CNS

Brain Regions and Organization

• Gray matter: short, nonmyelinated neurons and cell bodies.
• White matter: myelinated and nonmyelinated axons.
• Basic pattern in CNS: central cavity surrounded by gray matter, with white matter external to gray matter.
• Spinal cord exhibits this basic pattern, but it changes as you ascend into the brain stem.
• Brain stem has additional gray matter nuclei scattered within the white matter.
• Cerebral hemispheres (cerebrum) and cerebellum contain an outer layer of gray matter called the cortex.
• Cerebrum and cerebellum also have scattered areas of gray matter nuclei amid the white matter.

Pattern of Distribution of Gray and White Matter in the CNS

• The pattern of distribution of gray and white matter in the CNS can be seen in Figure 12.3.

Ventricles

• Ventricles are fluid-filled chambers that are continuous with each other and the central canal of the spinal cord.
• They are filled with cerebrospinal fluid (CSF) and lined by ependymal cells (neuroglial cells).
• Paired lateral ventricles: large, C-shaped chambers located deep in each hemisphere, separated by the membranous septum pellucidum.
• Each lateral ventricle connects to the third ventricle via the interventricular foramen.
• The third ventricle lies in the diencephalon.
• The third ventricle connects to the fourth ventricle via the cerebral aqueduct.
• The fourth ventricle lies in the hindbrain and is continuous with the central canal of the spinal cord.
• Three openings connect the fourth ventricle to the subarachnoid space:
  • Paired lateral apertures in the side walls.
  • Median aperture in the roof.

Ventricles of the Brain

• Lateral Ventricle
  • Anterior horn
  • Posterior horn
  • Inferior horn
• Third Ventricle
• Cerebral Aqueduct
• Interventricular Foramen
• Lateral Aperture
• Fourth Ventricle
• Median Aperture
• Septum Pellucidum
• Central Canal

Cerebral Hemispheres

• Cerebral hemispheres form the superior part of the brain, accounting for 83% of brain mass.
• Surface markings:
  • Gyri: ridges
  • Sulci: shallow grooves
  • Fissures: deep grooves
    • Longitudinal fissure: separates the two hemispheres
    • Transverse cerebral fissure: separates the cerebrum and cerebellum

Lobes, Sulci, and Fissures of the Cerebral Hemispheres

• The lobes, sulci, and fissures of the cerebral hemispheres are shown in Figure 12.5a, 12.5b and 12.5c.

Cerebral Hemispheres - Lobes

• Several sulci divide each hemisphere into five lobes:
  • Frontal lobe
  • Parietal lobe
  • Temporal lobe
  • Occipital lobe
  • Insula
• The first four are named after overlying skull bones.
• The insular lobe is buried under portions of the temporal, parietal, and frontal lobes.
• Major sulci that divide lobes:
  • Central sulcus: separates the precentral gyrus of the frontal lobe and the postcentral gyrus of the parietal lobe.
  • Parieto-occipital sulcus: separates the occipital and parietal lobes.
  • Lateral sulcus: outlines the temporal lobes.
• Each hemisphere has three basic regions:
  • Cerebral cortex of gray matter superficially.
  • Internal white matter.
  • Basal nuclei deep within white matter.

Cerebral Cortex

• The cerebral cortex is the "executive suite" of the brain.
• It is the site of the conscious mind: awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding.
• A thin (2–4 mm) superficial layer of gray matter composed of neuron cell bodies, dendrites, glial cells, and blood vessels, but no axons.
• 40% of total brain mass.
• Functional imaging (PET and MRI) shows specific motor and sensory functions are located in discrete cortical areas called domains.
• Higher functions are spread over many areas.
• Four general considerations of the cerebral cortex:
  • Contains three types of functional areas:
    • Motor areas: control voluntary movement.
    • Sensory areas: conscious awareness of sensation.
    • Association areas: integrate diverse information.
  • Each hemisphere is concerned with the contralateral (opposite) side of the body.
  • Lateralization (specialization) of cortical function can occur in only one hemisphere.
  • Conscious behavior involves the entire cortex in one way or another.

Cerebral Cortex - Motor Areas

• Located in the frontal lobe, motor areas act to control voluntary movement.
  • Primary (somatic) motor cortex in precentral gyrus.
  • Premotor cortex anterior to precentral gyrus.
  • Broca’s area anterior to inferior premotor area.
  • Frontal eye field within and anterior to premotor cortex; superior to Broca’s area.
• Primary (somatic) motor cortex:
  • Located in the precentral gyrus of the frontal lobe.
  • Pyramidal cells: large neurons that allow conscious control of precise, skilled, skeletal muscle movements.
  • Pyramidal (corticospinal) tracts: formed from long axons that project down the spinal cord.
  • Somatotopy: all muscles of the body can be mapped to an area on the primary motor cortex.
    • Motor homunculus: upside-down caricatures represent contralateral motor innervation of body regions.
• Premotor cortex:
  • Helps plan movements - staging area for skilled motor activities.
  • Controls learned, repetitious, or patterned motor skills.
  • Coordinates simultaneous or sequential actions.
  • Controls voluntary actions that depend on sensory feedback.
• Broca’s area:
  • Present in one hemisphere (usually the left).
  • Motor speech area that directs muscles of speech production.
  • Active in planning speech and voluntary motor activities.
• Frontal eye field:
  • Controls voluntary eye movements.

Functional and Structural Areas of the Cerebral Cortex

• The functional and structural areas of the cerebral cortex are shown in Figure 12.7a, 12.7b.

Cerebral Cortex - Sensory Areas

• Areas of cortex concerned with conscious awareness of sensation.
• Occur in the parietal, insular, temporal, and occipital lobes.
• Eight main areas include the primary somatosensory cortex, somatosensory association cortex, visual areas, auditory areas, vestibular cortex, olfactory cortex, gustatory cortex, and visceral sensory area.
  • Primary somatosensory cortex:
    • Located in the postcentral gyri of the parietal lobe.
    • Receives general sensory information from skin and proprioceptors of skeletal muscle, joints, and tendons.
    • Capable of spatial discrimination: identification of body region being stimulated.
    • Somatosensory homunculus: upside-down caricatures represent contralateral sensory input from body regions.
  • Somatosensory association cortex:
    • Posterior to the primary somatosensory cortex.
    • Integrates sensory input from the primary somatosensory cortex for understanding of objects.
    • Determines the size, texture, and relationship of parts of objects being felt.
  • Visual areas:
    • The primary visual (striate) cortex is located on the extreme posterior tip of the occipital lobe, mostly buried in the calcarine sulcus.
    • Receives visual information from the retinas.
    • The visual association area surrounds the primary visual cortex.
      • Uses past visual experiences to interpret visual stimuli (color, form, or movement).
      • Example: ability to recognize faces.
    • Complex processing involves the entire posterior half of the cerebral hemispheres.
  • Auditory areas:
    • Primary auditory cortex:
      • Superior margin of the temporal lobes.
      • Interprets information from the inner ear as pitch, loudness, and location.
    • Auditory association area:
      • Located posterior to the primary auditory cortex.
      • Stores memories of sounds and permits the perception of sound stimulus.
  • Vestibular cortex:
    • Posterior part of the insula and adjacent parietal cortex.
    • Responsible for the conscious awareness of balance (position of the head in space).
  • Olfactory cortex:
    • Primary olfactory (smell) cortex:
      • Medial aspect of the temporal lobes (in piriform lobes).
      • Part of the primitive rhinencephalon, along with olfactory bulbs and tracts.
      • The remainder of the rhinencephalon in humans becomes part of the limbic system.
      • Involved in the conscious awareness of odors.
  • Gustatory cortex:
    • In the insula just deep to the temporal lobe.
    • Involved in the perception of taste.
  • Visceral sensory area:
    • Posterior to the gustatory cortex.
    • Conscious perception of visceral sensations, such as upset stomach or full bladder.

Body Maps in the Primary Motor Cortex and Somatosensory Cortex of the Cerebrum

• The body maps in the primary motor cortex and somatosensory cortex of the cerebrum are shown in Figure 12.8.

Cerebral Cortex - Multimodal Association Areas

• Receive inputs from multiple sensory areas and send outputs to multiple areas.
• Give meaning to information received, store in memory, tie to previous experience, and decide on actions.
• Sensations, thoughts, and emotions become conscious: makes us who we are.
• Broadly divided into three parts: anterior association area, posterior association area, and limbic association area.
  • Anterior association area:
    • Also called the prefrontal cortex.
    • The most complicated cortical region.
    • Involved with intellect, cognition, recall, and personality.
    • Contains the working memory needed for abstract ideas, judgment, reasoning, persistence, and planning.
    • Development depends on feedback from the social environment.
  • Posterior association area:
    • A large region in the temporal, parietal, and occipital lobes.
    • Plays a role in recognizing patterns and faces and localizing us in space.
    • Involved in understanding written and spoken language (Wernicke’s area).
  • Limbic association area:
    • Part of the limbic system.
    • Involves the cingulate gyrus, parahippocampal gyrus, and hippocampus.
    • Provides the emotional impact that makes a scene important to us and helps establish memories.

Cerebral Cortex - Lateralization

• Lateralization of cortical functioning:
  • Lateralization: division of labor between hemispheres.
  • Hemispheres are not identical.
  • Cerebral dominance: refers to the hemisphere that is dominant for language.
    • 90% of humans have left-sided dominance.
    • Usually results in right-handedness.
    • In the other 10%, the roles of the hemispheres are reversed.
  • Left hemisphere:
    • Controls language, math, and logic.
  • Right hemisphere:
    • Visual-spatial skills, intuition, emotion, and artistic and musical skills.
  • Hemispheres communicate almost instantaneously via fiber tracts and functional integration.

Basal Nuclei

• Basal nuclei, or basal ganglia, are the third of the three basic regions of the cerebrum.
• Each hemisphere’s basal nuclei include:
  • Caudate nucleus
  • Putamen
    • Striatum = caudate nucleus + putamen
  • Globus pallidus
• Closely associated with the subthalamic nuclei (diencephalon) and substantia nigra (midbrain).
• Functions of basal nuclei:
  • Influence muscle movements, overlapping with the cerebellum, and play a role in cognition and emotion.
    • Regulate the intensity of slow or stereotyped movements.
    • Filter out incorrect or inappropriate responses.
    • Inhibit antagonistic or unnecessary movements.
  • Parkinson’s disease and Huntington’s disease are disorders of the basal nuclei.

Diencephalon

• The diencephalon consists of three paired gray matter structures:
  • Thalamus
  • Hypothalamus
  • Epithalamus
• All three enclose the third ventricle.

Thalamus

• The thalamus is a bilateral egg-shaped nuclei that forms the superolateral walls of the third ventricle.
• Makes up 80% of the diencephalon.
• Bilateral nuclei connected by the interthalamic adhesion (intermediate mass).
• Contains several nuclei, named for location.
• Nuclei project and receive fibers from the cerebral cortex.
• Main thalamic function is to act as a relay station for information coming into the cortex.
• Sorts, edits, and relays ascending input such as:
  • Impulses from the hypothalamus for regulating emotion and visceral function.
  • Impulses from the cerebellum and basal nuclei to help direct motor cortices.
  • Impulses for memory or sensory integration.
• Overall, it acts to mediate sensation, motor activities, cortical arousal, learning, and memory.

Selected Structures of the Diencephalon

• The selected structures of the diencephalon are shown in Figure 12.12a and 12.12b
• Includes Anterior nuclei, Reticular nucleus, Medial, Lateral dorsal dorsal posterior nucleus nucleus nucleus,Pulvinar, Medial geniculate body, Lateral, Ventral, Ventral, Ventral postero-anterior lateral lateral geniculate body, Ventral nuclei,anterior commissure,Preoptic nucleus,Anterior hypothalamic nucleus,Supraoptic nucleus,Suprachiasmatic nucleus,Optic chiasma,Infundibulum(stalk of the,Paraventricular nucleus,pituitary gland),Fornix,Dorsomedial nucleus,Posterior hypothalamic nucleus,Lateral hypothalamic area,Ventromedial nucleus,Arcuate nucleus, Mammillary body and Pituitary gland.

Hypothalamus

• The hypothalamus is located below the thalamus.
• Forms a cap over the brain stem and forms the inferolateral walls of the third ventricle.
• Contains many important nuclei such as:
  • Mammillary bodies: paired anterior nuclei that act as olfactory relay stations.
  • Infundibulum: stalk that connects to the pituitary gland at the base of the hypothalamus.
• The hypothalamus is the main visceral control and regulating center that is vital to homeostasis.
• Homeostatic roles:
  • Controls the autonomic nervous system:
    • Examples: blood pressure, rate and force of heartbeat, digestive tract motility, pupil size.
  • Initiates physical responses to emotions:
    • Part of the limbic system: perceives pleasure, fear, rage, biological rhythms, and drives (sex drive).
  • Regulates body temperature: sweating or shivering.
  • Regulates hunger and satiety in response to nutrient blood levels or hormones.
  • Regulates water balance and thirst.
  • Regulates sleep-wake cycles:
    • The suprachiasmatic nucleus of the thalamus sets our biological clock.
  • Controls endocrine system function:
    • Secretions of the anterior pituitary gland.
    • Production of posterior pituitary hormones.

Epithalamus

• The most dorsal portion of the diencephalon.
• Forms the roof of the third ventricle.
• Contains the pineal gland (body):
  • Extends from the posterior border.
  • Secretes melatonin that helps regulate the sleep-wake cycle.

Brain Stem

• The brain stem consists of three regions: the midbrain, pons, and medulla oblongata.
• Similar in structure to the spinal cord but contains nuclei embedded in white matter.
• Controls automatic behaviors necessary for survival.
• Contains fiber tracts connecting higher and lower neural centers.
• Nuclei are associated with 10 of the 12 pairs of cranial nerves.

Inferior View of the Brain Stem

• The inferior view of the brain stem, showing the three parts of the brain stem: midbrain, pons, and medulla oblongata is show in Figure 12.14

Midbrain

• The midbrain is located between the diencephalon and pons.
• Cerebral peduncles: two ventral bulges that contain pyramidal motor tracts, forming pillars that hold up the cerebrum.
• Cerebral aqueduct: a channel running through the midbrain that connects the third and fourth ventricles.
• Periaqueductal gray matter: nuclei that play a role in pain suppression and the fight-or-flight response.
  • Also includes nuclei that control cranial nerve III (oculomotor) and IV (trochlear).
• Midbrain nuclei scattered throughout the white matter include:
  • Corpora quadrigemina: paired dorsal protrusions
    • Superior colliculi: visual reflex centers
    • Inferior colliculi: auditory relay centers
  • Substantia nigra: functionally linked to the basal nuclei; Parkinson’s disease is a degeneration of this area.
  • Red nucleus: relay nuclei for some descending limb flexion motor pathways; part of the reticular formation.

Three Views of the Brain Stem and the Diencephalon

• Three Views of the brain stem (green) and the diencephalon (purple) are shown in Figures 12.13c

Cross Sections Through Different Regions of the Brain Stem

• Cross sections through different regions of the brain stem are shown in Figure 12.15a and 12.15b.

Pons

• The pons is located between the midbrain and medulla oblongata.
• The fourth ventricle separates the pons from the cerebellum.
• Composed of conduction tracts:
  • Longitudinal fibers connect higher brain centers and the spinal cord.
  • Transversal/dorsal fibers relay impulses between the motor cortex and cerebellum.
• Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial).
• Some nuclei play a role in the reticular formation, and some help maintain a normal rhythm of breathing.

Medulla Oblongata

• The medulla oblongata, or medulla, is the most inferior part of the brain stem.
• Blends into the spinal cord at the foramen magnum.
• Contains the fourth ventricle:
  • Continuation of the central canal of the spinal cord.
  • The medulla and pons form the ventral wall of the fourth ventricle.
• Structures of the medulla oblongata:
  • Pyramids: two ventral longitudinal ridges formed by pyramidal tracts from the motor cortex.
  • Decussation of the pyramids: the point where pyramidal tracts cross over to the opposite side of the body.
  • Olives: swellings caused by underlying inferior olivary nuclei that relay stretch information from muscles and joints to the cerebellum.
• Cranial nerves:
• Vestibular and cochlear nuclei: mediate responses that maintain equilibrium.
• Nucleus gracilis and nucleus cuneatus: relay ascending sensory information from the spinal cord.
• Functions of the medulla oblongata:
  • The medulla is an autonomic reflex center; many functions overlap with the hypothalamus.
  • The hypothalamus relays instructions via the medulla.
  • Functional groups of the medulla include:
    • Cardiovascular center:
      • The cardiac center adjusts the force and rate of heart contraction.
      • The vasomotor center adjusts blood vessel diameter for blood pressure regulation.
    • Respiratory centers:
      • Generate respiratory rhythm.
      • Control the rate and depth of breathing (with pontine centers).
    • Various other centers that regulate vomiting, hiccupping, swallowing, coughing, and sneezing.

Cerebellum

• The cerebellum accounts for about 11% of brain mass.
• Located dorsal to the pons and medulla.
• Processes input from the cortex, brain stem, and sensory receptors to provide precise, coordinated movements of skeletal muscles.
• Also plays a major role in balance.

Cerebellar Anatomy

• Cerebellar hemispheres connected by the wormlike vermis.
• Folia: transversely oriented gyri.
• Each hemisphere has three lobes: anterior, posterior, and flocculonodular lobes.
• Contains a thin cortex of gray matter with a distinctive treelike pattern of white matter called arbor vitae.
• Purkinje cells originate in the cortex and synapse with the cerebellum.

Cerebellum Structures

• The Cerebellum structures are shown in figure 12.16a, 12.16b, 12.16c and 12.16d

Cerebellar Peduncles

• All fibers in the cerebellum are ipsilateral—from and to the same side of the body.
• Three paired fiber tracts connect the cerebellum to the brain stem:
  • Superior cerebellar peduncles connect the cerebellum to the midbrain.
  • Middle cerebellar peduncles connect the pons to the cerebellum.
  • Inferior cerebellar peduncles connect the medulla to the cerebellum.

Cerebellar Processing

• The cerebellum fine-tunes motor activity as follows:
  • Receives impulses from the cerebral cortex of intent to initiate voluntary muscle contraction.
  • Receives signals from proprioceptors throughout the body, as well as visual and equilibrium pathways that continuously “inform” the cerebellum of the body’s position and momentum.
  • Cerebellar cortex calculates the best way to smoothly coordinate muscle contraction.
  • Sends a “blueprint” of coordinated movement to the cerebral motor cortex and brain stem nuclei.

Nonmotor Functions of the Cerebellum

• Neuroimaging suggests that the cerebellum plays a role in thinking, language, emotion, and food intake.
• As it does for motor processes, it may compare actual actions of these systems with expected actions and adjust accordingly.

Functional Brain Systems

• Networks of neurons that work together but span wide areas of the brain.
  • Limbic system
  • Reticular formation

Limbic System

• The limbic system is a group of structures on the medial aspects of the cerebral hemispheres and diencephalon.
  • Amygdaloid body: recognizes angry or fearful facial expressions, assesses danger, and elicits a fear response.
  • Fornix: a fiber tract that links limbic system regions.
  • Cingulate gyrus: plays a role in expressing emotions via gestures and resolves mental conflict.
• Includes parts of the diencephalon and some cerebral structures that encircle the brain stem.
• The limbic system puts emotional responses to odors. Example: skunks smell bad.
• Most output is relayed via the hypothalamus; the hypothalamus plays a role in psychosomatic illnesses.
• The limbic system interacts with the prefrontal lobes.
  • Allows us to react emotionally to things we consciously understand to be happening.
  • Makes us consciously aware of the emotional richness in our lives.
• The hippocampus and amygdaloid body also play a role in memory.

Limbic System components

• Figure 12.17 shows the components of the limbic system including Diencephalic structures, anterior thalamic nuclei (flanking 3rd ventricle) Hypothalamus, Mammillary body, Cerebral structures of the limbic system
  Cingulate gyrus,Septal nuclei,Amygdaloid body,Dentate gyrus,Hippocampus,Parahippocampal gyrus and Fiber tracts connecting limbic system structures Fornix and Anterior commissure.

Protection of the Brain - Meninges

• Meninges (singular meninx):
  • Cover and protect the CNS.
  • Protect blood vessels and enclose venous sinuses.
  • Contain cerebrospinal fluid.
  • Form partitions in the skull.
• Consists of three layers (from external to internal):
  • Dura mater
  • Arachnoid mater
  • Pia mater

Meninges: Dura Mater, Arachnoid Mater, and Pia Mater

• The Meninges are shown in Figure 12.22

Meninges - Dura Mater

• Dura mater:
  • The strongest meninx.
  • Made up of two layers of fibrous connective tissue:
    • The periosteal layer attaches to the inner surface of the skull (present only in the brain, not the spinal cord).
    • The meningeal layer is the true external covering of the brain; it extends into the vertebral canal as the spinal dura mater.
    • The two layers are mostly fused, but separate in certain areas to form dural venous sinuses, which collect venous blood from the brain and empty into the jugular veins of the neck.
  • The dura mater extends inward in several areas to form flat partitions that divide the cranial cavity (dural septa), which act to limit excessive movement of the brain.
    • Three main septa:
      • Falx cerebri: in the longitudinal fissure; attached to the crista galli.
      • Falx cerebelli: along the vermis of the cerebellum.
      • Tentorium cerebelli: a horizontal dural fold over the cerebellum and in the transverse fissure.

Dural Septa and Dural Venous Sinuses

• The Dural Septa and Dural Venous Sinuses are shown in Figures 12.23a and 12.23b.

Meninges - Arachnoid Mater

• Arachnoid mater:
  • The middle layer with spiderweb-like extensions.
  • Separated from the dura mater by the subdural space.
  • The subarachnoid space contains CSF and the largest blood vessels of the brain.
  • Recent evidence suggests there may be a fourth meningeal membrane that divides the subarachnoid space into two compartments called the subarachnoid lymphatic-like membrane (SLYM), which contains many immune cells and may be important for monitoring cerebrospinal fluid.
  • Arachnoid granulations protrude through the dura mater into the superior sagittal sinus and permit the reabsorption of CSF back into venous blood.

Meninges - Pia Mater

• Pia mater:
  • Delicate connective tissue that clings tightly to the brain, following every convolution.
  • Contains many tiny blood vessels that feed the brain.

Cerebrospinal Fluid (CSF)

• Cerebrospinal fluid (CSF) forms a liquid cushion of constant volume around the brain.
• Functions:
  • Gives buoyancy to CNS structures, reducing the weight of the brain by 97% by floating it so it is not crushed under its own weight.
  • Protects the CNS from blows and other trauma.
  • Nourishes the brain and carries chemical signals.
• Composed of a watery solution formed from blood plasma, but with less protein and different ion concentrations from plasma. CSF contains more $Na^+, Cl^-, H^+$; and less $Ca^{2+}$ compared with blood plasma.
• Choroid plexus: a cluster of capillaries that hangs from the roof of each ventricle, enclosed by the pia mater and a surrounding layer of ependymal cells. CSF is filtered from the plexus at a constant rate.
• Ependymal cells use ion pumps to control the composition of the CSF and help cleanse the CSF by removing wastes. Cilia of ependymal cells help to keep the CSF in motion.
• The normal adult CSF volume of 150 ml is replaced every 8 hours.

Blood-Brain Barrier

• The blood-brain barrier helps maintain a stable environment for the brain; chemical variations could lead to uncontrollable neuron firings.
• Substances from the blood must first pass through the continuous endothelium of capillary walls before entering neurons.
  • Tight junctions ensure substances pass through, not around, endothelial cells.
  • The feet of astrocytes and smooth muscle–like pericytes surround endothelial cells and help to promote tight junction formation in endothelial cells.
• Substances move through endothelial cells via:
  • Simple diffusion allows lipid-soluble substances to pass freely, as well as blood gases to pass freely through the cell membrane.
  • Specific transport mechanisms move substances important to the brain—facilitated diffusion moves substances important to the brain such as glucose, amino acids, and specific ions. Transcytosis moves larger substances into and out of the brain.
• A thick basement membrane surrounding capillaries is the last part of the barrier substances must pass through that contains enzymes that destroy certain chemicals that would activate brain neurons.
• It is absent in some areas, such as the vomiting center of the brain stem and the hypothalamus, which are necessary to monitor the chemical composition of the blood and body temperature.

The Blood Brain Barrier

• The Blood Brain Barrier is shown in Figure 12.26

Spinal Cord

• The spinal cord is enclosed in the vertebral column, beginning at the foramen magnum and ending at the L1 or L2 vertebra.
• Functions:
  • Provides two-way communication to and from the brain and body.
  • A major reflex center: reflexes are initiated and completed at the spinal cord.

Gross Structure of the Spinal Cord

• The Gross Structure of the Spinal Cord is shown in Figure 12.28 a

Gross Anatomy and Protection

• Protected by bone, meninges, and CSF.
• Spinal dura mater:
  • A single layer that does not attach to vertebrae.
• Epidural space: cushion of fat and network of veins in the space between the vertebrae and spinal dura mater.
• CSF fills the subarachnoid space between the arachnoid and pia maters.
• Dural and arachnoid membranes extend to the sacrum, beyond the end of the cord at L1 or L2, which is the site of lumbar puncture.
• The spinal cord terminates in a cone-shaped structure called the conus medullaris.
• The filum terminale extends to the coccyx (a fibrous extension of the conus covered with pia mater) and anchors the spinal cord.
• Denticulate ligaments are extensions of the pia mater that secure the cord to the dura mater.
• Cervical and lumbar enlargements: areas where nerves servicing the upper and lower limbs arise from the spinal cord. *Spinal nerves:
  • Part of the PNS that attaches to the spinal cord by 31 paired roots.
  • Each spinal cord segment is designated by the paired spinal nerves that arise from it.
• Cervical and lumbosacral enlargements: nerves serving the upper and lower limbs emerge here.
• Cauda equina: a collection of nerve roots at the inferior end of the vertebral canal.

Diagram of a Lumbar Puncture

• A Diagram of a Lumbar Puncture is given in Figure 12.29

Spinal Cord Cross-Sectional Anatomy

• Two lengthwise grooves that run the length of the cord partially divide it into right and left halves:
  • Ventral (anterior) median fissure
  • Dorsal (posterior) median sulcus
• Gray matter is located in the core, with white matter outside.
• The central canal runs the length of the cord and is filled with CSF.

Anatomy of the Spinal Cord

• The cross section of the spinal cord and vertebra is shown in Figure 12.31 a and b

Spinal Cord Gray and White Matter

• Gray matter and spinal roots:
  • The cross section of the cord resembles a butterfly or the letter “H”.
  • Three areas of gray matter are found on each side of the center and are mirror images:
    • Dorsal horns: interneurons that receive somatic and visceral sensory input.
    • Ventral horns: some interneurons; somatic motor neurons.
    • Lateral horns (only in thoracic and superior lumbar regions): sympathetic neurons.
  • Gray commissure: a bridge of gray matter that connects the masses of gray matter on either side and encloses the central canal.
  • Ventral roots: a bundle of motor neuron axons that exit the spinal cord.
  • Dorsal roots: sensory input to the cord.
  • Dorsal root (spinal) ganglia: cell bodies of sensory neurons.
  • Spinal nerves: formed by the fusion of dorsal and ventral roots.
  • Gray matter divided into four groups based on somatic or visceral innervation:
    • Somatic sensory (SS)
    • Visceral sensory (VS)
    • Visceral (autonomic) motor (VM)
    • Somatic motor (SM)
• White matter:
  • Myelinated and nonmyelinated nerve fibers allow communication between parts of the spinal cord, and spinal cord and brain.
  • Run in three directions:
    • Ascending: up to higher centers (sensory inputs)
    • Descending: from brain to cord or lower cord levels (motor outputs)
    • Transverse: from one side to the other (commissural fibers)
  • White matter is divided into three white columns (funiculi) on each side:
    • Dorsal (posterior)
    • Lateral
    • Ventral (anterior)
  • Each spinal tract is composed of axons with similar destinations and functions.

Major Ascending (Sensory) and Descending (Motor) Tracts of the Spinal Cord

• Figure 12.33 shows Major ascending (sensory) and descending (motor) tracts of the spinal cord, cross-sectional view.

Neuronal Pathways

• Major spinal tracts are part of multineuron pathways.
• Four key points about spinal tracts and pathways:
  • Decussation: Most pathways cross from one side of the CNS to the other at some point.
  • Relay: Consists of a chain of two or three neurons.
  • Somatotopy: precise spatial relationships in the CNS correspond to the spatial relationships in the body.
  • Symmetry: pathways are paired symmetrically (right and left).

Ascending Pathways

• Conduct sensory pathways upward through a chain of three neurons:
  • First-order neuron:
    • Conducts impulses from cutaneous receptors and proprioceptors.
    • Branches diffusely as it enters the spinal cord or medulla.
    • Synapses with the second-order neuron.
  • Second-order neuron:
    • Interneuron.
    • Cell body in the dorsal horn of the spinal cord or medullary nuclei.
    • Axons extend to the thalamus or cerebellum.
  • Third-order neuron:
    • Also an interneuron.
    • Cell bodies in the