Chapter 2: The Anatomy and Evolution of the Nervous System

Transcription

1 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.

2 -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.

3 -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.

4 -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.

5 - 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 forebrain. - The upper portion of the diencephalon consists of the thalamus. There are two thalamic nuclei, one on either side of the midline. - Inputs from most of our sensory systems converge on the thalamus, which then forwards info to the cerebral cortex for further processing. It filters information being passed on. Thalamus is also involved in states of arousal and consciousness. Damage results in coma and disturbances in circuits linking thalamus and cerebral cortex are involved in some seizures. - Below thalamus is hypothalamus. It is a regulatory center for behaviours like eating, drinking, sex, biorhythms and temperature control. It is a collection of nuclei. - The ventromedial nucleus of the hypothalamus participates in the regulation of feeding behaviour. - The suprachiasmatic nucleus receives input from the optic nerve and helps set daily rhythms according to the rising of the sun. - -Hypothalamus is directly connected to the pituitary gland which is a gland located just above roof of mouth that is connected to hypothalamus and serves as a major source of hormones. - Hypothalamus also directs the autonomic nervous system, which is part of the peripheral nervous system that controls our glands and organs. The Basal Ganglia -Participates in motor control. These nuclei include the caudate nucleus, the putamen, the globus pallidus and the subthalamic nucleus. Some people include the substantia nigra because it is closely connected. The nucleus accumbens is also associated with basal ganglia and plays role in experience of reward. -Basal ganglia are important to motor control. Degeneration would result in parkinson s or Huntington s disease. They are also included in disorders like ADHD and OCD. The Limbic System - The limbic system is a collection of forebrain structures that participate in emotional behaviour and learning. - The hippocampus, curves around within the cerebral hemispheres from close to the midline out to the tip of the temporal lobe. It participates in learning and memory. Damage to the hippocampus in both hemispheres results in anterograde amnesia. People with this have trouble forming new long term declarative memories (facts, language and personal experience). - The amydgala is an almond shaped structure in the rostral temporal lobes that is part of the limbic system - The amygdale plays role in fear, rage, and aggression. It also acts with hippocampus during the encoding and storage of emotional experiences. Damage to amygdala results in interference in organism s ability to respond to dangerous situations. - In rare cases, damage to amygdala can result in irrational violence - Hypothalamus considered part of the limbic system and is responsible for fight or flight response. - The cingulate cortex is a fold of cortical tissue on the inner surface of the cerebral hemispheres. It contains nerve cells (possibly newly evolved) called Von Economo Neurons. They are responsible for intelligent behaviour and are only found in humans and great apes.

6 - The congulate cortex is further divided into anterior and posterior sections. The anterior cingulated cortex (ACC) exerts some influence over autonomic functions but has received the attention for its role in decision making, error detection, emotion, anticipation of reward, and empathy. ACC also processes info about pain (physical as well as social pain, felt in the same area of the cingulated cortex). - The posterior cingulated cortex(pcc) participates in a variety of functions, including eye movements, spatial orientation, and memory. It is one of the first structures in the brain to be affected by Alzheimer s. - The septal area is located anterior to the thalamus and hypothalamus. When stimulation occurs= pleasure but lesion to area brings rage and attack behaviour. - Olfactory Bulbs also included in limbic system. They are a structure extending from the ventral surface of the brain that processes the sense of smell. - Parahippocampal gyrus is a fold of tissue near the hippocampus, mammillary bodies of the diencephalon and the fornix which is a fiber pathway connecting the hippocampus and mammillary bodies. The mammillary bodies are one of two bumps on the ventral surface of the brain that participate in memory. The Cortex -The cortex is the outer covering of the cerebral hemisphere. It is a thin layer of gray matter from 1.5mm to 4 mm thick. Unlike spinal cord, cerebral hemispheres are organized with gray matter on the outside and white matter on the inside. Beneath thin layers of cortical bodies are fiber pathways that connect the cortex with the rest of the nervous system. -Gyrus are the hills on the surface of the cerebral cortex while the valleys are call sulci. A very large sulcus is called a fissure. -The cortex is wrinkled so as to provide more surface area for the cortical cells. -In most parts of the cerebral cortex, there are 6 layers of cells (numbered from outermost to inner most). Layer 1 has no cell bodies but is made up of the nerve fibers of cells forming connections with other layers. Layers 2 and 4 contain large numbers of small cells known as granule cells. Layers 3 and 5 are characterized by large numbers of the triangular shaped pyramidal cells. These layers provide most of the output from an area of cortex to other parts of the nervous system. Layer 6 has many types of neurons, which merge into the white matter that lies below the cortical layers. -Brain has different ways of being divided. Korbinian Brodmann used the distribution of cell bodies within the 6 layers to distinguish between 52 separate areas of the cortex. But brain can also be divided into four sections called lobes. Lobes are named after the skill bones that lie above them. -The most rostral of the lobes is the frontal lobe. It is separated from the parietal lobe by the central sulcus and from the temporal lobe by the lateral sulcus. The caudal boundary of the frontal lobe is marked by the central sulcus which is a fissure. On the other side of the central sulcus is the parietal lobe. In the ventral direction, the frontal lobe is separated from the temporal lobe by the lateral sulcus. At the very back of the cortex is the occipital lobe. Separating the two cerebral hemispheres along the dorsal midline is the longitudinal fissure. -we can divide the functional areas of the cortex into three categories: sensory cortex, motor cortex, and association cortex. The sensory cortex processes incoming information from the sensory systems. Different areas of the sensory cortex are found in the occipital, temporal and parietal lobes. Occipital lobe contains primary visual cortex which provides initial cortical processing of visual information. -Primary auditory cortex is located in the temporal lobe. -The postcentral gyrus of the parietal lobe contains the primary somatosensory cortex, which is the highest level of processing for information about touch, pain, position and temperature.

7 -The motor areas of the cortex provide the highest level of command for voluntary movements. -The primary motor cortex is located in the precentral gyrus of the frontal lobe. The primary motor cortex is an area of the cortex located within the frontal lobe that provides the highest level of command to the motor systems. -The precentral gyrus is the fold of frontal lobe tissue just rostral to the central sulcus; the location of the primary motor cortex. -Some areas of the cortex have neither specific motor not specific sensory fuctions and are called association cortex. They link and integrate sensory and motor information. -The right and left cerebral hemispheres are linked by a special branch of white matter known as the corpus collosum and by the much smaller anterior commissure. -A wide band of axons connecting the right and left cerebral hemispheres. The anterior commissure is a small bundle of axons that connects structures in the right and left cerebral hemispheres. -The frontal lobe participates in planning of behaviour, attention and judgment. -Two important structures within the frontal lobes are the dorsolateral prefrontal cortex, located to the top and side of the areas of the frontal lobes and the orbitofrontal cortex located above and behind the eyes. The dorsolateral prefrontal cortex participates in executive functions such as attention and the planning of behaviour and orbitofrontal cortex is involved in impulse control...damage can result in antisocial behaviour. -Phineas Gage, frontal lobe damage. People with damage to the dorsolateral prefrontal cortex experience apathy, personality change and the lack of ability to plan. People with damage to the orbitofrontal cortex experience emotional disturbances and impulsivity. -underactivity in frontal lobes can cause a great deal of mental disorders. Extreme antisocialism (serial killers) have damage in the orbitofrontal cortex. - Frontal lobotomies were used because it was thought that frontal lobe damage caused a reduction on negative emotions...they were used to reduce fear and anxiety. In lobotomy, most rostral parts of the frontal lobe were separated from the rest of the brain -Lobotomies were discontinued because of the discovery of antipsychotic medication, not because of the negative side effects. -Frontal lobe is also home to important motor cortex known as Broca s area which is necessary for speech production. -Left hemisphere manages logical thought and basic math. It is rational. The right hemisphere is more emotional and intuitive. The Peripheral Nervous System -Peripheral nervous system perceives the outside world and tells the body to carry out its commands. Its role is to carry sensory information from the body to the spinal cord and brain and bring back to the body commands for appropriate responses. -The peripheral nervous system contains three structural divisions: the cranial nerves, the spinal nerves, and the autonomic nervous system -The cranial nerves and spinal nerves comprise the somatic nervous system. The somatic nervous system brings sensory input to the brain and spinal cord and returns commands to the muscles. The autonomic nervous system controls the actions of many glands and organs.

8 -12 cranial nerves enter and exit the brain directly to serve the region of the head and neck. Three of the cranial nerves carry only sensory information. They are 1. Olfactory nerve, 2. The optic nerve 3. Auditory nerve. -Five of the nerves carry only motor information. The muscles of the eyes are controlled by the oculomotor nerve, the trochlear nerve and the abducens nerve. The spinal accessory nerve controls the muscles of the neck and the hypoglossal nerve controls movement of the tongue. -The remaining nerves have mixed sensory and motor functions. -The trigeminal nerve controls chewing movements but also provides some feedback regarding facial expression. The facial nerve produces facial expressions and carries the sensation of taste. The glossopharyngeal nerve perdorms both sensory and motor functions for the throat. The long distance fibers of the vagus nerve provide input and receive sensation from the heart, liver and digestive tract. -31 pairs of spinal nerves exit the spinal cord to provide sensory and motor pathways to the torso, arms and legs. Each spinal nerve is also known as a mixed nerve because it contains a sensory or afferent ( a nerve that carries sensory info to the CNS) nerve and a motor or efferent nerve ( nerve that carries motor commands away from CNS). -Spinal nerve has the protection of the dura mater and pia mater. -Afferent roots arise from the dorsal part of the spinal cord and efferent roots aride from the ventral part. Once outside the cord, the dorsal afferent root swells into the dorsal spinal ganglion ( collection of cell bodies afferent nerves located just outside the spinal cord), which contains the cell bodies of the afferent nerves that process information about touch, temperature and other body senses from the periphery. Beyond the dorsal spinal ganglion, the dorsal and ventral roots join to form a mixed nerve. -afferent (sensory) nerves contain both myelinated and unmyelinated fibers whereas efferent (motor) nerves are all myelinated in the adult. -Myelin insulates nerve fibers and increases the speed in which they transmit messages. -Small unmyelinated afferent fibers responsible for dull achy feeling after injury. The Autonomic Nervous System -Heart, lungs, digestive system and other organs are commanded by the autonomic nervous system. -Biofeedback is a set of techniques that enable people to control typically unconscious or involuntary functions. -Autonomic nervous system is divided into two parts, the sympathetic and parasympathetic. The parasympathetic is typically activated by internal stimuli such as the arrival of food in the digestive system. The sympathetic is activated by external environmental cues such as the sensing of danger. -Sympathetic nervous system deals with coping with emergencies. It prepares the body for fighting or fleeing by shutting down low priority systems and putting blood and oxygen into the most necessary parts of the body. Heart and lungs operate to provide extra oxygen which is fed to muscles. Blood vessels near skin surface constrict to channel blood to muscles (and cause you to bleed less if cut). Increased blood flow to brain causes mental alertness. -axons from neurons in the thoracic and lumbar segments of the spinal cord communicate with a series of ganglia just outside the cord known as the sympathetic chain. It is a string of cell bodies outside the spinal cord that receive input from sympathetic neurons in the central nervous system and that communicate with target organs. Fibers from cells in the sympathetic chain then communicate with target organs. Because the messages from the spinal neurons reach the sympathetic chain through fibers of equal length, they arrive at the same time. Input from sympathetic chain arrives at all the target organs simultaneously.

9 -organs need a way of differentiating between the inputs of both sympathetic and parasympathetic systems. Both systems communicate with cells in ganglia outside the spinal cord, which then form a second connection with a target organ. Both systems use the chemical messenger acetylcholine (ACh) to communicate with their ganglia. At the target organ, the parasympathetic continues to use ACh but the sympathetic switches to norepinephrine to communicate with target organs. The only expection is between the sympathetic nerves and the sweat glands. -Hypothalamus plays greatest role in managing the autonomic nervous system. It connects with the midbrain tegmentum and to the reticular formation. -Natural selection is the process by which favourable traits would become more common and unfavourable traits would become less common in subsequent generations due to differences among organisms in their ability to reproduce successfully. -animals with spines are vertebrates or chordates. -body weight and brain weight must be taken into consideration to detect intelligence among different species -It is likely that new skills like tooluse, language, social behaviour, and ability to plan for future drove much of the increase in brain size among humans, -

10