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Principles of Physiology

Ashley Davidoff MD

Copyright 2010

Introduction

The recurring physiologic principle of receive process and export are applicable to the nervous system .

Receive (afferent – sensory)

A specialized sensory receptive neuron acts as the stimulus detector. Sensory neurons are highly specialized and capable of detecting only specific stimuli. They are so specialized in fact that they have lost the ability to divide through mitosis. Sensory neurons are located dorsally in the spinal cord. Each sensory neuron has a limited sensory field area over which its dendrites are capable of detecting stimuli. Activity level of the receptor cells depends on the intensity of the stimulus. Sub-threshold stimuli are too weak for sensory detection. There is a refractory period in which no stimulus will create a response and a relative refractory period in which only sufficiently large stimulus would create a response following the most recent response to an above threshold stimulus. Sensory neurons adapt and become less responsive to constant stimuli over time. All sensory stimuli carry information about time and location.

The stimulus is created into action potentials as signals for additional processing. An action potential is the disturbance of the electric field across the neuron membrane. The action potentials begin at the axon hillock and are propagated through the axon and the signal is transmitted via neurotransmitters across the synapse. The action potential propagation is driven by the Na+/K+ gradient. When the voltage across the membrane is disturbed, gated sodium channels open causing depolarization to occur. Potassium gated channels take longer to respond to the voltage change and cause repolarization following hyperpolarization. Axon myelination speeds action potential propagation. Axons are myelinated by oligodendrocytes and Schwann cells.

Sodium Potassium Pump Mechanism in the Transmission of a Nerve Impulse

Cell membranes in general and the neurons more specifically have resting membrane potentials of betweeen -70mV to -90mV The inside wall is relatively negative compared to the outside charge on the other side of the membrane which is relatively positive. This is due to the sodium and potassium pump. Sodium (orange) is the major player and under basal resting conditions it is forcefully and actively evicted from the cell by a pump (big orange arrow). To a lesser extent, potassium is pushed into the cell (big purple arrow). The chemical gradients that result cause passive but lesser movements of sodium into the cell (small orange arrow) and potassium out of the cell (small purple arrow).

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Ca2+ ion inflow releases stored neurotransmitters into synaptic cleft to bind postsynaptic receptors. Synaptic transmission is unidirectional. Neurotransmitters in the synaptic cleft diffuse via Brownian motion. Neurotransmitters bind only briefly before they are reabsorbed via active transport or degraded by enzymes in the synaptic cleft. Some of the most important neurotransmitters are: glutamate, aspartate, GABA, serotonin, acetylcholine, dopamine, and norepinephrine.

 Synaptic Cleft Requiring Chemical Transmitters eg Acetylcholine to Transmit Impulse Across Nerve Junctions
The synapse consists of a presynaptic ending, a synaptic cleft or space between the two connecting neurons, and a post synaptic ending. 

Within the presynaptic ending there are vesicles that contain the neurotransmitters and a voltage sensitive gate that releases calcium ions when stimulated called the voltage gated calcium channel. It also contains mitochondria and other organelles that are responsible for the production and packaging of the neurotransmitters. Within the post synaptic ending, there are gated ion channels with chemical receptors that enable the generation of electrical signals.

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Process

There are receiving centers where neurons receive information from sensory neurons and intermediate receiving centers. Interneurons transfer signals from neuron to neuron and compose 90% of the neurons in the body. Once the nature and importance of the stimulus has been determined by the receiving centers, an appropriate response is initiated. Ninety-nine percent of sensory input is discarded by the brain. Perception of the meaning and significance of the stimulus is determined by evaluative comparison to previous experiences of similar or the same stimulus. Processing of feedback from internally regulated systems functions identically to those we have conscious control over in that a signal is transmitted via neurons, axons, and neurotransmitters, the signal is processed, and an appropriate response to the communication is elicited.  The processing aspect which is the major function of the brain is complex and often beyond our understanding.  How does one explain for example explain emotion, or intellectual function on a mechanical or physical basis?  At this stage of our learning it is sufficient to know the variety of higher executive functions performed by the brain without delving to deep into their physiological basis.

Export (efferent – motor)

Motor neurons carry stimulus from the brain to the muscles to elicit the conscious or unconscious response to sensory stimuli. Motor signals are sent to the motor cortex, through the thalamus and travel down to the spinal cord. Motor response requires continuous sensory feedback for continued successful execution. In addition to sensory feedback, proprioception is also involved in guiding motor response. Motor neurons are located ventrally in the spinal cord. Motor neurons primarily rely on the hormone acetylcholine which signals the muscle to contract. The postsynaptic acetylcholine receptor is an ionophore receptor, which when activated is permeable to calcium and sodium ions. Motor neurons and the muscle fibers they innervate are referred to as motor units.

Voluntary motor response performed by striated muscle. Red striated muscle has a large blood supply, many mitochondria, and high concentration of  myoglobin. White muscle has less blood supply, mitochondria, and myoglobin but move more quickly than red muscle. Both types of striated muscle are innervated by motor neurons whose axons form motor nerves in the peripheral nervous system. Involuntary movements are performed by smooth muscle. An exception to this is cardiac muscle, which is characterized as unique striated muscle but still operates involuntarily.

Extenders and flexors are muscles that facilitate movement by opposing each others effect on the bone or organ. Reflex movements are wholly unconscious that occur before there is conscious awareness of their occurrence. Central program generators (CPG) regulate reflex response and consist of sensory and motor neurons that automatically produce a specific response and are self monitoring. CPGs are directed by the mesencephalic locomotor region in the brain stem.

The motor cortex is located on the cerebral hemisphere near the central sulcus. Neurons in the motor cortex exhibit vertical columnar organization and columns of neurons near each other control related muscle groups. The prefrontal and sensory cortices guide the motor cortex in urgent situations.

The basal ganglia are composed of the striatum, globus pallidus, subthalamic nucleus, and substantia nigra. The striatum receives sensory and motor information from the cerebral cortex. The striatum is also a pre-processing receptor prior to signals being processed by the sensory cortex. The substantia nigra is responsible for dopamine transmission which is the primary area effected in Parkinson?s disease. Dopamine is required for effective control of motor activity. The lack of dopamine affects the functioning of the globus pallidus, as well as other structures. The subthalamic nucleus regulates movement preventing its occurrence in excess without conscious control.

The cerebellum receives information from the cerebral cortex, brain stem, and spinal cord. Purkinje cells in the cerebellum are involved in proprioception and signals from these cells are received at the deep cerebellar nuclei which subsequently modifies the motor cortex. The cerebellum directs fine and complex motor tasks. It is also involved in cognitive functions unrelated to motor behavior.

Two parallel motor systems.

References

Web

Dowling How the Brain Works