Nervous System.docx
Transcript
Nervous System Organization Central Nervous System (CNS) – nervous tissue found making up the brain & spinal cord. Peripheral Nervous System – nervous tissue coming off of the CNS – Nerves Somatic Nervous System – sending information to voluntary muscle. Autonomic Nervous System – sending...
Nervous System Organization Central Nervous System (CNS) – nervous tissue found making up the brain & spinal cord. Peripheral Nervous System – nervous tissue coming off of the CNS – Nerves Somatic Nervous System – sending information to voluntary muscle. Autonomic Nervous System – sending information to involuntary muscles & glands. General Functions: Sensory functions: sensory receptors continuously monitoring internal & external environment, and relaying this information to CNS for interpretation. Integrative: bringing sensory information together for perception of sensations. Motor functions: consciously and subconsciously sending impulses to effectors (muscle cells of glands). Nerve Tissue: is made up of two types of cells. Neurons – the structural and functional units of the nervous system. React to changes in the environment or changes within the body. Conduct impulses to other neurons and to other cells outside the CNS. Neuron structure: Cell body – contains most of the organelles of other cells. Neurofibrils – fine threads which extend into the nerve supporting them. Nissl bodies (RER) are also seen here. Dendrites – a fiber of a neuron which carries nerve impulses (action potentials) towards the cell body. The axon can branch at its terminal end and contact numerous other cells. Axon – carries nerve impulses away from the cell body. Neuroglial cells – give structural support to CNS, produce myelin, and carry on phagocytosis. Schwann cells – cells which commonly surround large axons in the peripheral nervous system. In large axons, the membrane is composed of a lipid-protein called myelin, which acts as an insulator and also increase the speed of a nerve impulse. Smaller axons are enclosed in Schwann cells, but they are not wrapped in multiple layers and are therefore unmyelinated. Gaps between adjacent Schwann cells are called nodes of Ranvier. Are not found in the CNS. Astrocytes – give structural support and remove cellular debris. Responsible for information of scar tissue after CNS injuries. Oligodendrocytes – formation of myelin within the CNS. Microglia – phagocytosis of bacterial cells and cellular debris. Ependymal – lines the ventricle chambers of the brain. Neuron Classification Classified according to STRUCTURE Bipolar Neurons Unipolar Neurons Multipolar Neurons Classified according to FUNCTION Sensory Neurons Interneurons Motor Neurons Cell Membrane Potential Membrane Potential – a measurable difference in charge across a membrane. At rest, a neuron has an unequal distribution of ions on either side of the cell membrane. This causes the membrane to be electrically charged, or polarized. The unequal distribution of ions inside and outside the cell membrane is determined by the presence of specific ion channels and carrier molecules. In resting state: * Sodium is in a higher concentration outside the cell compared to the inside. * Potassium is in a higher concentration inside the cell compared to the outside. The measured potential difference in electrical charge across the membrane of a neuron is called resting membrane potential. Establishing and Maintaining Resting Membrane Potential There is an unequal permeability of the membrane for sodium and potassium. The membrane is more permeable (more channels) for potassium than sodium. This results in more cations (potassium) diffusing out of the cell than cations (sodium) diffusing into cells. The pumping ratio of sodium to potassium ions by active transport. Three sodium ions pumped out for every two potassium ions pumped in (3:2 ratio). Three cations pumped out for every two pumped in. At rest the potential difference between the inside and the outside of the cell membrane is -70 mv. (negative refers to the inside being negatively charged with respects to the outside.) Only muscle cells and neurons have the ability to manipulate (change) this difference in charge across the membrane (membrane potential). This manipulation of charge is known as a muscle or nerve impulse. To better understand impulses, we need to first look at how these cells respond to an external signal. Both neurons and muscle cells exhibit the characteristic excitability (irritability). This means they can respond to changes in their surroundings. These changes are called stimuli. Stimuli cause a change in the resting potential in a particular local region of the cell membrane. If, in response to a stimulus the resting membrane potential becomes: More negative ( > -70mv, for example -76mv), the membrane is hyperpolarizing. Less negative ( < -70mv, for example -57mv), the membrane is depolarizing. The amount of change in the membrane potential is graded, which means it is directly related to the strength of stimulus. A threshold stimulus is the minimal strength stimulus needed for the generation of a nerve impulse (action potentials). Threshold is reached when a stimulus causes a depolarization of the membrane (decrease in membrane potential) to -55mv. Threshold stimuli lead to the generation of an action potential. Events of an action potential: Gated Na+ channels open in the membrane at the point of stimulation Na+ diffuses into the cell Membrane potential decreases (depolarization) to +30mv (the outside of the membrane becomes negatively charged with respect to the inside.) Gated K+ channels open and K+ diffuses out of the membrane. Membrane potential increases (repolarizes) to -70mv. After repolarization, the Na/K pump moves Na+ back out of the membrane and moves K+ back in to return the membrane to its original resting potential. Nerve Impulses – the propagation of action potentials along a nerve fiber travelling to the end of the fiber. Unmyelinated nerve fibers conduct impulses over their entire surface. In myelinated fibers, the action potential jumps between node of Ranvier, which greatly accelerates the rate at which these impulses move along the fiber. All or None Response – if a threshold or greater stimulus is applied, the nerve impulse fires completely. This means with the formation of the first action potential, the propagation of action potential will continue down the entire neuron membrane. The action potentials will not stop halfway down. BEGINNING INFORMATION FOR TEST 4 Synapses – the junction between two neurons. Characteristics: Synaptic cleft – the space between two neurons. Presynaptic neuron Postsynaptic neuron Synaptic vesicles Synaptic Transmission Impulses move through dendrites toward the cell body Impulses move along an axon toward an end (synapse) When the impulse reaches the end (terminal knob) of the axon the following events occur: Impulse causes calcium to move inward and fuse to vesicles containing neurotransmitters. Vesicles fuse to postsynaptic membrane and release neurotransmitters into the synaptic cleft. Neurotransmitters attach to receptors on the postsynaptic membrane. If a threshold level is reached, then the impulse is generated in the post-synaptic neuron. Neurotransmitters – substances released at synapses. Most neurons release only one type. Stored in synaptic vesicles. 35 known different types. Types: Acetylcholine Monoamines – ex: epinephrine, norepinephrine, dopamine, serotonin. Amino acids – glycine, glutamic acid, aspartic acid, GABA Peptides – enkephalins, substance P NOTE: all neurotransmitters have specific enzymes that destroy them once they are released into the synaptic cleft. Examples: Acetylcholinesterase – breaks down Acetylcholine Monoaminoxidases – breaks down Monoamines. Neurotransmitters binding to receptors on the post synaptic neuron cause a change in the membrane potential at the local site. These changes are referred to as synaptic potentials. Synaptic potentials are graded. The direction in which the potential goes (depolarization or repolarization) depends on the type of neurotransmitter binding. Neurotransmitters can be excitatory or inhibitory. Excitatory Neurotransmitters – cause an decrease in membrane potential (depolarization) on the post-synaptic membrane. Excitatory Postsynaptic Potential (EPSP). Inhibitory Neurotransmitters – cause an increase in membrane potential (hyperpolarization) of the postsynaptic membrane. Inhibitory Postsynaptic Potential (ISPS). The integrated sum of the EPSP’s and IPSP’s determines whether an action potential is generated. Remember, in order to generate an action potential, threshold must be reached. Brain Principle parts: Cerebrum Diencephalon Brainstem Cerebellum Cerebrum – made of two cerebral hemispheres which are connected by corpus callosum. Gyri – convolutions (hills) Sulci – shallow grooves Fissures – deep grooves Cerebral Cortex (gray matter) – outer surface of brain, composed of nerve cell bodies and dendrites. Contains 75% of the neuron cell bodies in the nervous system. 1/8 inch thick. White Matter – made up of large groups of axons. Basal Nuceli – islands of gray matter found deep in white matter Sulci and fissures: Longitudinal fissures – separates the two cerebral hemispheres. Transverse fissure – separates cerebrum from cerebellum. Lateral Sulcus – separates frontal/parietal lobes from temporal. Central Sulcus – separates frontal and parietal lobes. Gyri Precentral gyrus – immediately in front of central sulcus and is the portion of the brain which initiates all nerve impulses for voluntary movement on the opposite side of the body. Postcentral gyrus – immediately behind the central sulcus and is part of brain which receives most of the sensory information from the opposite side of the body. Lobes of Cerebrum: Frontal Lobe – anterior portion of cerebrum found in front of the central sulcus. Part of cerebrum responsible for voluntary movement, speech, personality. Parietal Lobe – lobe just behind central sulcus. Responsible for experience of sensation, understanding speech. Temporal Lobe – lobe which lies below the lateral sulcus. Responsible for hearing and understanding what is heard. Occipital Lobe – posterior lobe of the brain. Responsible for vision and interpretation of what is seen. Insula – located deep inside the lateral sulcu. Functional Regions of Cerebral Cortex: Motor areas – controls voluntary motor function Sensory areas – perception of sensation Association areas – function in analysis, interpretation of sensory experiences, memory reasoning, judgement, emotions. 2. Diencephalon – composed of the thalamus and hypothalamus. Thalamus – relay station for sensory information coming to the cerebral cortex from the body. All descending fibers from the cortex to the body pass through the thalamus. Thalamus has crude awareness of pain, touch and temperature. Hypothalamus – is the critical importance in many homeostatic mechanisms: Regulates heart rate and blood pressure Regulates body temperature Regulation of water and electrolyte balance Regulation of hunger and thirst Regulation of sleep and wakefulness Controls pituitary gland functions Brainstem – connects the brain with the spinal cord. Composed of tracts (bundles of axons) and nuclei (collections of cell bodies) Midbrain – part of brainstem which connects the thalamus and pons. Tracts which ascend and descend from the cerebral cortex pass through here. Pons – between midbrain and medulla oblongata. Carries information to and from the cortex. Also sends impulses from the cerebrum to the cerebellum. Contains pnuemotaxic respiratory center. Medulla Oblongata – between pons and spinal cord. The lowest part of the brainstem. All ascending and descending pathways between brain and spinal cord must pass through the medulla. Also contains three important centers: cardiac center (heart rate), vasomotor center (blood center), and respiratory center (breathing). Cerebellum – consists of two hemispheres connected by the vermis. Like the cerebrum, it is composed of an outer layer of gray matter which covers the inner white matter. It connected to the brainstem by three pairs of cerebellar peduncles (nerve tracts). Function: Coordination of skeletal muscle contractions. Maintenance of posture. Meninges – protective membrane that protects the CNS. Three Layers Dura mater – outermost tough layer. Is attached to the skull and vertebrae. Parts of dura Falx cerebri – dura between cerebral hemispheres. Falx cerebelli – dura between cerebellar hemispheres. Tentorium cerebelli – dura between the cerebrum and cerebellum. NOTE: a blow to the head can result in damage to the blood vessels which supply the meninges. If bleeding is between the dura and the skull, it is called an epidural hematoma. If the bleeding is between the arachnoid and dura, it is called a subdural hematoma. Arachnoid – middle layer. Separated from dura by thin layer of fluid. Pia Mater – covers surface of brain and spinal cord. Subarachnoid space – contains cerebrospinal fluid and is the space between the arachnoid and pia meter. CSF acts as a shock absorber for the CNS. Spinal Cord – continuation of brainstem which is contained in the vertebra foramen. Structure: Composed of central core of gray matter surrounded by tracts of white matter. Ventral (anterior) horn – contains motor neurons which stimulate skeletal muscles to contract. Structure: Function: Spinal nerve Carries impulses into/out of spinal cord Dorsal root Carries sensory information into spinal cord Dorsal root ganglion Contains cell bodies of sensory neurons Ventral (anterior) horn Contains cell bodies of motor neurons Ventral root Carries impulses to skeletal muscles White matter Bundles of axons which carry impulses to and from the brain. Gray matter Collections of nerve cell bodies. Functions: Carry sensory information to brain from body (ascending tracts). Carry motor information muscles and glands from brain (descending tracts). Spinal reflexes. Cranial Nerves No. Name Type Sensory Function Motor Function I Olfactory Sensory Sense of Smell None II Optic Sensory Sense of Vision None III Oculomotor Motor None Eye muscles. Raises upper eyelid Contracts pupil IV Trochlear Motor None Eye muscle V Trigeminal Mixed Sensory to face & scalp Muscles of mastification VI Abducens Motor None Eye muscle VII Facial Mixed Taste buds ant. 2/3 tongue Muscles of facial expressions VIII Vestibulocochlear Sensory Sense of hearing & equilibrium None IX Glossopharyngeal Mixed Taste buds post. 1/3 tongue None X Vagus Mixed Taste buds pharynx/epiglottis Pharynx/Larynx muscles involved in swallowing XI Accessory Motor None Sternocleidomastoid Trapezius XII Hypoglossal Motor None Muscles of tongue
