Module 12.3 Protection of the Brain

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MODULE 12.3 PROTECTION OF THE BRAIN

Three features within protective shell of skull provide additional shelter for delicate brain tissue: • Cranial meninges – three layers of membranes that surround brain • Cerebrospinal fluid (CSF) – fluid that bathes brain and fills cavities • Blood-brain barrier – prevents many substances in the blood from gaining access to the cells of the brain

• Cranial meninges – three of them, are protective membranes of mostly dense irregular collagenous tissue • Structural arrangement from superficial to deep: dura mater, arachnoid mater, and pia mater(Figure 12.18)

• Cranial meninges (continued): • Epidural space – between inner surface of cranial bones and outer surface of dura mater; only a potential space as dura is normally tightly bound to bone only allowing for passage of blood vessels • Dura mater (dura) – outermost meninx; thickest and toughest of three meningeal layers; tough, leathery membrane with abundant collagen fibers and very few elastic fibers

• Cranial meninges (continued): • Dura mater (dura) (continued): • Has two layers(Figure 12.18a): • Periosteal dura – outer layer; attached to inner surface of bones of cranial cavity; functions as periosteum with extensive blood supply in epidural space • Meningeal dura – inner layer; avascular and lies superficial to arachnoid mater • Mostly fused, creating a single inelastic membrane except in regions where cavities called dural sinuses are found

• Cranial meninges (continued): • Dura mater (dura) (continued): • Dural sinuses – venous channels; drain CSF and deoxygenated blood from brain’s many veins; may also be found where meningeal dura folds over itself and courses between structures in brain

• Cranial meninges (continued): • Dura mater (dura) (continued): • Dural folds include falx cerebri, tentorium cerebelli, and falx cerebelli (Figure 12.14b) • Falx cerebri – within longitudinal fissure; partition between left and right cerebral hemispheres; superior sagittal sinus (large dural sinus) found superior to falx cerebri • Tentorium cerebelli – partition between cerebellum and occipital lobe of cerebrum • Falx cerebelli – partition between left and right hemispheres of cerebellum

• Cranial meninges (continued): • Subdural space – found deep to dura, thin serous fluid-filled space; houses veins that drain blood from brain • Arachnoid mater – middle meninx, deep to subdural space; named for its resemblance to a spider web, composed of dense irregular collagenous tissue, thinner and somewhat more elastic than dura (Figure 12.18c)

• Cranial meninges (continued): • Arachnoid mater (continued): • Arachnoid trabeculae – inward extensions, composed of collagen fiber bundles and fibroblasts; anchor arachnoid to deep pia mater • Arachnoid granulations (villi) – small bundles of arachnoid; project superficially through meningeal dura into the dural sinuses; play important role in the return of CSF to bloodstream

• Cranial meninges (continued): • Subarachnoid space – found deep to arachnoid mater and superficial to pia mater; contains CSF and the major blood vessels of brain • Pia mater – deepest meningeal layer; only meninx in physical contact with brain tissue

• Cranial meninges (continued): • Pia mater (continued): • Follows contour of brain, dives into sulci and fissures • Permeable to substances in brain extracellular fluid and CSF; allows substances to move between these two fluids; helps to balance concentration of different solutes in them

• Four ventricles within brain are linked cavities that are continuous with central canal of spinal cord (Figures 12.19, 12.20) • Lined with ependymal cells • Filled with cerebrospinal fluid

• Right and left lateral ventricles (first and second ventricles); within their respective cerebral hemisphere (Figure 12.19): • Resemble ram’s horns when observed in anterior view; horseshoe-shaped appearance in lateral view • Three regions: anterior horn, inferior horn, and posterior horn

• Third ventricle – narrow cavity found between two lobes of diencephalon; continuous with lateral ventricles by interventricular foramen • Fourth ventricle – between pons and cerebellum; connected to third ventricle by cerebral aqueduct (small passageway through midbrain) • Continuous with central canal of spinal cord • Contains several posterior openings that allow CSF in ventricles to flow into subarachnoid space (Figure 12.19a)

• Cerebrospinal fluid (CSF) – clear, colorless liquid similar in composition to blood plasma; protects brain in following ways: • Cushions brain and maintains a constant temperature within cranial cavity • Removes wastes and increases buoyancy of brain; keeps brain from collapsing under its own weight

• Most CSF is formed within each of the four ventricles by structures called choroid plexuses, located where blood capillaries come into direct contact with ependymal cells (which also produce some CSF) lining the ventricles. • The capillaries within the choroid plexuses are fenestrated, cells have small gaps between them and within their membranes that allow fluid and electrolytes to escape the blood and enter the ECF.

• Choroid plexuses (continued): • About 150 ml (about 2/3 cup) of CSF circulates through brain and spinal cord • 750–800 ml of CSF is produced daily so old CSF must be removed as choroid plexuses make new CSF • Process of CSF production and removal occurs constantly; roughly 50% of total CSF is completely replaced every 5–6 hours

• Pathway for formation, circulation, and reabsorption of CSF (Figure 12.20): • Fluid and electrolytes leak out of capillaries of choroid plexuses into ECF of ventricles • Taken up into ependymal cells; then secreted into ventricles as CSF • Circulated through and around brain and spinal cord in subarachnoid space; CSF circulates through ventricles with the help of the ciliated ependymal cells, some of this CSF then flows into the subarachnoid space around the spinal cord. • Some CSF is reabsorbed into blood in dural sinuses via arachnoid granulations

The brain is protects not only from outside threats, but also from inside. Most chemicals and disease-causing organisms (like bacteria and viruses) are denied access to the cells of the brain by the blood- brain barrier – keeps CSF and brain ECF separate from the blood(Figure 12.21) • Consists mainly of simple squamous epithelial cells (endothelial cells) of blood capillaries, their basal laminae, and astrocytes

• Unique features of endothelial cells found in barrier: • More tight junctions than cells of most capillaries • Limit endocytosis and exocytosis- many molecules in the blood cannot enter ECF of brain, the opposite is true as well, many substances in the ECF of brain cannot enter the blood.

• Substances that easily pass through plasma membranes are able to pass through blood-brain barrier; include water, oxygen, carbon dioxide, and nonpolar, lipid-based molecules • Protein channels or carriers allow for passage of other molecules, include glucose, amino acids, and ions • Most large, polar molecules are effectively prevented from crossing blood-brain barrier in any significant amount; while barrier is protective, it can hinder access of medications into brain

• No single structure is labeled “blood-brain barrier” on any figure because blood-brain barrier isn’t around brain; it’s within brain • Not located in one distinct place but found throughout entire brain • To understand this, we must first understand body’s tiniest blood vessels; capillaries are vessels that deliver oxygen and nutrients to body’s cells and remove any wastes produced by cells

• Capillaries found in most organs and tissues are fairly leaky; allow a wide variety of substances to move from blood to extracellular fluid (and vice versa) • Capillaries of brain are specialized to allow only selected substances to enter its extracellular fluid; effectively act as a “barrier”; prevents other substances from doing so (Figure 12.21) • Blood-brain barrier, therefore, is actually a property of capillaries found throughout brain rather than a distinct physical barrier

• Potentially life-threatening infection of meninges in subarachnoid space; inflammation occurs, causing classic signs: headache, lethargy, stiff neck, fever • Diagnosis – examination of CSF for infectious agents and white blood cells (cells of immune system); bacteria and certain viruses are most common causative agents: • Viral – generally mild; resolves in 1–2 weeks • Bacterial – can rapidly progress to brain involvement and death; aggressive antibiotic treatment necessary; some most common forms are preventable with vaccines

• Spinal cord – composed primarily of nervous tissue; responsible for both relaying and processing information; less structurally complex than brain but still vitally important to normal nervous system function; two primary roles: • Serves as a relay station and as an intermediate point between body and brain; only means by which brain can interact with body below head and neck • Processing station for some less complex activities such as spinal reflexes; do not require higher level processing

Brain’s meninges pass through foramen magnum to provide a continuous protective covering of spinal cord and distal nerves at base (Figure 12.22) • Three spinal meninges include dura, arachnoid, and pia mater; structurally similar to brain meninges except that spinal cord dura has only one layer and pia mater has some structural enhancements, helps to anchor the spinal cord in the vertebral cavity, thin pieces of spinal pia called denticulate ligaments extend outward through the arachnoid and attach to the spinal dura (Figure 12.22a)

• Actual or potential spaces between spinal meninges are same as those found between cranial meninges with following features (Figure 12.22b): • Epidural space – actual space due to absence of a periosteal dura; found between meningeal dura and walls of vertebral foramina; space is filled with veins and adipose tissue; cushions and protects spinal cord • Subdural space – only a potential space; normally dura and arachnoid adhere to one another • Subarachnoid space – found between arachnoid and pia mater; filled with thin layer of CSF; area inferior to the base of the spinal cord contains a larger volume of CSF, making it a useful place from which to sample CSF if needed.

• Epidural (spinal) anesthesia – local anesthetic medication is injected into epidural space through an inserted needle • Causes “numbing” (inability to transmit motor or sensory impulses) of nerves extending off spinal cord below level of injection • Commonly given during childbirth and other surgical procedures

• Lumbar puncture (spinal tap) – needle inserted into subarachnoid space generally between fourth and fifth lumbar vertebrae; avoids possibility of injuring spinal cord • CSF is withdrawn for analysis; used to assess conditions like meningitis, encephalitis and multiple sclerosis

Spinal cord extends proximally from foramen magnum to region between first and second lumbar vertebrae; following structural features can be seen on spinal cord (Figure 12.23): • Narrow posterior median sulcus on posterior side entire length of cord • Wider anterior median fissure can be seen on entire length of anterior side of cord • Conus medullaris is cone-shaped distal end of cord

• At the level of first and second lumbar vertebrae; spinal pia gathers into a very thin structure known as the filum terminale, which continues through the vertebral cavity and anchors into the first coccygeal vertebra • Spinal cord has two enlarged regions (cervical and lumbar enlargements); the nerve roots that fuse to form spinal nerves (serve upper and lower extremities) attach to these enlargements (Figure 12.23a)

• Spinal nerves are components of PNS; carry sensory and motor impulses to and from spinal cord • Projections off each side of spinal cord between vertebrae are posterior and anterior nerve roots • Roots of spinal nerves extend inferiorly from conus medullaris and fill remainder of vertebral cavity; this bundle of spinal nerve roots is called cauda equina due to its horsetail-like appearance

Butterfly-shaped spinal gray matter is surrounded by tracts of white matter; following features are seen on cross section of spinal cord (Figures 12.24, 12.25): • Central canal – filled with CSF; seen in center of spinal cord; surrounded by thin strip of gray matter (gray commissure); connects each “butterfly” wing

• Can tell anterior and posterior sides of spinal cord by different shapes of the wings • Anterior wings are broader while thinner posterior wings extend almost to outer surface of spinal cord

• Spinal gray matter makes up three distinct regions found within spinal cord; houses neurons with specific functions and includes (Figure 12.24): • Anterior horn (ventral horn) makes up anterior wing of gray matter and gives rise to anterior motor nerve roots; neurons of anterior horn are concerned with somatic motor functions (those of skeletal muscles)

• Spinal gray matter (continued): • Posterior horn (or dorsal horn) makes up posterior wing of gray matter, contains cell bodies of neurons that are involved in processing both somatic and visceral incoming sensory information. • Lateral horn- found only in spinal cord between first thoracic vertebra and lumbar region; contains cell bodies of neurons responsible for motor control of viscera via the autonomic nervous system.

• Spinal White Matter: Ascending and Descending Tracts • Recall that one of the main functions of the spinal cord is to act as a relay station. This function is carried out by its white matter, which contains the axons of neurons that travel to and from the brain. • Spinal white matter is organized into regions, each general region of spinal white matter is called a funiculus and three funiculi lie on each side of the spinal cord: posterior funiculus, lateral funiculus, and anterior funiculus • (Figure 12.25)

• Spinal White Matter (continued): • The white matter within each funiculus is further organized into tracts (columns) • Ascending and descending tracts bring information to and from a specific part of brain • Sensory pathways travel in posterior and lateral funiculi while motor pathways travel in anterior and lateral funiculi

• Spinal White Matter (continued): • Several ascending tracts carry various kinds of sensory information (Figure 12.25a) Major tracts include: • Posterior columns –carry somatosensory information, such as proprioception and touch; nucleus gracilis transmits signals from lower limbs and lower trunk while nucleus cuneatus transmits stimuli from upper limbs and upper trunk

• Spinal White Matter (continued): • Ascending tracts (continued): • Spinocerebellar tracts- carry information about joint position and muscle stretch from entire body to cerebellum • Anterolateral system includes spinothalamic tracts- send pain and temperature information from entire body to brain

• Spinal White Matter (continued): • Descending tracts (Motor) (Figure 12.25b) • Corticospinal tracts – largest of descending tracts; help control skeletal muscles below head and neck • Originate primarily from motor areas of cerebral cortex; descend as part of internal capsule then through brainstem where a majority of them decussate • Travel through lateral funiculi of spinal cord; bring motor information to appropriate places in the anterior horn

• We as humans have ability not only to perceive diferent sensory stimuli (things that cause senses to respond) but also to assemble multiple sensory stimuli into a single mental picture/idea • Each of these disparate stimuli reaches brain in two-part process: • Stimulus is detected by neurons in PNS and sent as sensory input to CNS • In CNS, sensory input is sent to cerebral cortex for interpretation

• Sensory stimuli (continued): • When CNS has received all different sensory inputs, it integrates them into a single perception (a conscious awareness of sensation) • Sensations can be grouped into two basic types: • Special senses – detected by special sense organs and include vision, hearing, equilibrium, smell, and taste • General senses – detected by sensory neurons in skin, muscles, or walls of organs; can be further subdivided into general somatic senses that involve skin, muscles, and joints and general visceral senses that involve internal organs

General somatic senses pertain to touch, stretch, joint position, pain, and temperature (Figures 12.26–12.28) • Two types of touch stimuli are delivered to appropriate part of cerebral cortex by different pathways: • Tactile senses – (fine or discriminative touch) include vibration, two-point discrimination, and light touch. Tactile senses allow you to discriminate between different shapes/textures without visual input. • Nondiscriminative touch (crude touch) lacks fine spatial resolution of tactile senses

• Most of general somatic senses are considered mechanical senses; neurons that detect them are responsive to mechanical deformation in the skin, joint, and/or organ (temperature sensation is exception) • Two major ascending tracts in spinal cord carry somatic sensory information to brain: posterior columns/medial lemniscal system and anterolateral system

• Basic pathway consists of following: • First-order neuron –the sensory neuron that detects initial stimulus in PNS; axon of this neuron then synapses on a second-order neuron • Second-order neuron – interneuron located in posterior horn of spinal cord or in brainstem; generally synapses on third-order neuron • Third-order neuron –interneurons in thalamus; delivers impulses to cerebral cortex

• Posterior columns/medial lemniscal system -axons of neurons that transmit tactile sensory information about discriminative touch travel with axons that convey information regarding proprioception (joint position) (Figure 12.26)

Figure 12.26 Ascending (sensory) pathways: the posterior column/medial lemniscal systems in the right and left sides of© the2016 Pearsonbody. Education, Inc. General Somatic Senses

• Anterolateral system –As with the tactile senses and proprioception, the fibers that carry the stimuli of pain, temperature, and nondiscriminative touch enter the posterior spinal cord. However, these stimuli ascend via pathways in the anterolateral spinal cord, referred to as the anterolateral system. (Figure 12.27) • First-order neurons synapse on second-order neurons in posterior horn; then decussate • Several tracts lie within the anterolateral system, the largest are right and left spinothalamic tract –send signals through the spinal cord to the sensory relay nuclei of the thalamus; third-order neurons from the thalamus then carry information to cerebral cortex

• Role of Cerebral Cortex in Sensation, S1 and Somatotopy: • Thalamus relays most incoming information to primary somatosensory cortex (S1) in postcentral gyrus • Each part of body is represented by a specific region of S1, a type of organization called somatotopy (Figure 12.28)

• Role of Cerebral Cortex (continued): • Map of S1 illustrates that different parts of body are unequally represented (Figure 12.28a) • A disproportionate amount of space is dedicated to the hands and face; reflects importance of manual dexterity, facial expression, and speech to humans • This unequal representation of body parts in S1 is exemplified by the sensory homunculus (literally “little man”) (Figure 12.28b)

• Role of the Cerebral Cortex in Sensation – Processing of Touch Stimuli: • Thalamic nuclei relay touch information from spinothalamic tracts and posterior columns primarily to S1 for conscious perception • Once sensory information has reached S1, it is processed/perceived, and passed along to cortical association areas

• Role of the Cerebral Cortex in Sensation – Processing of Touch Stimuli (continued): • One of main areas to which S1 axons send output is the somatosensory association cortex (S2), S2 in turn processes and sends information to structures of the limbic system (play a role in tactile learning and memory) • Neurons in S1 also send output to parietal and temporal association areas which integrate and relay information to motor areas of frontal lobe

• Role of the Cerebral Cortex in Sensation – Processing of Pain Stimuli: perception of pain stimuli is called nociception • Thalamus relays pain stimuli to multiple parts of the brain, including S1 and S2, where the sensory discriminative (localization, intensity, and quality) aspects of pain are perceived and analyzed • Such stimuli are also sent to basal nuclei, structures of limbic system, hypothalamus, and prefrontal cortex, where emotional, behavioral, and other aspects of pain are processed

• Role of the Cerebral Cortex in Sensation – Processing of Pain Stimuli (continued): • Cerebral cortex appears to have a significant influence over how pain is perceived; evident by a phenomenon called placebo effect, in which a patient is given a placebo “dummy treatment” with no actual therapeutic value, use of a placebo as an analgesic (pain reliever) can be surprisingly effective, in most studies, up to 30% of patients report relief from pain when a placebo is administered. • This does NOT mean the patient is “crazy” or that the pain is imagined, instead, it reflects the very real ability of the brain to modulate the PERCEPTION of pain.

• Role of the Cerebral Cortex in Sensation – Processing of Pain Stimuli (continued): • The explanation for the placebo effect appears to involve a descending pathway originating mostly in S1, amygdala, and a region of midbrain called periaqueductal gray matter • Neurons of the periaqueductal gray matter release neurotransmitters called endorphins, render the neurons of the posterior horn in the spinal cord less sensitive to pain input. • Thus, the stimulus causing the pain is still present and its intensity has not decreased, but the CNS neurons perceive it as being less intense or even absent. • Painkillers, like morphine, work by binding to the same receptors as endorphins.

• Phantom limb – occurs after amputation of limb, digit, or even breast; patients perceive body part is still present and functional in absence of sensory input; small percentage develop phantom pain (burning, tingling, or severe pain) in missing part • Very difficult to treat due to complex way CNS processes pain; supports idea that S1 has “map” of body that exists independently of PNS • Over time, map generally rearranges itself so body is represented accurately; phantom sensations decrease

• Special senses include vision, hearing (audition), taste (gustation), smell (olfaction), and balance (vestibular sensation) • Each involves neurons that detect a stimulus and send it to CNS for processing and integration • Thalamus – gateway for entry of most special sensory stimuli into cerebral cortex; interprets majority of this information

• Usual pathway for processing each type of special sensory stimuli: • Visual • Most stimuli are sent directly to thalamus; then relayed to primary visual cortex (processes stimuli and perceives an object’s depth, color, and detects rapidly changing stimuli) • Information is shared with association areas in temporal and parietal lobes; crucial for object recognition and spatial awareness, respectively

• Usual pathway for processing each type of special sensory stimuli (continued): • Auditory • Stimuli from sound waves first go to nuclei in brainstem where some stimuli processing occurs • Remainder of stimuli are routed to thalamus and then primary auditory cortex (superior temporal lobe), for processing sounds with complex tempos such as speed and music • Information is relayed to many association areas such as Wernicke’s area for language comprehension

• Usual pathway for processing each type of special sensory stimuli (continued): • Gustatory • Stimuli are sent to nucleus in medulla and relayed to thalamus • Then sent to gustatory cortices (insula and parietal lobes) to analyze different components of taste • Information is sent to hypothalamus and limbic system which presumably influence taste preferences and food-seeking behavior

• Usual pathway for processing each type of special sensory stimuli(continued): • Olfactory • Stimuli enter cerebral cortex of limbic system for initial processing, bypassing thalamus • Then sent to several regions of brain including thalamus (which then transmits them to prefrontal cortex), hypothalamus, and other limbic system components • Allows smell stimuli to influence a number of behaviors including those relating to feeding, emotion, and cognition.

• Usual pathway for processing each type of special sensory stimuli(continued):

• Balance • Processing vestibular stimuli involves multiple brainstem nuclei, cerebellum, descending pathways through spinal cord, and pathways through thalamus to cerebral cortex