Conduction functions
Nerve fibers connecting different parts of the nervous system pass through the medulla oblongata and pons. They can be divided into three groups:
- Transit pathways. Pass all the way through. These include the ascending and descending tracts of the spinal cord, the pyramidal, rubrospinal, and spinothalamic tracts.
- Terminating pathways. End in the brainstem structures. For example, the tracts of Goll and Burdach (fasciculus gracilis and cuneatus).
- Originating pathways. Originate in the medulla oblongata itself: vestibulospinal, reticulospinal, and olivospinal tracts.
The role of the reticular formation (RF)
The reticular formation of the brainstem has a pronounced effect on motor activity and reflexes. The effect depends on which part of the RF is activated:
- Ventromedial regions: their stimulation leads to inhibition of the knee-jerk reflex, suppression of movements caused by the cortex, and blocking of excitation transmission along $\gamma$-afferent fibers.
- Dorsolateral areas: stimulation causes facilitation of the knee-jerk reflex and cortically induced movements, and also increases muscle tone (both flexors and extensors).
Intrinsic functions and reflex activity
These structures house the cranial nerve (CN) nuclei and the centers of somatic and autonomic reflexes. The nuclei of CN VII, IX, X, XI, and XII are localized in the medulla oblongata, while CN V and VI are located in the pons.
Neurons perform primary analysis of sensory information: vestibular, tactile, auditory, and gustatory. Bulbar centers are under the descending control of the cerebellum and the red nuclei of the midbrain.
Reflexes are characterized by complex coordination, often including both somatic and autonomic components. The main groups are:
- Feeding: motor (sucking, chewing, swallowing) and secretory (secretion of saliva and gastric juice).
- Protective: coughing, sneezing, vomiting, blinking, lacrimation.
Each reflex is strictly specific and is triggered from a specific reflexogenic zone. An interesting example: mechanical irritation of the pharynx causes vomiting, while the entry of food into the same zone initiates swallowing.
Autonomic and metabolic functions
The medulla oblongata is responsible for regulating vital processes.
- Respiratory center: is paired and consists of inspiratory (inspiration) and expiratory (expiration) divisions. Respiratory neurons are capable of automatism—spontaneous generation of impulses. This automatism depends on the humoral influence of $CO_2$ and signals from the vagus nerves.
- Vasomotor center: maintains blood vessel tone.
- Cardiac center: includes the vagus nerve nucleus (N. vagus), which exerts an inhibitory effect on the heart, and neurons connected to spinal centers, which produce an excitatory effect.
The brainstem is also involved in the regulation of metabolism, as proven by Claude Bernard's classic experiment ("sugar puncture").
Briefly: answers to topical questions
Which specific nuclei and groups of neurons form the respiratory center of the medulla oblongata?
The respiratory center of the medulla oblongata includes inspiratory and expiratory divisions consisting of two main groups of neurons.
- Dorsal group — consists of 90% inspiratory neurons.
- Ventral group — contains the majority of expiratory neurons, which are located near the nucleus ambiguus between the zones of inspiratory cells, as well as in the region of the facial nerve posterior nucleus.
What functional zones does the vasomotor center of the medulla oblongata consist of?
The vasomotor (vasopressor) center of the medulla oblongata includes pressor and depressor divisions, as well as specific functional zones.
- Vasoconstrictor zone — designated as C-1.
- Cardioacceleratory zone — responsible for accelerating heart rate.
- Sensory zone — represented by the nucleus of the solitary tract, which collects impulses from vagus nerve fibers.
Additionally, there is a depressor zone (division) which, when stimulated, inhibits the activity of the pressor zone.
Class and exam questions
What are "bulbar animals"?
These are animals in which the brain is transected along the anterior border of the pons. They retain independent breathing and stable blood pressure, but lose the ability to thermoregulate and move independently.
What is the purpose of the pneumotaxic center?
It is located in the pons and is responsible for switching activity between the inspiratory and expiratory centers, ensuring the rhythmicity of breathing.
Where are the neurons of the serotonergic system located?
In the dorsal and medial raphe nuclei of the medulla oblongata, pons, and midbrain. Their axons innervate the cerebral cortex, hypothalamus, amygdala, hippocampus, and globus pallidus.
What else to study on this topic
- Mechanisms of respiratory center automatism
- Detailed structure of the raphe nuclei and serotonergic system
- Specific reflexogenic zones of protective reflexes
- Interaction of the brainstem RF with the cerebral cortex
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