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Physiological overview of nervous system
Central Nervous System: Controls most functions of the body and comprises the brain and spinal cord.
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Sensory part- Sensory Receptors
- Sensory experiences excite sensory receptors.
- So the visual receptors in the eyes, auditory receptors in ears, tactile receptors on in body surface, etc are some examples.
- There are 2 fates of this excitation-
- Cause immediate response from the brain, or
- Brain stores memories of the experiences for a certain period of time and this determines bodily reactions at some future excitation.
- There are multiple sensory areas in the spinal cord, medulla, pons, mesencephalon, cerebrum, thalamus, cerebral cortex.
Motor part- Effectors
- The most important role of nervous system is controlling various body actions and this is done via:
- contraction of skeletal muscles
- contraction of smooth muscle in the internal organs
- secretion of active chemical substances by glands
- Thus muscles and glands are called effectors because they are actual anatomical structures that perform functions.
Processing of information:
- CNS processes input information so that appropriate mental and motor response occurs.
- Henceforth human body is deliberately made to discard many sensory informations.
- For instance, a sense of clothing, etc. and only occasional objects draw attention in one’s field of vision.
- However, whenever the Central Nervous System detects important sensory information, it channels them into proper integrative and motor regions so that desired response is generated.
- This is the integrative function of the nervous system.
Role of synapses in processing information:
- The synapse is the junction point from one neuron to the next neuron.
- The neuron is the basic structural and functional unit of the nervous system.
- Synapses determine the directions in that the nerve signals will spread.
- Some synapses transmit signals with ease whereas others with difficulty.
- Also, facilitatory and inhibitory signals controls the synaptic transmission.
Storage of information: Memory
- Most storage occurs in the cerebral cortex, but even the basal regions of the brain and the spinal cord can store small amounts of information.
- The storage of function is also a function of synapses.
- Each time certain types of sensory signals pass through sequences of synapses, these synapses become more capable of transmitting the same type of signal the next time, a process called facilitation.
- After the sensory signals pass through the synapses a large number of times, the synapses become so facilitated that signals generated within the brain itself can also cause transmission of impulses through the same sequences of synapses, even when the sensory input is not excited.
- This gives the person a perspective of experiencing the original sensations, although the perceptions are only memories of the sensations.
- The precise mechanisms by which long-term facilitation of synapses occurs in the memory process are still uncertain.
- Once the nervous system stores the memories, they constitute part of the brain processing mechanism for future “thinking.”
- That is, the thinking processes of the brain compare new sensory experiences with existing memories; the memories then help to select the important new sensory information and to channel this into appropriate memory storage areas for future use or into motor areas to cause immediate bodily responses.
The opening of sodium channels allows large numbers of positive electrical charges to flow to the interior of the postsynaptic cell. This is by far the most common means of causing excitation.
Negatively charged chloride ions from outside of postsynaptic neurons diffuse rapidly and this increases the negativity inside the cell. This is inhibitory.
Synapses or synaptic cleft separates two consecutive neurons. So the neuron which passes the signal is a presynaptic neuron and the one which receives the signal is a postsynaptic neuron.
Chemical substances functioning as synaptic transmitters:
1st group:- small-molecule, rapidly acting transmitters
Acetylcholine, Norepinephrine, etc
2nd group:- large number of neuropeptides of a much larger molecular size that are usually much slower in action
Vasopressin, Insulin, etc
- Ganong’s review of medical physiology- 26th edition
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