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However we do understand, to some degree, how age affects it, as well as the nervous system as a whole.

Basic Structure and Function of the Nervous System – Anatomy and Physiology

This fifth article in our series on the anatomy and physiology of ageing describes the age-related changes occurring in the brain, spinal cord and peripheral nerves. These changes lead to a gradual decline in cognitive function and a range of other issues, such as reduced bladder control or postural hypotension, but in health the brain normally continues to function adequately throughout life.

Nursing Times [online]; 6, Authors: John Knight is senior lecturer in biomedical science; Yamni Nigam is associate professor in biomedical science, both at the College of Human Health and Science, Swansea University. It receives and processes sensory input from organs such as the eyes, ears and skin, and responds through a variety of effector organs. Ageing leads to a progressive loss of neurons and depletion of neurotransmitters Mather, , these changes are usually associated with a gradual decline in cognitive function and influenced by environmental, genetic and lifestyle factors Nyberg et al, The ageing brain gradually loses neurons and supporting neuroglial cells Fig 1.

Between the ages of 20 and 60, the brain loses around 0. The remaining tissues harbour an increased concentration of potentially harmful materials such as iron, aluminium and free radicals. Aged neural tissues also show increasing pigmentation, largely due to the deposition of two pigments: one brown, lipofuscin Ottis et al, , and one black, neuromelanin Clewett et al, Lipofuscin is linked to amyloid protein deposition and the formation of neurofibrillary tangles. The loss of neurons is most apparent in the cerebral cortex.

The grooves sulci that mark the surface convolutions gyri of the cerebral cortex are visibly deeper in brains of older people Fig 1. It was originally thought that the frontal lobes were particularly vulnerable to neural loss, but similar losses have been observed in other cortical regions such as the parietal lobes Fjell et al, Structural changes in the frontal and parietal lobes are related to poor memory. Many people in their 80s have modest levels of amyloid protein deposition and retain their memory, while individuals with higher levels typically have a poorer working short-term memory Nyberg et al, However, the role of amyloid deposition in impairing memory has recently been questioned and other factors, such as accumulation of tau proteins, may play a more important role Brier et al, The hippocampus has a key role in memory and the acquisition of new skills.

With age, it loses a significant amount of neural tissue Burke and Barns, , which may explain why activities such as learning a new language become more difficult with advancing age. Virtual reality computer programs could therefore potentially be used to reduce shrinkage in this vital brain area. The somatic motor cortex — located in the frontal lobes of the brain — controls the movement of muscles involved in walking.

From middle age onwards the neurons in this region show signs of atrophy Manini et al, , which can contribute to gait problems, potentially reducing mobility in older people Rosso et al, Ageing is associated with a gradual increase in the size of the ventricles fluid-filled chambers in the brain Fig 1.

This is likely to result from a progressive loss of the cells that line the ventricles. Enlarged ventricles fill with more cerebrospinal fluid, and are readily observable using techniques such as magnetic resonance imaging. On average, the volume of the ventricular system increases by around 2. Although ventricular expansion is seen in most older people, the cognitive impact of this remains unclear. The medulla oblongata and other areas of the brainstem lose fewer neurons than other regions of the brain.

The brainstem is probably the best preserved region of the brain, which probably reflects its essential role in supporting life: it controls breathing, peristalsis, heart rate and blood pressure. Both branches of the autonomic nervous system ANS — the parasympathetic and sympathetic branches — are compromised with age Parashar et al, These changes can negatively affect older people. For example, the blunting of baroreceptor responses increases the risk of postural hypotension, so standing up suddenly can lead to falls and injury.

Another negative consequence is the gradual loss of bladder control. To control micturition, the body relies on the interplay of sensory stretch receptors and the ANS which together monitor bladder filling and the conscious areas of the cerebral cortex which signals when the bladder is full. To initiate micturition, the body needs motor control of the urinary sphincter.

All these elements function less well with age, and these age-related changes combine with those in other body systems — such as prostate enlargement in men and weakened pelvic floor muscles in women — to reduce bladder sensitivity and control Hotta and Uchida, , which can lead to continence problems.

Cerebral blood flow decreases by around 0. This is a direct consequence of the age-related changes in the cardiovascular system, and may be exacerbated in patients with atherosclerotic occlusion of the carotid arteries.

Digital nervous system

The blood-brain barrier BBB is formed primarily of tight junctions between adjacent endothelial cells within the blood vessels in the brain. Additionally, specialised neuroglial cells called astrocytes wrap around the cerebral vessels, forming a further physical barrier between the blood and neural tissues.

The BBB is essential to prevent most pathogens and many toxic materials crossing into the neural networks and pathways of the brain, but its integrity appears to diminish with age. A recent study indicates that, during normal ageing, the BBB is first weakened in the hippocampus, thereby allowing harmful substances and pro-inflammatory mediators to cross into this vital region of learning and memory. This breaching of the BBB may contribute to hippocampal shrinkage, and therefore to cognitive decline Montagne et al, Ageing is associated with a declining production of many neurotransmitters, including noradrenaline, glutamate, dopamine and serotonin.

The number of dopamine-producing neurons decreases as part of the normal ageing process, and this can adversely affect the ability to learn from past experiences. Few studies have examined age-related changes to the spinal cord.

Nervous system

A recent animal-model study shows an increase of cholesterol content in the ageing spinal cord, and the authors suggest this may potentially impair cord function Parkinson et al, Age-related changes to neurons and neuroglial cells appear to have little effect on spinal cord function. However, age-related changes to the vertebrae and intervertebral discs may increase pressure on the spinal cord and its branching nerve roots.

This can slow down the conduction of nerve impulses along motor neurons, contributing to reduced muscular strength Manini et al, Reduced sensory and motor conduction will increase the risk of injury due to poor coordination, poor balance and poor fine motor control. Some of this damage may be caused by a rise in the concentrations of pro-inflammatory mediators in the body. The ageing body becomes less effective in clearing toxic metabolites and, as peripheral nerves are not afforded the protection of the BBB, this may contribute to peripheral nerve damage Manini et al, In health, this reduction in conductivity causes few problems, but in older people with diabetes it may contribute to, and exacerbate, diabetic neuropathy.

Damaged peripheral nerves are not repaired as efficiently in older people as in their younger counterparts, and some of these nerves are never repaired. This can contribute to reduced sensation and motor control.

In the absence of disease, intellectual ability can be retained throughout life. However, the gradual loss of neurons, depletion of neurotransmitters and slowing of nerve conduction may act together to slow down the processing of information. As a result, older people may take longer to complete certain tasks, and commonly experience the functional brain changes described below. The loss of short-term and episodic memory is probably the earliest indication of age-related changes in the brain.

The Nervous System In 9 Minutes

Episodic memory that is, remembering autobiographical events and their timings and sequence also gradually declines in many older people Fjell et al, The ability to quickly name a common object usually remains stable up to the age of 70, but then declines with advancing years Harada et al, The progressive loss of neurons, reduction in impulse velocity and minor changes in the spinal cord lead to a slowing down of reaction times Spirduso, This can create problems, particularly when a fast reaction is essential for example, to step out of the way of oncoming traffic.

It is almost impossible to determine whether depression in older people occurs as a normal consequence of ageing or as a result of chemical imbalances seen in types of depression that also affect younger people. Concentrations of neurotransmitters involved in lifting mood particularly serotonin diminish with age and this can contribute to symptoms of depression Fidalgo et al, Depression can often produce symptoms that mimic dementia pseudo-dementia and this often causes great anxiety.

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On the whole, older people are less prone to emotional outbursts than younger people. Depression and anxiety.


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