The Brainstem

The brainstem controls numerous vital body functions (breathing, swallowing, and vasomotor control over BP). Additionally, except for vision (CN2) and smell (CN1), all of the cranial nerves originate from the brainstem. The brainstem also has nuclei that regulator sympathetic and parasympathetic autonomic functions. All efferent and afferent nerve tracts between the cerebrum and the cerebellum travel through the brainstem. The brainstem exits the bottom of the skull through a hole called the foramen magnum.

The basic parts of the brainstem include:

• the medulla oblongata,
• pons,
• midbrain,
• cerebellum (usually considered separately), and
• hypothalamus (although the thalamus and its basal ganglia--key components of the brainstem--are usually considered separately).

MEDULLA OBLONGATA (the "Medulla"): the posterior/lower half of the brain stem. It connects directly to the spinal cord at the inferior end and to the pons at the superior end. The medulla houses the

• cardiac,
• respiratory and
• vasomotor centers

that deal with heart rate, breathing, and blood pressure, respectively.

PONS: located inferior to the midbrain, superior to the medulla, and anterior to the cerebellum.

It includes neural pathways (tracts) that

1. transmit signals from the brain to the cerebellum and medulla; and
2. tracts that carry signals from the spine up into the thalamus.

Several nuclei are involved in sleep, respiration, swallowing, bladder control, equilibrium, hearing, eye movement, taste, facial expressions/sensation, and posture.

The Hering-Breuer reflex, provoked by overstretching of smooth muscle stretch receptors in the airway, stimulates a vagal parasympathetic inhibition of respiration; this prevents damage via overstretching. 

MIDBRAIN ("mesencephalon"): two separate structures--

1. the tectum and the
2. tegmentum.

They control/regulate motor movement (especially of the eyes), and the processing of auditory and visual stimuli.

The tectum makes up the posterior portion of the midbrain and receives input from the retina and visual cortex, primarily for tracking objects in the visual field.

The tegmentum is located in front of the tectum and consists of nerve tracts and three distinct regions known as the red nucleus, periaqueductal gray, and the substantia nigra.

• The red nucleus is mainly responsible for the coordination of gait.
• The periaqueductal gray is mainly responsible for pain suppression and has a high concentration of endorphins.
• The substantia nigra mainly assists in movement and motor coordination it is the area that is damaged in Parkinson's disease.

CEREBELLUM: [discussed in "STRUCTURE OF THE CEREBELLUM" unit]

LIMBIC SYSTEM

The limbic system is a group of brain structures involved in emotion, motivation, and memory. It includes the hippocampus, which is located in the most inferior sections of the temporal lobes and is a vital structure for the formation and processing of memories. The limbic system also contains other structures such as the amygdala, olfactory bulbs, anterior thalamic nuclei, fornix, mammillary body, cingulate gyrus, and limbic midbrain.

The limbic system allowed our survival as a species by generating and orchestrating the innate drives, desires, and emotions, the ability to remember and learn, and sexual inclinations--crucial features of living and surviving. The amygdala, for example, is involved in the processing of emotions such as fear and aggression, while the cingulate gyrus is involved in decision-making and emotion regulation.

HYPOTHALAMUS: small region below the thalamus that coordinates the autonomic nervous system and the pituitary gland. Through these mechanisms it controls:

1. body temperature,
2. thirst and hunger; and is also involved in
3. sleep and
4. emotional activity.

Most of the nerve fibers within the hypothalamus are bidirectional, allowing transmissions in two directions.

Hypothalamic nuclei have clear differences in structure and function between males and females.

Hypothalamic control of the anterior pituitary gland is achieved by releasing hormones. These compounds are produced in the hypothalamus and transmitted along axons to the posterior pituitary gland and either start or stop secretions of pituitary hormones, or they are stored until needed.

THALAMUS: At the top of the brainstem "most cephalad", is the thalamus. It is located in the midline, between the parietal lobes.

The thalamus relays sensory information from the body to the cerebral cortex and acts as a center for pain perception and regulation of

1. consciousness,
2. sleep, and
3. alertness.

The thalamus serves as the intermediary between sensory nerves and the cerebral cortex. The thalamus takes these impulses and creates the sensations of touch, pain, and temperature.

The thalamus is connected to the cerebral cortex by the thalamocortical radiations and to the spinal cord by the spinothalamic tract.

• THALAMOCORTICAL RADIATIONS: tree-like fibers that extend into the cerebral cortex from the thalamus, carrying sensory information up into the cerebral lobes.

• SPINOTHALAMIC TRACTS: lateral transmission paths beginning in the spinal cord that transmit sensory information to the thalamus:

1. Pain,
2. temperature, and
3. itch.

Crude touch and pressure are sensed in the brain from signals traveling up the anterior (ventral) spinothalamic tracts.

BASAL GANGLIA: a group of subcortical (below the cortex) nuclei surrounding and lateral to the thalamus, some of which are contained in the other brainstem structures mentioned previously.

They are strongly interconnected with the cerebral cortex, thalamus, and brainstem. Many of the CNS functions rely on these structures. They and are associated with

1. control of voluntary movements,
2. procedural learning,
3. memory,
4. routine behaviors,
5. eye movements,
6. cognition,
7. motivation, and
8. emotion.

The basal ganglia are several discrete structures named the:

• dorsal striatum,
• ventral striatum,
• globus pallidus,
• ventral pallidum,
• substantia nigra, and
• subthalamic nucleus.