note: the use of synthetic heroin and blows to the head have created the signs of Parkinson's disease) ## F. Hemispheric dominance (lateralization) 1. The left hemisphere - houses language functions, and can communicate awareness better than the right side - deals with analytical and sequential processing of nonlinguistic information (numbers, music, logic etc.) 2. The right hemisphere - deals with spatial patterns and visualizing things as a whole, i.e... identifying something by feel, identifying faces, appreciating a symphony (it deals with pictures not words) - is responsible for intuition, emotions and creativity - has some verbal functions, but it isn't as densely wired as the left side ## G. The ventricles of the brain (introduced here so they can be used as landmarks for the regions associated with the diencephalon and brain stem) 1. cavities within the cerebral hemispheres and brain stem 2. Locations of the 4 ventricles - The 2 lateral ventricles are found within the cerebral hemispheres (one ventricle in each) - The 3rd ventricle is in the center of the diencephalon and connects with the lateral ventricles via the interventricular foramen - The 4th ventricle is located in the brain stem (medulla oblongata) - is connected to the 3rd ventricle via the cerebral aqueduct - is continuous with the central canal of the spinal cord and with the subarachnoid space - are filled with cerebrospinal fluid - Note: the ventricles will be covered in more detail in conjunction with our discussion of the meninges ## III. Diencephalon (mammalian brain) ### A. The thalamus 1. consists of two masses of nuclei (gray matter) joined by a bridge that crosses the 3rd ventricle 2. forms the bulk of the 3rd ventricle wall (lateral and superior) and makes up about 80% of the diencephalon 3. is the gateway to the cortex for sensory impulses and motor commands - The thalamus is a relay station for sensory impulses (except olfaction) on their way to the primary sensory cortices - The thalamus is organized into clusters of nuclei with each cluster having a functional specialty, e.g., the medial geniculate body is where auditory impulse relay on their way to the primary auditory cortex. - medial geniculate body relays impulses to the primary auditory cortex - the lateral geniculate body relays impulses to the primary visual cortex - The ventral posterior body relays general sensations and taste to the primary gustatory cortex - A crude determination of the type and source of sensation being experienced, i.e. pain, touch, etc., occurs here - The basal nuclei and cerebellum relay impulses to the motor cortex through the thalamus (which can facilitate and/or inhibit motor impulses) - is connected to the limbic and prefrontal area where emotions and moods are modified - Note: consciousness has been defined as "a dialogue between the thalamus and the cortex" ### B. The pineal gland 1. is the size of a small pea and attaches to the epithalamus, the roof of the 3rd ventricle superior and posterior to the thalamus 2. contributes to your biological clock by producing the hormone melatonin - which targets the sleep center in the hypothalamus - The amount of melatonin rises at night and falls during the day - more is produced during winter months because of the shorter days - Light enters the eyes → impulses are sent to the hypothalamus → the hypothalamus sends impulses to the pineal gland and inhibits melatonin production - induces sleepiness, reduces alertness and slows reaction time - SAD (Seasonal Affective Disorder/Winter depression) - occurs when there is a delayed drop in melatonin levels during the day - The use of light (10,000 lux), for a half hour after dawn, can be used to suppress melatonin production ### C. The hypothalamus 1. consists of about a dozen nuclei 2. is located under the thalamus and forms the floor of the 3rd ventricle 3. ultimately receives all sensory data 4. contains cell bodies that extend their axons - to the brain stem and spinal cord where they synapse with preganglionic neurons of the autonomic nervous system - to the posterior pituitary gland where they control the release of 2 hormones 5. The hypothalamus is a major regulator of homeostasis - The hypothalamus contains receptors that monitor solutes in the blood, sugar levels, temperature and hormone levels - It integrates the involuntary motor activities of the autonomic nervous system (controls smooth and cardiac muscle and glandular activity) - either directly or indirectly controls the release of many hormones - it controls normal body temperature (contains the body thermostat) - regulates food intake via the feeding and satiety centers - it contains a thirst center to create the sensation of thirst when dehydrated - The hypothalamus is associated with feelings of rage and aggression and triggers sexual responses to the opposite sex - It is the pacemaker that drives biological clock rhythms (it is richly supplied with receptors for melatonin, changing levels of which can reset the clock) - It contains a sleep center ### D. The pituitary gland 1. is the master endocrine gland 2. is attached to the hypothalamus by the infundibulum - The anterior pituitary produces and stores 6 hormone. It has a vascular connection to the hypothalamus such that releasing factors (hormones) produced by the hypothalamus can travel to the anterior pituitary and trigger the release of its 6 hormones - Neurosecretory cells extend from the hypothalamus into the posterior pituitary - Neurosecretory cells produce 2 different hormones within the cell bodies (found in the hypothalamus) that are packaged in vesicles and transported to the terminal knobs within the posterior pituitary. - If either hormone is needed the hypothalamus initiates a nerve impulse that travels down the axons of the neurosecretory cells causing the hormone to be released from the terminal knobs in the posterior pituitary to enter the blood. ## IV. The brain stem ### A. The midbrain (mesencephalon) 1. is located around the cerebral aqueduct between the 3rd and 4th ventricles 2. is the location of several nuclei from which cranial nerves emerge 3. is the location of the cerebral peduncles (little foot) - which are on the ventral surface of the midbrain - and are the main motor and sensory connection between the forebrain and the hindbrain (cerebellum, pons and medulla oblongata) 4. The corpora quadrigemina - are on the dorsal surface of the midbrain just anterior to the cerebellum - includes the 2 superior colliculi, visual reflex centers and - the 2 inferior colliculi - Relay station for impulses on their way to the primary auditory cortex - Deal with reflexes that occur when the head must be turned to hear clearly ### B. The hindbrain (rhombencephalon) 1. The pons (bridge) - forms part of the anterior wall of the 4th ventricle - Ascending sensory and descending motor tracts (projection fibers) pass through here - links the cerebellum with the midbrain (cerebellar peduncles) and medulla oblongata - contains several nuclei from which cranial nerves emerge - contains the Pontine Respiratory Center that assists in respiration 2. The cerebellum - is located on the dorsal surface of the brain stem just beneath the occipital lobe - coordinates the contractions of skeletal muscles - The cerebellum receives information from - the motor cortices which indicate their intent regarding beginning movement - the vestibular apparatus in the inner ear which provides information regarding balance - proprioceptors in muscles and joints which indicate the amount of muscle contraction and joint movement - the eyes which provide visual information regarding what the body is doing - The cerebellum assesses the sensory and motor data then, if needed, initiates impulses to the motor cortex and/or sends impulses to the synapses between upper and lower motor neurons in the spinal cord to control the number of impulses stimulating skeletal muscle. - Helps to coordinate the force, direction and strength of muscle contraction to maintain posture and smooth, coordinated movements. Ensures the intended movement matches the actual movement. - stores memories of learned patterns of movement and sends out programmed commands - Dysfunctions of the cerebellum - Dysmetria (ataxia) - When a person can't automatically regulate the amount of muscular contraction and/or joint movement - results in an abnormal strength, rate, direction or range of movement - Intention tremor, when shaking becomes more intense the closer one gets to one's objective or goal (the body is over correcting for poor direction control) 3. The medulla oblongata - forms part of the anterior wall of the fourth ventricle and extends from the inferior boarder of the pons to the foramen magnum where it is continuous with the spinal cord - contains several important nuclei that control reflex activity associated with smooth and cardiac muscle - The cardiac center controls heart rate - The vasomotor center controls the dilation of blood vessels - The medullary rhythmicity center regulates the rate of breathing - The vomiting, swallowing, coughing and sneezing centers are also located here - The anterior surface has 2 ridges known as the pyramids. This is where neurons supplying skeletal muscles cross over to the opposite side of the body (known as the decussation of the pyramids). - The major ascending sensory and descending motor pathways that connect the brain with the spinal cord pass through the medulla ## V. Brain Systems ### A. The reticular formation 1. is distributed through the pons, midbrain and medulla oblongata 2. consists of 3 columns of loosely organized neurons the axons of which carry impulses to the hypothalamus cerebellum and spinal cord 3. helps maintain the brain in an alert state via the Reticular Activating System (RAS) - Neurons from all the major ascending sensory pathways synapse in the RAS providing impulses that help the RAS neurons send "alert" impulses to the cortex - RAS acts as a filter for weak, common, repetitive sensations (Note: about 99% of all sensations are considered to be unimportant and are ignored by the brain) - RAS is inhibited by sleep centers found in the hypothalamus (norepinephrine is associated with the awake state and serotonin is associated with sleep) - If this system is damaged the result is unconsciousness 4. The motor part of the reticular formation helps coordinate motor activity, modifies reflex activity in the spinal cord and contributes to muscle tone ### B. The limbic system 1. consists of regions of the cerebrum and diencephalon that encircle the top of the brain stem 2. includes parts of the hippocampus, amygdala, hypothalamus, thalamus and fornix 3. the primitive emotional brain - Especially concerned with strong emotions of rage, pleasure, fear, sex drives and sorrow - Deals with emotional behaviors that play a part in survival instincts (searching for food and water, reproduction etc.) - Interacts with higher centers - communicates with the prefontal cortex - emotions can override logic - logic can control emotions 4. plays an important role in establishing memory - Memory is the process by which we retain and retrieve knowledge - The hippocampus - is associated with the primary area for smell and was originally responsible for the short term memory of smell - is now responsible for very short-term (a few seconds), and short-term (a few hours) memories which, if reinforced through repetition, are sent to the cortex for long-term storage and consolidation (placing new facts into a variety of categories) - Note: there is no localization of long-term memory, it is stored wherever processing occurs - the hippocampus may determine if information is to be stored or discarded - produces norepinephrine and serotonin which are essential for printing memories - If inadequate amounts of these neurotransmitters are generated, impaired learning and depression may occur - Synaptic fatigue involving these neurotransmitters may mark the limit of one's attention span - If the hippocampus is damaged, anterograde amnesia will occur (you can't lay down certain types of new memory). - Immediate shock will keep long-term memories from forming; delayed shock will impair it; shock after 24 hours won't have an effect - The hippocampus is also involved in the recall of facts and events (working memory) ### C. The amygdala (almond) 1. is located at the tip of the caudate nucleus (basal ganglia) 2. facilitates long-term memory - by stamping sensory input with any strong emotions associated with the input (memories laced with emotion are stored and recalled best) - plays a role in emotional responses especially fear. Helps recognize fearful stimuli/situations and express the fear response (feel fear). ### D. The nucleus basalis (basal forebrain) 1. is located in the forebrain anterior to the hypothalamus and is connected to the hippocampus and the cortex 2. monitors sensory data and increases its activity if the sensation is pleasurable, relevant, useful, vivid etc. 3. sends "take note" directions to prepare neurons of the cortex to accept long-term memories and neurons of the hippocampus to acquire short-term and recent memories 4. deteriorates during Alzheimer's disease - There is an accelerated loss of neurons that produce acetylcholine in the nucleus such that - cells of the cortex become starved for excitement and begin to die - The disease is characterized by the presence of neurofibrillary tangles (clumping of neurofibrils within the nerve cell) and the formation of senile plaques (axons, microglia cells and astrocytes wrap around a waxy, translucent, beta-amyloid protein) - Beta-amyloid is a fragment of a larger, incorrectly cut protein, coded by a gene on chromosome #21 that - accumulates in the cortex and hippocampus - Beta-amyloid is also found in the skin, blood vessels and intestines of victims and may provide a test, i.e., a skin test for Alzheimer's - Signs of Alzheimer's disease include - deterioration of left cerebral activities resulting in the loss of language skills - memory loss of recent events - personality changes ## VI. The cranial meninges A. are the three membranes that surround the brain (the same membranes also become the spinal meninges around the cord) ### B. The dura mater 1. is the outermost of the three meninges 2. is composed of dense irregular connective tissue 3. except where the dural sinuses form, the dura consists of two fused layers - periosteal layer that is equivalent to the internal periosteum of the skull bones - It adheres to bones of the skull - and stops at the foramen magnum - a meningeal layer - is independent of the cranial layer only where they separate from each other to form the four dural sinuses - The dural sinuses are cavities that collect blood from the brain, (blood that is returning to the heart via the jugular veins in the neck) - The sinuses also collect cerebrospinal fluid - sends three extensions inward, one separates the two cerebral hemispheres from one another, one separates the two cerebellar hemispheres from one another and one separates the cerebellum from the cerebral hemispheres ### C. The arachnoid 1. is the middle meninges 2. consists of a serous membrane-like layer that secretes serous fluid into the subdural space (the space between the dura mater and the arachnoid) and dips into the longitudinal fissure 3. The subarachnoid space - is filled with cerebrospinal fluid that escapes from the 4th ventricle of the brain via the medial and lateral apertures - contains delicate connective tissue strands that extend from the arachnoid into the space 4. Arachnoid Villi (granulations) extend from the arachnoid into the dural sinuses so cerebrospinal fluid can be reabsorbed. (note: cerebrospinal fluid is reabsorbed via the villi at the same rate at which it is generated in the ventricles) ### D. The pia mater 1. is the innermost meninges and adheres to the surface of the brain 2. is a vascular membrane consisting of blood vessels and areolar connective tissue that dips into folds (convolutions) on the surface of the brain 3. forms the choroid plexuses - The plexuses are clusters of capillaries that represent pia mater that protrude through the medial wall of the lateral ventricles and the roofs of the 3rd and 4th ventricles - The capillaries are covered by ependymal cells - The plexuses secrete cerebrospinal fluid (CSF) - CSF consists about 150ml of blood plasma that continuously filters from the capillaries and is replaced every 8 or 9 hours - Secretion is assisted by the ependymal cells which can use facilitated diffusion and active transport to contribute ions to the filtrate - Tight junctions between ependymal cells create a barrier that makes the formation of cerebrospinal fluid somewhat selective (note: blood pressure of 160-170 can distend the tight junctions and breech the barrier) - Cerebrospinal fluid provides - protection and buoyancy for the brain (reduces the brain's weight by about 97%) - a specialized, chemically stable environment for the brain and spinal cord - nutrients and waste removal - The flow of cerebrospinal fluid - Fluid generated in the lateral, 3rd and 4th ventricles exits from the 4th ventricles via the lateral and medial apertures - The apertures drain fluid from the 4th ventricle into the subarachnoid space - Once in the subarachnoid space, CSF circulates around the brain and spinal cord - CSF circulates until it is reabsorbed via the arachnoid villi that extend into the dural sinuses - hydrocephalus occurs when blocked flow of CSF generates enough pressure to cause the brain to swell and potentially crush the cortex against the inner surface of the skull ### E. Meningitis is an inflammation of the meninges 1. that is usually caused by a microbial infection (bacterial or viral) 2. that is commonly limited to the pia mater and the arachnoid