Physiology of the Nervous System PDF

Physiology of the Nervous System Dr. Hiwa S. Namiq 15-4-2024 1  GENERAL DESIGN OF THE NERVOUS SYSTEM A-CENTRAL NERVOUS SYSTEM NEURON: THE BASIC FUNCTIONAL UNIT 1.The central nervous system is estimated to contain 80 to 100 billion...

Physiology of the Nervous System Dr. Hiwa S. Namiq 15-4-2024 1  GENERAL DESIGN OF THE NERVOUS SYSTEM A-CENTRAL NERVOUS SYSTEM NEURON: THE BASIC FUNCTIONAL UNIT 1.The central nervous system is estimated to contain 80 to 100 billion neurons 2.Incoming signals enter a typical brain neuron (figure) through synapses located mostly on the neuronal dendrites, but also on the cell body. 3.There may be only a few hundred or as many as 200,000 such synaptic connections from input fibers 4.The output signal travels by way of a single axon leaving the neuron. This axon may have many separate branches to other parts of the nervous system or peripheral body 5.The signal normally passes only in the forward direction (from the axon of a preceding neuron to dendrites of subsequent neurons). 2 Structure of a large neuron in the brain showing its important functional parts. 3  B-Sensory part of CNS: Sensory receptors Most activities of the nervous system are initiated by exciting sensory receptors Visual receptors in the eyes, Auditory receptors in the ears, Tactile receptors on the surface of the body. Types of responses: Immediate reaction from the brain Stored (as memory for minutes, weeks, or years and determine bodily reactions at future date. 4  The information enters the central nervous system through peripheral nerves and is conducted immediately to multiple sensory areas in (1)the spinal cord at all levels; (2) the reticular substance of the medulla, pons, and mesencephalon of the brain; (3) the cerebellum (4) the thalamus (5) areas of the cerebral cortex. Somatosensory axis of the nervous system. 5  C-Motor part of CNS – Effector organ The most important eventual role of the nervous system is to control the various bodily activities. The motor part of the CNS controls body activity through: 1.Controlling contraction of appropriate skeletal muscles throughout the body 2.Contraction of smooth muscle in the internal organs 3.Secretion of active chemical substances by both exocrine and endocrine glands in many parts of the body. 6 Skeletal motor nerve axis of the nervous system. 7  levels of contraction of skeletal muscles (1) the spinal cord; (2) the reticular substance of the medulla, pons, and mesencephalon (midbrain) (collectively called brain stem); (3) the basal ganglia; (4) the cerebellum; and (5) the motor cortex. Integrative function of nervous system (Processing of Information) 1. The incoming signal to the CNS is processed to cause appropriate mental and motor response. 2. More than 99% of all sensory information is discarded by the brain as irrelevant and unimportant. 3. Others are immediately channeled into proper integrative and motor regions of the brain to cause desired responses. 8  Storage of Information—Memory New information is stored (future control of motor activities and for use in the thinking processes) Storage occurs: 1.Cerebral cortex (mainly) 2.Basal regions of the brain and spinal cord (spinal cord stores small amounts of information). 9  Major Levels of Central Nervous System Function The three major CNS levels are; (1)The spinal cord level (2)The lower brain or subcortical level (3)The higher brain or cortical level. 10  Spinal cord Neuronal circuits in the cord can cause: (1)Walking movements. (2)Reflexes that withdraw portions of the body from painful objects. (3)Reflexes that stiffen the legs to support the body against gravity. (4)Reflexes that control local blood vessels, gastrointestinal movements, or urinary excretion. 11  Lower brain or subcortical level Subconscious activities of the body are controlled in the lower areas of the brain (medulla, pons, mid-brain (mesencephalon), thalamus and hypothalamus (diencephalon), cerebellum, and basal ganglia). For instance, 1. Control of arterial pressure and respiration (medulla and pons). 2. Control of equilibrium is a combined function of the cerebellum and the reticular substance of brainstem. 3. Feeding reflexes (salivation and licking) controlled by areas in the medulla, pons, mesencephalon, amygdala, and hypothalamus. 4. Emotional patterns such as anger, excitement, sexual response, reaction to pain, and reaction to pleasure can still occur after destruction of much of the cerebral cortex. 12  Higher Brain or Cortical Level The cerebral cortex is an extremely large memory storehouse. The cortex never functions alone but always in association with lower centers of the nervous system. In fact, it is the lower brain centers, not the cortex, that initiate wakefulness in the cerebral cortex, thus opening its bank of memories to the thinking machinery of the brain. The cortex is the “seat of intelligence”. It provides us with the ability to read, write and speak; to make calculations and to compose music; to remember the past and plan for the future and to produce. 13  The synapse A neuron is composed of three major parts: the soma, which is the body of the neuron; a single axon, which extends from the soma into a peripheral nerve; and the dendrites, which are great numbers of branching projections of the soma. The presynaptic terminals of the first neuron synapse with the dendrite or the soma of the second neuron. These terminals are either excitatory (secreting excitatory neurotransmitter) or inhibitory (secreting inhibitory neurotransmitter) depending on the type of postsynaptic receptor. 14  Types of Synapses A. Chemical synapse. Almost all the CNS synapses are of this type. These synapses always transmit the signals in one direction. B. Electrical synapse. Here the impulse is directly conducted from one cell to the next through gap junctions 15 Physiological anatomy of a chemical synapse (A) and an electrical synapse (B). 16  Action of the transmitter substance on the postsynaptic neuron The membrane of the postsynaptic neuron contains large numbers of receptor proteins. The receptor either acts as 1- ion channel for passage of anions or cations 2- second messenger activator. Cation channels allow positively charged ions (Na ion) Anion channels allow entering negatively charged ions - (Cl ion) and opening of potassium channel causes potassium efflux The second messenger system is essential for prolonged changes in neurons for seconds to months after the initial transmitter substance is gone, such as the process of memory. 17  Postsynaptic excitation and postsynaptic inhibition  Sensory Receptors Input to the nervous system is provided by sensory receptors that detect such sensory stimuli as touch, sound, light, pain, cold, and warmth. Each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to other types of sensory stimuli. 19  Types of Sensory Receptors There are five basic types of sensory receptors: 1. Mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor. 2. Thermoreceptors, which detect changes in temperature. 3. Nociceptors (pain receptors), which detect damage occurring in the tissues, whether physical damage or chemical damage. 4. Electromagnetic receptors, which detect light on the retina of the eye. 5. Chemoreceptors, which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration. 20 Transduction of Sensory Stimuli into Nerve Impulses Whatever the type of stimulus is, its immediate effect is to change the membrane electrical potential of the receptor. This change in potential is called a receptor potential. When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor, then action potentials occur. Receptors can adapt partially or completely to any constant stimulus after a period of time. Some receptors adapt within seconds (pacinian corpuscle) while pain receptors never adapt completely. 21 Physiologic classifications of nerve fibers 22 Sensory Pathways for Transmitting Somatic Signals into the CNS ►Almost all sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves. ►There are two main pathways for transmitting sensory information from the SC to the sensory cortex: (1) Dorsal column–medial lemniscal system (2) Anterolateral (spinothalamic) system  Dorsal column (medial Anterolateral (spinothalamic) lemniscal system) system 1. Carries signal upward to the 1. On entering the S.C, they medulla in the dorsal synapse in the dorsal horn of columns of white matter. gray matter then cross to the 2. The velocity of transmission opposite site. ranges from 30 to 110 m/sec 2. The velocity of transmission 3. The degree of spatial ranges between 8 and 40 localization is high m/sec. 4. The gradations of intensities 3. The degree of spatial are accurate. localization of signals is poor. 5. The ability to transmit rapidly 4. The gradations of intensities changing or repetitive signal are far less accurate. is good (e.g. vibration) 5. The ability to transmit rapidly changing or repetitive signals It transmits the following: It transmits the following: 1. Touch sensations requiring 1. Pain a high degree of localization 2. Thermal sensations, 2. Touch sensations requiring including both warmth and transmission of fine cold sensations gradations of intensity 3. Crude (simple) touch and 3. Vibration pressure sensations capable 4. Sensations that signal only of crude localizing movement against the skin ability on the surface of the 5. Position sensations from body the joints (proprioception). 4. Tickle and itch sensations 6.Pressure sensations needing 5. Sexual sensations. fine degrees of judgment of pressure intensity  Dorsal column-medial lemniscal system: 1.First-order neuron 2.Second-order neuron 3.Third-order neuron Anterolateral Pathway Cross immediately to the opposite anterior and lateral white columns, where they ascend toward the brain by way of the anterior spinothalamic and lateral spinothalamic tracts. The spinothalamic tracts end at reticular nuclei of the brain stem and thalamic nuclei The signals are then transmitted to the somatosensory cortex along with the signals from the dorsal column pathway.  Somatosensory Cortex ►Brodman’s areas (50 areas based on histological differences). ►The central fissure (central sulcus) extends horizontally across the brain. ►In general, sensory signals from all modalities of sensation terminate in the cerebral cortex immediately posterior to the central fissure. And, generally, the anterior half of the parietal lobe is concerned almost entirely with reception and interpretation of somatosensory signals. ►But the posterior half of the parietal lobe provides still higher levels of interpretation. ►Visual signals terminate in the occipital lobe, and auditory signals in the temporal lobe.  Central sulcus Two somatosensory cortical areas, somatosensory areas I and II. Somatosensory area I of the cortex  Motor pathways and autonomic nervous system  Motor cortex The motor cortex controls the motor activity. It is divided into three subareas: (1)Primary motor cortex. (2)Premotor area. (3)Supplementary motor area. The functions of the motor cortex are controlled by nerve signals from multiple cortical and non-cortical brain regions. Degree of representation of the different muscles of the body in the motor cortex.  Primary Motor Cortex It lies in the first convolution of the frontal lobes anterior to the central sulcus. More than one half of the entire primary motor cortex is concerned with controlling the muscles of the hands and the muscles of speech. Premotor Area The premotor area lies anterior to the primary motor cortex. Nerve signals generated in the premotor area cause much more complex “patterns” of movement than the discrete patterns generated in the primary motor cortex. E.g. shoulder and arm positioning so that hands can perform specific tasks.  Supplementary Motor Area In general, this area functions with the premotor area to provide attitudinal and fixation movements of the different segments of the body, positional movements of the head and eyes as a background for finer control of arms and hands by premotor and primary MC.  Representation of the different muscles of the body in the motor cortex and location of other cortical areas responsible for specific types of motor movement  Transmission of Signals from the Motor Cortex to the muscles Motor signals are transmitted from the cortex: directly to the spinal cord through the corticospinal tract (pyramidal tract) and indirectly through multiple accessory pathways that involve the basal ganglia, cerebellum, and various nuclei of the brain stem (extra pyramidal system).  Corticospinal (Pyramidal) Tract The most important output pathway from the motor cortex is the corticospinal tract, (pyramidal tract). The corticospinal tract originates about 30 percent from the primary motor cortex, 30 percent from the premotor and supplementary motor and 40% from somatosensory cortex After leaving the cortex, it passes through the posterior limb of the internal capsule and then downward through the brain stem, forming the pyramids of the medulla.  The majority of the pyramidal fibers then cross in the lower medulla to the opposite side and descend into the lateral corticospinal tracts of the cord, finally terminating principally on the interneurons of the cord gray matter. A few of the fibers pass ipsilaterally down the cord in the ventral corticospinal tracts. Many of these fibers eventually cross to the opposite side of the cord either in the neck or in the upper thoracic region. Corticospinal (pyramidal) tract  Brain Stem Control of Motor Functions The brain stem consists of the medulla, pons, and midbrain (mesencephalon). It contains motor and sensory nuclei that perform motor and sensory functions for the face and head regions. It also provides many special control functions, such as the following: 1. Control of respiration 2. Control of the cardiovascular system 3. Partial control of gastrointestinal function 4. Control of many stereotyped (spontaneous) movements of the body 5. Control of equilibrium 6. Control of eye movements  Cerebellum and its motor functions The cerebellum helps to sequence the motor activities and also monitors and makes adjustments in the body’s motor activities while they are being executed. It is especially vital during rapid muscular activities such as running, typing, playing the piano, and even talking. Autonomic Nervous System  The autonomic nervous system controls most visceral functions of the body including control of arterial pressure, gastrointestinal motility and secretion, urinary bladder emptying, sweating, body temperature, and many other activities. The autonomic nervous system is activated mainly by centers located in the spinal cord, brain stem, and hypothalamus. In addition, portions of the cerebral cortex, especially of the limbic cortex, can transmit signals to the lower centers and in this way can influence autonomic control. The efferent autonomic signals are transmitted to the various organs of the body through two major subdivisions called the sympathetic nervous system and the parasympathetic nervous system  Physiologic Anatomy of Sympathetic N. System: The sympathetic nerve fibers originate in the spinal cord along with spinal nerves between cord segments T-1 to L-2 (Thoraco-lumbar) and pass first into the sympathetic chain and then to the effector tissues. The peripheral portion of the sympathetic N.S is composed of the following: 1. Two paravertebral sympathetic chains of ganglia. 2. Prevertebral ganglia (the celiac, superior mesenteric, aortico-renal, inferior mesenteric, and hypogastric) 3. Nerves extending from the ganglia to the different internal organs.  The postganglionic sympathetic neuron thus originates either in one of the sympathetic chain ganglia or in one of the peripheral sympathetic ganglia. From either of these two sources, the postganglionic fibers then travel to their destinations in the various organs. Each sympathetic pathway from the cord to the stimulated tissue is composed of two neurons, a preganglionic neuron and a postganglionic neuron. Regarding the adrenal medulla, the preganglionic sympathetic nerve fibers pass directly, without synapsing, from spinal cord into the two adrenal medullae where they cause secretion of epinephrine and norepinephrine into the blood stream. Sympathetic nervous system. The black lines represent postganglionic fibers, and the red lines show preganglionic fibers.  Parasympathetic system (1973-S23): The parasympathetic nervous system leave the central nervous system through cranial nerves III, VII, IX, and X (1973); and also from second and third sacral spinal nerves (S2,S3) (Cranio-sacral). About 75 per cent of all parasympathetic nerve fibers are in the vagus nerves, passing to the entire thoracic and abdominal regions of the body. The preganglionic fibers pass uninterrupted all the way and synapse with the postganglionic neurons which is located in the wall of the effector organ.  Preganglionic and Postganglionic Parasympathetic Neurons. The parasympathetic system, like the sympathetic system, has both preganglionic and postganglionic neurons. However, except in the case of a few cranial parasympathetic nerves, the preganglionic fibers pass uninterrupted all the way to the organ that is to be controlled. The postganglionic neurons are located in the wall of the organ. The preganglionic fibers synapse with these neurons, and extremely short postganglionic fibers, a fraction of a millimeter to several centimeters in length, leave the neurons to innervate the tissues of the organ. Parasympathetic NS  The sympathetic and parasympathetic nerve fibers secrete one of the two main synaptic transmitter substances, acetylcholine or norepinephrine. All preganglionic sympathetic and parasympathetic neurons are cholinergic. All postganglionic parasympathetic neurons are also cholinergic. The postganglionic sympathetic nerve fibers to the sweat glands, piloerector muscles of the hairs, and very few blood vessels are cholinergic. Most postganglionic sympathetic neurons are adrenergic.  Almost all the systemic arterioles are kept partially constricted. This is called the sympathetic tone. By increasing the degree of sympathetic stimulation above normal, these vessels can be constricted even more; conversely, by decreasing the stimulation below normal, the arterioles can be dilated. Parasympathetic tone is very important for the gastrointestinal function. Surgical removal of the parasympathetic supply causes serious and prolonged gastric and intestinal atony followed by constipation.  Sympathetic Responses Dominance by the sympathetic system is caused by physical or emotional stress -- “E situations” emergency, embarrassment, excitement, exercise Alarm reaction = flight or fight response dilation of pupils increase of heart rate, force of contraction & BP decrease in blood flow to nonessential organs increase in blood flow to skeletal & cardiac muscle airways dilate & respiratory rate increases blood glucose level increase Long lasting (effects) are due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland  Parasympathetic Responses Enhance “rest-and-digest” activities Mechanisms that help conserve and restore body energy during times of rest Normally dominate over sympathetic impulses SLUDD type responses = salivation, lacrimation, urination, digestion & defecation 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil Paradoxical fear when there is no escape route or no way to win causes massive activation of parasympathetic division loss of control over urination and defecation