3.3—structure and organization of the nervous system Flashcards
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3.3 Learning Objectives
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3.3 Focus Questions
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- how do the different divisions of the nervous system work together when you’re startled?
- how does the brain control movement?
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Central Nervous System (CNS)
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- central nervous system (CNS): consists of the brain and the spinal cord.
- the spinal cord receives information from the brain and stimulates nerves that extend out into the body, producing movements, and from sensory nerves in the body and transmits it back to the brain.
- in the case of reflexes, your spinal cord organizes rapid movements without the help of the brain.
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Peripheral Nervous System (PNS)
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a division of the nervous system that transmits signals between the brain and rest of the body and is divided into two subcomponents, the somatic system and autonomic system. (figure 3.20)
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Somatic Nervous System | PNS
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consists of nerves that control skeletal muscles, which are responsible for voluntary and reflexive movement; also consist of nerves that receive sensory input from the body.
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Autonomic Nervous System | PNS
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- autonomic nervous system: the portion of the peripheral nervous system responsible for regulating the activity of organs and glands. (figure 3.21)
- contains the sympathetic and parasympathetic nervous systems.
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Sympathetic Nervous System | PNS
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responsible for the fight-or-flight response of an increased heart rate, dilated pupils, and decreased salivary flow—responses that prepare the body for action.
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Parasympathetic Nervous System | PNS
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maintains homeostatic balance in the presence of change; following sympathetic arousal, it works to return the body to a baseline, nonemergency state.
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The Brain and Its Structures
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- the brain appears to be divided into two symmetrical halves known as cerebral hemispheres.
- the human brain, as well as that of other animals, can be subdivided into three main regions: the hindbrain, midbrain, and forebrain. (table 3.2)
- entire brain is composed of highly integrated circuitry and feedback loops.
- although the forebrain may perform complex thinking processes like decision making, its activity is influenced by (and influences) structures in the midbrain and hindbrain.
- research has shown that exercise improves functioning of the prefrontal cortex, and can increase the number of cells in the hippocampus.
- many studies have also shown a “significant positive relationship” between physical activity and academic performance; perhaps due to reduction in stress levels, a positive association with play, or a combination of several factors.
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Brain Stem | Hindbrain
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- brain stem: the “stem” or bottom of the brain and consist of two structures: the medulla and the pons. (figure 3.22)
- nerve cells in the medulla connect with the body to perform basic functions such as regulating breathing, heart rate, sneezing, salivating, and even vomiting—things your body does with little conscious control.
- the pons is connected to structures in the brain and spinal cord, helping to control things like balance, eye movements, and swallowing.
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reticular formation: a hindbrain structure that extends from the medulla upwards to the midbrain.
- influences attention and alertness.
- communicates with cells in the spinal cord involved with movements related to walking and posture.
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Cerebellum | Hindbrain
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- cerebellum: the lob-like structure at the base of the brain that is involved in the monitoring of movement, maintaining balance, attention, and emotional responses.
- involved with coordinating and timing ongoing movements, rather than generating responses on its own.
- patients with damage to the cerebellum have difficulty controlling their attention and emotions, including personality changes and impulsivity (cognitive affective behavioural syndrome).
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Midbrain
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- midbrain: resides just above the hindbrain, primarily functions as a relay station between sensory and motor areas. (figure 3.22)
- superior colliculus: (plural colliculi) influences the ability of your brain to capture your attention (e.g. when you detect a sudden movement out of the corner of your eye).
- inferior colliculus: influences the ability to move your auditory attention (e.g. when someone’s phone rings in class and your head turns in that direction).
- substantia nigra: a network of dopamine-releasing cells involved with the control of movements.
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Forebrain
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- forebrain: the most visibly obvious region of the brain, consist of all the neural structures that are located above the midbrain, including all of the folds and grooves on the outer surface of the brain; the multiple interconnected structures in the forebrain are critical to such complex processes as emotion, memory, thinking, and reasoning.
- has ventricles containing cerebrospinal fluid that eliminates waste and provides nutrition and cushioning for many parts of the brain. (figure 3.23)
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Basal Ganglia | Forebrain
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- basal ganglia: a group of three structures that are involved in facilitating planned movements, skill learning, and integrating sensory and movement information with the brain’s reward system. (figure 3.24)
- people who are very practiced at a specific motor skill have actually modified their basal ganglia through practice to better coordinate engaging in the activity.
- damage to the basal gangling can lead to movement disorders like Parkinson’s disease (resting tremors and unable to coordinate movements) and Huntington’s disease (uncontrollable movements of the body, head, and face).
- excess dopamine in the basal ganglia is related to Tourette’s syndrome—marked by erratic and repetitive facial and muscle movements (tics), heavy eye blinking, and frequent noise making.
- forms a network with a nearby structure, the nucleus accumbens: activity accompanies many kinds of pleasurable experiences, including sexual excitement and satisfying a food craving.
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Limbic System | Forebrain
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an integrated network involved in emotion and memory. (figure 3.25)
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Cerebral Cortex | Forebrain
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- cerebral cortex: the convoluted, wrinkled outer layer of the brain that is involved in multiple higher functions, such as thought, language, and personality.
- the wrinkled surface increases the surface area of the cortex while fitting into our skull.
- consists primarily of the cell bodies and dendrites of neurons.
- the axons of these neurons extend throughout the brain and allow communication between different neural regions to occur.
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white matter and grey matter (figure 3.26)
- the grey matter of the brain consists of approximately 100 billion neurons.
- the white matter of 20 year-olds could go on for over 100,000km.
- healthy adults have 100-500 trillion synapses, each able to fire several times a second.
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The Four Lobes
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- the four lobes are: occipital, parietal, temporal, and frontal. (figure 3.27)
- nerve cells are interconnected and networked with regions of the midbrain and hindbrain.
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Parietal Lobes | Four Lobes
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- parietal lobes: involved in our experiences of touch as well as our bodily awareness.
- somatosensory cortex: at the anterior (front) edge; a band of densely packed nerve cells that register touch sensations.
- the amount of neural tissue is condensed in regions that we use to gain information from touch (e.g. face and hands, since people don’t go around touching things with their stomach or thigh). (figure 3.28)
- right parietal lobe damage leads to neglect, meaning a person can’t see anything in the left half of their visual field, sometimes even in their imagined images.
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Frontal Lobes | Four Lobes
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- frontal lobes: important in numerous higher cognitive functions, such as planning, regulating impulses and emotion, language production, and voluntary movement.
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primary motor cortex: a thick band of neurons rear of the frontal lobes; involved in the control of voluntary movement.
- similar to the somatosensory cortex, where body parts that perform fine-motor control (e.g. fingers) require more space in the motor cortex. (figure 3.28)
- frontal lobes are active in the planning of movements, which is useful to deal with threats.
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prefrontal cortex: the front two-thirds of the frontal lobes; performs many higher-order cognitive functions such as decision making and controlling our attention.
- executive functions: regulating the function of other areas of the brain.
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Corpus Callosum | Four Lobes
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- corpus callosum: a collection of neural fibres connecting the two hemispheres. (figure 3.29)
- allows the two hemispheres to work together, and opens the possibility for each hemisphere to become specialized.
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hemispheric specialization: the two sides of the cortex perform different functions; right side for cognitive tasks (involving visual and spatial skills, recognition of visual stimuli, and musical processing), left side for language and math.
- however, most cognitive functions are spread throughout multiple regions, with one hemisphere being sometimes dominant to the other. (table 3.3)
- the degree to which personalities are linked to different hemispheres is very limited (referring to being “left-brained” or “right-brained” as perpetuated by pop culture).
- split-brain patients: patients who had their corpus callosum split to cure severe epileptic seizures; these patients behave normally, but respond differently to visual input. (figure 3.30)
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Neuroplasticity and Recovery from Brain Activity
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- some animals with relatively simple brains and spinal cords can regenerate damaged areas of their central nervous system; humans can also do this to an extent.
- trophic factors: (chemicals, also known as growth factors) can stimulate the growth of new dendrites and axons.
- our brain contains chemicals that inhibit the growth of new axons around an injured area to prevent forming incorrect connections between brain areas.
- studies have found that patients with damage to Broca’s area (left frontal lobe; involved with production of speech) can’t speak, but can sing the same words. (figure 3.31)
- patients who go under Melodic Intonation Therapy (MIT) regain significant language function; it doesn’t “heal” damaged nerve cells in Broca’s area (left hemisphere) but transfers function to the right area.
- when nerve cells are damaged, associated cells in the neighbouring area can take over and attempt to form new connections.
- limits to neuroplasticity; it works better in younger people, and could be contributed to a number of other factors like hormone levels or the brain’s metabolism.
