Anatomy and Physiology for Nursing and Healthcare Students at a Glance. Ian Peate

11 The structures of the brain

Figure 11.1 The cerebrum.

       Source: Peate I, Wild K & Nair M (eds). Nursing Practice: Knowledge and Care (2014).

Figure 11.2 The cerebellum.

Figure 11.3 The limbic system.

      The four major regions of the brain are:

       the cerebrum

       the diencephalon

       the brainstem

       the cerebellum.

Cerebrum

The cerebrum, also known as the telencephalon, is the largest and most highly developed part of the brain. It covers around two‐thirds of the brain mass and lies over and around most of the structures of the brain (Figure 11.1). The cerebrum sits on top of the brainstem, with the cerebellum underneath the rear portion.

      The cerebrum of an adult is divided into two large hemispheres (the left and right hemispheres). The surfaces of the cerebral hemispheres are highly folded and covered by a superficial layer of grey matter known as the cerebral cortex. The functions of the cerebrum include regulation of muscle contraction, memory storage and processing, production of speech, interpretation of taste, sound and memory for storage and processing.

Diencephalon

      The diencephalon provides a functional link between the cerebral hemispheres and the rest of the central nervous system (CNS). It contains three paired structures: the thalamus, hypothalamus and epithalamus. Each component of the diencephalon has specialised functions that are integral to life.

      Thalamus

      The thalamus acts as a relay station for sensory impulses going to the cerebral cortex for integration and motor impulses entering and leaving the cerebral hemispheres. It also plays a role in memory.

      Hypothalamus

      The hypothalamus is closely associated with the pituitary gland and produces two hormones: antidiuretic hormone (ADH) and oxytocin. It is also the chief autonomic integration centre and is part of the limbic system, which is the emotional brain.

      Epithalamus

      The epithalamus is a small structure that is linked to the pineal gland, which secretes the hormone melatonin responsible for sleep/wake cycles.

Brainstem

      The brainstem regulates vital cardiac and respiratory functions and acts as a vehicle for sensory information.

      The structures that form the brainstem are involved in a range of activities that are essential for life. The brainstem is associated with the cranial nerves. The structures of the brainstem include the midbrain, pons and medulla oblongata (see Figure 11.1).

      Midbrain

      The midbrain contains nuclei that deal with auditory and visual information and reflexes. It also maintains consciousness and provides a conduction pathway connecting the cerebrum with the lower brain structures and spinal cord.

      Pons

      The pons connects and communicates with the cerebellum. It works with the medulla oblongata to control the depth and rate of respiration and contains nuclei that function in visceral and somatic motor control.

      Medulla oblongata

      The medulla oblongata is a relay station for sensory nerves going to the cerebrum. The medulla contains autonomic centres such as the cardiac centre, respiratory centre, vasomotor centre and coughing, sneezing and vomiting centres. The medulla is also the site of decussation of the pyramidal tract; this means that the right side of the body is controlled by the left cerebral hemisphere and vice versa.

Cerebellum

The second largest structure of the brain (Figure 11.2), partially hidden by the cerebral hemispheres, is the cerebellum. The cerebellum is responsible for the co‐ordination of voluntary muscle movement, motor learning, cognitive functions and balance and posture. It ensures that muscle movements are smooth, co‐ordinated and precise. Motor commands are not initiated in the cerebellum; the cerebellum modifies the motor commands of the descending pathways to ensure that movements are more adaptive and accurate. Even though the cerebellum accounts for approximately 10% of the brain’s volume, it contains over 50% of the total number of neurons in the brain.

Limbic system

The limbic system is a complex set of brain structures located on both sides of the thalamus, under the cerebrum. The limbic system includes the hippocampus, amygdala, anterior thalamic nuclei, septum, habenula, limbic cortex and fornix. It supports a variety of functions, including emotion, behaviour, motivation, long‐term memory and olfaction. The limbic system acts on the endocrine and autonomic nervous systems (Figure 11.3).

Ventricles of the brain

      These are a series of interconnected, fluid‐filled cavities that lie within the brain, a communicating network filled with cerebrospinal fluid (CSF) and located within the brain parenchyma. The ventricular system is composed of two lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle. The choroid plexuses located in the ventricles produce CSF, filling the ventricles and subarachnoid space, following a cycle of constant production and reabsorption.

Cerebrospinal fluid

      Cerebrospinal fluid is a clear body fluid occupying the subarachnoid space and the brain ventricular system around and inside the brain and spinal cord. It acts as a cushion or buffer for the cortex, providing basic mechanical and immunological protection to the brain inside the skull. It is produced by modified ependymal cells of the choroid plexus found in all components of the ventricular system except for the cerebral aqueduct and the posterior and anterior horns of the lateral ventricles. CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen (also called the foramen of Monro). The third ventricle and fourth ventricle are connected to each other by the cerebral aqueduct (also called the aqueduct of Sylvius). CSF then flows into the subarachnoid space through the foramina of Luschka (there are two