Phineas Gage Case Study & Nervous System Overview PDF

# Chapter 10: Nervous System ## Phineas Gage Phineas Gage's case is a well-documented early case of a person to have survived severe damage to the brain, making his case rather famous. He is also the first patient from whom we learned something about the relation between personality and the functi...

# Chapter 10: Nervous System ## Phineas Gage Phineas Gage's case is a well-documented early case of a person to have survived severe damage to the brain, making his case rather famous. He is also the first patient from whom we learned something about the relation between personality and the function of the front parts of the brain. As the first newspaper account of the accident reported, Phineas Gage was the foreman of a railway construction gang. In September 1848, an accidental explosion of a charge he had set blew his tamping iron (3'7" long iron pole weighing 13.5 lbs and 1.25 inches in diameter) through his head. The tamping iron went in, point first, under his left cheekbone and completely out through the top of his head, landing about 25 to 30 yards behind him. Phineas was knocked over but may not have lost consciousness even though most of the front part of the left side of his brain was destroyed. He was medically treated and with success he returned home 10 weeks later. Some months after the accident, probably in about the middle of 1849, Phineas felt strong enough to resume work. But because his personality had changed so much, the contractors who had employed him would not give him his place again. Before the accident he had been their most capable and efficient foreman, one with a well-balanced mind, and who was looked on as a shrewd smart businessman. He was now fitful, irreverent, and grossly profane, showing little deference for his fellows. He was also impatient and obstinate. His friends said he was "No longer Gage." He lived another 11and a half years working in a horse livery and on a farm. In February 1860, he began to have epileptic seizures and was buried in May of 1860. ## Overall Function and Organization of the Nervous System The overall function of the nervous system is to: 1. receive and monitor sensory input inside and outside of the body 2. process and interpret the sensory input and make decisions, this is called integration 3. effect a response by activating glands or muscles, this is called motor output. ## The Central and Peripheral Nervous Systems - **Peripheral Nervous System (PNS)**: Consists of cranial nerves and spinal nerves. Nerves function as communication lines between the CNS and the rest of the body. - **Afferent Division:** Consists of somatic and visceral sensory nerve fibers. Conduct impulses from sensory receptors to the CNS. - **Efferent Division:** Consist of visceral and somatic motor nerve fibers. Conduct impulses from the CNS to effectors (muscles and glands). - **Central Nervous System CNS**: Consists of brain and spinal cord. Processes and interprets sensory input and makes decisions about what should be done at each moment. ## The Autonomic and Somatic Nervous Systems - **Autonomic Nervous System:** Visceral motor fibers (involuntary). Conduct impulses from the CNS to cardiac muscles, smooth muscles, and glands. - **Sympathetic Division:** Visceral motor fibers (involuntary). Mobilizes body systems during activity ("fight or flight" responses). - **Parasympathetic Division:** Visceral motor fibers (involuntary). Conserves energy and promotes "housekeeping" functions during rest. - **Somatic Nervous System:** Somatic motor fibers (voluntary). Conduct impulses from the CNS to skeletal muscles. ## Functional Areas of the Cerebral Hemispheres Many of the functional areas of the cerebral hemispheres have been identified, but there is much more than what is shown below. - **Primary motor area:** Impulses from this region of the brain travel to the skeletal muscles and allow us to consciously move our bodies. *Crossed pathways (left hemisphere interprets for right side of body, etc.)*. - **Broca's area:** Ability to speak; You know what you want to say, but can't vocalize the words. (one hemisphere only) - **Frontal association area:** Higher intellectual reasoning. Well developed only in primates, gives us a sense of our relationship to the rest of the world, enabling us to think about it and to plan and execute appropriate behaviors. - **Central Sulcus:** This divides the frontal and parietal lobes. - **Somatic sensory area:** Impulses traveling from the body's sensory receptors (non-special senses) are interpreted here (pain, cold, touch). *Crossed pathways (left hemisphere interprets for right side of body, etc.)*. - **Gustatory area:** Interprets taste - **Wernicke's area:** Ability to understand spoken and written language. - **Visual area:** Interprets what you see - **Auditory area:** Interprets what you hear - **Olfactory area:** Interprets what you smell (deep in the temporal lobe) ## Connecting the Hemispheres - **Corpus callosum:** A very large fiber tract (bundles of nerve fibers) that allows the hemispheres to communicate with each other. This is important because, as already noted, some of the functional areas are only in one hemisphere. ## Location and Substructures Associated with the Diencephalon, Brain Stem, and Cerebellum - **Diencephalon (Interbrain):** - **Thalamus:** Large left and right mass of nuclei that functions as relay center. - **Hypothalamus:** Mass under thalamus. - **Midbrain:** Most superior region of the brain stem. Contains fiber tracts and reflex centers involved in vision and hearing. - **Optic Chiasma:** Where optic nerve enters the brain from the eye. - **Pituitary gland:** Major endocrine gland; secretes: growth hormone, reproductive hormones, antidiuretic hormones, etc. - **Pons:** Middle of the brain stem, consists of mostly fiber tracts and involved in control of breathing. - **Medulla oblongata:** Most inferior part of brain stem. Controls heart rate, blood pressure, breathing, swallowing, and vomiting, among others. - **Spinal Cord:** Continuation of brain stem. Provides a two-way conduction pathway to and from the brain. - **Cerebellum:** Provides precise timing for skeletal muscle activity and controls our balance and equilibrium, giving us smooth coordination of movements. ## Functional Anatomy of the Central Nervous System: Brain and Spinal Cord - **Cerebral Hemispheres**: Paired, left and right, superior structures of the brain that most people think of when they think of the brain. - **Gray matter:** Outermost layer of the cerebral hemispheres, consists of the cell bodies of nerve cells. Referred to as the cerebral cortex. - **Gyri:** Elevated ridges of tissue - **Sulci:** Shallow grooves - **Fissures:** Deep grooves - **White matter:** Underlying area of the hemispheres, is composed of tracts (bundles of nerve fibers) carrying impulses to or from the cortex. It is white because the axons of the nerve fibers are surrounded by an insulating fatty substance, which gives the region a white appearance. ## Anatomy of the Spinal Cord - Approximately 17 inches long. - Functioning as a two-way conduction pathway to and from the brain. - A major reflex center that can integrate sensory signals and effect a motor response without communicating with the brain. ## Protection of the Central Nervous System Nervous tissue is very soft and delicate, and even the slightest pressure can injure the irreplaceable neurons. Remember that neurons are amitotic. Protection includes: bone, membranes, fluid, and barriers. - **Bone:** Skull and vertebral column. - **Membranes:** Meninges - **Dura Mater:** Outermost layer, thick and tough. - **Arachnoid Mater:** Middle layer - **Pia Mater:** Innermost layer, affixed to brain and spinal cord. - **Cushioning Fluid:** Cerebrospinal fluid (CSF) - **Cerebrospinal fluid:** Continually moving and returns to the blood at the same rate it is being produced. This keeps the pressure and volume constant. - **Exchange Barrier:** The brain is dependent on a constant internal environment and cannot be subject to even the slightest fluctuations that occur with changes in eating or exercising, etc. The neurons of the brain are kept separated from blood-borne substances by a so-called blood-brain barrier. ## Nervous Tissue: Supporting Cells and Neurons - **Neuroglia:** Supporting cells that support, insulate, and protect the delicate neurons. - **Astrocytes:** Form a living barrier between neurons and capillaries (function in blood-brain barrier). - **Microglial Cells:** Phagocytes that dispose of substances such as dead brain cells and bacteria. - **Ependymal Cells:** Line the cavities of the brain and help circulate CSF by beating their cilia. - **Oligodendrocytes:** Form fatty coverings called myelin sheaths around nerve fibers in the CNS. - **Schwann cells:** Form fatty myelin sheaths around nerve fibers in the PNS. Speeds up impulses. - **Satellite cells:** Small cushioning cells that protect the cell bodies of PNS neurons. - **Neurons:** Individual nerve cells, have a variety of structural differences, but all neurons have: - **Cell body:** Contains the nucleus and is the metabolic center of the cell. - **Processes:** Slender processes extending from the cell body, to receive and send impulses. - **Dendrites:** Receive and send impulses toward the cell body (Depending on the neuron type, there may be hundreds of dendrites on a single neuron). - **Axon:** Extends from the cell body and generates electrical impulses and conducts them away from the cell body (each neuron can only have one axon which may branch). All axons branch profusely at their ends forming hundreds-to-thousands of axon terminals. ## Neuron Physiology (An Electrochemical Event) - **Resting membrane:** Inactive and polarized, which means that there are more positive ions (Na+) outside the neuron than inside (K+). - **Stimulus:** Pressure, light, sound, chemicals) causes Na channels in the membrane to open making the inside of the neuron more positive at the site of stimulation. This is depolarization. - **Action potential:** If enough Na+ floods in reaching a threshold of -55mV inside the cell, then an action potential (nerve impulse) is generated. - **Propagation:** Action potential is propagated along the entire neuron in an all-or-none response. - **Repolarization:** K+ ions flood out of the cell repolarizing it. ATP is used to pump the Na and K+ ions back to their starting locations (a), so that another impulse may occur, if needed. - **Myelin Sheaths:** Conduct impulses much faster, because the impulses literally jump from node to node (nodes are small unmyelinated areas) along the length of the fiber. ## Physiology of Chemical Transmission of Neurotransmitters to Other Neurons, Muscles, or Glands - **Axon terminal:** Tiny vesicles containing chemicals called neurotransmitter, are stimulated to fuse with the plasma membrane. - **Synaptic cleft:** Area between the axon terminal and the dendrite of another neuron. - **Receptor ion channels:** Neurotransmitter chemical diffuses across the synaptic cleft and binds to receptor ion channels on the next neuron's membrane. The Na+ ions flood into this next neuron, causing action potential along this second neuron. - **Breakdown:** Neurotransmitters are quickly broken down and taken back up by the axon terminals. This causes the Nat ion channels to close (6) so that another stimulus may be received. ## Major Neurotransmitters in the Body - **Acetylcholine:** Used by the spinal cord neurons to control muscles and by many neurons in the brain to regulate memory. In most instances, acetylcholine is excitatory. - **Dopamine:** Produces feelings of pleasure when released by the brain reward system. Dopamine has multiple functions depending on where in the brain it acts. It is usually inhibitory. - **GABA (gamma-aminobutyric acid):** Major inhibitory neurotransmitter in the brain. - **Glutamate:** Most common excitatory neurotransmitter in the brain. - **Glycine:** Used mainly by neurons in the spinal cord. It probably always acts as an inhibitory neurotransmitter. - **Norepinephrine:** Acts as a neurotransmitter and a hormone. In the peripheral nervous system, it is part of the fight-or-flight response. In the brain, it acts as a neurotransmitter regulating normal brain processes. Norepinephrine is usually excitatory, but is inhibitory in a few brain areas. - **Serotonin:** Involved in many functions including mood, appetite, and sensory perception. In the spinal cord, serotonin is inhibitory in pain pathways. ## Neurological Disorders - **Alzheimer's Disease:** Progressive degenerative disease of the brain that ultimately results in dementia (mental deterioration and memory loss). - **Parkinson's Disease:** Usually striking people in their 50s and 60s, attacks particular dopamine-releasing neurons in the area of the brain called the substantia nigra. - **Huntington's disease:** Autosomal dominant genetic disease that strikes during middle age and leads to massive degeneration of the basal ganglia region of the brain and later the cerebral cortex of the brain. - **Concussions vs. Contusion:** - **Concussion:** More microscopic and more widespread over the brain. - **Contusion:** Macroscopic and localized, shows marked tissue destruction or bruising. - **Cerebrovascular accident (strokes):** 3rd leading cause of death in US. CVAs occur when blood circulation to the brain is blocked, maybe by a blood clot or a ruptured blood vessel, and vital brain tissue dies. - **Meningitis:** Inflammation of the meninges. Bacterial or viral meningitis can be a serious threat to the brain, because the inflammation may spread into the nervous tissue of the CNS. - **Multiple Sclerosis:** The myelin sheaths around the fibers are gradually destroyed, and converted to hardened sheaths called scleroses. As this happens, the current of the nerve impulse is short-circuited, and the affected person loses the ability to control his or her muscles and becomes increasingly disabled. MS is an autoimmune disease in which a protein complex of the myelin sheath is attacked.