Central Nervous System Chapter 5 Part II Fall 2024 PDF

Central Nervous System Spinal Cord and Brain Chapter 5 part II Protection of the CNS Bony structures Cranium (skull) encases brain Vertebral column surrounds spinal cord Meninges Three membranes between bone and nervous tissue (dura mater, arachnoid mater, pia...

Central Nervous System Spinal Cord and Brain Chapter 5 part II Protection of the CNS Bony structures Cranium (skull) encases brain Vertebral column surrounds spinal cord Meninges Three membranes between bone and nervous tissue (dura mater, arachnoid mater, pia mater) Cerebrospinal fluid (CSF) Formed by selective transport across choroid plexus Blood-brain barrier Tight junctions between capillary endothelial cells Meninges The brain and spinal cord are enveloped within three layers of membrane collectively known as the meninges functions: 1. Separate and support soft tissues of the brain from the bones of the cranium 2. enclose and protect some blood vessels that supply the brain 3. contain and help circulate cerebrospinal fluid (CSF) https://doi.org/10.1016/B978-0-12-813088-  Figure 13.5 Meninges  Figure 13.6 Dural Folds Cerebrospinal Fluid Clear, colorless liquid surrounding CNS Circulates in ventricles and subarachnoid space Functions: Environmental stability – transport of nutrients / wastes and protects against fluctuations Buoyancy – reduces brain’s apparent weight by 95% Protection – provides a liquid cushion Choroid Plexus CSF formed by choroid plexus Layer of ependymal cells and blood capillaries Blood plasma is filtered through capillary and modified by ependymal cells Compared to plasma CSF has more: Na+, Cl– CSF has less: Ventricular System Production and Circulation of Cerebrospinal Fluid Blood-Brain Barrier Functions of blood- brain barrier (BBB) Regulates which substances enter brain’s interstitial fluid Helps prevent neuron exposure to harmful substances Drugs, wastes, abnormal solute concentrations some drugs can pass and affect the brain (e.g., alcohol) Spinal Cord  Spinal Cord The spinal cord is a long, slender cylinder of nerve tissue that retains fundamental segmental organization. Protected by bone and meninges Spinal Cord The number of spinal cord segments differs between species. Cervical Thoracic Lumbar Sacral Conus medullaris Cauda Equina comparative-spinal-cords-nieuwenhuys-1964-fig-1.jpg (761×768) (svpow.com) Spinal nerves (31 pairs in humans) Cervical (8) Thoracic (12) Lumbar (5) Sacral (5) Coccygeal (1) Each region of the body surface, supplied by a particular spinal nerve, is called a dermatome.  Spinal Nerves contain both afferent and efferent fibers enclosed in connective tissue Epinerium Perinerium Endonerium  Spinal nerves Afferent fibers enter the spinal cord through the dorsal root Cell bodies of afferent neurons are clustered in dorsal root ganglion Efferent fibers leave the spinal cord through the ventral root Central gray matter is surrounded by white matter. Central gray matter is surrounded by white matter.  Anatomical Organization Gray Matter Clusters of cell bodies “Nuclei” Processes and integrates signals White Matter Bundles of Myelinated axons “Tracts” Transmits signals  Spinal Cord: Gray matter Dorsal horn contains cell bodies of interneurons on which afferent neurons terminate Ventral horn contains cell bodies of efferent motor neurons Lateral horn contains cell bodies of autonomic neurons  Spinal Cord: White matter Bundles of myelinated fibers (tracts) Ascending tracts transmit afferent signals to the brain Descending tracts relay messages from brain to efferent neurons Spinal Cord Functions of the spinal cord: 1. Transmits information between the brain and the body Tracts 2. Integrates reflex activity between afferent input and efferent output spinal reflex White Matter: Overview of Conduction Pathways Sensory vs Motor Tracts Pathways are paired: there is a left and a right tract Each pathway is made of a chain of two or more neurons Most pathways decussate: axons cross midline so brain processes information for contralateral side Uncrossed pathways work on the ipsilateral side of body Spinal Cord: Sensory Tracts originates from general sense receptors two categories: 1. Somatic sensory (somatosensory) receptors Tactile receptors – detect characteristics of an object Proprioceptors – detect stretch in joints, muscles, tendons 2. Visceral sensory receptors Detect changes (e.g., stretch) in an organ Spinal Cord: Motor Tracts Control effectors such as skeletal muscles Start in brain and include at least two neurons: Upper motor neuron in brain  contacts lower motor neuron Lower motor neuron in spinal cord anterior horn excites muscle  Gray Matter: Overview of Spinal Reflexes Simplest response to a stimulus involuntary, stereotyped responses to stimuli Components Sensory receptor Afferent fiber Processing center (CNS) Efferent fiber Effector Reflex arc: neural pathway responsible for generating the response  Withdrawal reflex Withdrawal of a limb from a painful stimulus Afferent neurons stimulate: excitatory interneurons that stimulate efferent motor neurons to flexor muscles (polysynaptic reflex) inhibitory interneurons that inhibit efferent neurons supplying extensor muscles (reciprocal innervation) Other interneurons ascend to carry the signal to a sensory area of the brain  Stretch reflex Reflexive contraction of a muscle after it is stretched Stretch is detected by a muscle spindle proprioceptor Sensory neuron synapses with motor neuron innervating same muscle Monosynaptic and ipsilateral Crossed extensor reflex Extension of the opposite limb during the withdrawal reflex Ensures that opposing limb will be in position to bear weight when the injured limb is withdrawn Polysynaptic and bilateral Brain  Organization of the vertebrate brain Classically organized into: Hindbrain, Midbrain, Forebrain These form from successive portions of embryonic neural tube These regions do not follow functional aspects of brain Organization of the vertebrate brain: Functional Regions 1. Brainstem -- smallest Cerebral cortex and most ancient part Basal nuclei Medulla oblongata and (lateral to thalamus) pons control many life- sustaining processes Thalamus Midbrain coordinates reflex (medial) responses to sight and sound Hypothalamus 2. Cerebellum -- changed little with evolution Cerebellum Maintains proper position of the body in space Midbrain Coordinates motor activity Brain stem Pons Medulla Spinal cord Organization of the vertebrate brain: Functional Regions 3. Forebrain -- most Cerebral cortex changed during vertebrate Basal nuclei evolution (lateral to thalamus) Diencephalon Thalamus Hypothalamus controls (medial) many homeostatic functions Thalamus is a relay station Hypothalamus Cerebrum -- larger and more Cerebellum highly convoluted in advanced vertebrate species Midbrain Basal nuclei Brain stem Pons Cerebral cortex Medulla Spinal cord Brainstem Consists of the midbrain, pons, and medulla oblongata Midbrain Connects cerebrum, Brain stem Pons Medulla diencephalon, and cerebellum to spinal cord Contains ascending and descending tracts Contains autonomic nuclei, nuclei of cranial nerves, and reflex  Medulla Oblongota White Matter Ascending & descending tracts Midbrain Brain stem Pons Medulla Gray Matter Cardiac center Vasomotor center blood vessel diameter Medullary respiratory center controls breathing rate Other nuclei for varied functions Coughing, sneezing, vomiting, salivating, swallowing Pons Bulge in the brainstem, anterior surface Bridge Middle cerebellar peduncles axons connecting pons to cerebellum Nuclei concerned with hearing, balance, taste, eye movements, facial expression, facial sensation Pontine respiratory center Helps regulate skeletal muscles Midbrain Tectum Dorsal midbrain Contains four mounds making a tectal plate corpora quadrigemina superior colliculi control visual reflexes inferior colliculi control auditory reflexes Tegmentum Ventral midbrain Involved in postural motor control Contains red nuclei (pigmented) Substantia nigra Midbrain  Reticular Formation Circuits scattered throughout pons, midbrain & medulla oblongata Descending Tracts: major component of pain control systems that allow us to suppress pain in certain situations Ascending Tracts: Reticular Activating System Processes sensory information, sends signals to cortex to bring about alertness Alertness helps bring about awareness which is necessary for highest states of consciousness  Cerebellum Important in balance and coordination Three parts Vestibulocerebellum -- maintains balance and controls eye movements Spinocerebellum -- enhances muscle tone and coordinates skilled movements Cerebrocerebellum -- plans voluntary muscle activity The spinocerebellum compares the “intentions” of higher motor centers with the “performance” of the muscles and corrects any “errors” by making the necessary adjustments to accomplish the intended movement. Figure 5-25 p186 Diencephalon Thalamus Hypothalamus Epithalamus Thalamus (medial) Hypothalamus Diencephalon: Hypothalamus Integrating center for homeostatic functions Body temperature Thirst and urine output Food intake Thalamus Controls anterior pituitary hormone secretion (medial) Produces posterior pituitary hormones Hypothalamus Stimulate uterine contraction and milk ejection Autonomic nervous system coordination Emotional and behavioral patterns Sleep-wake cycle  Diencephalon: Thalamus Bilateral oval masses of gray matter Relay Center Receives nearly all sensory information on its way to Thalamus cerebral cortex (medial) integrate & directs information to appropriate Hypothalamus area Epithalamus Forms posterior part of roof of diencephalon, covers third ventricle Pineal gland Endocrine gland secreting melatonin Helps regulate day- night cycles, circadian rhythm  Forebrain: Limbic system Ring of forebrain structures functionally linked: ancient portions of cerebral cortex, diencephalon, and basal nuclei Functions: Emotion Amygdala -- gives rise to fear, avoidance of danger, activates fight or flight responses Sociosexual behavior Motivation Highly motivated activities are eating, drinking, and sexual behavior Learning Hippocampus -- memory  Mammalian Cerebrum Divided into two cerebral hemispheres Gyri vs sulci Cerebral cortex -- outer shell of gray matter Six layers organized into functional vertical columns Fiber tracts in white matter transmit signals from one area to another Corpus callosum connects the two cerebral hemispheres  Mammalian Cerebral Cortex Each hemisphere of the cerebral cortex is divided into four lobes. Relative sizes of these lobes vary between species Occipital lobe -- vision Temporal lobe -- hearing Parietal lobe -- body sensory (e.g. touch) Frontal lobe -- motor activity, speech, memory, planning Mammalian Cerebral Cortex: PET scans Fig 5-29. Different areas of the brain light up on positron emission tomography (PET) scans as a person performs different tasks. PET scans detect the magnitude of blood flow in various regions of the brain.  (a) Top view of brain Somatosensory cortex Top Anterior part of parietal lobe behind the central sulcus Initial processing and perception of somasthetic Left hemisphere (body surface) and proprioceptive (body position) sensations Cross-sectional view Receives sensory information from the opposite side of the body Temporal lobe Different body areas are (b) Sensory homunculus  Rabbit Cat Monkey Human FIGURE 5-31 Somatosensory projections of four mammals. Each is distorted to represent the extent of innervation to different body parts. Disproportionately large projection areas in the somatosensory cortex are associated with densely innervated tissue. Figure 5-31 p193  Primary motor cortex Posterior part of frontal lobe in front of the central sulcus Nonreflex control over movement produced by skeletal muscles Controls the opposite side of the body Different muscle groups are mapped and unequally represented on the surface (motor homunculus)  Higher motor areas Readiness potential occurs about 750 msec before electrical activity is detectable in motor cortex Motor association areas program and coordinate complex movements Supplementary motor area Premotor cortex Posterior parietal cortex Cerebellum is also involved in anticipatory planning and timing of some movements Figure 5-29a p191 Specialization of cerebral hemispheres Left cerebral hemisphere Speech Fine motor control (in right-handed people) Logical, analytical, sequential tasks Right cerebral hemisphere Nonlanguage skills Spatial perception, artistic, musical skills Left hemisphere dominant individuals are “thinkers”, while right hemisphere dominant individuals are “creators”