Structure, Function and Homeostasis of the Nervous System Presentation By: Leigh Stonerook

Structure and Function of a Neuron

The Parts to the Neuron… Dendrites are extensions at the beginning of a neuron that help increase the surface area of the cell body and are covered with synapses. They receive information from other neurons and transmit electrical stimulation to the soma. The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. The myelinated axons transmit information much faster than other neurons. The cell body or soma is the factory of the neuron. It creates all the proteins for the dendrites, axons and synaptic terminals and contains specialized organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes and polysomes to provide energy and make the parts, as well as a production line to assemble the parts into completed products. The soma is where the signals from the dendrites are joined and passed on. The soma and the nucleus do not play an active role in the transmission of the neural signal. Instead, these two structures serve to maintain the cell and keep the neuron functional. The Myelin Sheath of a neuron consists of fat-containing cells that insulate the axon from electrical activity. This insulation acts to increase the rate of transmission of signals. A gap exists between each myelin sheath cell along the axon. Since fat inhibits the “propagation” of electricity, the signals jump from one gap to the next. Multiple sclerosis is a condition characterized by patches of demyelination (destruction or loss of the myelin sheath) in the central nervous system. The symptoms that result from this demyelination are determined by the functions normally contributed by the affected neurons. Disruption of muscle control, speech and visual disturbances are most common. Many axons are covered in a shiny fatty sheath called the myelin sheath. It is the greatly-expanded plasma membrane of an accessory cell, the Schwann cell. Schwann cells are spaced at regular intervals along the axon. Their plasma membrane is wrapped around and around the axon forming the myelin sheath. Schwann cells act as support cells in peripheral nerves. They form the myelin around myelinated peripheral axons.  Schwann cells also envelop unmyelinated axons, but without the dense membrane wrapping which characterizes myelin.  

Neuron Transmitting Electrical Current A nerve impulse is an electrical current that travels along dendrites or axons due to ions moving through voltage-gated channels in the neuron's plasma membrane. Voltage gated channels open and close in response to an electrical voltage to they are affected by changes in electrical charge around them. When a neuron is at rest a charge difference is maintained between the inside and outside of the cell. This charge difference is produced and maintained largely by active transport using sodium-potassium pumps. These pumps send sodium ions out of the cell and bring potassium ions in! While some potassium ions flow back out of the cell, sodium ions can't get back in to replace the lost positive charges. This results in the outside of the cell having a net positive charge and the inside of the cell having a net negative charge. This difference in charge is called the resting membrane potential. Now a nerve impulse begins when a stimulus disturbs the plasma membrane on a dendrite causing sodium gates to open. Sodium ions come into the cell lessening the charge difference at that location. if the change is great enough, this will cause voltage-gated sodium channels to open allowing the incoming sodium ions to depolarize the membrane! Neighboring voltage-gated sodium gates open and the depolarization travels down the membrane. this movement is called an action potential. changes occur behind the action potential to restore the resting membrane potential. the voltage-gated sodium channels close and the voltage-gated potassium channels open. Potassium ions flood out of the cell, "depolarizing" the membrane so that the inside is again negative and the outside is again positive. Finally, sodium potassium pumps fully restore the resting membrane potential and reestablish proper concentrations of sodium and potassium inside and outside the cell!

Running with Parkinson’s Disease Parkinson's disease (PD) belongs to a group of conditions called motor system disorders, which are the result of the loss of dopamine-producing brain cells. The four primary symptoms of PD are trembling in hands, arms, legs, jaw, and face; rigidity, or stiffness of the limbs and trunk; slowness of movement; and postural instability, or impaired balance and coordination. All of these would make any running exercise very challenging. As these symptoms become more pronounced, patients may have difficulty walking, talking, or completing other simple tasks. PD usually affects people over the age of 50.  Early symptoms of PD are subtle and occur gradually. In some people the disease progresses more quickly than in others.  As PD worsens, the shaking, or tremor, which affects the majority of PD patients may begin to interfere with daily activities.  Other symptoms may include depression and other emotional changes; difficulty in swallowing, chewing, and speaking; urinary problems or constipation; skin problems; and sleep disruptions.  Currently no blood or laboratory tests that have been proven to help in diagnosing sporadic PD.  Therefore the diagnosis is based on medical history and a neurological examination.  Parkinson's disease can be difficult to diagnose accurately. Doctors may request brain scans or lab tests in order to rule out other diseases. PD is chronic (it persists over a long period of time) and progressive (its symptoms grow worse over time).  Although some people become severely disabled, others experience only minor motor disruptions. Tremor is the major symptom for some patients, while for others tremor is only a minor complaint and other symptoms are more troublesome.  No one can predict which symptoms will affect an individual patient, and the intensity of the symptoms also varies from person to person. Because running is such an involved activity, patients with Parkinson's disease would find this task to be nearly impossible. The loss of muscle control and balance/ coordination are two major factors that would hinder any runner that is affected by this condition.

Running and Serotonin, Dopamine and Acetylcholine… Serotonin is a neurotransmitter in the brain that has a great influence over many brain functions. It is synthesized from amino acids in brain neurons and stored in vesicles. Serotonin is found in three main areas of the body: the intestinal wall; large constricted blood vessels; and the central nervous system. The most widely studied effects have been those on the central nervous system. There are numerous functions of serotonin including control of appetite, sleep, memory and learning, temperature regulation, mood/ behavior cardiovascular function, muscle contraction, endocrine regulation, and depression. Such activities like running and many others forms of exercise would be greatly impacted by the lack of serotonin. If a person's appetite, sleep abilities, and muscle contraction abilities were disturbed, running would not be possible for that person being that all three of these things are vital in being able to run and exercise in general. Dopamine is another neurotransmitter in the brain that heavily impacts one’s day to day life. Dopamine has many functions in the brain, including important roles in behavior and cognition, voluntary movement, motivation and reward. Dopamine is also responsible for inhibition of prolactin production (involved in lactation), sleep, mood, attention, and learning. Dopaminergic neurons (i.e., neurons whose primary neurotransmitter is dopamine) are present in the ventral tegmental area of the midbrain, the Substantia Nigra pars Compacta, and the arcuate nucleus of the hypothalamus. The physical parts of running would most heavily be impacted by the affects that Dopamine has on voluntary movement. Lack of motivation and reward could also influence a person’s ability to run simply because there is no drive to exercise. Acetylcholine in the Peripheral Nervous System acts to stimulate muscle movement. Acetylcholine receptors on the muscles accept acetylcholine and cause skeletal muscles to contract. Interestingly, they cause heart muscles to relax. In the Central Nervous System it has a range of effects including arousal and reward, along with learning and short-term memory. If this neurotransmitter was at all affected in the brain, muscle contraction and therefore all movement would be impossible to perform. Running, being an activity that requires complete body movement would become very challenging.

The Brian…A deeper look

The Brain Stem The Brainstem functions as our own private "auto-pilot“ device. The brain stem is located at lower part of the brain, directly connected to the spinal cord. This structure is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. The majority of cranial nerves originate in the brainstem. The brainstem is the pathway for all fiber tracts passing up and down from peripheral nerves and spinal cord to the upper areas of the brain. The Medulla Oblongata, Midbrain and Pons are all part of the brain stem that help this structure carry out its functions! The Medulla Oblongata is a relay station for the crossing of motor tracts between the spinal cord and the brain. It also contains the respiratory, motor and cardiac functions, as well as several mechanisms of reflex activities such as coughing, swallowing and vomiting. The midbrain serves as the nerve pathway of the cerebral hemispheres and contains auditory and visual reflex centers. The Pons main purpose is to act as a relay station from the medulla to the higher cortical structures of the brain. The Pons also is unique in that it contains the respiratory center making breathing possible for humans. If the brainstem were to get damaged, a person would not be able to have regulated breathing, heartbeat, and blood pressure. If any of these things are not at a stable state, running would only aggravate the body system as a whole and the body would not be able to perform this task.

The Cerebellum The cerebellum, sometimes referred to as the "little brain," lies on top of the Pons, behind the Brain Stem. The cerebellum is comprised of small lobes and receives information from the balance system of the inner ear, sensory nerves, and the auditory and visual systems. This structure is associated with regulation and coordination of movement, posture, and balance. It is similar to the cerebrum in that it has two hemispheres and has a highly folded surface or cortex. If this structure were to be damaged, the patient’s sensory system would be greatly impaired and movements involving coordination would become very challenging. Running would be difficult because the patient’s sense of balance and coordination of movement would have been dramatically affected!

Thalamus The thalamus is a large, dual lobed mass of grey matter buried under the cerebral cortex. It is primarily involved in sensory perception and regulation of motor functions. Being a limbic system structure, it connects areas of the cerebral cortex that are involved in sensory perception and movement with other parts of the brain and spinal cord that also have a role in sensation and movement. As a regulator of sensory information and processing, the thalamus also controls sleep and the many states of consciousness. The thalamus is involved in several functions including Motor Control, Receiving Auditory signals, Visual Sensory Signals, Relaying Sensory Signals to the Cerebral Cortex, Controlling Sleep/Awake States. The thalamus is located at the top of the brainstem, between the cerebral cortex and midbrain and is superior to the hypothalamus! If the thalamus were to be damaged, the patient could suffer irregular sleep patterns and would have a difficult time processing the world around them due to the fact that the receiving of the auditory and visual sensory signals had been damaged. Simple tasks such as walking and movements involving coordination (such as running) would become a challenge as the body and the sensory receptors (or operating system of body) are not working together.