Neuroanatomy PDF - Nervous System Classification and Structure

Here is the converted text from the images provided, formatted in markdown. ### INTRODUCTION **CLASSIFICATION OF THE NERVOUS SYSTEM** **Anatomically**, it is divided into: 1. **Central nervous system (CNS):** * Includes the brain and spinal cord (fig.1) that are not capable of regeneratio...

Here is the converted text from the images provided, formatted in markdown. ### INTRODUCTION **CLASSIFICATION OF THE NERVOUS SYSTEM** **Anatomically**, it is divided into: 1. **Central nervous system (CNS):** * Includes the brain and spinal cord (fig.1) that are not capable of regeneration, thus are protected by: * Bones (skull & vertebral column). * Three meninges (pia mater, arachnoid mater and dura mater). * Cerebrospinal fluid (CSF) in the subarachnoid space. * The brain is subdivided into (fig.2): * Two cerebral hemispheres (right & left). * Between them is a diencephalon (the thalamus, hypothalamus, subthalamus, metathalamus & epithalamus). * Below it is the brainstem (midbrain, pons & medulla oblongata). * Behind this is the cerebellum. 2. **Peripheral nervous system (PNS)** which includes: * 12 pairs of cranial nerves. * 31 pairs of spinal nerves. * All ganglia. The details of the PNS are described in the regional anatomy books. **Physiologically**, it is divided into: 1. **Somatic nervous system** which is voluntary and includes a motor system and a sensory system. 2. **Autonomic (visceral) nervous system** which is involuntary and includes a sympathetic system and a parasympathetic system. **Embryologically,** the neural tube gives the spinal cord and three brain vesicles (fig.3): 1. Forebrain (prosencephalon) which gives a median diencephalon (its cavity is the 3rd ventricle) and 2 telencephalons or cerebral hemispheres (each contains a cavity is the lateral ventricle). 2. Midbrain (mesencephalon) whose cavity is the cerebral aqueduct of Sylvius. 3. Hind brain (rhombencephalon) which includes the pons, medulla oblongata and cerebellum (its cavity is the 4th ventricle). ### CELLS OF THE NERVOUS SYSTEM I. Neuron = nerve cell It is the building unit of the nervous system; it consists of: 1. cell body (soma) containing the nucleus and cell organelles 2. processes: * Many short *dendrites* (receiving inputs). * One long *axon* (conducting outputs) that terminates by making synapses with dendrites of other neurons. The axons are generally called nerve fibers. * **Types of neurons:** * According to the number of processes: (fig.4) 1. Unipolar (pseudounipolar): as in posterior root ganglion. 2. Bipolar: as in the retina, cochlear & vestibular ganglia 3. Multipolar: as in most parts of the brain & spinal cord. * According to the length of the axon: 1. Golgi type I neuron: of long axon as in long tracts of brain &spinal cord as in (pyramidal cells of cerebral cortex, Purkinje cells of cerebellar cortex & motor cells of spinal cord). 2. Golgi type II neuron: of short axon (inhibitory in function), numerous in all parts of the CNS. II. **Glial cells:** are non-excitable, supporting, protecting and nourishing cells representing half the total volume of the CNS. There are 4 types of glia (fig.5): * **Astrocytes:** These are cells with many branches, forming the main support for the nerve cells & nerve fibers as well as electrical insulators. They are of 2 types: fibrous astrocytes &protoplasmic astrocytes. * **Oligodendrocytes:** These are small cells with few processes, responsible for the formation of the myelin sheath of the nerve fibers of the CNS. * **Microglia:** The smallest glial cells (the only glial cells of mesodermal origin while other glial cells are of ectodermal origin). They act as phagocytes in degenerative and inflammatory conditions. * **Ependyma:** These are cuboidal ciliated cells that line the cavities of the brain & spinal cord. They also form the cells of choroid plexus. They assist in the formation of CSF. ### DEFINITIONS ##### Grey matter & white matter: In the CNS, the cell bodies form the grey matter while the nerve fibers form the white matter. * In the spinal cord, the white matter surrounds the grey matter which is H-shaped. * In the brain stem, the grey matter collects into nuclei embedded in the white matter. * In the cerebral hemispheres and the cerebellum, part of the grey matter collects into deep nuclei and another part spreads on the surface forming the cortex. **Nucleus =** a collection of cell bodies that have the same function within the CNS. Outside the CNS, a collection of cell bodies is called ganglion. **Tract (fasciculus) =** a bundle of nerve fibers *within* the CNS having the same origin, same termination and same function. Bundles outside the CNS constitute nerves. **Pathway =** a chain of successive tracts having the same function (e.g. carrying pain sensation). **Lemniscus =** collection of ascending fibers in the brainstem. **Commissure =** a band of white or grey matter connecting the right & left sides of the CNS across the midline. **Decussation** = a point at which an ascending or descending tract crosses the midline. **Afferent**= input i.e., going towards a certain structure. **Efferent** = output i.e., going away from a certain structure. **Synapse** = site of contact between two neurons; usually between the axon of one neuron and the dendrites of the other neuron. ### SPINAL CORD **EXTERNAL FEATURES** (fig.6) * **Site:** * Occupies the upper 2/3 of the vertebral canal. * Begins opposite the upper border of Atlas vertebra, as a continuation of the medulla oblongata. * Ends opposite the disc between L1 & 2 in adults. At birth, it ends opposite L3. Later, the vertebral column grows faster than the spinal cord. * **Length** 45 cm. * **Shape:** cylindrical but shows:2 enlargements * Cervical enlargement gives origin to brachial plexus (C4-T1). * Lumbar enlargement gives origin to lumbar & sacral plexuses (L1-S4). A tapering lower end called **conus medullaris.)** From its apex, a thin filament of pia mater called *filum terminale* extends down to be attached to the back of coccyx. **31 segments** = 8 cervical, 12 thoracic, 5 lumbar, 5 sacral & I coccygeal. Segments are not demarcated externally but each segment gives origin to a pair of spinal nerves. **Longitudinal grooves:** (fig.7) an anterior median fissure & a posterior median septum divide the cord almost completely into right and left halves. In addition, 2 posterolateral & 2 anterolateral sulci give attachment to posterior & anterior roots of spinal nerves respectively. ### SPINAL CORD **Levels:** spinal cord segments do not lie opposite the corresponding vertebrae * In the cervical region, subtract one from the number of spinal cord segment to get the corresponding vertebra (e.g., C6 segment is opposite C5 vertebra). * In the upper 6 thoracic segments subtract 2 (e.g., T6 segment is opposite T4 vertebra). * In the lower 6 thoracic segments subtract 3 (e.g., T12 segment is opposite T9 vertebra). * In the lumbar segments subtract 4 (e.g., L5 is opposite L1 vertebra). 2 * All sacral & coccygeal segments are opposite L1-L2 vertebrae. **Direction of roots:** C1&2 are horizontal, C3-T12 are oblique, L1-Co are vertical **Exit of spinal nerves from the vertebral canal:** * C1-7 pass above corresponding vertebrae. * C8 passes below C7 vertebra. * T1-L5 pass below corresponding vertebra. (C2-L5 exit via intervertebral foramina) * S1-4 pass via the ant. & post. sacral foramina. * S5 & Co pass via the sacral hiatus. The collection of spinal nerves that surround the film terminale below the termination of the spinal cord (i.e., below L2) is called **cauda equinal)** because it resembles, a horse tail. They occupy the lower 1/3 of the vertebral canal & the sacral canal. **Applied anatomy: Nerve compression** The size of the spinal nerves increases gradually from above downwards. The largest spinal nerve is the 1st sacral. Meanwhile, the size of the intervertebral foramina decreases from above downwards. The 4th & 5th lumbar nerves are the most liable to compression. ### SPINAL CORD □ **3 meningeal coverings of spinal cord:** (fig10) 1. The **dura mater** (outer layer) & **arachnoid mater** (middle layer) form one tube together. Above, it continues with the cerebral meninges at the foramen magnum. Below, it ends opposite S2 2. **Pia mater** (inner layer) is adherent to the cord and continues below as the *filum terminale* which pierces the tube of dura & arachnoid to be attached to the back of coccyx. **Spaces between meninges:** (fig11) 1. **Extradural (epidural) space:** between the dura and walls of vertebral canal. Contains fat, small arteries, venous plexus & lymphatics. 2. **Subdural space:** between the dura & arachnoid. Contains a thin film of fluid 3. **Subarachnoid space:** between the arachnoid & pia. Contains CSF + 3 ligaments supporting the spinal cord: * *Filum terminale.* * *Ligamentum denticulatum:* one on each side of the cord, extending laterally between the anterior and posterior roots of spinal nerves. Laterally, it has 21 teeth connecting the pia (on one side) to the arachnoid and dura (on the other side). * *Subarachnoid septum:* extends from the posterior median septum to the arachnoid mater. A needle is introduced to the spinal subarachnoid space below the end of the spinal cord * Applied: Lumbar Puncture (fig10) * Site:Just above or just below the tip of 4th lumbar spine (which lies opposite an imaginary line connecting the highest points of iliac crests). **Uses:** 1. **Diagnostic:** to measuring CSF pressure, obtain sample for analysis (meningitis), or inject air for contrast X-ray (air, encephalography). 2. **Therapeutic:** to remove some CSF to relieve increased intracranial tension, or to inject antibiotics or spinal anesthesia. ### SPINAL CORD **INTERNAL STRUCTURE** * The spinal cord is formed of a central H-shaped gray matter surrounded by white matter (fig.12). | | | | :------ | :------------------------------------------------------- | | **Parts** | | | Gray matter | White matter | | 1. Anterior horn [motor] | 1. Anterior column (funiculus). | | 2. Posterior horn [sensory] | 2. Lateral column (funiculus). | | 3. Lateral horn [autonomic] | 3. Posterior column (funiculus). | | Structures | Structures | | 1. Nerve cells (nuclei) | Myelinated nerve fibers (tracts) | |2. Unmyelinated nerve fibers. | Neuroglia. | |3. Neuroglia. |Capillaries. | |4. Capillaries. | | Its center contains a narrow **central canal** extending throughout the length of spinal cord. The cord is divided into right & left halves by an **anterior median sulcus** & a **posterior median septum.** The two halves are connected by 3 commissures: 1. White commissure: behind the anterior median sulcus. 2. Anterior grey commissure: infront of the central canal. 3. Posterior grey commissure: behind the central canal. ### SPINAL CORD **ATTACHMENT OF SPINAL NERVES (**fig.14**) A. Each spinal nerve arises from a spinal cord segment by 2 roots:** 1. Anterior root: contains * Motor fibers (from the anterior horn) * Autonomic fibers (from the lateral horn) 2. Posterior root: purely sensory. Carries. post. root ganglion (spinal ganglion) containing pseudounipolar neurons. * Their peripheral processes (dendrites) pass peripherally * Their central processes (axons) enter the cord at the posterolateral sulcus. **B. Both roots unite forming a mixed nerve, which exits the vertebral canal through the intervertebral foramen IVEY and soon divides into 2 rami (both are mixed):** 1. Anterior ramus (large): * All anterior rami form plexuses except the 12 thoracic (intercostal nerves) * Only 14 anterior rami (12 thoracic + upper 2 lumbar) send white rami communicants [preganglionic] to sympathetic ganglia. * All 31 anterior rami receive grey rami communicants [postganglionic] from sympathetic ganglia. 2. Posterior ramus (small): supplies muscles & skin of the back. Note: (Both rami of CL spinal nerve are purely motor * Recurrent meningeal nerves It is the first branch of the mixed spinal nerve, just outside IVF. it re-enters the spinal canal via IVF to supply the dura, periosteum, blood vessels & I.V. discs. It plays a role in referred pain or occipital headache. ### SPINAL CORD **Nuclei of Grey matter of Spinal Cord (**fig.15**) * In Dorsal Horn:Nuclei are mainly sensory 1. Substantia Gelatinosa of Rolandi: Present at tip of dorsal horn in all segments of spinal cord. Function: pain modulation. Temp 2. Nucleus Proprius: Present anterior to Substantia Gelatinosa in all segments of spinal cord. Function: relays exteroception. Touch 3. Nucleus Dorsalis "Clarke's Nucleus": Present at the base of dorsal horn in C8 to L3 segments of the spinal cord. Function: relays unconscious proprioception. 4. Visceral Afferent Nucleus: Present in C8 to L3 segments of the spinal cord lies lateral to Clarke's Nucleus. Function: relays visceral sensations. * In Lateral Horn: Contains the intermediate nucleus present in thoracic & upper 3 lumbar segments. It is further divided into Intermedio-medial & intermedio-lateral nuclei. These are sympathetic neurons whose axons pass in the ventral root of the corresponding spinal nerves to reach the ganglia of the sympathetic trunk. * A similar group of autonomic neurons “Sacral Parasympathetic” is present in S2, 3, 4 segments of the spinal cord but these do not form a lateral horn. * In Ventral Horn: Nuclei are mainly motor neurons which are either 1. Alpha-motor neurons (anterior horn cells- AHC): Large, their axons pass in ventral root to supply extrafusal muscle fibers. 2. Gamma- motor neurons: Small, their axons also pass in ventral root to supply intrafusal muscle fibers (muscle spindles) The nuclei in the ventral horn are arranged in three groups: 3. Medial Group: present throughout the whole length of the spinal cord and supply trunk muscles 4. Central Group: present only in some cervical segments e.g. Phrenic Nucleus C3,4,5 & spinal accessory nucleus (C1-5). 5. 43. Lateral Group: present in cervical & lumbosacral segments and supply limb muscles. ### SPINAL CORD **Grey matter Laminae ‘of Rexed'** Rexed (1964) described 10 laminae in the grey matter of the spinal cord depending on neurons size, density, shape & cytological features. 1. 6 * Laminae I -VI: are sensory & occupy the posterior horn. * L I = marginal layer of Waldeyer * L II + part of LIII = Substantia gelatinosa of Rolandi. * The rest of LIII + L IV = Main sensory nucleus. * Lamina VII occupies the lateral horn & extends into the middle part of the anterior horn. * Clarke's nucleus. * Lateral horn nuclei (intermediolateral & internediomedial). * Middle part of anterior horn (between L VIII & IX), contains Renshaw cells. 7. Laminae VIII- IX occupy the anterior horn. * L IX is lateral. It contains the motor neurons. * LVIII is medial. It controls the muscle tone. 8. Lamina X surrounds the central canal. ### SPINAL CORD **TRACTS OF WHITE MATTER (**fig.16**)** I. Ascending tracts (sensory):includes3 main groups: A. Lemniscal system: : lie in the dorsal column - carries conscious proprioception) (from deep structures such as muscles & joints) to the cerebral cortex: I. Gracile tract [lower body proprioception] II. Cuneate tract [upper body proprioception] B. Unconscious proprioceptive tracts(to cerebellum): lie superficially in the lat. column I. Two spino-cerebellar tracts (Post. & Ant.) II. Spino-olivary tract C. Anterolateral system: lie in the ant. and lat. columns - carries exteroception Lat. Spinothalamic tract [pain & temp] Ant. spinothalamic tract [crude touch] II. Spino-reticular tract III. Spino-tectal tract II. Descending tracts (motor): A. Pyramidal: lateral & anterior corticospinal tracts. B. Extrapyramidal: * From the midbrain: rubro-spinal tract & tecto-spinal tract * Reticulo-spinal tracts: medial & lateral III Vestibulo-spinal tracts: medial & lateral Olivo-spinal tract. * Intersegmental tracts (propriospinal): Surround the grey matter forming the fasciculus proprius anterior, lateralis & posterior. Contains ascending and descending short axons of interneurons between adjacent segments of spinal cords ### SPINAL CORD **ASCENDING TRACTS** Somatic sensory pathways Form parts of the sensory pathways. A sensory pathway is formed of three neurons Vs First-order Neuron is always a pseudounipolar cell of the Dorsal Root Ganglion. It or carries The sensation by its peripheral process from receptors & conveys this sensation by its or central processes to cranial to the dorsal root to Location Cranial DRG, Cranial nerve the spinal_cord in nerve in nerves Second-order neuron is always a cell in the CNS (spinal cord or medulla oblongata). Its axon always decussates to the opposite side and ascends in the brainstem as lemniscus to end in the thalamus. interneuron within CNS on & cord neuron to the neuron. Synapse brain in Cord locate in spinal Spinal Third-order Neuron is always cells of the Ventral Postero-Lateral Nucleus of cord that brain cells. Thalamus (VPLN). Their axons pass through posterior limb of internal capsule, then through the area cerebrum cerebral to thalamus reach cortex cortex through cerebral cerebral cortical So 3rd corona radiata, cord. - to 2nd So 1st Gracile & Cuneate tracts (Posterior column, Dorsal column tract) (fig.17) Function: carries proprioception (deep sensations) & fine touch. Gracile tract from the lower part of body (below T6). Cuneate tract from the upper part of body (above T6). Beginning: They are formed by the central processes of the dorsaI roo pseudo unipolar cells. They enter spinal cord via ganglion The dorsal. Spinal cord: they occupy the with cord in dorsal the in Position cord dorsal column the Gracile segment the Cuneate segments to lies to it. The in sacral for the segments & tracts to is above the in dorsal colum lamination Dorsal. lateral lie most cervical fibers the tracts the In that The cervical fiber the colum. cuneate tracks dorsal in spinal cord reach ends the spinal cords dorsal horn of in cuneate reach ### Proprioception Pathways. Spinal Cord Pathway of conscious proprioception& fine touch (fig.18) | Proprioception (deep sensations) | Fine touch (complex touch) | | :------------------------------------------ | :-------------------------------------- | | 1. Sense of position. | 1. Tactile discrimination. | | 2. Sense of movement. | 2 Tactile localization. | | 3. Sense of vibration. | Stereognosis. | * First Neuron Dorsal which pro carry deep in their to the Fibers part body from tract ascend 1. the Fibers from upper (above column form 2. cuneate lamination fibers tracts cervical * N: during, that these these end the that form the tract & tract some end the fiber for the these fibers (to by cerebellum * Second tracts Gracile of oblongate Axons these the fiber axons to the lemnis thalamus. : dorsal of the through limb, reach Third that SPINAL CORD ### posterior spinocerebellar Tract. SPINAL CORD **lateral white blumn. Spinal Cord-Four tracts carry unconscious proprioception to the cerebellum (**fig.20**)**. Four tracts muscle carry tracts four carries cerebellum. muscles that no. third newton 1. Carries proprioception from the lower limb & trunk. The that enter the the reach that Clarke the . in the tracks , in the 2 the the to via in 21 from . the that a. Carries proprioception from the lower trunk tracts. b. The center enters the spinal cord and to form that must . fibers the through ,to reach for for that 3. spinal . tracts tract ,. . carry tracts trunk tracts spinal that in is reach to for reaches the with lateral ### SPINAL CORD Lateral Spinothalamic Tract (**fig.23**) Function: it is the 2nd-order neuron of pain &temperature from the opposite side of the body to the thalamus. Begins from neurons of Laminae I& IV-VIII. Their axons decussate in the ventral white commissure. Position in spinal cord: ascends i-n the lateral white column medial to the anterior spinocerebellar tract. Lamination cervical fibers are most medially and sacral fibers most laterally. Termination: The tract ascends in the brain stem as the spinal lemniscus to reach the VPLN of thalamus. 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