Task 1 - Anatomy PDF

Task 1 - Anatomy Learning Goals What are the different parts of the brain and their functions? What types of visualizations of the brain exist (views, not methods)? What are the developmental stages of the brain? What can different types of brain damage tell about the biological basis of behavior?...

Task 1 - Anatomy Learning Goals What are the different parts of the brain and their functions? What types of visualizations of the brain exist (views, not methods)? What are the developmental stages of the brain? What can different types of brain damage tell about the biological basis of behavior? General layout of the nervous system The vertebrate nervous system is composed of 2 divisions: Central nervous system (CNS) - the parts of the nervous system in the skull (the brain) and the parts in the spine (the spinal cord). Peripheral nervous system (PNS) - all the parts outside of skull and spine. The PNS is divided further into 2 divisions: Somatic nervous system (SNS) - the part of PNS that interacts with the external environment. ○ Afferent nerves carry sensory signals from the skin, muscles, joints, eyes, ears etc. ○ Efferent nerves carry motor signals from CNS to the muscles. Autonomic nervous system (ANS) - the part of PNS that regulates the body's internal environment. ○ Afferent nerves carry sensory signals from organs to CNS. ○ Efferent nerves carry motor signals from CNS to organs. There are 2 types of efferent nerves: ▪ Sympathetic nerves project from the CNS in the lumbar and thoracic regions of spinal cord (see spinal cord further below in order to understand the words lumbar, thoracic and sacral). They mobilize energy resources in threatening situations. Sympathetic changes are indicative of psychological arousal. ▪ Parasympathetic nerves project from the brain and sacral region of spinal cord. They tend to act to conserve energy. Parasympathetic changes are indicative of psychological relaxation. These principles of (para)sympathetic activity are generally accepted, but there are exceptions to them. Each organ receives opposing sympathetic and parasympathetic input, and its activity is determined by the relative levels of these inputs. The PNS also includes 12 pairs (one left-sided and one right-sided nerve in each pair) of cranial nerves, which project from the ventral surface of the brain. They pass through small openings in the skull, directly leaving the brain without ever joining the spinal cord. Functions and locations of cranial nerves are quite specific, which is why disruptions in cranial nerves functions provide excellent cues about the location and extent of tumors or other brain pathology. Some important terms Body and Behavior Page 1 Some important terms Nerve - a collection of nerve fibers (axons) in the peripheral nervous system, bundled together and extending throughout the body. Tract - a collection of nerve fibers (axons) in the central nervous system. Nucleus - a collection of cell bodies in the central nervous system. Ganglia - a collection of cell bodies in the peripheral nervous system. Gray matter - mostly cell bodies and unmyelinated interneurons. White matter - mostly myelinated axons (more about these in next topic). A simplified view is that gray matter primarily receives and processes information, while white matter mostly transmits information to other locations. Directions in the nervous system Anterior/rostral - towards the nose end (rostrum=nose). Posterior/caudal - towards the tail end (cauda=tail). Dorsal - towards the back or top of the head (dorsum=back). Ventral - towards the chest or bottom of the head (venter=abdomen). Medial - towards the middle Lateral -away from the middle Superior - (in the brain) towards top of head Inferior - (in the brain) towards bottom of head Directions used for PNS: Proximal - close to CNS Distal - far from CNS Other terms: Ipsilateral - structures on the same side of the body Contralateral - structures on opposite sides of the body Slicing the brain We can slice the nervous system in 3 different ways: Coronal slicing, which separates the brain into a front and back part. The plane where the brain is sliced is called coronal/frontal/transverse plane. The resulting view is called a cross/front section. Because of our upright posture, cross sections of the spinal cord are parallel to the ground. Sagittal slicing, which separates the brain into a left and right part. The plane where the brain is sliced is called sagittal/lateral plane. Horizontal/axial slicing, which separates the brain into a lower and upper part. The plane where the brain is sliced is called horizontal/axial plane. Meninges and ventricles Meninges Body and Behavior Page 2  Meninges The brain and the spinal cord are covered by 3 protective membranes (meninges). The outermost meninx is called dura mater - a thick, tough and flexible but not stretchable membrane. Arachnoid membrane - the middle layer - soft and spongy. Pia mater - the innermost layer, which adheres to the brain and spinal cord's surface and follows all surface convolutions. Beneath the arachnoid membrane is a space called the subarachnoid space, which contains many large blood vessels and cerebrospinal fluid (CSF). Ventricles Apart from the subarachnoid space, the CSF fills: The central canal (a thin central channel that runs the length of the spinal cord); 4 large internal chambers of the brain called the cerebral ventricles: the lateral ventricles, third and fourth ventricle. The cerebral aqueduct connects the 3rd and 4th ventricles. The subarachnoid space, central canal and cerebral ventricles are interconnected by a series of openings and form a single reservoir. Cerebrospinal fluid (CSF) CSF is produced by choroid plexuses (networks of capillaries that protrude into the ventricles from the pia mater). The excess CSF is absorbed from the subarachnoid space into large blood-filled spaces - dural sinuses. Too little CSF can cause headaches. Too much CSF can cause the entire brain to expand, leading to obstructive hydrocephalus (water head). Blood-brain barrier Cerebral blood vessels have a special structure: blood vessel walls are tightly packed, forming the blood-brain barrier, which prevents many molecules (especially proteins & large molecules) to enter the brain. The strength of a drug depends on how easily it penetrates the blood-brain barrier. Some large molecules that are critical for normal brain function (e.g. glucose) are actively transported through cerebral blood vessel walls. Structure and function of the central nervous system Spinal cord The spinal cord is a long, conical structure, approximately as thick as one's little finger. Its main functions are distributing motor fibers to muscles and glands and collecting somatosensory information to be passed on to the brain. The vertebral column that protects the spinal cord consists of 24 vertebrae of the cervical (neck), thoracic (chest), lumbar (lower back) and sacral & coccygeal (fused) portions of the column. The Body and Behavior Page 3 back) and sacral & coccygeal (fused) portions of the column. The spinal cord passes through a hole in each of the vertebrae. The spinal cord consists of 2 different areas: an inner H-shaped core of gray matter and a surrounding area of white matter. Dorsal horns - the 2 dorsal arms of the spinal gray matter. Ventral horns - the 2 ventral arms of the spinal gray matter. 31 pairs of spinal nerves are attached to the spinal cord. Each of them divide as they near the spinal cord and join the cord via the dorsal root or the ventral root. All dorsal root neurons are sensory, which group together just outside the cord to form the dorsal root ganglia. Many of their synaptic terminals are in the dorsal horns. All ventral root neurons are motor neurons with their cell bodies in the ventral horns. Development-based brain divisions In the vertebrate embryo, the tissue that eventually develops into the CNS looks like a fluid-filled tube (neural tube). A few weeks after conception, it swells and develops into the adult forebrain, midbrain and hindbrain. Before birth, the 3 initial swellings become 5: the telencephalon, diencephalon, mesencephalon (midbrain), metencephalon and myelencephalon Encephalon means brain, comes from the Greek en, "in", and kephale, "head." An easy way to remember them: Telencephalon is at the Top of the brain, and the other four are arranged in alphabetical order. In humans, as in other higher vertebrates, the telencephalon (left and right cerebral hemispheres) undergoes the greatest growth during development. The other 4 divisions of the brain (some scientists exclude the diencephalon) are referred to collectively as the brain stem. In the brain stem, there are lots of nuclei, which are important for controlling functions such as heart rate, blood pressure, respiration, as well as level of consciousness, wakefulness and arousal. The myelencephalon is also referred to as the medulla. Body and Behavior Page 4 Myelencephalon and metencephalon The myelencephalon and metencephalon are composed largely of tracts carrying signals between the rest of the brain and the body. Reticular formation - a complex network of ~100 tiny nuclei that occupies the central core of the brain stem. It spans from the posterior boundary of the myelencephalon to the anterior boundary of the mesencephalon. The nuclei in the reticular formation play a role in sleep, attention, movement, maintenance of muscle tone, arousal, and various cardiac, circulatory and respiratory reflexes. In the metencephalon, the bulge on the brain stem's ventral surface is called the pons. The other large structure in the metencephalon is the cerebellum (large & convoluted, has 2 hemispheres, sits on the brain's dorsal surface, means 'little brain'). The cerebellum receives visual, auditory, vestibular and somatosensory information and information about individual muscle movements being directed by the brain. It integrates this information and modifies the motor outflow, applying a coordinating and smoothing effect on the movements. Apart from motor coordination and control, the cerebellum also participates in certain aspects of cognition, including learning. Although the cerebellum accounts for roughly 10% of the total brain weight, it contains more neurons than the rest of the brain combined. Damage to the cerebellum impairs standing, walking, performance of coordinated movements, and a variety of cognitive deficits (e.g. in decision making and language use) Mesencephalon The mesencephalon surrounds the cerebral aqueduct and also has 2 divisions: Tectum - the dorsal surface of the midbrain, and; Tegmentum - the rest of the midbrain that is ventral to the tectum. In mammals, the tectum is composed of 2 pairs of bumps (colliculi): The inferior colliculi have an auditory function. The superior colliculi have a visual-motor function (directing the body's orientation toward/away from particular visual stimuli). The tegmentum contains a part of the reticular formation and tracts of passage. The tegmentum also contains 3 colorful structures with psychological significance: The periaqueductal gray is the gray matter surrounding the cerebral aqueduct. Opiates such as morphine decrease an organism's sensitivity to pain by stimulating receptors on neurons located in this region. The substantia nigra (black substance) and the red nucleus are both important components of the Body and Behavior Page 5  nucleus are both important components of the sensorimotor system. Loss of the substantia nigra causes Parkinson's disease. Diencephalon The diencephalon is composed of 2 structures: the thalamus and hypothalamus. The thalamus is a large, 2-lobed structure that constitutes the brain stem's top. One lobe sits on each side of the third ventricle, and the 2 lobes are connected by a bridge of gray matter (the massa intermedia), which pierces the middle of the ventricle. Visible on the surface of the thalamus are white lamina (layers of myelinated axons). The thalamus is a gateway to the cerebral hemisphere. It consists of many different pairs of nuclei, most of which project into the cortex. Such are the sensory relay nuclei, which receive signals from sensory receptors, process them, and then transmit them to the appropriate areas of sensory cortex. Sensory relay nuclei receive feedback from the cortex to control which sensory information is transmitted. Most neural input to the cerebral cortex is received from the thalamus. The hypothalamus is located below the anterior thalamus. It controls the autonomic nervous system and the endocrine system and organizes several motivated behaviors (e.g. eating, drinking, sleep and sexual behavior). The hypothalamus regulates the release of hormones from the pituitary gland, which dangles from it on the ventral surface of the brain. It also regulates body temperature and blood pressure. The optic chiasm is the point at which the optic nerves from each eye come together and change into the optic tracts. That is where some axons of the optic nerve cross over (decussate) to the other side of the brain. The mamillary bodies are a pair of spherical nuclei located on the inferior surface of the hypothalamus, behind the pituitary. Telencephalon The telencephalon mediates the brain's most complex functions: initiation of voluntary movement, sensory input interpretation, complex cognitive processes (learning, speaking, problem solving etc.) The cerebral hemispheres are covered by the cerebral cortex - a layer of mainly small, unmyelinated neurons (often referred to as the gray matter). The layer beneath the cortex is mainly composed of large myelinated axons, which are often referred to as the white matter. Body and Behavior Page 6 matter. The convolutions of the cerebral cortex increase the amount of cerebral cortex without increasing the volume of the brain. The large furrows are called fissures, and the small ones sulci. The bulges between fissures and sulci are called gyri. The cerebral hemispheres are almost completely separated by the longitudinal fissure. They are connected by the cerebral commissures, the largest of which is called the corpus callosum. Each lateral surface has a central fissure (or central sulcus) and a lateral/Sylvian fissure, which divide the hemisphere into 4 lobes: frontal, parietal, temporal and occipital lobe. Among the largest gyri are the precentral gyri, the postcentral gyri, and the superior temporal gyri in the frontal, parietal and temporal lobes, respectively. Sensory cortex The primary visual cortex (back of the brain in the occipital lobes, on the inner surfaces of the cerebral hemispheres) receives visual input from the eyes. The primary auditory cortex (in the temporal lobes, on the lower surface of the lateral fissure) receives auditory information. The primary somatosensory cortex - S1 (located on the postcentral gyrus in the parietal lobe) mediates the sense of touch. Different regions of this cortex receive information from different regions of the body (see figure to the right). The base of the somatosensory cortex and a portion of the insular cortex (hidden from view) receive taste information. All sensory information, except smell and taste is sent to the primary sensory cortex of the contralateral hemisphere. Sensory association cortex Every primary sensory area of the cerebral cortex sends information to adjacent regions, called the sensory association cortex - SAC (parietal lobe). The regions of the SAC that are close to S1 or the primary visual cortex receive information only from those sensory systems (e.g. region closest to primary visual cortex analyzes visual information and stores visual memories). The regions of the SAC, far from the primary sensory areas receive and integrate information from more than 1 sensory system (e.g. we can learn the connection between the sight of a face and the sound of a voice). Damage to the SAC causes deficits related to the environment in general (e.g. difficulty perceiving shapes of object that one can touch but not see or naming parts of one's body). Destruction of the primary visual cortex causes blindness. Damage to the visual association cortex does not cause blindness, but may cause difficulties in recognizing objects by sight. Damage to the auditory association cortex may cause difficulties in perceiving speech/producing meaningful speech. Damage to regions of the association cortex at the junction of the 3 posterior lobes, where the somatosensory, visual and auditory functions overlap, may cause difficulties in reading or writing. Motor and motor association cortex Body and Behavior Page 7 Motor and motor association cortex The primary motor cortex (M1), located just anterior to the primary somatosensory cortex is the region of the cerebral cortex most directly involved in the control of movement. Neurons in different part of M1 are connected to different muscles of the body. The connections are contralateral. The motor association cortex (a.k.a. premotor cortex) is located just rostral to M1. The motor association cortex controls M1. The rest of the frontal lobe, rostral to the premotor cortex, is known as the prefrontal cortex. It is a critical part of the executive system, which refers to planning, reasoning and judgement. It is also involved in personality and emotion by contributing to the assessment and control of appropriate social behaviors. Damage to the prefrontal cortex may result in impairments in initiating voluntary behavior, the inability to inhibit inappropriate social behaviors and memory dysfunction. Subcortical regions 90% of the cerebral cortex is 6-layered, which differ from one another in terms of the size and density of their cell bodies and relative proportion of types of cell bodies (pyramidal cells or stellate cells) that they contain. This 6-layered layout of cerebral cortex is evolutionary recent and it is called neocortex. Many long axons and dendrites course vertically (i.e. at right angles to the cortical layers). This vertical flow of information is the basis of the neocortex's columnar organization and neurons in a given vertical column of neocortex often form a mini-circuit that performs a single function. The hippocampus is an important cortex area that is not neocortex - it only has 3 layers. The hippocampus is located at the medial edge of the cerebral cortex (between the thalamus and the cortex itself) and folds back on itself in the medial temporal lobe. This folding results in a shape that resembles a seahorse. The hippocampus plays a major role in some types of memory, particularly memory for spatial location. Although most of the subcortical portion of the telencephalon consists of axons projecting to and from the neocortex, there are several large subcortical nuclear groups (clusters of neurons). Some of them are considered part of either the limbic system or the basal ganglia system. The word 'system' here does not mean that the limbic system and the basal ganglia are separate functional units. The classification is only used for organization purposes. The limbic system is a circuit of structures that circle the thalamus. Apart from the mamillary body and the hippocampus, structures of the limbic system include: ○ Amygdala - almond-shaped nucleus in the anterior temporal lobe. Cingulate cortex - large strip of cortex in the cingulate gyrus Body and Behavior Page 8  ○ Cingulate cortex - large strip of cortex in the cingulate gyrus on the medial surface of the cerebral hemispheres, just superior to the corpus callosum. It encircles the dorsal thalamus. ○ Fornix - also encircles the dorsal thalamus; it leaves the dorsal end of the hippocampus and sweeps forward in an arc coursing along the superior surface of the third ventricles and terminating in the septum and mammillary bodies. ○ Septum - located at the anterior tip of the cingulate cortex. Several tracts connect the septum and mammillary bodies with the amygdala and hippocampus, thereby completing the limbic ring. The limbic system is involved in the regulation of motivated behaviors - including the 4 F's (fleeing, fighting, fighting and fucking). The amygdala is involved in emotion, particularly fear. Out of each amygdala, the long tail-like caudate sweeps first in a posterior direction and then in an anterior direction. Each caudate almost form a circle, in the center of which, connected by a series of fiber bridges, is the putamen. The caudate and the putamen are known as the striatum (striped structure). The nucleus accumbens, which is in the medial portion of the ventral striatum, is thought to play a role in the rewarding effects of addictive drugs and other reinforcers. The remaining structure of the basal ganglia is the pale circular structure known as the globus pallidus (pale globe). It is located medial to the putamen between the putamen and thalamus. The basal ganglia play a role in the performance of voluntary motor responses and decision making. For example, Parkinson's disease is caused by degeneration of neurons in the midbrain that send axons to the caudate nucleus and the putamen. Lateralization in the cerebral cortex Some functions are lateralized - they are performed primarily on one side of the brain. In general, the left hemisphere analyses information by extracting elements that make up the whole of an experience. This makes the left hemisphere good at recognizing serial events and controlling sequences of behavior. These include verbal activities (e.g. talking, understanding others' speech, reading and writing). In contrast, the right hemisphere is specialized for synthesis (putting isolated elements together to perceive things as a whole). Abilities that rely on such synthesis are e.g. drawing sketches, reading maps, constructing complex objects out of smaller elements etc. The unification of our perceptions and memories is accomplished by the corpus callosum (a large band of axons that connects corresponding parts of the cerebral cortex of the left and right hemispheres). More functional areas Body and Behavior Page 9 More functional areas Broca's area is related to speech production and it is located in the inferior frontal gyrus. Wernicke's area is located in the left hemisphere, near the junction between the temporal and parietal lobes. It is thought to play a role in speech comprehension and production. Wernicke's aphasia: a disorder, caused by damage in Wernicke's area, causing patients to produce fluent, but meaningless language (e.g. use made-up words or similar-sounding words substituted for one another to produce speech that makes little sense). Comprehension of language is also impaired. What can different types of brain damage tell about the biological basis of behavior? Lesion - a region of damage within the brain. Lesion studies - a study of an individual (usually animal)'s behavior after receiving brain damage. The goal of lesion studies is to discover what functions are performed by different regions of the brain and to understand how these functions are combined to accomplish particular behaviors. Brain function ≠ behavior. A behavior includes several functions performed by the brain's circuits. If a lesion to one specific brain structure impairs a particular behavior, it does not mean that the behavior is produced by that area. It could be that the lesion interferes with the normal operation of a different structure in the brain. Body and Behavior Page 10