CNS Material by Prof Eze Nwanni PDF

27/4/23 CENTRAL NERVOUS SYSTEM (NEUROPHYSIOLOGY) (Professor Ezenwanne) Development of the brain and spinal cord This starts with a primitive neural tube (simple neural tube). This is a tubular structure composed of a cell layer and a central canal. Then, there is expansion in 3 ways, of...

27/4/23 CENTRAL NERVOUS SYSTEM (NEUROPHYSIOLOGY) (Professor Ezenwanne) Development of the brain and spinal cord This starts with a primitive neural tube (simple neural tube). This is a tubular structure composed of a cell layer and a central canal. Then, there is expansion in 3 ways, of unequal growth. This gives rise to 3 areas: the forebrain, midbrain and hindbrain. The lining cells multiply and give rise to nervous tissue of the developing brain and spinal cord, while the canal gives rise to the ventricles of the brain and the central canal of the spinal cord. Further development causes more complication and differentiation of the Central Nervous System into areas such as: Forebrain (Prosencephalon) ○ Telencephalon Basal ganglia Neopallium (cerebral hemisphere) (newer part) Rhinencephalon (older part) (vital for olfaction) ○ Diencephalon Thalamus Hypothalamus Midbrain (Mesencephalon) ○ Anterior part Cerebral peduncles Tegmentum ○ Posterior part Tectum Superior corpora quadrigemina (superior colliculus) Inferior corpora quadrigemina (inferior colliculus)  Hindbrain (Rhombencephalon) ○ Cephalic part (metencephalon) Pons Cerebellum Cerebellar peduncles (superior, middle and inferior) ○ Caudal part (myencephalon) Medulla The tail of the neural tube also develops into the spinal cord. The neural tissues of the nervous system have overall functions, categorized under the following headings: Provision of sensation or sensory information Integration of sensory information Provision of coordination of voluntary and involuntary response Regulation or control Along with the endocrine system, these responses are boiled down into control, coordination and communication. The two systems have a lot of differences though, such as speed, mode of transmission, intensity of effect, intended response (endocrine is for metabolism) and sphere of influence. Integration refers to the collation and coalescing of input, for precise computation, interpretation and analysis, used to determine the appropriate response(s) to that input. For convenience of study, the nervous system is divided into 4 areas: Sensory division - gathering of sensory information from all the sensory systems of the body, and transmitting it to the integration center Motor division - to control and coordinate bodily motor activities/motor activities of effector tissues Processing division - integration of incoming signals and determination of appropriate response Storage division - relevant for memory and thought. Synapses are very important here General organization and functional plan The brain is made up of 3 different regions: Brain stem - made of the medulla, pons and midbrain Cerebellum Cerebrum - cortex, basal ganglia, thalamus, hypothalamus The nervous system as a whole is divided into central (brain and spinal cord) and peripheral (sensory/afferent and motor/efferent nerves). A nerve impulse is the basis for the communication of information in the nervous system. It is the language of the nervous system. A nerve cell is the building block of the nervous system; the structural unit Peripheral nervous system This consists of two divisions, 12 pairs of cranial nerves and 31 pairs of spinal nerves. These nerves form the link between the CNS and all organs and systems of the body. Spinal nerves - each of these is in communication with the spinal cord via two Roots: the posterior root and ventral root, from the dorsal and ventral horns of the spinal cord respectively. The sensory nerves make use of the dorsal root, but they never go into the spinal cord directly. Rather, they reside in the dorsal root ganglion and send their fibers into the spinal cord. Motor nerves, on the other hand, use the ventral root and reside in the gray matter of the spinal cord. There's no ventral root ganglion Cranial nerves - these have no definite root system, and are identified using roman numerals (Cranial Nerve I to Cranial Nerve XII) Cranial Nerve Type Functions I - Olfactory nerve Sensory Smell nerve II - Optic nerve Sensory Sight nerve III - Oculomotor nerve Motor nerve Eye movement for focusing IV - Trochlear nerve Motor nerve Eye movement (up and down V - Trigeminal nerve Mixed nerve Facial sensations, chewing VI - Abducens nerve Motor nerve Eye movement (lateral), proprioception VII - Facial nerve Mixed nerve Taste, facial expressions, eye  movement, salivary secretion VIII - Vestibulocochlear Mixed nerve Hearing, balance (equilibrium) nerve IX - Glossopharyngeal Mixed nerve Taste, salivary secretion, nerve swallowing, other sensations X - Vagus nerve Motor nerve Movement and secretion of abdominal and thoracic organs (visceral reflexes), swallowing XI - Accessory nerve Motor nerve Head, shoulder, larynx movement, speaking XII - Hypoglossal nerve Motor nerve Tongue movement The brainstem is a part that's posterior to the brain and continuous with the spinal cord. It consists of 3 parts: Midbrain Pons Medulla oblongata It connects the brain to the spinal cord. Although it is very small, or is a very important area of the brain as it helps to relay information between the two parts of the CNS. It has many functions, such as: It houses many control centers for involuntary activities The relay between brain and spinal cord Coordination of motor and sensory function Regulation of cardiac function Regulation of breathing Regulation of sleep Regulation of consciousness Regulation of eating and swallowing Regulation of respiration Regulation of blood pressure Neural tract carries signals which are relayed from the cerebrum to the cerebellum and vice versa Integrative functions like alertness, arousal, digestion, pain, sleep Diseases of the brain stem can result in many abnormalities, such as those of cranial nerve function, including visual and hearing difficulty, muscle weakness, vertigo, dulling of sensations, coordination problems, dysphagia, speech problems and voice changes, breathing difficulty or apnea, changes in or cessation of heart rate, etc 3/5/23 Draw and label a typical motor nerve and typical sensory nerve (assignment) Nerve cells These are highly modified cells, for the transmission of electrochemical signals within the nervous system. They are the functional unit. Even within nerve cells, there are also modifications such as length of dendrites or presence of dendrites. Neurons of the CNS are profusely dendritic. The soma themselves also vary in size. As for the axons, some of them have branches, called collaterals, with some neurons having several collaterals. Even dendrites sometimes have a major branch which forms its own branches, acting as a collateral The difference in arborization leads to a difference in sensory field, which is the area of tissue subserved by dendrites Myelination of fiber tracts in the nervous system Studies show that various tracts (pathways) receive myelination at different stages. These include: The sensory pathway, which are usually the first to become myelinated (from the early embryonic stage)  The posterior columns of the spinal cord, which are the second (4th - 5th month of fetal life) The spinocerebellar tract, third The motor pathways/corticospinal pathways/pyramidal tract. These become myelinated from the second month of fetal life, but take up to two years to complete myelination, making them the last Functions of the myelin sheath A school of thought says that they function to confine impulses to individual nerve fibers Another says that it ensures greater speed of conduction of the nerve fiber Functional organization of the nerve cell The neuron can be split into 4 zones: Receptor zone or dendrite zone. There's multiple local potential changes constantly taking place here. This is called graded electrogenesis. They are not conducted Initial segment zone. This is where action potential is initiated from the integration of the graded potential Axonal zone. This is where transmission of the action potential occurs. It ensures the all or none transmission of impulse, and maintains the unidirectional nature of transmission; towards the nerve endings Nerve ending zone. This region ensures the secretion of the appropriate neurotransmitter substance. The arrival of action potential at this zone triggers the release of neurotransmitters from the storage vesicles into the synaptic cleft via exocytosis Classification of nerve fibers The greater the diameter of a nerve fiber, the greater the speed of conduction Conduction velocity is higher in myelinated fibers than in non myelinated  There are larger axons which are mostly concerned with proprioceptive sensation. They are also concerned with somatic motor function The larger the diameter of a nerve fiber, the greater the magnitude of its electrical response The larger the diameter, the lower the excitation threshold and the shorter the duration of it's response Basis of system of classification Erlanger and Gasser classify based on diameter and conduction velocity in mammalian nerve cells. Based on this, there are A, B and C group fibers. But this was not a satisfactory system of classification, so they further split group A into A𝜶, A𝜷, A𝜹 and A𝜸 Fiber Type Diameter Conduction Velocity Nature A𝜶 12 - 20 µm 70 - 120 m/s They are mostly for proprioception A𝜷 5 - 12 µm 30 - 70 m/s Tactile sensations, pressure and other motor functions A𝜸 3 - 6 µm 15 - 30 m/s Motor fibers to muscle spindles A𝜹 2 - 5 µm 12 - 30 m/s Pain, temperature and touch B