Human Nervous System PDF

- The nervous system is one of the two control systems in the human body. - Regulates all activities along with the endocrine system. - Without the nervous system, we would not be able to respond to the internal or external environment Contr...

- The nervous system is one of the two control systems in the human body. - Regulates all activities along with the endocrine system. - Without the nervous system, we would not be able to respond to the internal or external environment Control Center Receptors/ Sensors Stimulus Response Effectors There are two types of nervous tissue (a) Neurons/ Nerve cells- send electrochemical impulses in response to stimuli received and to effect an action as a result of stimuli- send and receive signals from the brain (b) Neuroglia- support and protect the neurons  Soma- cell body with a large nucleus. Why large? Axon- a long slender projection that conducts electrical impulses (action potential) away from the cell body Axon is surrounded by Schwann cells forming the myelin sheath Dendrites- tree-like structures that receive messages from other neurons Axon terminals/ Nerve ending- contains synaptic vesicles holding neurotransmitters. Synapses- specialized junctions between neurons In In terms of function, neurons are classified into three broad types: A. Sensory- triggered by physical (sound, touch, heat, and light) and chemical (smell and taste) inputs from your environment. For ex, stepping on hot sand activates sensory neurons in the soles of your feet → neurons send a message to brain → makes you aware of the heat. B. Motor neurons play a role in movement, including voluntary and involuntary movements. These neurons allow the brain and spinal cord to communicate with muscles, organs, and glands all over the body. For ex, when eating, lower motor neurons in spinal cord send signals to smooth muscles in esophagus, stomach, and intestines → muscles contract → allows food to move through GI tract. C. Interneurons are neural intermediaries found in brain and spinal cord, most common type of neuron, pass signals from sensory neurons and other interneurons to motor neurons and other interneurons. For ex, when you touch something hot, sensory neurons in fingertips send a signal to interneurons in spinal cord → interneurons pass the signal on to motor neurons in your hand, which allows you to move your hand away. Other interneurons send a signal t o the pain center in your brain, and you experience pain. Enteric Division  Neuroglia or glial cells or glia= nerve glue Functions - Form myelin, which wraps around axons to speed up electric impulse conduction - Provide nutrients to neurons, including oxygen - Destroy pathogens - Provide a general support structure on which neurons can sit How is information transmitted? Action Potential Action Potential = when the membrane potential changes rapidly, has 2 main parts: Depolarization → Negative to Positive and Repolarization → Positive to Negative Neurons have a potential difference of -70 mV and neurons are capable of changing this electrical charge difference, enabling them to produce an action potential, and then return it to its original stage. Saltatory Conduction Propagation of action potentials along myelinated axons. Here the signal jumps from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. The uninsulated nodes of Ranvier are the only places along the axon where membrane depolarization and ion exchange occurs across the axon membrane. On the contrary, in an unmyelinated neuron, the signal is transmitted from one neuronal segment to another as depolarization and repolarization, making the process a lot slower. Synaptic Transmission Biological process by which a neuron communicates with a target cell across a synapse. Chemical synaptic transmission involves the release of a neurotransmitter from the pre-synaptic neuron, and neurotransmitter binding to specific post-synaptic receptors. Electrical synapse transmission involves the transfer of electrical signals through gap junctions. Chemical Synapse Gap Junction Steps in Synaptic Transmission Parts of a brain Forebrain- largest section, includes; (A) diencephalon and (B) cerebrum/ cerebral cortex, associated with body temperature control, eating, sleeping, cognition, emotions, reproduction. A. Diencephalon Includes: Thalamus Thalamus- Deep inside brain, major sensory-motor relay station for nerve impulses moving to and from cerebrum; gatekeeper, decides importance- whether or not you should pay attention to it. Hypothalamus- Links the CNS to the endocrine system controlling the Epithalamus immune system- controls autonomic system (involuntary functions), Hypothalamus neuro-endocrine system and limbic system- control and command center Pituitary gland (small extension below HT)- key link between nervous and endocrine systems. Epithalamus- Links the limbic system (primitive/crude brain) to other CNS systems; Made up primarily of Pineal gland- secretes serotonin during the day, melatonin at night, regulates sleep-wake cycle/ circadian rhythms, larger in childhood, calcifies with age. Habenula- sleep, stress, pain, reinforcement processing B. Cerebrum or Cerebral Cortex Includes grey matter, underlying white matter, basal ganglia/ nuclei and limbic system, folded into gyri, five functional lobes. Frontal, Parietal, Occipital, Temporal lobes Insular Lobe- deep location, difficult to access, little understanding (may be perception of pain, touch, taste, temperature, etc.) Insular Lobe B. Cerebrum or Cerebral Cortex Includes grey matter, underlying white matter, basal ganglia/ nuclei and limbic system, folded into gyri, five functional lobes. Basal ganglia or basal nuclei- important role in motor and thought control and inhibit muscle tone. Included parts Limbic system- controls 90% of our unconscious thoughts (next slide) B. Cerebrum or Cerebral Cortex Includes grey matter, underlying white matter, basal ganglia/ nuclei and limbic system, folded into gyri, five functional lobes. Limbic system- includes the amygdala, hippocampus, hypothalamus, olfactory cortex, and thalamus. Responsible for regulating our emotional lives, and perceptions, and higher mental functions and thought processes such as learning, motivation, formulating, and storing memories, controlling adrenaline and autonomic response, and regulating hormones and sexual response, sensory perception (optical and olfactory), and motor function. Limbic System has links to Hypothalamus, midbrain, reticular formation and olfactory system  Forebrain Midbrain Midbrain Includes tracts of nerve fibers and nuclei of cranial nerves. Serves important functions in motor movement, particularly movements of the eye, and in auditory and visual processing.  Forebrain Midbrain Hindbrain Hindbrain Includes pons, medulla oblongata and cerebellum Pons- controls the sleep-wake cycle, responsible for generating dreams, regulates respiration, involuntary actions, bladder control, hearing, posture, maintaining equilibrium, chewing, swallowing, taste, eye movement, facial expressions. Injury to the pons region may result in difficulty in walking, touch, swallowing, etc. Cerebellum- coordination and fine-tuning of motor activity, helps in the precision of muscle movements, regulates equilibrium, posture and muscle tone, stores implicit memories and motor skills like swimming, cycling, etc. Medulla- controls various autonomic functions, regulates respiration, cardiovascular reflexes, heartbeat, swallowing, vomiting, coughing, etc. Grey matter and White matter of the spinal cord  The white matter contains the ascending and descending pathways connecting the brain and spinal cord. The gray matter is divided into (a) dorsal/posterior horn contains neurons that receive afferent somatosensory information from the body, which is then transmitted via the ascending pathways, to the brain. (b) ventral/anterior horn contains motor neurons that exit the spinal cord to innervate skeletal muscle. (c) intermediate column and lateral horn contains autonomic neurons that innervate visceral and pelvic organs, such as smooth and cardiac muscle and glands.  PNS= nerves and ganglia outside the CNS → includes cranial and spinal nerves Carries information to and from the CNS Has two divisions- (i) sensory/ afferent- transmits impulses from peripheral organs to the CNS (ii) motor/ efferent or motor division- transmits impulses from the CNS out to the peripheral organs to cause an effect or action. Sensory/Afferent division- to CNS Receptors (classified based on stimulus detected or environment monitored) (a) Enteroceptors- internal environment (b) Exteroceptors- external environment, located in skin (c) Proprioceptors- parts of the body, can sense when tissues are stretched or experience tension and pressure. For example, the proprioceptors in muscles are called muscle spindles. Sensory information is carried to brain through three sets of neurons. Motor/Efferent division- from CNS A. Somatic Nervous System- responsible for voluntary (conscious) control of body movement through stimulating contraction of skeletal muscles. The axons of upper motor neurons extend from the brain to lower motor neurons via two types of somatic motor pathways. 1. Direct pathway - provide input to lower motor neurons via axons that extend directly from the cerebral cortex. - collections of upper motor neurons with cell bodies in the motor cortex that project axons into the spinal cord, where they synapse with lower motor neurons or interneurons in the anterior horns. - also called the pyramidal pathways. 2. Indirect pathway - provide input to lower motor neurons from motor centers in the basal nuclei, cerebellum, and cerebral cortex. - convey information from the brain down the spinal cord for automatic movements, coordination of body movements with visual stimuli, skeletal muscle tone and posture, and balance. - Slower and more complex…. also known as extrapyramidal pathways. 3. Spinal Cord - Motor part of spinal cord or lower motor neuron (LMN) also called final common pathway - Cell body of LMN receives and integrates impulses from: - (a) direct and indirect pathways - (b) sensory nerves carrying information from the tissues at the spinal level - (c) association neurons carrying information from higher and lower segments of spinal cord Reflex arc - Example of integration and coordination at the level of the spinal cord - Reflex arc begins with a sensory neuron at a receptor (e.g., a pain receptor in the fingertip) and ends with a motor neuron at an effector (e.g., a skeletal muscle). Stimulus Response B. Autonomic Nervous System - Subconscious control system for visceral organs including those of the circulatory, digestive and respiratory systems  Read list of activities from pages 126- 127 Speeds up activities- “Fright, Flight or Fight” system Slows down activities- “Rest and Digest” system Cranial and Spinal Nerves Cranial nerves (12 in all) are the nerves that emerge directly from the brain (including brain stem) and relay information between the brain and parts of the body, primarily to and from regions of the head and neck. Read the mnemonic on Page 127 bottom for the names. Spinal nerves (31 pairs) are peripheral nerves that emerge from spinal cord segments and transmit messages between the spinal cord and the rest of the body, including muscles, skin, and internal organs. Each spinal nerve is dedicated to certain regions of the body. Nutrition and Protection of the Nervous System Cerebral Circulation The brain derives its arterial supply from the paired carotid and vertebral arteries. Every minute, about 600-700 ml of blood flow through the carotid arteries and their branches while about 100-200 ml flow through the vertebral-basilar system. Circle of Willis- circle of blood vessels supplying blood to different parts of brain, enables blood flow and blood pressure regulation within the brain and facilitates collateral circulation if arteries in the brain are narrowed and blocked. Nutrition and Protection of the Nervous System Meninges Three layers of connective tissue that surround the brain and spinal cord Dura matter- tough to prevent friction against skull Arachnoid matter- contains large blood vessels for maximal circulation and arachnoid villi to reabsorb CSF as it circulates through CNS Pia matter- thin innermost layer, ensures the CSF remains in subarachnoid space Nutrition and Protection of the Nervous System Cerebrospinal Fluid (CSF) Protects and nourishes the CNS. Contains glucose, protein and white cells but no red blood cells. Flows in one direction and washes out waste products and metabolites. Flow of CSF: 1. Created in choroid plexus- 400-500 ml formed daily 2. Moves through the foramen of Monro to the third ventricle 3. Passes through the cerebral aqueduct to the fourth ventricle 4. Passes through three apertures to the cerebromedullary cistern. 5. Circulates over spinal cord and enters subarachnoid space where it is reabsorbed. Nutrition and Protection of the Nervous System Blood Brain Barrier (BBB) Structural barrier- capillary endothelial cells (CEC) packed very tightly with no gaps in cell wall, capillary lumen has reduced surface area Chemical barrier- CEC have several mitochondria for high energy production, specific proteins inhibit lipid-soluble drugs from crossing into brain by transporting them into blood stream, if at all they pass, CEC contain drug-metabolizing enzymes to eliminate / deactivate them before reaching CNS. Caffeine and alcohol cross the BBB freely- hence acute effect on the nervous system. Structural Water soluble substances pass through barrier ion channels or transport proteins. Fats move across by diffusion. Larger items by transcytosis Sleep and Consciousness What is sleep? It is a dynamic activity and neurotransmitters control whether we are asleep or awake by acting on different groups of cranial neurons. Serotonin and norepinephrine keep some parts of the brain active while we are awake. Other neurons at the base of the brain begin signaling when we fall asleep. Research also suggests that a chemical called adenosine builds up in our blood while we are awake and causes drowsiness. Aden osine gradually breaks down while we sleep. Sleep and Consciousness Brain waves Different stages of sleep can be differentiated from one another by the patterns of brain wave activity that occur during each stage. Sleep can be divided into two different general phases: - Rapid eye movement (REM) sleep is characterized by darting movements of the eyes under closed eyelids. - In contrast, non-REM (NREM) sleep is subdivided into stages distinguished from each other and from wakefulness by characteristic patterns of brain waves. Sleep and Consciousness What is a sleep cycle? Children and adults spend almost 50% of their total sleep time in stage 2 sleep, about 20 % in REM sleep, and the remaining 30% in the other stages. Infants, by contrast, spend about half their sleep time in REM sleep. Brain areas regulating mental health Hippocampus, amygdala, and areas of the prefrontal cortex regulate physiological and behavioral stress processes, which can be adaptive in the short-term and maladaptive in the long-term.