Nervous System 2-CNS PDF

Nervous System 2 CNS Associate Prof. Sara Wahdan Pharmacology and Toxicology Department [email protected] Structural Classification Nervous Tissue: Structure and Function Nervous tissue is made up of j...

Nervous System 2 CNS Associate Prof. Sara Wahdan Pharmacology and Toxicology Department [email protected] Structural Classification Nervous Tissue: Structure and Function Nervous tissue is made up of just two principal types of cells: Supporting cells (Neuroglia) and neurons.  Supporting Cells (Neuroglia) Supporting cells in the CNS are “lumped together” as neuroglia, which literally means “nerve glue”. Neuroglia includes many types of cells that generally support, insulate, and protect the delicate neurons 1- Astrocytes. Star-shaped cells that account for nearly half of the neural tissue Form a barrier between the capillaries and neurons protect neurons from harmful substances that might be in the blood 2- Microglia. Spiderlike phagocytes that dispose of debris, including dead brain cells and bacteria. 3- Oligodendrocytes. Producing fatty insulating coverings called myelin sheaths. 4- Schwann cells. Schwann cells form the myelin sheaths around nerve fibers that are found in the PNS. 5-Satellite cells. Satellite cells act as protective, cushioning cells 6- Ependymal cells. Theyline the central cavities of the brain and the spinal cord; the beating of their cilia helps to circulate the cerebrospinal fluid Supporting Cells (Neuroglia) Myelin Sheath A whitish, fatty material, waxy in appearance, protects and insulates the fibers and increases the transmission rate of nerve impulses.  Neuroglia Glial cells may contribute to the development of neurodegenerative diseases such as multiple sclerosis, Alzheimer’s disease, and Parkinson’s disease Multiple sclerosis results from the loss of myelin in the central nervous system. It is an autoimmune disease, meaning that the immune system attacks oligodendrocytes. Alzheimer’s disease is caused by the loss of cholinergic neurons, while Parkinson’s Disease is caused by loss of dopaminergic neurons. During neurons degeneration astrocytes and microglia become overly active. These glial cells release inflammatory chemicals that enhance further degeneration of the neurons Excitatory and Inhibitory synapses Physical support of CNS Cranium (Bony skull): surrounds the brain Vertebral column: surrounds the spinal cord The meninges: Three connective tissue membranes that separate the soft tissue of the CNS from the surrounding bone. The three meningeal membranes are the dura mater, the arachnoid mater, and the pia mater. The space between the pia mater and arachnoid mater, called the subarachnoid space, is filled with cerebrospinal fluid. Cerebrospinal fluid (CSF): Clear, watery fluid that surrounds the CNS and fills its cavities (ventricles), providing protection of the nervous tissue. It is similar (but not identical) in composition to plasma. Blood supply to CNS Although the CNS accounts for only some 2% of body weight (the adult brain and spinal cord weigh approximately 3–4 pounds), it receives almost 15% of the blood that the heart pumps to all of the body’s organs and tissues under resting conditions. This large blood supply is necessary because CNS tissue has a high rate of metabolic activity compared to most other body tissues and, therefore, has a high demand for glucose and oxygen to meet its energy needs. Blood Brain Barrier A physical barrier that exists between the blood and CSF In the CNS, most hydrophobic molecules can diffuse across the endothelial cells of capillaries (simple diffusion e.g. ethanol). The movement of hydrophilic molecules across capillary walls is restricted by the blood-brain barrier. The existence of this barrier is due to the presence of tight junctions between the capillary endothelial cells, which eliminate capillary pores, thereby restricting the diffusion of hydrophilic molecules between the cells. Astrocytes are critical to the formation of the blood-brain barrier. The brain is composed of three main parts: Forebrain, Brainstem and Cerebellum The forebrain It is the largest and most superior part of the brain and divided into left and right halves, or hemispheres. The forebrain Cerebrum Diencephalon A large, roughly C-shaped structure Consists of the thalamus and containing both gray (cell bodies) and hypothalamus, two midline structures white (axons) matter. located near the base of the forebrain The gray matter areas include the cerebral cortex at the surface and deep subcortical nuclei. The white matter of the cerebrum is located beneath this layer Cerebrum: Cerebral Cortex The cerebral cortex is the outermost portion of the cerebrum, comprising a thin, highly convoluted layer of gray matter. The convolutions, which consist of grooves called sulci (singular: sulcus) and ridges called gyri (singular: gyrus), allow for a greater volume of cerebral cortex to be accommodated within a given cranial volume. The cerebral cortex is responsible for higher brain functions including motor control, sensory perception, language, emotions, learning, and memory. The cerebral cortex is divided into four lobes: the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. Functional areas of the cerebrum  Cerebrum: The subcortical nuclei Regions of gray matter in the cerebrum including: Nuclei of the limbic system (later) The basal nuclei (Part of Basal Ganglia) are important in motor control. The major components of the basal nuclei are: Caudate, the putamen (together form striatum) and the globus pallidus Diencephalon: Thalamus The thalamus is a cluster of nuclei that functions as a relay center to the cerebral cortex. Much sensory input is filtered by and refined in the thalamus before being transmitted to the cortex. In this manner, the thalamus seems to be important in directing attention. The thalamus also relays information from the cerebellum and basal ganglia to the motor cortex to provide feedback in controlling movement  Diencephalon: Hypothalamus The hypothalamus roles in regulating homeostasis: Link between the endocrine and nervous systems. Releases tropic hormones that regulate the release of anterior pituitary hormones. Controls the release of hormones from the posterior pituitary, including antidiuretic hormone, which regulates plasma volume and oxytocin, which regulates uterine contractions and milk ejection. It contains satiety and hunger, thirst and thermoregulatory centers The hypothalamus is part of the limbic system, it affects emotions and behaviors in response to emotions. The hypothalamus exerts many of its responses through communication with the autonomic nervous system The brainstem It is located at the base of the brain It connects the forebrain and cerebellum to the spinal cord. It is responsible for carrying signals that control body functions. The brainstem It consists of three main regions: (1) Midbrain connects to the forebrain (2) Pons: coordination center for signals between the two hemispheres Both midbrain and pons regulate sensory and motor information such as movement of eyes, upper limbs, vision, hearing, pain perception, sleep/wake cycle regulation, arousal (alertness), and temperature regulation, facial movements. (3) Medulla oblongata which connects to the spinal cord. Regulate vital processes such as heartbeat, breathing, blood pressure, swallowing, coughing, vomiting, and sneezing. Brainstem Functions: A) Origin of 12 pairs of cranial nerves: peripheral nerves that originate directly from the brain. B) The reticular formation system: a diffuse network of nuclei that plays important roles in sleep-wake cycles, arousal of the cerebral cortex, and consciousness. C) Regulation of many involuntary functions controlled by the autonomic nervous system, such as cardiovascular function and digestion. NB: The midbrain contains an important nucleus called substantia nigra that is rich in dopamine neurons and considered part of the basal ganglia. Basal ganglia The basal nuclei in the cerebrum (striatum and globus pallidus) are connected with a few more nuclei in the brain stem (substantia nigra and subthalamic nucleus) that together act as a functional group that forms a motor pathway 2 Motor pathways control the movement The switch between the two pathways is the substantia nigra pars compacta, which projects to the striatum and releases the neurotransmitter dopamine Direct Pathway: Thalamus excite the cortex, promoting movement This pathway is activated by dopamine released from Substantia nigra (excitatory) Indirect Pathway: Thalamus inhibit the cortex, inhibiting movement Dopamine released from Substantia nigra reinforce the thalamic inhibition on cortex (inhibitory) Parkinson’s Disease Progressive neurodegenerative disorder affecting motor control. Degeneration of dopaminergic neurons in nigrostriatal pathway (Loss of the inhibitory dopaminergic neurons in the substantia nigra) Signs of this disease include involuntary tremors, rigidity, gait abnormalities  The Limbic system Collection of brain areas associated with autonomic functions, motivation, memory, and emotions. It includes the amygdala, hippocampus, fornix, parts of the cerebral cortex, portions of the basal nuclei, prefrontal cortex, thalamus, and hypothalamus. Amygdala: involved in aggression and fear. The hippocampus: a major component of the limbic system, is involved in learning and memory. Disorders in the limbic system  The cerebellum It is bilaterally symmetrical, located inferior to the forebrain and posterior to the brainstem. The cerebellum functions in motor coordination and balance Cerebellum acts much as a “guidance system” for movement of the human body. After the cortex formulates plans of action and commands the muscles to execute them, the cerebellum compares the actual movements as they are occurring with the plans, making corrections in the force and direction of movement It plays a role in maintaining muscle tone The cerebellum may also store memories of motor activities  Spinal cord The spinal cord is a cylinder of nervous tissue that is continuous with the lower end of the brain and is surrounded by the vertebral column. In the spinal cord, the white matter is on the outside, whereas the gray matter is on the inside 31 pairs of spinal nerves branch off the spinal cord at regular intervals are 8 pairs of cervical nerves (C1–C8) emerge from the neck region. 12 pairs of thoracic nerves (T1–T12) emerge in the chest region 5 pairs of lumbar nerves (L1–L5), emerge in the region of the lower back 5 pairs of sacral nerves (S1–S5), emerge from the region of coccyx 1 coccygeal nerve (C0), emerges from the tip of the coccyx. Spinal transmission Dorsal and Ventral horns (Gray matter) The two functional halves of spinal cord grey matter: dorsal and ventral horns. Axons of afferent neurons enter the spinal cord and terminate in the dorsal horn through the dorsal root; their cell bodies are located in dorsal root ganglia. Axons of efferent neurons originate in the ventral horn and exit through the ventral root. Efferent neurons synapse at the periphery with effector organ (Skeletal muscles) Spinal transmission Spinal transmission Ascending and Descending tracts (White matter) The white matter of the spinal cord consists of tracts that provide communication either between the different levels of the spinal cord or between the brain and various levels of the spinal cord. Ascending tracts transmit information from spinal cord to brain: Lateral spinothalamic tract Descending tracts transmit information from brain to spinal cord: Pyramidal tract The ascending and descending tracts effectively link the peripheral nerves to the brain.  Pathways of selected ascending and descending tracts. Lateral spinothalamic Pyramidal tracts (Anterior & Lateral) Ascending pathway that Descending tracts, originates with sensory originate in the primary receptors in the motor cortex. periphery. It travels up Both tracts terminate in the spinal cord. the ventral horn of the It communicates spinal cord. sensory information to They communicate to the thalamus and then motor neurons innervating to the cerebral cortex. skeletal muscles