How is an impulse transmitted from one neuron to another?

The marked structure is the medulla oblongata, the most inferior portion of the brainstem, continuous with the spinal cord at the foramen magnum. It lies between the pons superiorly and the spinal cord inferiorly and contains both ascending and descending neural tracts. The medulla plays a vital role in autonomic control centers regulating respiration, heart rate, and blood pressure. It also contains nuclei of several cranial nerves (IX–XII), making it essential for life-sustaining reflexes.

A. Pons: The pons is the middle segment of the brainstem located superior to the medulla and inferior to the midbrain. It serves as a bridge (“pons” meaning bridge) connecting the cerebrum with the cerebellum via transverse pontine fibers. It also plays a role in regulating respiration and relaying motor and sensory information. Compared to the medulla, it is more rounded and positioned superiorly in the brainstem.

B. Medulla oblongata: The medulla oblongata is the most caudal part of the brainstem, continuous with the spinal cord. It contains vital autonomic centers controlling respiration, cardiac rhythm, and vasomotor tone. It houses nuclei for cranial nerves IX, X, XI, and XII and coordinates reflexes such as swallowing, coughing, and vomiting. Its location just above the spinal cord and below the pons makes it the correct answer.

C. Midbrain: The midbrain is the superior portion of the brainstem located between the pons and the diencephalon. It is involved in visual and auditory reflexes and contains structures such as the superior and inferior colliculi. It is more rostral and smaller compared to the medulla and does not extend into the spinal cord region.

D. Cerebellum: The cerebellum is located posterior to the brainstem in the posterior cranial fossa. It is responsible for coordination of voluntary movement, balance, posture, and motor learning. Unlike the medulla, it is not part of the brainstem’s autonomic control system and does not regulate vital life-sustaining functions like respiration or cardiac output.

Neuronal communication occurs at specialized junctions called synapses, where signals are transmitted from one neuron to another. Most synapses in the nervous system are chemical synapses, relying on neurotransmitters rather than direct electrical continuity. When an action potential reaches the end of a presynaptic neuron, it triggers a cascade of events that leads to neurotransmitter release. These chemical messengers then cross the synaptic cleft and bind to receptors on the postsynaptic membrane, initiating a new electrical signal.

A. A neurotransmitter traveling from postsynaptic axons crosses the synapse to presynaptic dendrites or a cell body: neurotransmitters are released from the presynaptic neuron, not the postsynaptic neuron. Additionally, signaling does not occur in reverse direction in a typical chemical synapse. The postsynaptic neuron receives, rather than sends, the chemical signal.

B. An impulse stimulating a postsynaptic axon causes the release of neurotransmitters into a synaptic cleft: neurotransmitter release is triggered by an action potential arriving at the presynaptic axon terminal, not the postsynaptic axon. The postsynaptic neuron receives chemical signals rather than initiating them at the synapse.

C. An impulse stimulates a presynaptic axon, causing release of neurotransmitters into the synaptic cleft: when an action potential reaches the axon terminal of the presynaptic neuron, voltage-gated calcium channels open. Calcium influx triggers synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic membrane, generating a new electrical response. This sequence ensures one-directional and regulated neural communication.

D. Electrical current flows directly from dendrite to dendrite across the synaptic cleft: most synapses are chemical, not electrical, and do not involve direct cytoplasmic continuity between neurons. Electrical synapses do exist (via gap junctions), but they are not the primary mechanism described here. Furthermore, synaptic transmission does not occur directly between dendrites across a cleft in typical neural signaling.