Medicine I Neurophysiology Lecture Notes 2024 PDF

THE CENTRAL NERVOUS SYSTEM  LECTURER INFORMATION Mrs M van Hoogland-van Heerden BMS Building 4th Floor Room: N412 Office number: 012 521 4332 Email: [email protected] NERVOUS SYSTEM ORGANISATION Important:...

THE CENTRAL NERVOUS SYSTEM  LECTURER INFORMATION Mrs M van Hoogland-van Heerden BMS Building 4th Floor Room: N412 Office number: 012 521 4332 Email: [email protected] NERVOUS SYSTEM ORGANISATION Important: How everything fits together in the bigger picture FUNCTIONAL CLASSES OF NEURONS Afferent neurons Inform the CNS about conditions in both the internal and external environment At its peripheral ending there is a sensory receptor that generates action potentials in response to stimulus Efferent neurons Carry instructions from CNS to effector organs, muscles and glands Lie primarily in the PNS Interneurons (Most abundant) Found entirely in the CNS Lie between afferent and efferent neurons Responsible for:  Integrating afferent information and formulating an efferent response  Interconnections between interneurons are associated with higher mental functions NEUROGLIA Neuroglia/Glial Cells Unlike neurons neuroglia/glial cells do not initiate or conduct nerve impulses Neuroglia/glial cells do communicate with neurons and among themselves via chemical signals Serve as connective tissue of the CNS Support interneurons physically, functionally and metabolically 4 Types of Neuroglia/Glial cells Know the 1. Astrocytes functions of each 2. Oligodendrocytes type of glial cell – 3. Microglia refer to table on 4. Ependymal cells next slide  FUNCTIONS OF NEUROGLIA GLIAL CELL TYPE FUNCTIONS Astrocytes Most abundant glial cell type and act as the main ‘glue’ of the CNS (Holding neurons together) Serve as a scaffold that guides neurons during fetal brain development Responsible for the formation of the BBB (Induce capillary changes both anatomical/functional) Helps with transfer of nutrients from the blood to the neurons Brain injury repair by forming neural scars Take up/degrade locally released NTs (Bringing chemical messenger actions to a halt): Glutamate and GABA Take up excess K+ from the brain ECF (Excitability) * Refer back to action potentials Extracellular communication with neurons and via jap junctions * Refer to textbook Enhance synapse formation and modify synaptic transmission Oligodendrocytes Insulative myelin sheaths around the axons in the CNS Later in fetal life – Nerve growth inhibiting proteins (Nogo) Microglia Immune defence cells of the CNS (stationary until activated by infections/injuries) Release destructive chemicals for assult against foreign invaders Nerve growth factor release in resting state (helps neurons and glial cells survive) Ependymal Line the internal, fluid filled cavities of the CNS Help to form CSF Neural stem cells (Able to form other types of glial cells and neurons)  NEUROGLIA MENINGIOMAS (Tumour of the meninges which are the covering of the brain and the spinal cord) The most common kind of brain tumour Mostly benign Does not infltrate the brain but can exert pressure on the brain NON-NEURAL TUMOURS NEURAL TUMOURS PROTECTION OF THE CNS Enclosed by hard, bony structures  Cranium (Skull) (brain)  Vertebral column (spinal cord) Wrapped by 3 protective and nourishing membranes- meninges  Dura mater  Arachnoid mater  Pia mater Brain floats in cushioning fluid – CSF BBB limits access of blood-borne materials into brain tissue  CSF COMPOSITION CSF has the same density as the brain CSF is lower in potassium compared to blood CSF has a slightly higher sodium content than blood Necessary for the movement of ions down concentration gradients Almost no proteins in CSF (Plasma proteins cannot exit the brain capillaries to leave the blood during the formation of CSF) 125mL – 150mL (replaced more than 3x per day) CSF (FUNCTION) Surrounds and cushions the brain and spinal cord Major function: Serves as a shock-absorbing fluid to prevent the brain from bumping against the hard skull Other functions:  Exchange of materials between neural cells and interstitial fluid surrounding the brain  Formed primarily by choroid plexuses  Richly vascularised masses of pia mater tissue that dip into pockets formed by ependymal cells  CSF forms as a result of selective transport mechanisms across membranes of choroid plexuses CSF CONDITIONS Hydrocephalus: Excess CSF - Causes increased CSF pressure - Brain damage can occur (mental retardation) - Treatment: Surgical shunting to drain excess CSF to veins  BBB Highly selective Protects the brain from chemical fluctuations in the blood  Strictly limits exchange between blood and brain Minimizes the possibility that harmful blood-borne substances might reach the central nervous tissue Prevents certain circulating hormones that could also act as NTs from reaching the brain Limits the use of drugs for the treatment of brain and spinal cord disorders as many drugs cannot penetrate the BBB  BBB Certain areas of the brain (portion of the Hypothalamus) are not subject to the BBB “sampling of blood” to make appropriate adjustments (homeostasis) Output – in the form of water soluble hormones that must enter the hypothalamic capillaries Hypothalamic capillaries are not sealed by tight junctions 4 areas of the brain are not protected by the BBB. These areas include the posterior pituitary gland, pineal gland, the median eminence of the hypothalamus and the area postrema  O2 & GLUCOSE The brain depends on constant delivery of oxygen and glucose by the blood The brain cannot produce ATP in the absence of oxygen (metabolism) Neuroglobin (oxygen binding protein in the brain) Brain primarily uses glucose for energy (starvation – ketone bodies produced by the liver) Resting: 20% of oxygen and 50% glucose 15% of blood pumped out by the heart  BRAIN DAMAGE Oxygen / glucose deprivation More than 4-5 min without oxygen More than 10-15 min without glucose Common cause: Stroke Treatment option: clot dissolving drugs FUNCTIONS OF THE CNS CNS consists of the brain and the spinal cord CNS enables you to:  Subconsciously regulate your internal environment by neural means  Experience emotions  Voluntarily control your movements  Be consciously aware of your own body and your surroundings  Engage in other higher cognitive processes such as thought and memory ANATOMY OF THE BRAIN Forebrain Diencephalon - Thalamus - Hypothalamus Cerebrum - Basal nuclei - Cerebral cortex  FUNCTIONS OF THE BRAIN COMPONENTS n t rta n s p o ti o Im nc fu BRAIN STEM Oldest region of the brain Continuous with spinal cord Controls many life-sustaining processes, such as respiration, circulation, and digestion Concerned with maintaining proper position of the body in space and subconscious coordination of motor activity (movement) Consists of the midbrain, pons and medulla CEREBELLUM Subcortical* region of the brain Attached at top rear portion of the brain stem Important role in planning, initiating, and timing of movements by sending input to the motor areas of the cortex Maintains proper position of the body in space Subconscious coordination of motor activity (movement) Plays a key role in learning skilled motor tasks *Subcortical: Below the cortex DIENCEPHALON CONSISTS OF 2 BRAIN COMPONENTS:  Thalamus  Performs primitive sensory processing  Serves as a relay station  Screens out insignificant signals and routes important sensory impulses to appropriate areas  Helps to direct attention to stimuli of interest  Hypothalamus  Controls many homeostatic functions  Plays a role in emotional and behavioural patterns  Regulation of the internal environment  CEREBRUM Cerebrum Corpus Callosum Highly developed 80% of total brain weight Divided into 2 halves:  R and L Cerebral hemispheres (connected via the Corpus Callosum) Inner core houses the basal ganglia Outer surface is highly convoluted cerebral cortex  Caps inner core that houses basal nuclei  Highest, most complex integrating area of the brain  Plays key role in most sophisticated neural functions Pons Cerebellum Highest, most complex integrating area of the brain Medulla  CEREBRAL CORTEX Thin outer shell of grey matter that covers each hemisphere Covers a thick central core of white matter Cerebral Organised into 6 well defined layers Cortex Layers are organised into functional vertical columns Each half of cortex divided into 4 major lobes:  Occipital  Temporal  Parietal  Frontal CEREBRAL CORTEX (4 LOBES) OCCIPITAL LOBE  Carries out initial processing of visual input TEMPORAL LOBE  Initial reception of auditory (sound) sensation PARIETAL LOBE  Receive and process sensory input  Somatosensory processing FRONTAL LOBE  Responsible for 3 main functions:  Voluntary motor activity t  Speaking ability r t an po ns Im c t io  Elaboration of thought fun CEREBRAL CORTEX (LANGAUGE AREAS) Primary areas of cortical specialisation for language  Broca’s area (speaking)  Wernicke's area (language comprehension) Language disorders  Aphasias (mostly due to stroke)  Speech impediments (defect in the mechanical aspect of speech – vocal cords)  Dyslexia (inappropriate interpretation of words) BASAL NUCLEI (BASAL GANGLIA) Consists of several masses of grey matter located deep within the white matter Act by modifying ongoing activity in motor pathways Primary functions:  Inhibiting muscle tone throughout the body  Selecting and maintaining purposeful motor activity while suppressing useless or unwanted patterns of movement  Helping to monitor and coordinate slow, sustained contractions (posture)  PARKINSON’S DISEASE Associated with a gradual destruction of neurons that release the NT Dopamine (Basal nuclei) 3 Types of motor disturbances:  Increase in muscle tone, or rigidity  Involuntary, useless, or unwanted movements such as resting tremors  Slowness in initiating and carrying out different motor behaviours Treat with L-Dopa (precursor of dopamine) – BBB THALAMUS Positively reinforces voluntary motor behaviour initiated by the cortex Part of the diencephalon Serves as a ‘relay station’ and synaptic integrating center for processing sensory input on its way to cerebral cortex Along with brain stem and cortical association areas, important in ability to direct attention to stimuli of interest Capable of crude awareness of various types of sensation but cannot distinguish their location or intensity HYPOTHALAMUS Brain area most involved in directly regulating internal environment Functions:  Controls body temperature  Controls thirst and urine output  Controls food intake  Controls anterior pituitary hormone secretion  Influence sleep-wake cycle  Plays a role in emotional and behavioural patterns  Many more functions … (Refer to textbook) LIMBIC SYSTEM Not a separate structure but a ring of forebrain structures that surround the brain stem Includes portions of the hypothalamus and other forebrain structures that encircle the brain stem Responsible for:  Emotion  Basic, inborn behavioural patterns related to survival and perpetuation of the species  Plays important role in motivation and learning Self-study: Reward vs punishment, motivation and homeostatic drives p125 CEREBELLUM Important in balance and in planning and executing voluntary movement 3 DIFFERENT PARTS  Vestibulocerebellum  Important in maintaining balance and controls eye movements  Spinocerebellum  Enhances muscle tone and coordinates skilled, voluntary movements  Cerebrocerebellum  Plays a role in planning and initiating voluntary activity by providing input to cortical motor areas  Stores procedural memories BRAIN STEM Critical in connecting the link between spinal cord and higher brain regions Oldest region of the brain Continuous with spinal cord All incoming and outgoing fibers transversing between periphery and higher brain centers must pass the brain stem Consists of:  Medulla  Pons  Midbrain ANATOMY OF THE SPINAL CORD/ NERVES Extends from brain stem through vertebral canal 31 pairs of spinal nerves emerge from spinal cord through spaces formed between arches of adjacent vertebrae  Named for region of vertebral column from which they emerge  8 pairs cervical (neck) nerves  12 pairs thoracic (chest) nerves  5 pairs lumbar (abdominal) nerves  5 pairs sacral (pelvic) nerves  1 pair coccygeal (tailbone) nerves FUNCTIONS OF THE SPINAL CORD 2 Vital functions  Neuronal link between brain and PNS  Integrating center for spinal reflexes THE PERIPHERAL NERVOUS SYSTEM PNS: EFFERENT DIVISION Communication link by which CNS controls activities of muscles and glands Two divisions of PNS  ANS  Involuntary branch of PNS  Innervates cardiac muscle, smooth muscle, most exocrine glands, some endocrine glands, and adipose tissue  Somatic nervous system  Subject to voluntary control  Innervates skeletal muscle  ANS Autonomic nerve pathway  Extends from CNS to an innervated organ Two-neuron chain  Preganglionic fiber (synapses with cell body of second neuron)  Postganglionic fiber (innervates effector organ) BASIC STRUCTURE: IMPORTANT  ANS Two subdivisions of the ANS: Kn Sympathetc nervous system ow tab this Parasympathetc nervous system le Sympathetic Nervous System Parasympathetic Nervous System Fibers originate in thoracic and lumbar regions of the Fibers originate from cranial and sacral areas of CNS spinal cord Most preganglionic fbers are short Preganglionic fbers are longer Long postganglionic fbers Very short postganglionic fbers Preganglionic fbers release Ach Preganglionic fbers release Ach Most postganglionic fbers release noradrenaline Postganglionic fbers release Ach ANS  ANS Most visceral organs are innervated by both sympathetic and parasympathetic fibers In general produce opposite effects in a particular organ Dual innervation of organs by both branches of ANS allows precise control over organ’s activity Sympathetic system: Dominates in emergency or stressful (‘fight-or-flight’) situations Promotes responses that prepare the body for strenuous physical activity Parasympathetic system: Dominates in quiet, relaxed (‘rest-and-digest’) situations Structures innervated by Promotes body-maintenance activities sympathetic and parasympathetic nervous systems  ADVANTAGE OF DUAL INNERVATION 1. Innervated blood vessels: receive only sympathetic nerve fibers (most arterioles and veins) Accomplished by decreasing or increasing the firing rate above/below tone level in these fibers *Only blood vessels that have both sympathetic/parasympathetic fibers are those supplying the penis and clitoris 2. Sweat glands: innervated only by sympathetic nerves Postganglionic fibers secrete Ach instead of NE 3. Salivary glands: innervated by both sympathetic and parasympathetic activity that is not antagonistic Both stimulates salivary secretion (volume, composition differs)  NEUROTRANSMITTER RECEPTORS Tissues innervated by the ANS have one or more of several different receptor types for postganglionic chemical messengers  Cholinergic receptors – bind to Ach  Nicotinic receptors – found on postganglionic cell bodies of all autonomic ganglia  Muscarinic receptors – found on effector cell membranes  Adrenergic receptors – bind to norepinephrine and epinephrine  Alpha (α) receptors  Beta (β) receptors  ANS Adrenal medulla is a modified part of the sympathetic nervous system  Modified sympathetic ganglion that does not give rise to postganglionic fibers  Stimulation of preganglionic fiber prompts secretion of hormones into blood  About 20% of hormone release is norepinephrine  About 80% of hormone release is epinephrine ANS Exceptions to general rule of dual reciprocal innervation by the two branches of autonomic nervous system  Most arterioles and veins receive only sympathetic nerve fibers (arteries and capillaries are not innervated)  Most sweat glands are innervated only by sympathetic nerves  Salivary glands are innervated by both ANS divisions but activity is not antagonistic – both stimulate salivary secretion  AUTONOMIC AGONISTS AND ANTAGONISTS Agonists Drugs that act selectively at α- and β-  Bind to same receptor as neurotransmitter adrenergic receptor sites  Elicit an effect that mimics that of  Salbutamol: Selectively activates β2- neurotransmitter adrenergic receptors at low doses. (Dilates Antagonists bronchioles for asthma)  Bind with receptor  Metoprolol: Selectively blocks β1-  Block neurotransmitter’s response adrenergic receptors (BP medication)  SOMATIC NERVOUS SYSTEM Consists of axons of motor neurons that originate in spinal cord or brain stem and end on skeletal muscle Motor neuron releases neurotransmitter, Ach, which stimulates muscle contraction Motor neurons are final common pathway by which various regions of CNS exert control over skeletal muscle activity  These areas of CNS include spinal cord, motor regions of cortex, basal nuclei, cerebellum, and brain stem  SOMATIC NERVOUS SYSTEM The cell bodies of motor neurons may be selectively destroyed by polio virus – Paralysis ALS / Lou Gehrig’s disease is the most common motor neuron disease – Degeneration / death of motor neurons (progressive disease with death 3-5 years after onset)