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

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Название Anatomy and Physiology for Nursing and Healthcare Students at a Glance
Автор произведения Ian Peate
Жанр Медицина
Серия
Издательство Медицина
Год выпуска 0
isbn 9781119757221



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The meninges.

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

Schematic illustration of the neuron.

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

Schematic illustration of the cranial nerves.

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

      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 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 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.

      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.

      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