Lecture 5 Human Central Nervous System.pdf

Medical Physiology I Lecture 5 ‫ ســــتار جابـر‬.‫د‬ The Human Central Nervous System (CNS) Brain : The Diencephalon [The diencephalon and telencephalon together make up the...

Medical Physiology I Lecture 5 ‫ ســــتار جابـر‬.‫د‬ The Human Central Nervous System (CNS) Brain : The Diencephalon [The diencephalon and telencephalon together make up the forebrain] The diencephalon contains two major structures: the thalamus and hypothalamus A. Thalamus Sensory information from the periphery passes through the thalamus for processing before reaching a conscious level. Output from the olfactory system is the single exception, insofar as it bypasses the thalamus and feeds raw olfactory data to the cortex directly. The thalamus also controls sleep and wakefulness and is required for consciousness. Damage to the thalamus can result in deep coma. The thalamus is also involved in motor control and has areas that project to the cortical motor regions. Figure / Thalamus and Hypothalamus location 1 B. Hypothalamus The hypothalamus is a major autonomic nervous system control center. Its functions include control of body temperature, food intake, thirst and water balance, and blood pressure, and it also controls aggression and rage. The hypothalamus exerts control through direct neural connections to autonomic centers in the brainstem, but it also controls the endocrine system. Endocrine control occurs directly through hormonal synthesis and release (oxytocin and antidiuretic hormone) and indirectly by secreting hormones that affect release of pituitary hormones. Figure / Conceptual overview of hypothalamic function. DM = dorsomedial nucleus The telencephalon, or cerebrum, is the seat of human intellect. It is organized into two cerebral hemispheres comprising the basal ganglia and the cerebral cortex. A. Basal ganglia The basal ganglia are a group of functionally related nuclei that work closely with the cerebral cortex and thalamus to effect motor control. Major structures within the basal ganglia include the caudate nucleus and putamen (together forming the striatum) and the globus pallidus. B. Cerebral cortex The cerebral cortex is involved in conscious thought, awareness, language, learning and memory. 2 1.Anatomy: The cortex comprises a sheet of neural tissue organized in six layers that is folded to accommodate the 15 to 20 billion neurons contained within. The folds (gyri) are separated by sulci (grooves). Deep fissures separate the cortex into four lobes: frontal, parietal, occipital, and temporal. The lobes contain discrete areas that can be distinguished on a cytoarchitectural basis and that correlate with regions of specialized function. 2. Function: The cortex can be functionally divided into three general areas that stretch across both hemispheres: sensory, motor, and associative. Figure / Cerebral cortex 1. Sensory: Sensory regions process information from the sensory organs. Primary sensory regions receive and process information directly from the thalamus. Spatial information is preserved as data flows from the senses to the sensory areas and then accurately maps onto the cortex (topographic mapping). Thus, the pattern of light falling on the retina is faithfully replicated in the pattern of excitation within the primary visual cortex. 2.Motor: Motor areas are involved with planning and executing motor commands. Primary motor areas execute movements. 3 Axons from these areas project to the spinal cord, where they synapse with and excite motor neurons. Supplementary motor areas are involved with planning and fine control of such movements. C. Associative: The majority of cortical neurons are involved in associative functions. Each cortical sensory region feeds information to a corresponding association area. Here, patterns of color, light, and shade are recognized as a human face, for example, or a series of notes can be recognized as coming from a songbird. Other associative areas integrate sensory information from other parts of the brain to allow for higher mental functions. These include abstract thinking, acquisition of language, musical and mathematical skills, and the ability to engage in social interactions. The Brain Ventricles : The brain contains four ventricles: two lateral ventricles and a third and fourth ventricle. They are all connected by foramina that allow CSF to flow caudally to the spinal cord and through its central canal. a. Lateral: The two lateral ventricles are the largest of the four. They are symmetrical C shapes and are located at the center of the two cerebral hemispheres. They connect with the third ventricle via two interventricular channels called the foramina of Monro. b. Third: The third ventricle lies on the midline at the level of the thalamus and hypothalamus. It connects with the fourth ventricle via the cerebral aqueduct (of Sylvius). c. Fourth: The fourth ventricle is located within the brainstem. The caudal end communicates with the spinal cord’s central canal. The ventricle also provides a pathway for CSF to flow into the subarachnoid space via three openings. The foramen of Magendie is located at the midline. Two foramina of Luschka are located laterally 4 Figure / Location of the cerebrospinal fluid-filled ventricles and cerebral aqueduct. The spinal cord The spinal cord is a thick communications tract that relays sensory and motor signals between the peripheral nervous system (PNS) and the brain. The cord also contains intrinsic circuits that support certain muscle reflexes. The spinal cord is housed within the vertebral canal. It extends from the foramen magnum at the base of the skull caudally to the second lumbar vertebra. The vertebral column consists of a series of stacked vertebrae divided anatomically into five regions: cervical, thoracic, lumbar, sacral, and coccygeal 5  Figure / Central nervous system The cervical, thoracic, and lumbar vertebrae are separated by intervertebral disks that allow the bones to articulate, but the sacral and coccygeal vertebrae are fused to form the sacrum and coccyx, respectively. The spinal cord can be divided into 31 named segments. Thirty-one pairs of spinal nerves (one on each side of the body) emerge from corresponding segments. Although the spinal cord terminates before it reaches the sacrum, spinal nerves continue caudally within the vertebral canal until they reach an appropriate exit level. Spinal nerves are a component of the PNS. The nerves contain sensory afferent and motor efferent fibers (spinal nerves are sometimes called mixed spinal nerves for this reason) that generally serve tissues on the same level as the nerves. 6 a. The spinal cord passes through the vertebral canal formed by the vertebrae.It gives off spinal nerves that project through openings between the vertebrae. b. The spinal cord has a central canal filled with cerebrospinal fluid, graymatter in an H-shaped configuration, and white matter elsewhere. The white matter contains tracts that take nerve impulses to and from the brain. 7 1.Sensory: Somatic and autonomic sensory fibers travel to the spinal cord via peripheral nerves.They relay sensations of pain, temperature, and touch from the skin; proprioceptive signals from muscle and joint receptors; and sensory signals from numerous visceral receptors. Multiple peripheral nerves come together to form the posterior root of a spinal nerve and enter the vertebral canal via an intervertebral foramen. The cell bodies of these nerves cluster within a prominent spinal ganglion located within the foramen. The posterior root then divides into a number of rootlets and 8 joins the spinal cord. Sensory nerves travel rostrally to synapse within nuclei en route to the brain. Branches of sensory afferents may also synapse directly with motor neurons or on interneurons that synapse with motor neurons, which makes local spinal cord– mediated reflexes possible 2. Motor: Motor efferents from the brain travel caudally and synapse with peripheral motor nerves within the spinal cord. These nerves include both somatic and autonomic motor efferents. They leave the spinal cord via anterior rootlets, which join to form an anterior root and then travel out to the periphery alongside sensory fibers in spinal nerves. The spinal cord’s interior is roughly organized into a butterfly-shaped central area of gray matter surrounded by white matter. The white matter contains bundles of nerve fibers with common origins and destinations that relay information between the PNS and the brain. Sensory nerve fibers from the periphery travel rostrally to the brain in discrete ascending tracts. Descending tracts carry bundles of motor efferents from the CNS en route to the periphery Figure / Cross section of spinal cord 9 The tracts (also known as fasciculi) are grouped in posterior, lateral, and anterior columns (also known as funiculi). The tracts are named according to their origin and destination. The spinothalamic tract carries pain fibers from the spine upward to the thalamus. The corticospinal tract carries motor fibers from the cortex downward to the spine. The “wings” of the gray butterflies are divided into posterior and anterior horns and act as synaptic relay stations for information flow between neurons. They contain neuronal cell bodies, which may be clustered in functionally related groups, or nuclei. The gray matter on either side of the cord is connected by commissures containing bundles of fibers that allow for information flow across the midline. The spinal cord reflexes Reflex arcs are simple neuronal circuits in which a sensory stimulus initiates a motor response directly. Classic examples include withdrawal reflexes triggered by touching a hot stove or stepping on a sharp object. Such arcs are often mediated by the spinal cord, where a sensory neuron synapses with and activates a motor neuron. More complex arcs involve synapses with multiple neurons, at least one of which may be inhibitory. The neurons involved are relatively short so as to further minimize signal transmission and processing times. Motor sensory and control neurons are specialized to conduct signals at up to 120 m/s, representing some of the fastest nerve cells in the body. This ensures that sensory information is relayed to the CNS and compensatory commands executed in the shortest time possible. Reaction times are enhanced further by using local reflexes mediated by the spinal cord to make many routine adjustments to gait. The spinal cord mediates a number of important reflex arcs, including the myotatic reflex, the inverse myotatic reflex, and the flexion reflex. Each end of a skeletal muscle is attached to a tendon that typically tethers it to a bone. The musculotendinous junction contains Golgi tendon organs [GTOs] , which are sensory organs that monitor the amount of tension that develops in a muscle when stretched passively or when it contracts. 10 Control sequences ensure that pairs of muscle groups whose actions typically oppose one another (e.g., extensors and flexors, abductors and adductors, and external and internal rotators) These muscle groups contract and relax in a coordinated fashion to effect smooth limb movements. The central nervous system (CNS) is informed about limb movements relative to the torso, made possible by the sense of kinesthesia. Kinesthesia is a form of proprioception and one of the somatic senses. Kinesthesia relies primarily on two sensory systems that sense muscle length (muscle spindles) and tension (Golgi tendon organs [GTOs]). Each end of a skeletal muscle is attached to a tendon that typically tethers it to a bone. The musculotendinous junction contains GTOs, which are sensory organs that monitor the amount of tension that develops in a muscle when stretched passively or when it contracts. The role of joint receptors in kinesthesia is minimal. Slow-adapting Ruffini endings in skin do have an important role. Skin that covers joints is stretched whenever a limb or digit is retracted, causing Ruffini endings to fire. The importance of sensory data from Ruffini endings is increased in the fingers, where layering of the various muscles and tendons required for execution of fine movements may impede acquisition of sensory information from spindles and GTOs. I- Myotatic reflex (also known as a stretch reflex or deep-tendon reflex) is initiated by stretching a muscle and causes contraction of the same (“homonymous”) muscle. Reflex contraction of the thigh (quadriceps) muscles caused by tapping the patellar ligament is a familiar example. Tapping the patellar ligament stretches the quadriceps and activates spindles buried within. Sensory signals are carried by Ia nerve afferents to the spinal cord, where they synapse with and excite -motor neurons innervating the same muscle. The muscle contracts reflexively, the leg extends, and the foot jerks forward. Forward foot movement stretches the hamstring muscles at the back of the thigh and stimulates their spindles also. This might be expected to initiate a second reflex that opposes the actions of the first, but the arc is interrupted by a Ia inhibitory spinal interneuron. The Ia interneuron is activated by the same Ia afferent signal that caused the quadriceps to contract. The interneuron synapses with and inhibits the α- motor 11 neurons that innervate the hamstring muscles (e.g., semitendinosus) and, thereby, allows the leg to extend without resistance. This circuitry is referred to as reciprocal innervation and is used commonly in situations in which two or more sets of muscles oppose each other around a joint (e.g., flexors and extensors). Figure / Myotatic reflex 12 II. The inverse myotatic reflex, also known as a Golgi tendon reflex, activates whenever a muscle contracts and GTOs are stretched. Type Ib afferents from GTOs synapse with Ib inhibitory interneurons upon entering the spinal cord. When activated, they inhibit α-motor output to the homonymous muscle. Excitatory interneurons simultaneously activate α-motor output to the heteronymous muscle. The Golgi tendon reflex is believed to be important for fine motor control and for maintaining posture, acting synergistically with the myotatic reflex above. 13  Figure / Inverse myotatic reflex (Golgi tendon reflex) III. flexion and crossed-extension reflexes Stepping on a thorn or other injurious object precipitates two urgent actions. The first withdraws the foot from the source of pain (leg flexion). The second braces the opposing limb so that weight can be transferred while still maintaining balance. This complex motion is mediated by flexion and crossed-extension reflexes. Similar reflexes can be induced in the upper limbs. The action sequence can be broken down into three stages: 14  Figure / Flexion and crossed-extension reflexes 1. Sensation: Flexion and crossed-extension reflexes are usually initiated as a response to a noxious, painful stimulus. Pain fibers project to and synapse with interneurons in the spinal cord. 2. Flexion: The sensory afferents synapse on the ipsilateral side with excitatory motor neurons that innervate flexor muscles. Extensor muscles are inhibited simultaneously, and the limb retracts from the pain source. 3.Crossed-extension: Sensory fibers also cross the spinal cord’s anterior fissure and synapse with motor neurons controlling contralateral limb movement. Extensors are excited and contract, whereas flexors are inhibited and relax. This is known as a cross-extension reflex and braces the contralateral limb for the sudden weight transfer caused by raising the wounded limb. End Dr. Sattar Jabir 2023 15