Cellular Communication, Reflexes, and Sensory Receptors: A Comprehensive Overview
Cell-to-cell communication works through synapses. Two parts: the axon terminal of the presynaptic cell and the membrane of the postsynaptic cell. Postsynaptic cells may be neurons or non-neuronal cells. Presynaptic axons are next to either the dendrites or the cell body of the postsynaptic neuron.
Electrical synapses pass signals directly from the cytoplasm of one cell to another through gap junctions. They can flow in both directions. They occur mainly in the CNS. They are found in glial cells, cardiac and smooth muscles.
Chemical synapses are the majority of the nervous system. They use neurotransmitters to carry information. During a chemical synapse, an electrical signal of the presynaptic cell is converted into a chemical signal that crosses the synaptic cleft. This can take place in the nerve cell body or in the axon terminal. The axon terminal does not have the organelles needed for protein synthesis.
Excitation is a change in the properties of a cell membrane or a change in the cell metabolism caused by the extracellular environment.
VII. Relative Refractory Period is the interval immediately following the absolute refractory period during which initiation of a second action potential is inhibited but not impossible. It appears gradually in the repolarizing phase of an action potential and corresponds to the period when the potassium channels are open (several milliseconds). The return to the equilibrium resting potential marks the end of the relative refractory period. During this period, a second action potential can be evoked, but the minimal stimulus necessary for activation must be stronger or longer than normal.
The Golgi tendon organ excites inhibitory interneurons in the spinal cord. This reflex slows muscle contraction as the force of contraction increases to prevent excessive contraction that might injure the muscle.
Spinal Reflexes – Flexor Reflex The sensory neuron then synapses with interneurons that connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal; some motor neurons send inhibitory impulses to the extensors so flexion is not inhibited – this is referred to as reciprocal innervation. While all of this occurs, other interneurons relay the sensory information up to the brain so that the person becomes aware of the pain and what happened.
The patellar reflex, stretching of the tendon pulls on the muscle spindle, exciting the primary sensory afferents, which convey their information through group Ia axons. These axons make monosynaptic connections to the α motor neurons that innervate the quadriceps resulting in the contraction of this muscle. There is no interneuron in the pathway leading to contraction of the quadriceps muscle. Instead, the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord. However, there is an inhibitory interneuron used to relax the antagonistic hamstring muscle which makes it polysynaptic.
Adequate stimulus The minimum stimulus that is required to activate a receptor is known as the threshold.
Tonic receptors are slowly adapting receptors that fire rapidly when first activated, then slow and maintain their firing as long as the stimulus is present.
Phasic receptors are rapidly adapting receptors that fire when they first receive a stimulus but cease firing if the strength of the stimulus remains constant.
Physiologic nystagmus is a form of involuntary eye movement that is part of the vestibulo-ocular reflex (VOR). It is characterized by alternating smooth pursuit in one direction and saccadic movement in the other direction.
Caloric nystagmus is induced by irrigating the ears with warm or cold water or air. Cold or warm water or air is irrigated into the external auditory canal. The temperature difference between the body and the injected water creates a convective current in the endolymph of the nearby horizontal semicircular canal. Hot and cold water produce currents in opposite directions and therefore a horizontal nystagmus in opposite directions.
Postrotatory nystagmus If one spins in a chair continuously and stops suddenly, the fast phase of nystagmus is in the opposite direction of rotation, known as the”post-rotatory nystagmus” while the slow phase is in the direction of rotation.
Opticokinetic nystagmus is a nystagmus induced by looking at moving visual stimuli, such as moving horizontal or vertical lines, and/or stripes.
Pathologic nystagmus When nystagmus occurs without fulfilling its normal function, it is pathologic (deviating from the healthy or normal condition). Pathological nystagmus is the result of damage to one or more components of the vestibular system, including the semicircular canals, otolith organs, and the vestibulocerebellum.
Gate theory of pain
AB fibers carrying sensory information about mechanical stimuli help block pain transmission by enhancing the activity of inhibitory interneurons.
A. Acetylcholine
B. Norepinephrine
C. Cholinergic nicotinic receptors
D. Adrenergic receptors
E. Cholinergic muscarinic receptors
a. Parasympathetic tissue receptors – E
b. Target receptor for preganglionic neurons – C
c. Released by all autonomic preganglionic neurons – A
d. Primary sympathetic neurotransmitter – B
Sympathetic tissue receptor – D
Olympic When the tension produced by the flexing muscles (triceps) decreases to a point where they can no longer work against the resistance of the mass, the weight lifter will drop the weights involuntarily.
eights involuntarily.