A person wearing a cast on an arm or leg experiences extra fatigue. Explain this on the basis of.....?
The cast is made of plaster, which is typically quite thick and heavy. The muscles on the arm or leg is trained through day to day life to move a certain amount of weight (the weight of the limb), but once you put a cast on suddenly it has to move a much greater amount of weight. As the muscles need time to adapt and train for the additional amounts of limb weight, initially there will be a period where the limb will fatigue or tire easily. Think of it as jogging in runners as opposed to heavy iron clad boots. The extra weight makes a difference when you move around a lot. In terms of newton's first law, the increased mass means that a greater unbalanced force (suppled by the arm/leg muscles) needs to act on the limb to get it to move, or to stop it to moving. In either case the muscle has to generate more force by invoking more sacromeres/ motor units, which means it'll tire more easily. Newton's second law states that Force = mass* acceleration. Rearrange to get acceleration = force/mass. This means the lighter the object, the less force required to accelerate it to the same degree as a heavier object. As the limb is now heavier, to move it normally greater forces need to be used, which again, is drawn from the muscles. After a certain period (6-8 weeks, maybe less) the muscles to adjust to the extra weight and gain strength. If at this point the cast is removed then the person will have to readjust again to a not so heavy limb. This process part of motor learning and involves the cerebellum and how its neurons (Purkinje Cells) relate information to one another.
What is the structure of a nerve cell?
That depends. First of all, I will assume that by ‘nerve cell’ you are referring to ‘neurons’. But remember that the Nervous System has also another important cell: Glial cells.So, about neurons structure: Have you ever heard someone says that a snowflake has a unique shape? ‘There is no equal snowflake’. Well, the same is true for neurons. Although there is a general structure, there is no equal neuron. The common structures of neurons are:Dendrites: Is a structure responsable for receiving signals from other neurons.Soma: It’s the cell body, where occurs most of metabolic pathways that keep this cell alive and is where the synthesis of neurotransmitters takes place.Axons: Long extensions of the neurons, it is responsible for the emission of signals to be received by other neurons.Although this general structure, neurons have a different shape depending on the area they are located. Neurons from the cerebellum are very different from the ‘classical’ neuron.Figure: Purkinje cells found in the cerebellum.
What is the anatomy and function of the different types of neurons?
The anatomy of neurons and their functions are correlated.First we will take the example of sensory neurons where this correlation is very clear. A typical sensory neuron has branched dendrites which act as receptors for stimuli. Pain fibres have simple sensory endings, called free nerve endings which detect noxious stimuli. Pacinian corpusles have layered bulbous endings and detect pressure. Hair cells, which detect auditory or vestibular stimuli are not neurons.Muscle afferents detect length (spindles), velocity (Spindles) and force (GTOs) (figures for the spindle and GTOs are modified from Kandel’s book).Axon size: Many neurons which bring messages from the cortex to the spinal cord have long axons, so do the neurons which carry messages from the spinal cord to the brain (brain stem, cerebellum, thalamus). Most of the sensory neurons also have long axons, mostly in the periphery. Local interneurons on the other hand are small with very short axons. The projection interneurons can relay messages over long distances and hence have long axons.Shapes of dendritic tree: For central neurons shapes of dendrites can be very different and complex (D part of the figure above). Motoneurons in the spinal cord have elaborate structure compared to dendrites of sensory neurons. However, they are less elaborate compared to pyramidal cells in the cortex and Purkinje cells in the cerebellum. There are no spines on the dendrites of motoneurons while pyramidal cells and Purkinje cells do have spines. Spines are very plastic, and they contribute to plasticity of the nervous system. The dendrites of Purkinje cells (P-cell) in the cerebellum are very beautifully organized. The dendrites of each P-cell lie in one plane, and dendrites of consecutive P-cells lie in parallel planes. This facilitates systematic orthogonal inputs from other types of cells (figure from Kandel below).
Is the human brain analog or digital?
The brain is neither analog nor digital, but works using a signal processing paradigm that has some properties in common with both. Unlike a digital computer, the brain does not use binary logic or binary addressable memory, and it does not perform binary arithmetic. Information in the brain is represented in terms of statistical approximations and estimations rather than exact values. The brain is also non-deterministic and cannot replay instruction sequences with error-free precision. So in all these ways, the brain is definitely not "digital". At the same time, the signals sent around the brain are "either-or" states that are similar to binary. A neuron fires or it does not. These all-or-nothing pulses are the basic language of the brain. So in this sense, the brain is computing using something like binary signals. Instead of 1s and 0s, or "on" and "off", the brain uses "spike" or "no spike" (referring to the firing of a neuron).Internal to the neuron, everything works via biochemical pathways, which are somewhat similar to analog. Neurons also perform internal electrical signal integration in an analog fashion. Analogously, the digital logic gates used by computers are implemented internally using transistors and resistors, which are also analog.This recording of neural spikes over time shows that the spatiotemporal pulses of the neural code looks a lot like digital signaling:The bottom line is that the brain processes information using a representation strategy that is neither analog nor digital. It is a different type of computation, involving circuits and networks composed of spiking neurons. One of the central tasks of neuroscience is to figure out how this information processing paradigm works.Related:Do human brains store information in binary?Is the human brain just another information processing device?Is it possible the brain is completely deterministic?Is there anything required to define a mind that cannot be modeled as a computation?What is the clock speed equivalent of the human brain?How does an individual neuron learn?How is the human brain so energy-efficient?What are some good introductory materials on computational neuroscience?Why have neurons evolved to send all-or-nothing action potentials over communicating with graded responses?
Sequence of cardiac impulses?
ormally, the only pathway available for action potentials to enter the ventricles is through a specialized region of cells (atrioventricular node, or AV node) located in the inferior-posterior region of the interatrial septum. The AV node is a highly specialized conducting tissue (cardiac, not neural in origin) that slows the impulse conduction considerably (to about 0.05 m/sec) thereby allowing sufficient time for complete atrial depolarization and contraction (systole) prior to ventricular depolarization and contraction. The impulses then enter the base of the ventricle at the Bundle of His and then follow the left and right bundle branches along the interventricular septum. These specialized fibers conduct the impulses at a very rapid velocity (about 2 m/sec). The bundle branches then divide into an extensive system of Purkinje fibers that conduct the impulses at high velocity (about 4 m/sec) throughout the ventricles. This results in rapid depolarization of ventricular myocytes throughout both ventricles. The conduction system within the heart is very important because it permits a rapid and organized depolarization of ventricular myocytes that is necessary for the efficient generation of pressure during systole. The time (in seconds) to activate the different regions of the heart are shown in the figure to the right. Atrial activation is complete within about 0.09 sec (90 msec) following SA nodal firing. After a delay at the AV node, the septum becomes activated (0.16 sec). All the ventricular mass is activated by about 0.23 sec.
Does the cardiac conduction system work more like nerves or more like wires?
Heart has pacemaker cells [Sino-Atrial (SA) node] which generate action potentials spontaneously; this sets the rhythmic heart rate. The signal for the cardiac muscle fibres (cardiac myocytes) to contract comes not directly from the nervous system , but from pacemaker cells of SA node. Cells of the SA node depolarize the atrial myocytes. Cardiac myocytes which generate force are connected to each other by gap junctions, so depolarization of one cell quickly spreads to the neighbouring cells, action potentials are generated by membranes of atrial myocytes.The green fibres shown around the atria in the figure below, conduct action potentials to the atrio-ventricular node (AV node). In ventricals, myocytes are also depolarized by the conducting fibres. Purkinje fibres are like heart cells but with very little contractile material. They are larger and conduct action potentials faster, more like nerves. Purkinje fibers act as axons designed to propagate an AP rapidly without depolarizing surrounding ventricular septal tissue significantly. There are specific areas where the conducting system interacts with contractile muscle fibres to depolarize contractile cells.All myocytes generate action potentials. Essentially, all cells in the heart generate action potentials, wires do not generate action potentials. However, when current flows from one cell to the others via gap junctions, that flow is more like one along a wire.In conclusion: the pacemaker cells, atrial myocytes, ventricular myocytes, AV bundle branches and Purkinje fibres— all conduct action potentials and do not act as wires. But the current flow through gap junctions is more like flow along a wire.