Neural Physiology - AP Biology

Monosynaptic reflexes do not require a neuron between the pre-synaptic and post-synaptic neuron, and do not require input from the brain. These reflexes can be triggered even in brain-dead individuals. The knee-jerk reflex is an example of a monosynaptic reflex.

The accommodation reflex is used to adjust the focus of the eye. The acoustic reflex reduces sound intensity by adjusting the bones of the middle ear. The suckling reflex is the complicated reflex of an infant mammal being able to breast feed. Somatic reflexes are a broad category simply involving muscle reflexes. Some of these reflexes involve input from the brain, while others (like the knee-jerk reflex) do not.

The sympathetic nervous system is activated during times of stress, and is responsible for initiating the "fight or flight" response. Part of this response in an increase in heart rate, allowing better conduction of blood and delivery of oxygen throughout the body.

The other answer options are effects of parasympathetic stimulation, which allows for rest. During this time, the body stores energy from glucose into glycogen, and allows for digestion and reproduction.

The brain is often divided into four lobes based on anatomy and physiology: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. Each lobe controls various aspects of cognition and motor skills. The frontal lobe is associated with reasoning, speech, movement, and emotions. The parietal lobe is associated with orientation and recognition. The occipital lobe is associated with visual processing. The temporal lobe is associated with auditory processing and memory.

Broca's area is a small region of the frontal lobe located in the left hemisphere. This region of the brain is responsible for generating speech and articulation. Damage to this region of the frontal lobe could cause speech impairment. In contrast, Wernicke's area is located in the temporal lobe and is associated with comprehension of speech.

Spastic muscle activity is not related to the brain, but results from injury to motor neurons spanning from the spinal cord to the limbs.

Synapses are special regions where a neuron releases a signal to its target cell. Most commonly this signal is chemical (neurotransmitters), but it can also be electrical. The synapse is a gap between the neurons, and does not allow for direct contact. Signals are released from the axon of one neuron and must traverse the synaptic cleft before interfacing with receptors on the target cell.

Neurons do not directly release hormones into the blood stream and synapses do not offer protection to neurons.

Phototaxis is movement (taxis) in response to light (photo). Movement towards a source is positive; movement away from a source is negative. "Positive phototaxis" would be used to describe movement toward a light source.

Glial cells are non-neuronal cells that support neuron activity. Their functions include physical support of neurons, supply oxygen and nutrients, and take up excess neurotransmitters.

Neurotransmitters and hormones are both chemical signals, but hormones are used in the endocrine system, released from glands into the blood, while neurotransmitters are released from the axon terminals of neurons to signal other neurons. Signals travel across neurons as electrical signals caused by the movement of large numbers of atomic ions across the membrane via protein channels. Charged proteins would be too large to quickly move through the channels in such large numbers.

Neurotransmitters in the human body are under tight control. Many drugs, such as anti-depressants or drugs for ADHD, limit neurotransmitter responses. Antagonistic molecules will inhibit neurotransmitters and are used in many drugs. These molecules structurally interact with receptor proteins, either blocking the active site or binding allosterically to alter the binding site shape. Antagonists can be competitive or uncompetitive.

In contrast, agonists are molecules that structurally resemble the ligand for a certain receptor and can bind to the active site to trigger a response. Nicotine, for example, is an agonist to certain acetylcholine receptors and can trigger these receptors.

The SNARE proteins are responsible for allowing vesicles filled with neurotransmitter to fuse with the cell membrane at the synaptic cleft, resulting in exocytosis. Without these proteins, the neurotransmitter cannot propagate the signal to any other cells.

Neurotransmitter synthesis occurs via translation or synthesis in the smooth endoplasmic reticulum, depending on the identity of teh molecule. Resting potential is determined by the sodium-potassium pump, and action potential propagation relies heavily on voltage-gated sodium channels and myelin.

Neurotransmitters, such as acetylcholine, are chemical signals that transmit action potentials from one neuron to another. This process occurs at the synapse, where a neurotransmitter is released from the presynaptic neuron. This neurotransmitter travels across the synaptic cleft and binds to a receptor on the postsynaptic neuron. Statement I is thus false.

The rate of propagation of a nerve signal is limited by the synapse because neurotransmitters must diffuse across the gap; statement II is true.

Calcium ions are very important in the release of neurotransmitters. Voltage-gated calcium channels are located on the axon of the presynaptic neuron. When an action potential reaches the synapse, calcium ions are allowed to enter into the presynaptic neuron. This influx of calcium ions interacts with vesicles containing neurotransmitters and causes them to release their contents into the synaptic cleft. Statement III is false because calcium ion channels are located on the membrane of presynaptic neuron, not postsynaptic neuron.

All of the choices could potentially be neurotransmitters.

Peptide neurotransmitters are proteins. An example of a peptide neurotransmitter is somatostatin. Nitric oxide is the most well-known gaseous neurotransmitter. Monoamines are molecules that contain an amine group connected to an aromatic ring. These molecules are derived from aromatic amino acids. Dopamine, norepinephrine, and epinephrine are very well-known monoamine neurotransmitters.

Receptors on postsynaptic neurons are connected to ion channels. When the neurotransmitter binds to the receptor, the channel opens, making that neuron more or less likely to have an action potential depending upon which type of ion the channel allows to enter or exit the neuron. The result is either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).

The parasympathetic division of the autonomic nervous system promotes the "rest and digest mode." The somatic nervous system controls voluntary skeletal muscles, but the parasympathetic nervous system controls involuntary smooth & cardiac muscles. The neurons of the parasympathetic nervous system release acetylcholine, which is a neurotransmitter that leads to a decrease in heart rate and blood pressure. Results of increased parasympathetic activity include: decreasing blood flow to skeletal muscles, increasing blood flow to the gut, constricting pupils, and glycogenesis.

The correct answer is ACh (acetylcholine) because it is involved with muscle contraction. It is released at the neuromuscular junction, the site where the neuron and muscle meet.

During the generation of an action potential, the cell will undergo two refractory periods. The first is referred to as the absolute refractory period, during which no stimulus, regardless of size, will generate another action potential. This is followed by the relative refractory period, during which an action potential will be generated only if an abnormally large stimulus is encountered. During the relative refractory period, the cell is hyperpolarized due to the removal of potassium ions from the cell interior, which results in a more negative membrane potential than the cell would have at rest.

By pumping two positively-charged potassium molecules in for every three positively-charged sodium molecules that are pumped out of the cell, the sodium-potassium pump maintains a resting potential of –70mV relative to outside of the cell. This function is important for creating an electrochemical gradient along the neuron.

Remember that sodium flows down its gradient to enter the cell during depolarization, while potassium flows down its gradient to exit a cell after an action potential, causing hyperpolarization during the refractory period.

After the sodium channel is opened, sodium rushes into the cell down its concentration gradient (as previously created by the sodium-potassium pump). This causes depolarization of the membrane as its potential reaches a value of +35mV, which is eventually lowered by the opening of the potassium channels. This leads to hyperpolarization, which prevents the signal from traveling backwards.

The refractory period, a phase in which action potentials cannot be fired, is the result of hyperpolarization, during which the membrane potential drops below –70mV. The membrane potential is at this –70mV level while the threshold, which needs to be reached to fire action potential, is slightly higher at –50mV. During the period of extreme hyperpolarization, an action potential will not form.

Threshold potential is defined as the potential that must be reached in order for an action potential to be initiated by a neuron. Threshold potential is around -55mV in humans, which is slightly higher than the resting potential of -70mV. Once this threshold is reached, the electrical signal will propagate as the membrane depolarizes to a positive potential.

Sub-threshold stimuli, such as stimulus causing depolarization to -65mV, will not trigger action potentials. Muscle contractions can result from action potentials or provide sensory feedback, but the contractions themselves do not play a role in initiating action potentials.

The refractory period is the span of time during which the neuron "recovers" and generally does not respond to a second stimulus as strongly as it did to the first. As the name implies, the absolute refractory period refers to the time when no stimulus, no matter how strong, can provoke a second action potential. This occurs because sodium channels, the opening of which causes depolarization, are sealed by a gating mechanism.

The relative refractory period follows the absolute refractory period. During the relative refractory period, the cell will not respond to normal stimuli, but can generate an action potential if an exceptionally large stimulus occurs. During this period the sodium channels are closed, but not sealed by the gating mechanism; they are essentially normal. The relative refractory period is caused by hyperpolarization as potassium rushes out of the cell after the action potential. Because the potential is lower than normal, only a very large stimulus can overcome the threshold.

Though threshold, saltatory conduction, and depolarization do relate to nervous system potentials, they do not refer to this specific period. Apoptosis is a completely unrelated process referring to a type of cell death.