Midterm PDF - Neuroscience

1. What are the basic components of the nervous system and what is their function? Neurons: Neurons are the functional elements of the nervous system. Unlike other cell types present in the body, neurons are unique in several ways and primarily communicate with other cells through synapses. In a bro...

1. What are the basic components of the nervous system and what is their function? Neurons: Neurons are the functional elements of the nervous system. Unlike other cell types present in the body, neurons are unique in several ways and primarily communicate with other cells through synapses. In a broad sense, a neuron is a basic functional unit of the nervous system. It contains a cell body with a nucleus – called the ‘perikaryon’. Glial Cells: Glia are cells found in the nervous system that support neurons. They provide nutrition to neurons and help with ‘neurotransmission’. However, they do not directly participate in neurotransmission. The glial cells form ‘myelin’, which surrounds nerve bers and provide insulation, protection and support. Endothelial Cells: Endothelial cells for a thick layer lining the inside of the blood vessels called ‘endothelium’. They have di erent functions depending on their origin. For example, in the central nervous system, endothelial cells maintain the blood-brain-barrier and protect the CNS against detrimental agents. 2. How can you classify the neurons based on its processes? Neurons can be broadly classified into 3 types based on its processes: 1. Unipolar (Pseudounipolar): The dendrite and axon come together to form a single process to the cell body. These neurons are also called ‘pseudounipolar’. 2. Bipolar: The dendrites join together to form a common trunk before they reach the cell body at a site that is different from that of the origin of the axon. 3. Multipolar: A number of dendrites join the cell body at different points. Most neurons are multipolar. ff fi Neuroscience 3. What are the di erent types of connections (synapses)? ff Several different types of connections are known to exist: Axosomatic – between an axon of one neuron and a cell body of another neuron. Axodendritic – between an axon of one neuron and a dendrite of another neuron. Axoaxonic – between an axon of one neuron and an axon of another neuron. Dendrodendritic between dendrites of different neurons. 4. What are the di erent types of glia and what is their function? In general, there are three types of glial cells in the CNS: astrocytes, oligodendrocytes and microglia (please keep in mind that this is a general classification – there are other sub-classifications that have been proposed). Astrocytes: The astrocytes are typically star shaped (‘astron’ in Greek is Star) and are found in the brain and the spinal cord. They are histologically identified by the presence of a protein called ‘glial fibrillary acidic protein (GFAP)’. They are involved in multivarious functions that are critical for CNS function. Some of these include providing metabolic support for neurons, maintenance of blood-brain barrier, repair of injury to the nervous system, regulating ion concentrations, etc. Oligodendrocytes: The oligodendrocytes mainly provide support to axons of the CNS by wrapping around axons and forming the myelin sheath. ff Microglia: The microglia are cells of immune origin and are very critical for the immune defense mechanism of the CNS. There are several types of microglia and they have the ability to undergo changes in their structure to enable them to participate in immune functions in the CNS. 5. What is axoplasmic ow and how is it classi ed? Axoplasmic ow - Movement of materials inside axons. Anterograde (orthograde) axoplasmic flow - Movement away from the cell body. Retrograde axoplasmic flow - Movement toward the cell body. 6. Understand the regional di erentiation of the neural tube Cranial end of neural tube expands into series of dilated vesicles which is the rudimentary brain. 1. Initially three vesicles develop: a. Prosencephalon - most rostral b. Mesencephalon – intermediate c. Rhombencephalon - more caudal 2. Subsequent subdivision produces five vesicles: a. Telencephalon - paired dorsolateral evaginations from rostral end of prosencephalon b. Diencephalon - remaining midline portion of prosencephalon c. Mesencephalon - essentially unchanged from 3-vesicle stage d. Metencephalon - expanded rostral portion of rhombencephalon e. Myelencephalon - tapered terminal portion of rhombencephalon fi ff fl fl Each of these vesicles develops into a different part of the CNS as shown in Fig 2.2. 7. How is neural tube divided functionally? The primitive neural tube is subdivided by a longitudinal groove called the sulcus limitans which runs the length of the tube on each side of the lumen. A horizontal plane through each sulcus limitans separates the neural tube into dorsal and ventral portions. Fig 2.3. Cross section of the neural tube. 1. The dorsal portion is called alar plate and the neurons of the gray matter of the alar plate are primarily sensory in function. This sensory zone can be subdivided into specific regions for specialized sensory activity: a. General somatic afferent (GSA) - neuron cell bodies in this area are influenced by receptors in the body wall sensitive to touch, pressure, temperature, and pain (Fig. 2.4; 2.5). b. Special somatic afferent (SSA) -neurons related to special senses of vision and hearing (located in the brain). c. Special visceral afferent (SVA) -neurons related to special senses of olfaction and taste (located in the brain). d. General visceral afferent (GVA) - neurons in this area are influenced by sensory receptors in the viscera and blood vessels (Fig. 2.4; 2.5). 2. Ventral portion is called basal plate and neurons of the gray matter of the basal plate are predominantly motor in function. This motor zone can also be subdivided into specific regions for specialized motor activities: a. General visceral efferent (GVE) - neurons in this area innervate smooth muscle of viscera, blood vessels and other body structures, and cardiac muscle; therefore, GVE is the autonomic nervous system (Fig. 2.4; 2.5). Fig 2.4. Cross section of the spinal cord with sensory and motor zones. b. Special visceral efferent (SVE) -these neurons innervate skeletal muscles derived from branchial arches and includes muscles of jaws, face, pharynx, larynx, and esophagus, and the trapezius muscle (Fig. 2.5). c. General somatic efferent (GSE) - these neurons innervate skeletal muscles of the trunk and limbs and non-branchial skeletal muscles (extraocular and tongue muscles) (Fig. 2.4; 2.5). 8. Understand the regional anatomy of the brain and understand that major structures associated with speci c areas. A. Brain stem is the segmental, roughly cylindrical portion of the adult brain derived from the following: 1. Myelencephalon 2. Ventral metencephalon 3. Mesencephalon 4. Diencephalon B. The remainder of the adult brain is composed of the two following expanded suprasegmental portions: 1. Dorsal metencephalon (Cerebellum) 2. Telencephalon fi Fig 2.6. Schematic division of the brain. The hindbrain (rhombencephalon = Metencephalon + Myelencephalon), midbrain (Mesencephalon), forebrain (Diencephalon+Telencephalon). The hindbrain (Rhombencephalon) consists of the medulla oblongata (myelencephalon), pons (ventral metencephalon) and cerebellum (dorsalmetencephalon). The medulla oblongata caudally continues as the spinal cord. The cerebellum is a globular mass of neural tissue connected to the brain stem by three peduncles (like the stem of an inflorescence). The cerebellum is separated from the cerebral hemispheres by a transverse fissure and is physically separated by ‘tentorium cerebelli’. The cerebellum consists of two lateral hemispheres and a narrow middle portion (median ridge) called ‘vermis’. The cerebellum controls balance and is responsible for coordinating postural and locomotor activities. The midbrain is also known as the mesencephalon and is made of the crura cerebri, tectum, tegmentum and cerebral peduncle. The forebrain comprises of the diencephalon and the telencephalon: The diencephalon is the anterior most portion of the brainstem. The most ventral part of the diencephalon is the hypothalamus. The hypothalamus itself is further subdivided into number of discrete regions and regulates most of the central and neuroendocrine functions in the body. Along with hypothalamus, the diencephalon also includes the thalamus and the epithalamus. The epithalamus includes the pineal gland. The thalamus functions as the relay and integration center of the brain. The telencephalon is formed by the two cerebral hemispheres. II. External anatomy A. Myelencephalon 1. This embryonic vesicle forms the medulla oblongata of the adult brain 2. The medulla is similar in external appearance to the spinal cord: a. On the dorsal surface the following features are visible from midline to lateral surface: i. Dorsal median fissure ii. Fasciculus gracilis (terminates as nucleus gracilis) iii. Fasciculus cuneatus (terminates as nucleus cuneatus) b. On the dorsal surface of the rostral medulla is a diamond-shaped depression called the rhomboid fossa which, when covered by a thin roof, is part of the ventricular system of the brain (4th ventricle). A number of cranial nerves arise from here. Fig 2.7 Schematic of the myelencephalon. Figure on left: Courtesy- Hitachi healthcare. Figure on right courtesy: Scientific e-books. B. Metencephalon 1. Dorsal metencephalon a. Consists of cerebellum, a highly-convoluted enlargement composed of paired lateral hemispheres and a single midline vermis 2. Ventral metencephalon a. Consists of the pons, a ventral bulge composed of transverselyrunning fibers. Pons means "bridge." 3. Connecting the pons (ventral metencephalon) and cerebellum (dorsal metencephalon) on each side are three pairs of stalks or cerebellar peduncles 4. The space between the pons ventrally, cerebellum dorsally and the three cerebellar peduncles laterally is the rostral extent of the 4th ventricle, seen already to occupy most of the rostral portion of the medulla. Fig 2.8. Schematic on left depicts the metencephalon. Courtesy: Scientific American. Figure on right depicts the Mesencephalon and the Metencephalon. Courtesy: Brainmind.com. C. Mesencephalon 1. This embryonic vesicle remains rather simple, forming the midbrain, which extends from the pons and cerebellum caudally to the diencephalon rostrally. 2. Ventrally the midbrain features a pair of large fiber bundles called cerebral peduncles which converge toward the pons. 3. Dorsally, the midbrain displays four swellings called the corpora quadrigemina a. The more rostral pair consists of the rostral colliculi b. The caudal colliculi comprise the caudal pair. D. Diencephalon 1. This portion lies rostral to the midbrain, and except for the ventral surface, is hidden by the cerebral hemispheres 2. Ventrally the diencephalon has the following features from caudal to rostral: a. Mamillary bodies - paired enlargements just rostral to midbrain b. Tuber cinereum - a single midline swelling which is continuous with the infundibulum (hypophyseal stalk – pituitary stalk). c. Optic chiasm - this x-shaped structure represents the junction and partial decussation of the optic nerves (Cranial Nerve II). 3. Dorsally the diencephalon has a small caudally-projecting enlargement called the pineal body, which is completely hidden by the cerebral hemispheres Fig 2.9 Schematic depicting the diencephalon in relation to other structures. E. Telencephalon 1. The major portion of this part of the brain is visible as the greatly expanded cerebral hemispheres 2. Exposed dorsal and lateral surface of the hemisphere is convoluted, forming raised folds (gyri) and deep grooves (sulci). 3. Lobes of the brain a. Frontal lobe b. Occipital lobe c. Temporal lobe d. Parietal lobe e. Pyriform lobe - ventral to rhinal sulcus, this area appears as a single large gyrus intimately connected with the more rostral olfactory peduncle and bulb. Thus, the pyriform lobe is part of the phylogenetically older "olfactory portion" of the cerebrum. 4. Covering the ventral surface of the rostral cerebral hemispheres are paired olfactory peduncles and bulbs (essentially, tracts of the brain). The olfactory bulbs lie over the cribriform plate of the ethmoid bone of the skull and are connected with the numerous olfactory nerves (CN I) which penetrates the cribriform plate and are distributed to nasal epithelium. Fig 2.10. Schematic showing the telencephalon in relation to other structures. Fig on left courtesy: Scientific American. FIg on right courtesy: Medicine for all 9. Know the cranial nerves Cranial nerves can be classified into groups based on 3 specific types of innervation: 1. Nerves, that are responsible for special senses; 2. Nerves that provide innervation to head muscles; 3. Nerves, that provide innervation to structures that originate from the pharyngeal arch. In spite of this possible grouping, traditionally cranial nerves have been dealt with in a numerical order from anterior to posterior (rostrocaudally) based on their origin from the brain. The following are the 12 cranial nerves and their origins (Fig. 2.12): 1. Olfactory nerves (CN I): Fibers of the olfactory nerves pierce through the cribriform plate of the ethmoid bone of the nasal cavity. The cell bodies of these fibers are located in the olfactory bulb of the brain. 2. Optic nerve (CN II): Optic nerve is responsible for vision. It arises from the diencephalon and connects the diencephalon with the retina of the eye. The optic tracts decussate (cross over) at the optic chiasm. The optic nerve exits through the optic foramen in the skull. 3. Occulomotor nerve (CN III): The occulomotor nerve contains both somatic and autonomic fibers (visceral efferent – parasympathetic) (Please review the somatic vs autonomic nervous system in the powerpoint that has been provided). Occulomotor nerve exits from the midbrain (both the somatic and the parasympathetic component) close to the midline. The somatic component of the occulomotor nerve provides motor innervation to some of the muscles of the eyeball (Dorsal, medial and ventral recti muscles; levator palpebrae superioris and the ventral oblique). Note: recti are straight muscles. Levators are involved in elevating or lifting structures. Palpebrae are eyelids. So levator palpebrae superioris is the muscle that lifts the upper eyelid- hence opens the eye. 4. Trochlear nerve (CN IV): It is one of the smallest nerves of the 12 cranial nerves and arises from the midbrain and innervates the dorsal oblique muscle of the eyeball and exits through the orbital fissure of the skull. 5. Trigeminal nerve (CN V): It is the largest sensory nerve among cranial nerves. This also provides motor innervation to muscles of mandibular (lower jaw) origin. The nerve has three divisions: Ophthalmic (pertaining to eye), Maxillary (pertaining to upper jaw) and Mandibular (pertaining to lower jaw). The mandibular nerve exits through the oval foramen of the skull. The ophthalmic and maxillary divisions exit through the orbital fissure and round foramen of the skull respectively. 6. Abducent nerve (CN VI): This nerve arises from the caudal brainstem and exits the skull through the orbital fissure. It innervates the lateral rectus and retractor bulbi (draws the eyeball into the socket) muscles of the eyeball. 7. Facial nerve (CN VII): This nerve originates from the brain stem and exits the skull through the stylomastoid foramen. It also has a parasympathetic component. The somatic component of this nerve provides innervation to muscles of facial expression. 8. Vestibulocochlear nerve (CN VIII): This nerve originates from the brain stem and is responsible for the special senses related to vestibular (location and movement perception functions of the ear) and cochlear ( auditory part of the ear) function. 9. Glossopharyngeal nerve (CN XI): This nerve has both parasympathetic and somatic sensory and motor components. This nerve originates from the brainstem. It provides sensory innervation to the mucosa of the tongue and the pharyngeal regions. It also provides some motor innervation to the pharyngeal region. 10. Vagus nerve (CN X): The vagus nerve contains visceral afferent, parasympathetic and somatic efferent (motor) fibers. It provides parasympathetic innervation to visceral organs in the thoracic and abdominal area. Most of the fibers in vagus provide afferent information from the viscera to the brain. The small motor component is responsible for providing innervation to the laryngeal muscles (details will be dealt with later). 11. Accessory nerve (CN XI): The accessory nerve originates from the brain stem and divides into dorsal and ventral branches. The dorsal branch provides innervation to muscles in the neck (brachiocephalicus, omotransversarius, trapezius) and the ventral branch innervates sternocephalicus. 12. Hypoglossal nerve (CN XII): This nerve arises from the brain stem and exits the skull through hypoglossal canal and provides motor innervation to muscles of the tongue. Fig 2.1. Schematic of the ventral surface of the brain showing the emergence of various cranial nerves. 10. Know the membranes of the brain and spinal cord Three membranes, namely, dura mater, arachnoid and pia mater surround the brain and the spinal cord. The pia mater is the innermost layer and closely adheres to the brain and the spinal cord. The dura mater serves as the outermost layer and the arachnoid is situated in between dura and pia mater. Dura mater is composed of dense connective tissue and because of this is the thickest of the three layers. In the cranium, it closely lines the inner aspect of the skull, however, in the vertebral canal, there is space between the periosteum that cover the vertebrae and the dura. This space is referred to as the ‘epidural space’. The arachnoid is pushed against the dura mater because of the presence of cerebrospinal fluid. Innumerable filaments from the arachnoid attach to the pia mater. The arachnoid is the thinnest of the layers. Pia mater (inner surface) is fused with the brain and the spinal cord. It is thinner than dura mater but thicker than the arachnoid. 11. Know the ow of CSF fl The ventricular system plays an important role in circulating the CSF. The following are the critical steps. I. Choroid plexus of the lateral ventricle - Ependymal cell layer and capillaries - site of formation of cerebral spinal fluid II. CSF travels through the Lateral ventricles - located in the cerebral hemispheres and enters the third ventricle through “Foramen of Monro”. III. The flow of CSF is then increased by the plexus in the third ventricle, which is located in the diencephalon between the two halves of the thalamus and hypothalamus IV. Then, the CSF passes caudally through the Cerebral aqueduct (also called as mesencephalic aqueduct or Cerebral aqueduct of Sylvius) located in the midbrain and reaches the Fourth ventricle V. The plexus in the Fourth ventricle, which is located in the pons and medulla is further increased by the plexus here. A. Lateral foramina - allow passage of cerebrospinal fluid (CSF) into the subarachnoid space B. Median foramen - dorsal midline opening into the subarachnoid space (not found in some species) VI. From the Fourth ventricle, CSF enters the Central canal - extends for varying distances into the spinal cord VII. Clinically-important areas of the subarachnoid space A. Cisterna magna - between cerebellum and medulla; preferred area to draw CSF. B. Spinal cistern - located caudal to L6 vertebral level; contains cauda equina VIII. Hypocephalus A. Non-communicating (obstructive) - caused by a block within the ventricular system; most commonly seen; may result in disruption and atrophy of white and gray matter - cerebrum (cortex and underlying white matter) may be reduced to 3-4 m thick. There may be a loss of gyral patterns and herniation of the medulla cerebellum B. Communicating - CSF is not absorbed from the subarachnoid space; may result in enlargement of entire ventricular system; not commonly seen. fi fi ff 12. Understand the organization of gray matter – speci cally, know the di erences and similarities between nuclei, cortices and reticular formation; know the important nuclei in speci c areas of the brain such as telencephalon, diencephalon, mesencephalon and metencephalon. Gray matter - contains cell bodies and is present in the brain in three basic organizations: 1. Nuclei - discrete clusters of neurons 2. Cortices - sheets of neurons, usually folded 3. Reticulum - diffuse loosely organized network of neurons There are also nuclei within the rostral portions of the brain which are not related to any specific cranial nerve, but rather have broad sensory and/or motor functions. With the exception of the red nucleus, we will not spend much time in discussion of this area. a. Telencephalon i. Corpus striatum - this is a large cluster of nuclei in the base of each cerebral hemisphere. Individual nuclei in this complex are: - caudate nucleus - a long nucleus composed of an enlarged head rostrally and a tapered body and tail extending caudad which lies in the lateral wall of the lateral ventricle. - putamen - ovoid nucleus lying lateral to head of caudate nucleus in rostral cerebral hemisphere. - globus pallidus - small ovoid nucleus lying medial to putamen ii. Amygdala - nuclear mass in rostral portion of each pyriform lobe b. Diencephalon i. Thalamus - this is a large egg- shaped group of nuclei occupying most of diencephalon on either side of the third ventricle. The paired thalami are connected across the ventricle by the massa intermedia (interthalamic adhesion). ii. Hypothalamus - this paired group of nuclei lies, as the name suggests, below the thalamus on either side of 3rd ventricle (but unconnected). c. Mesencephalon i. Red nucleus (nucleus ruber) - a pair of small nuclei in rostral midbrain, lying ventral to cerebral aqueduct on either side of midline. ii. Substantia nigra - a pair of flattened nuclei ventral to red nuclei in rostral midbrain. Characterized by presence of melanin pigment (hence the name). d. Cerebellum - several nuclei are found in this brain region, clustered at the dorsal end of the cerebellar peduncles. e. A wealth of other nuclei are found throughout the brain, most quite small and poorly understood. 13. White matter organization – tracts vs commissures Located in areas of brain not occupied by gray matter or cells; thus, white matter of brain is more irregular in its organization than that of spinal cord. Grouped into two basic categories: 1. Tracts - these are the long bundles of nerve fibers with white matter which interconnect brain and spinal cord, and shorter bundles which interconnect different brain regions, usually on same side of midline. 2. Commissures - these white matter bundles interconnect similar brain regions across the midline. 14. Organization of the spinal cord – white matter vs gray matter (dorsal, ventral and lateral horns) The spinal cord consists of an H-shaped core of gray matter (in transverse/cross section) which is primarily neuron cell bodies and glia, and a peripheral layer of white matter which is primarily nerve cell processes and glia. 15. Understand the regional variations in spinal cord. Externally the spinal cord varies in gross appearance at different cord levels. The outline of the spinal cord in transverse section changes from distinctly ovoid in the cervical region to circular in the thoracic region, then to ovoid or roughly triangular in the lumbar region, and circular once again in the sacral and coccygeal region. The diameter of the transverse section is greatest in the cervical region, smaller in the thoracic region, increasing slightly in the lumber region, and smallest in the sacral and coccygeal region. Although some spinal root fragments may be seen in all transverse sections of the cord, the greatest accumulation of roots would be seen surrounding the sacral and coccygeal cord segments. This is called the auda equina. 1. Cervical enlargement - lower cervical and upper thoracic region 2. Lumbar enlargement - lower lumbar and upper sacral region 3. Conus medullaris - formed as the lower sacral and coccygeal region tapers to the cone-shaped terminus of the spinal cord 4. Cauda equina - formed caudal to the end of the spinal cord by the numerous sacral and coccygeal nerves as they fan out like the hairs of a "horses tail”. Internally the spinal cord also varies in cross-sectional appearance at different cord levels. Gray matter variation 1. Dorsal horn - variably sized and shaped at different levels. 2. Lateral horn - present only in thoracic and upper lumbar regions (location of origin of sympathetic nn). 3. Ventral horn - greatly enlarged at lower cervical/upper thoracic and lower lumber/upper sacral regions due to increased efferent output to the limbs, which are located at those levels. White matter variation 1. Dorsal funiculus - Thicker in the more cranial portion of the cord because in the upper thoracic and cervical region both the fasciculus gracilis and fasciculus cuneatus are present, whereas in the caudal areas only the fasciculus gracilis is present. 2. Ventral and lateral funiculi - Also thicker in the more cranial portions because the ascending and descending tracts contain more fibers here. 16. Anatomy of the lower motor neurons – its location Lower motor neuron: The somatic efferent neuron which begins in the brainstem or ventral horn of the spinal cord and leaves to innervate skeletal muscle. LMN’s are the last site of synaptic integration of all afferent input and descending pathways which modify skeletal muscle activity. Location of the lower motor neurons in the spinal cord Cell bodies of LMN's in the spinal cord are located in the ventral horn of the gray matter 1. LMN's innervating trunk (axial) skeletal muscle are located within the medial nucleus. 2. LMN's innervating limb (appendicular) skeletal muscle are located within the lateral nucleus Just as a group of LMN's supplying skeletal muscle in a specific region of the body may be localized within the ventral horn, so may a group of LMN's innervating a certain individual skeletal muscle be localized. These clusters of LMN's innervating specific skeletal muscles are called motor pools. Example: Within the ventrolateral nucleus of the ventral horn in the lumbar enlargement would be individual groups of LMN's supplying the rectus femoris, vastus lateralis, vastus intermedius and vastus medialis mm, all proximal mm of the thigh which extend the stifle joint. Each individual group of LMN's, immediately adjacent but distinct from one another, would be a motor pool, one for the rectus femoris, one for the vastus lateralis, etc. A single LMN innervates many skeletal muscle fibers (the motor unit). Therefore, the motor pool represents the central location for the motor neurons supplying all of the muscle fibers of a given skeletal muscle. 3.5.2. Location of the lower motor neurons in the brainstem LMN's are also located within the brainstem. Somatic efferent neurons innervating skeletal muscles of the head (extraocular muscles, tongue muscles, etc) are located in nuclei within the brainstem. These somatic efferent neurons are also LMN’s. Motor pools for individual muscles can also be described within these brainstem nuclei. Example: The oculomotor nucleus can be subdivided into individual motor pools supplying the dorsal rectus muscle, medial rectus muscle, levator palpebrae superioris muscle and so forth. 17. General organization of primary a erents – di erent components, receptors, cell bodies, conductile segments and transmissive regions. The general organization of primary afferents, which are sensory neurons in the peripheral nervous system, involves several key components: Receptors: These are specialized structures that detect changes in the environment, such as mechanical, thermal, and chemical stimuli. They transduce this information into electrical signals. Cell Bodies: The cell bodies of primary afferents are typically located in dorsal root ganglia for those that innervate the trunk and limbs. These cell bodies give rise to two processes: one peripheral and one central. Peripheral Process: This extends from the cell body to the receptors in tissues like skin or muscle, where it terminates. Central Process: This enters the spinal cord from the cell body and carries the sensory information to the central nervous system. Conductive Segments: These include the axons or nerve bers that transmit the electrical signals from the peripheral receptors to the central nervous system. Transmissive Regions: These are the synapses where the primary afferents connect with secondary neurons, typically in the spinal cord or brainstem. This is where the initial processing of sensory information occurs. The primary afferent neurons are pseudounipolar, meaning they have a single axon that splits into two branches: one that communicates with the periphery and one with the central nervous system. The information carried by these neurons is then processed through various pathways, ultimately leading to the perception of sensations. fi ff ff fi 18. Classi cation of receptors by structure, by function, by energy that stimulates the receptors, by location Classification of receptors by structure 1. Encapsulated - surrounded by distinct connective tissue capsule (e.g. muscle spindle, Pacinian corpuscles, etc): i.e.: sensation of touch/pressure. The Pacinian corpuscle is surrounded by many layers, or laminae, similar to the layers of an onion peel. These layers cause it to quickly regain its shape after being compressed, thus it functions well in responding to quick changes in pressure. For example, Pacinian corpuscles located in the heel of a horse’s foot is activated during walking and can regain its shape as soon as there is no pressure in the horse’s heel. 2. Unencapsulated - lacks connective tissue capsule. Free nerve endings are the most common type: i.e.: pain sensations. Classification of receptors by function By modality (modality = sensation) 1. Touch receptor 2. Pressure receptor 3. Heat (or cold) receptor 4. Sound receptor, etc. By specific energy which stimulates receptor 1. Mechanoreceptor - elicited by any form of mechanical energy 2. Thermoreceptor - receptor responds to heat or absence of heat 3. Photoreceptor - Receptor sensitive to electromagnetic radiation (light) 4. Chemoreceptor - Receptor sensitive to various chemical stimuli 5. Nociceptor - Receptor which responds to a variety of different energies which are so intense as to be harmful Classification of receptors by location 1. Exteroceptor - Located superficially (i.e. in skin, retina, inner ear) so that receptor responds to external environment. 2. Proprioceptor - Located in body wall (i.e. in skin, inner ear, muscles, bones etc) This type of receptor detects body position, muscle tone, joint angle and the like. 3. Interoceptor - Located in viscera and blood vessels, responds to internal environment. It is not unusual for one receptor to function in such a way as to belong to more than one of these categories. For example, an encapsulated mechanoreceptor in the skin can respond to "touch" by some external stimulus, hence be classified as an exteroceptor. However, the same receptor may also provide essential sensory information regarding body position and also be classified as a proprioceptor. 19. Learn the di erence between re ex and reaction. fl ff Reflex: A motor response to some stimulus which is mediated entirely within the spinal cord and does not require higher centers for manifestation. In contrast, a reaction is a response which does require higher brain centers in order to take place. Thus, it is important to understand the difference between reflex and reaction. Ex: Patellar reflex – does not require higher center for manifesting the reflex. However, the quality of the reflex is monitored by the brain. On the other hand, proprioceptive reactions require higher centers for manifesting them. 20. Di erence between monosynaptic vs polysynaptic re ex. 6.2.1. Monosynaptic reflexes Only the largest afferent fibers form this type of synapse, where a collateral of the primary afferent terminates directly upon a LMN. This reflex pathway is the mechanism by which muscle tone is regulated. Since the muscle spindle is embedded in the skeletal muscle, the spindle can detect the tension present in the muscle, thus keeping the CNS apprised of the amount of muscle tone, thus reflexly regulating muscle tone. Rapid stretch of the muscle will also stretch the muscle spindle, causing excitation of the receptor. This burst of activity in the primary afferent fiber stimulates the LMN to that muscle, causing a brief contraction. This forms the basis for the tendon tap reflex. fl ff There are small motor neurons (GSE) in the ventral horn called gamma motor neurons which innervate the contractile intrafusal muscle fibers of the muscle spindle. These motor neurons are controlled by descending pathways to be considered later. Muscle tone can be directly regulated by these descending fibers. 6.2.2. Polysynaptic reflexes All other primary afferent fibers form this type of reflex connection, in which one or more interneurons is (are) interposed between the termination of the primary afferent fiber and the LMN, i.e., more than one synapse. The presence of the interneurons in this type of reflex pathway is significant for two reasons: 1. Since each synapse requires a finite amount of time to transmit the nerve impulse from presynaptic to postsynaptic cells (about.5 msec) the polysynaptic reflexes are slower to occur. 2. "The interneurons can send out collateral branches themselves, so that polysynaptic reflexes can incorporate a large number of LMN's and produce a more widespread response. A good example of such a polysynaptic reflex is the flexor withdrawal reflex: application of an appropriate stimulus (pin prick, pinch, etc) to an animal's toe will excite receptors in the skin and deeper tissues. This afferent activity is carried centrally by the primary afferents which synapse with interneurons in the dorsal horn (in substantia gelatinosa and nucleus proprius). Chains of interneurons will subsequently be excited, and these interneurons will eventually excite large numbers of flexor LMN's. Contraction of flexor muscles results, pulling the limb away from the stimulus. Remember that the primary afferent, before entering the gray matter, sends collaterals up and down the cord. Therefore, flexor LMN's in several adjacent cord segments can be recruited into this reflex. This spread is important for moving the entire limb, rather than simply producing a localized contraction of flexor muscles which would be less effective in "escaping" the painful stimulus. Reflex excitation can even spread across the cord to the opposite side. Here, however, extensor LMN's are excited. This contralateral reflex involvement is called a crossed extensor reflex and ensures that while lifting the stimulated leg the animal (or the human being) doesn't fall down! 21. What is proprioception? Proprioception: The category of afferent input from mechanoreceptors in the joints, muscles, tendons, and skin (cutaneous mechanoreceptors often double as proprioceptors and exteroceptors) which provide the CNS with information about body position, state of muscle contraction, joint angle, etc. This afferent input is essential for coordinating body movements. 22. Proprioceptive pathways – conscious vs unconscious Unconscious proprioception pathways refer to pathways that go to the cerebellum that helps us perform complex activities such as walking without thinking about which joints to involve performing these activities. A wide variety of afferent input is collected from a large group of receptors in the periphery and then transmitted to higher centers (cerebral cortex) which are consciously perceived. These inputs project via two main systems namely gracile/cuneate (related to limbs) and spinothalamic pathways which then come together in a single pathway called ‘medial lemniscal system’. Another system of afferent information that transmits the afferent input is the ‘ascending reticular formation’ which is to some extent consciously perceived and remains distinct from the medial lemniscus system. 23. Names of unconscious proprioceptive pathways from the legs and trunk Proprioceptive afferents from trunk and legs (pelvic limb and trunk in animals) 1. Dorsal spinocerebellar tract - collaterals of large primary afferents synapse in the dorsal horn. Second order fiber (i.e. axon or interneuron in relay nucleus) enters dorsal part of ipsilateral lateral funiculus to ascend to brain. Fibers enter cerebellum via caudal cerebellar peduncle. 2. Ventral spinocerebellar tract - collaterals of large primary afferents synapse in lateral portion of dorsal horn; second order fibers cross spinal cord to ascend contralaterally in lateral funiculus. Fibers enter cerebellum (via rostral cerebellar peduncle) (fibers decussate after entering cerebellum; therefore, ventral spinocerebellar fibers are distributed to ipsilateral cerebellum). 24. Names of unconscious proprioceptive pathways from the arms and neck B. Proprioceptive afferents from cervical region and arms (thoracic limb in animals and neck) 1. Cuneocerebellar tract - collaterals of large afferents run in lateral edge of fasciculus cuneatus to synapse I the caudal medulla (accessory (lateral) cuneate nucleus) (located in medulla just rostrolateral to nucleus cuneatus). Second order fibers enter cerebellum (via caudal cerebellar peduncle). 2. Rostral spinocerebellar tract - collaterals of large afferents synapse in lateral portion of dorsal horn. Second order fibers ascend in lateral funiculus (deep to dorsal and ventral spinocerebellar tracts) to enter cerebellum (via both rostral and caudal cerebellar peduncles). C. Proprioceptive afferents from head (muscle, joint and skin mechanoreceptors, and probably afferents from tongue, facial and extraocularmuscles as well) - collaterals of these large proprioceptive afferents synapse in trigeminal nuclei (mesencephalic and/or pontine sensory nuclei) and send secondary fibers (via caudal cerebellar peduncle to cerebellum). 25. Di erent conscious proprioceptive pathways ff Recognizable conscious sensations can be grouped into four major groups: 1. Sensations comprising of touch, pressure and joint proprioception – these are transmitted through the gracile/cuneate bundles. 2. Sensations comprising of pinprick pain, heat and cold – transmitted through the spinothalamic tract in humans. 3. Sensation related to ‘true pain’ – conveyed through the ascending reticular formation in humans. 4. Sensations of special senses such as vision, hearing, balance, taste and olfaction A. Proprioceptive afferents from body and limbs 1. This pathway occupies the dorsal funiculus and is subdivided into two adjacent tracts or fasciculi a. Fasciculus gracilis - as mentioned previously this fiber bundle is composed of ascending and descending collateral branches of large primary afferents from the caudal half of the animal including the pelvic limb. b. Fasciculus cuneatus - this fiber bundle carries collateral branches of large afferents from the cranial half of the animal (except the head) including the thoracic limb. As above, these afferents ascend via this fasciculus to synapse in the nucleus cuneatus. c. Second order fibers from these relay nuclei decussate in the caudal medulla and ascend contralaterally as a fiber bundle called the medial lemniscus to reach the thalamus. As these fibers cross over to opposite side of body, they collectively form the medial lemniscus. d. In contrast to most other ascending tracts, the dorsal column-medial lemniscal pathway is very highly organized in that afferent fibers from specific body regions are grouped into distinct layers within the dorsal funiculus and medial lemniscus - within the fasciculus gracilis caudal (coccygeal) afferents lie most medially, with sacral afferents intermediate in position and lumbar afferents immediately adjacent to the fasciculus cuneatus. - thoracic afferent fibers occupy the medial portion of the fasciculus cuneatus while cervical afferents lie most laterally. This specific somatotopic arrangement is maintained (although rearranged slightly) throughout the medial lemniscus and, as we shall see, in the thalamus and cerebral cortex as well. This precise organization is the mechanism by which the dorsal column-medial lemniscal pathway provides specific information to the brain regarding location of stimulus and even the quality of stimulation (ie different types of tactile stimulation: firm, gentle, hard, soft, smooth, rough, etc) Spinocervicothalamic tract- collaterals of primary afferents from cutaneous and deep mechanoreceptors synapse in dorsal horn; secondary fibers ascend in ipsilateral lateral funiculus to synapse again in lateral cervical nucleus in upper cervical cord. Third order fibers decussate in caudal medulla and run with medial lemniscus to thalamus. This pathway carries much of the same information as the dorsal column tract, but not as precisely organized. B. Proprioceptive afferents from head, collaterals from cutaneous and joint afferents synapse in trigeminal nuclei (pontine and/or spinal sensory nuclei); second order fibers decussate in brainstem and ascend to thalamus with medial lemniscus (sometimes called trigeminothalamic tract). Primary afferent fibers in this tract are also somatotopically arranged: 1. The most ventral layer of fibers in trigeminal (V) nerve and spinal tract of V are from ophthalmic branch of V, which is sensory to upper head and face and part of nasal mucosa. 2. Intermediate layer composed of fibers from maxillary branch of V, which innervates face ventral to orbit, part of nasal mucosa and upper teeth. 3. Most dorsal layer composed of fibers from mandibular branch of V which supplies skin over side of face, head and lower jaw, most of oral mucosa 26. Examples of proprioceptive reactions A. Positive supporting reactions - this category is an entire constellation of normal motor activities mediated by mechanoreceptors in muscles, joints and skin. 1. Contact with a solid surface will excite receptors in foot pads and joints of digits, producing contraction of extensor muscles of the stimulated limb (and possibly of all limbs), preparing to support animal's weight in standing position. 2. While animal is standing, gently abducting one limb and planting the foot a few inches lateral to its previous location will usually result in a rapid adduction of the limb back in line with the body. This complex reaction is a mechanism. B. Righting reactions - Placing an animal in an abnormal position (e.g. on side, on back) excites receptors in many different joints and over a large area of skin. This afferent input, coupled with afferent activity from vestibular and visual systems, elicits a complex of muscle contractions which will return the animal to an upright posture. C. Placing reactions - inverting a standing animal's paw will normally result in very rapid replacement of the paw in normal position. Cutaneous and joint mechanoreceptors in the paw inform the brain of the abnormal placement and motor responses quickly rectify the situation. The foregoing are examples of motor responses mediated by cutaneous and deep mechanoreceptors via ascending afferent pathways. Such proprioceptive tracts also inform higher centers of the state and extent of muscle contraction, very important during cyclical repetitive muscle activity as in locomotion. One can appreciate how all of these proprioceptive pathways must be active if one considers a horse galloping across a rocky, uneven field: not only must rapid bursts of muscle contraction and relaxation be coordinated for simple limb movement, but the animal must respond to the rapidly changing terrain underfoot in such a way as to balance and support not only its own weight but that of the rider as well.