Source: Peate I, Wild K & Nair M (eds). Nursing Practice: Knowledge and Care (2014).
Figure 10.3 The neuron.
Source: Peate I, Wild K & Nair M (eds). Nursing Practice: Knowledge and Care (2014).
Figure 10.4 The cranial nerves.
Source: Peate I, Wild K & Nair M (eds). Nursing Practice: Knowledge and Care (2014).
The brain
The brain is one of the largest and most complex organs in the body. It is responsible for the integration of sensory information (for example, it interprets the senses); it also directs motor responses (it is the initiator of body movement and controller of behaviour) and is the centre of learning.
The brain weighs around 1400 grams. It is protected inside a bony shell (the cranium or skull) and washed by protective fluid (cerebrospinal fluid). It is the source of all those qualities that define us as humans (Figure 10.1).
The brain receives 15% of the cardiac output. It has a system of autoregulation making sure that its blood supply is constant regardless of positional changes. Most of the expansion comes from the cerebral cortex, a convoluted layer of neural tissue covering the surface. The frontal lobes are particularly expanded and are involved in executive functions, such as self‐control, planning, reasoning and abstract thought.
The meninges
As nervous tissue can be easily damaged by pressure, it needs to be protected. The hair, skin and bone provide an outer layer of protection. Closest to the nervous tissue are the meninges which cover the delicate nervous tissue. They also protect the blood vessels that serve nervous tissue and they contain cerebrospinal fluid. The meninges are made up of three connective tissue layers: dura, arachnoid and pia maters (Figure 10.2).
The cerebrospinal fluid
The cerebrospinal fluid (CSF) is produced by the choroid plexus located within the ventricles of the brain. Approximately 150 mL of CSF circulates around the brain, in the ventricles and around the spinal cord. Every 8 hours the CSF is replaced. The CSF provides a cushion to the brain to protect it from damage, it maintains a uniform pressure between the brain and spinal cord, and plays a small role in fluid and waste exchange between the brain and spinal cord.
The neuron
The functional unit of the brain is the neuron or nerve cell (Figure 10.3). A neuron has a number of features in common with other cells, including a nucleus and mitochondria. However, because of its key role, it is well protected with some specialist modifications.
Neurons are composed of an axon, dendrites and a cell body. Their function is to transmit nerve impulses. Nerve impulses only ever travel in one direction: from the receptive area – the dendrites – to the cell body and down the length of the axon.
Axon
Each neuron has only one axon, but the axon can branch to form an axon collateral (see Figure 10.3). The axon will also branch at its end into several axon terminals. The axon length may vary quite significantly, from very short to 100 cm long. The axon is much thicker and longer than the dendrites of a neuron. Larger neurons have a noticeably expanded region at the start of the axon, known as the axon hillock, which is the site of summation for incoming information. At any given moment, the combined influence of all axons that conduct impulses to a given neuron determines whether or not an action potential will be initiated at the axon hillock and disseminated along the axon.
Dendrites
Dendrites are the short branching processes that receive information. Generally, dendrites are very thin appendages, becoming narrower as they extend further away from the cell body (see Figure 10.3). Dendritic spines are short outgrowths that further increase the receptive surface area of a neuron. The surface of dendrite branches is covered with junctions designed for the reception of incoming information. Their branching processes provide a large surface area for this function. In sensory neurons, the dendrites frequently form part of the sensory receptors and in motor neurons they can be part of the synapse between one neuron and the next.
Cell body
The cell body (soma) is the central part of the neuron. It contains the nucleus of the cell and as such it is where most protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter. Most of the neuron cell bodies (see Figure 10.3) are found inside the central nervous system, forming the grey matter. When there are clusters of cell bodies grouped together in the central nervous system, they are known as nuclei. Cell bodies found in the peripheral nervous system are called ganglia.
Myelin sheath
Oligodendrocytes and Schwann cells form the myelin sheaths that insulate axons in the central and peripheral nervous systems, respectively. Peripheral nerve axons and long or large axons are covered in a myelin sheath (see Figure 10.3). Myelin is a fatty material that protects the neuron as well as electrically insulating it, which speeds up impulse transmission. Schwann cells, wrapped in layers around the neuron within the peripheral nervous system, form the myelin sheath. The outermost part of the Schwann cell is its plasma membrane, called the neurilemma. There is a regular gap (about 1 μm) between adjacent Schwann cells called the nodes of Ranvier. Collateral axons can occur at the node. Some nerve fibres are unmyelinated and nerve impulse transmission in these fibres is significantly slower.
Cranial nerves
There are 12 pairs of cranial nerves emerging from the brain and supplying various structures, most of which are associated with the head and neck (Figure 10.4). The cranial nerves vary in their functions; some are sensory nerves, containing only sensory fibres, some are motor nerves, containing only motor fibres, and some are mixed nerves, containing both sensory and motor fibres.
Clinical practice point
Multiple sclerosis is a disease that affects the central nervous system. The immune system attacks the myelin, the protective layer around nerve fibres, and causes inflammation and lesions. This makes it difficult for the brain to send signals to the rest of the body. The most common symptoms of multiple sclerosis include eye problems, numbness or tingling feelings, fatigue and pain. The symptoms can come and go and may change over time. They can be mild or more severe. The healthcare provider must ensure an individual assessment of needs is carried out and individual care plans are implemented