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Chapter 2

Chapter 2 of Physiological Psychology.docx

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Steve Joordens

Chapter 2: The Anatomy and Evolution of the Nervous System -Structures that are located toward the head end of the animal are rostral or anterior. Structures located toward the tail end of the animal are caudal or posterior. Structures located toward the belly side are inferior or ventral and structures toward the back are superior or dorsal. -Anatomical directions are different in people because our two legged stance puts a 90 degree bend in the neuraxis which is an imaginary line that runs the length of the spinal cord to the front of the brain. -In a four legged animal, the neuraxis forms a straight line running parallel to the ground, in humans the dorsal parts of our brain form a 90 degree angle with the dorsal parts of the spinal cord. -The midline is an imaginary line that divides us into equal halves. -If two structures are ipsalateral, they are both on the same side of the midline. If structures are on the opposite side of the midline, they are contralateral. Structures close to the midline are referred to as medial and structures to the side of the midline are lateral. -Proximal = close to the center and Distal= far away from the center -Traditionally, the coronal, sagittal and horizontal sections are used to display the cuts of the brain’s nervous system. -Coronal sections (frontal sections) divide the nervous system from front to back. Sagittal sections are parallel to the midline (side view) and the midsagittal section divides the brain into two equal halves. The Horizontal or axial section divides the brain from top to bottom. -The soft spot on a baby’s head is also known as the fontanel and it takes 18 months for the baby’s skull bones to fuse completely. -Layers of membranes (meninges) surround the nervous system. -There are three layers of the meninges. The outermost layer is called the dura mater (which means hard mother. It is composed of leather like tissue that follows the outlines of the skull bones. -Below the dura mater is the arachnoid layer. The inner most layer is the pia mater (pious mother). It is a nearly transparent membrane and sticks closely to the outside of the brain. -Between the arachnoid and pia mater layers is the subarachnoid space (sub= below). -All three layers cover the brain and spinal cord. Only the dura mater and pia mater cover nerves that exit the brain and spinal cord. These nerves are referred to as the peripheral nervous system. -Cerebrospinal fluid (CSF) is secreted within hollow spaces in the brain known as ventricles. Within the lining of the ventricles, the choroid plexus converts material from the nearby blood supply into CSF. -CSF is close in composition to clear plasma of the blood. Due to its weight and composition it floats the brain within the skull. -CSF acts as a cushion to soften falls and blows to the head and also causes neurons to respond to appropriate input and not pressure on the brain. Pressure can cause neurons to fire incorrectly...eg. when a tumor causes seizures by pressing down on a part of the brain. - CSF circulates through the central canal of the spinal cord and four ventricles in the brain: the two lateral ventricles, one in each hemisphere and the third and fourth ventricles in the brainstem. -The fourth ventricle is continuous with the central canal of the spinal cord which runs the length of the cord at its midline. Below this ventricle, three small openings allow the CSF to flow into the subarachnoid space that surrounds both the brain and spinal cord. -New CSF is constantly being made, and the entire supply is turned over three times a day. Old CSF is reabsorbed into the blood supply at the top of the head. -CSF is produced by the choroid plexus that lines the walls of the ventricles. From the lateral ventricles, the CSF flows through the third and fourth ventricle and into the central canal of the spinal cord. At the base of the cerebellum, CSF exits into the subarachnoid space and is reabsorbed by veins near the top of the head. -Blockage of narrow sections in this circulation system are called hydrocephalus (water in the brain). The large quantity of CSF in the brain prevents normal growth of the brain. Hydrocephalus can be treated with a shunt installed to drain off excess fluid. Babies can be born with this condition. -The brain receives its nutrients through the carotid arteries on either side of the neck as well as through the vertebral arteries that travel up through the back side of the skull. Once inside the skull, these major arteries branch to form the anterior, middle and posterior cerebral arteries, which serve most of the brain. -Because the brain cannot store energy, any interruption of the blood supply produces damage very quickly. Brain damage can occur three minutes after the stop of a beating heart. -We divide the entire nervous system into two components, the central nervous system and the peripheral nervous system. -Central nervous system includes the brain and spinal cord. The peripheral nervous system contains all the nerves that exit the brain and spinal cord, carrying sensory and motor messages to and from the other parts of the body. The tissue of the CNS is encased in bone, but the tissue of the PNS is not. -The spinal cord is a long cylinder of nerve tissue that extends from the medulla (most caudal structure of the brain) down to the first lumbar vertebra (a bone in the spine, or vertebral column). -The neurons making up the spinal cord are found in the upper two thirds of the vertebral column. -The spinal cord is shorter than the vertebral column because the cord stops growing before the bones in the vertebral column do. Running down the center of the spinal cord is the central canal. -The spinal nerves exit between the bones of the vertebral column. The bones are cushioned from one another with disks. If any of these disks degenerate, pressure is exerted on the adjacent spinal nerves, producing a painful pinched nerve. -Based on the points of exit, the spinal cord is divided into 31 segments. Starting closest to the brain, there are eight cervical nerves that serve the area of the head, neck and arms. Below the cervical nerves are the 12 thoracic nerves, which serve most of the torso. Five lumbar nerves come next, serving the lower back and legs . (Lower back pain=lumbar problems). -The Five sacral nerves serve the backs of the legs and the genitals. -When the spinal cord is viewed in a horizontal section, much of the cord appears white. White matter is made up of nerve fibers known as axons, the parts of neurons that carry signals to other neurons. Material looks white due to the myelin. -Large bundles of axons are responsible for carrying information to and from the brain. Axons from sensory neurons that carry information about touch, position, pain and temperature travel up the dorsal parts of the spinal cord. Axons from motor neurons, responsible for movement, travel in the ventral parts of the cord. -Gray matter (area of neural tissue primarily made up of cell bodies) consists of areas primarily made up of cell bodies. It appears gray because the cell bodies absorb some of the chemicals used to preserve the tissue, which stains them gray. -Neurons found in the dorsal horns of the H receive sensory input, wheras neurons in the ventral horns of the H pass motor information on to the muscles. These ventral horn (gray matter in spinal cord that contains motor neurons) cells participate in either voluntary movement or spinal reflexes (involuntary action or response). -The knee jerk is called a patellar reflex and is an example of a spinal reflex. This reflex is managed by two neurons. One processes sensory information coming to the cord from muscle stretch receptors. This neuron communicates with a spinal motor neuron that responds to input by contracting a muscle, causing foot to kick. -The withdrawal reflex pulls a body part away from a source of pain. Three neurons are involved in this: sensory, motor and interneuron. Because so few neurons are involved, there is rapid movement. -Damage to spinal cord produces loss of sensation. Muscles can still be used, but involuntarily. Cervical damage= quadriplegic. Lumbar level damage is a paraplegic- use of arms and torso maintained, lower torso and legs lost. -Spinal injury = no control over bladder and bowel functions because input from brain to sphincter muscles does not occur. -The hindbrain is located just above the spinal cord. The brain divides into three parts: the hindbrain, midbrain (or mesencephalon) and forebrain. Together, the hindbrain and midbrain make up the brainstem. The hindbrain is the most caudal division of the brain, including the medulla, pons and cerebellum. The midbrain is the division of the brain lying between the hindbrain and forebrain. The Forebrain is the division of the brain, containing the diencephalon and the telencephalon. The brainstem is the lower two thirds of the brain. -Later in embryological development, the midbrain makes no further divisions, but the hindbrain divides into the myelencephalon or medulla and the metencephalon. -Cephalon refers to the head -The gradual swelling of tissue above the cervical spinal cord marks the most caudal portion of the brain, the medulla. It contains large quantities of white matter. Vast majority of info passing to and from higher structures of the brain must still pass through the medulla. -The gray matter in the spinal cord has a butterfly like appearance but the medulla’s gray matter contains a number of nuclei (collections of cell bodies that share a function). These nuclei are suspended within the white matter of the medulla. Some of the nuclei contain cell bodies whose axons make up several of the cranial nerves serving the head and neck, while others manage essential functions like breathing. Damage to the medulla is fatal. -Along the midline of the upper medulla is the caudal portion of a structure called the reticular formation. It is a complex collection of nuclei that runs along the midline of the brainstem from the medulla up into the midbrain. It plays an important role in the regulation of sleep and arousal. -The Metencephalon contains two major structures, the pons and the cerebellum. The pons is a structure located in the metencephalon between the medulla and midbrain; part of the brainstem located in the hindbrain. The cerebellum is a structure located in the metencephalon that participated in balance, muscle tone, muscle coordination, some types of learning and possibly higher cognitive functions. -The pons (bridge) lies rostral to the medulla. One of the roles is to form connections between the medulla and higher brain centers as well as with the cerebellum. -large fiber pathways with embedded nuclei are found in the pons and among the nuclei are the cochlear nucleus and the vestibular nucleus. The fibers communicating with these nuclei arise in the inner ear. -The cochlear nucleus is a nucleus found in the pons that receives information about sound from the inner ear. The vestibular nucleus is a group of cell bodies in the pons that receive input about the location and movement of the head from sensory structures in the inner ear. -Cochlear=info about sound; Vestibular= info about position and movement of the head (helps keep balance, or feel motion sickness) -The reticular formation begins in the medulla, extends through the pons and on into the midbrain. Nuclei located in the pons are necessary for the production of rapid eye movement sleep. -The raphe nuclei (nuclei located in the pons that participate in the regulation of sleep and arousal) and the locus coeruleus (structure in the pons that participates in arousal) project widely to the rest of the brain and influence mood, states of arousal and sleep. -The second major part of the metencephalon is the cerebellum. It contains more nerve cells (neurons) than the rest of the brain combined. When viewed in sagittal section, internal section looks like a tree; white matter or axons forms the trunk and branches, while gray matter or cell bodies forms the leaves. -the cerebellum has a role in coordinating voluntary movements, maintaining muscle tone, and regulating balance. -Inputs from spinal cord tell cerebellum about location of the body. Input from cerebral cortex by way of the pons tells the cerebellum about the movement you intend to make. Cerebellum then processes the sequences and timing of muscle movements required to carry out the plan. -The cerebellum is one of the first structures affected by alcohol. It affects skilled movements and speech production. -cerebellum is also involved in processing information. During the course of evolution, the size of the cerebellum has kept in pace with cerebral cortex. One of the embedded nuclei of the cerebellum, the dentate nucleus has become large in monkeys and humans. Part of the denate nucleus, the neodenate is found only in humans. -People with cerrebellar damage experience language difficulties as well as deficits in cognition and perception as well as learning. -People with autism have an abnormal cerebellum. Midbrain - The midbrain or mesencephalon has a top half known as the tectum which is the roof of the midbrain. The bottom half or ventral is known as the tegmentum. - CSF is contained in a small cannel at the midline known as the cerebral aqueduct. The cerebral aqueduct separates the tectum from the tegmentum and links the third and fourth ventricles. - Surrounding the cerebral aqueduct are cell bodies known as periaqueductal gray This plays an important role in our perception of pain. Electrical stimulation to this area provides relief from pain - Midbrain contains most rostral portion of the reticular formation and a number of nuclei associated with cranial nerves. Several important motor nuclei are also found at this level of the brainstem, including the red nucleus and the substanianigra. The Red nucleus which is located within the reticular formation, communicated motor information between the spinal cord and the cerebellum. The substanianigra (black in pigment) is closely connected with the basal ganglia of the forebrain. - Degeneration of the substantia nigra occurs in Parkinson’s disease which is characterized by difficulty moving. - On the dorsal surface of the midbrain are four prominent bumps. The upper pair are known as the superior colliculi. The SC receive input from the optic nerves leaving the eye. They are part of the visual system but are unable to tell you what you are seeing. Instead, they allow us to make visually guided movements. They also are involved in visual reflexes. - The other pair of bumps is known as the inferior colliculi. These are involved in hearing. They are involved with auditory reflexes such as turning the head in one direction of a loud noise and in localizing the sounds of an environment by comparing the timing of the arrival sounds at the two ears. Forebrain - Divides later in embryological development. Two resulting divisions are the diencephalon and the telencephalon. - The diencephalon contains the thalamus and hypothalamus which are located at the midline just above the mesencephalon or midbrain (rostral end of brainstem). The telencephalon contains the bulk of the symmetrical left and right cerebral hemispheres. The cerebral hemispheres are large globular structures that make up the telencephalon of the
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