Chapter 5 Nervous system and special senses
Nervous tissue
Neuron structure
Each neuron consists of the following parts:
Neurons vary in size – the diameter of the axons and dendrons may be a few micrometres and the length depends on the destination of the nerve fibre. This may be anything from a few millimetres to more than a metre long. Neurons also vary in shape (Fig. 5.2).
Generation of a nerve impulse
Nerve impulses transmitted along axons and dendrites can be considered to be electrical phenomena. An impulse changes the electrical charge of the neuron by altering the relationship between the negative charge of the cell contents and the positive charge of the cell membrane. This results from a change in the permeability of the cell membrane to sodium and potassium ions, causing an exchange of ions between the inside and the outside of the neuron (Fig. 5.4).
Classification of nerves
Peripheral nerves can be classified according to their function and to the structures they supply.
Nerve fibres may be further classified according to the organ with which they are associated.
The central nervous system
During embryonic development of the CNS, a hollow neural tube forms from the ectodermal layer of the inner cell mass of the embryo. This neural tube runs along the dorsal surface of the embryo and, as it develops, nerve fibres grow out laterally extending to all parts of the body and eventually forming the peripheral nervous system. The anterior end of the tube becomes the brain and the remaining tube becomes the spinal cord. The brain and spinal cord are hollow and are filled with cerebrospinal fluid (CSF).
The brain
Forebrain
This consists of the cerebrum, thalamus and hypothalamus.
The tissue of the cerebral hemispheres consists of:
The hypothalamus lies ventral to the thalamus and has several functions:
On the ventral surface of the forebrain (Fig. 5.6) the following structures can be seen:
Midbrain
This is a short length of brain lying between the forebrain and the hindbrain. It is overhung by the cerebral hemispheres and is not easy to see in the gross specimen. It acts as a pathway for fibres running from the hindbrain to the forebrain carrying the senses of hearing and sight.
Hindbrain
This consists of the cerebellum, pons and medulla oblongata.
The cerebellum lies on the dorsal surface of the hindbrain (Fig. 5.5). Its name means ‘little brain’ as it was originally thought to be a smaller version of the cerebrum. It has a globular appearance and is covered in deep fissures. In cross-section, the tissue is very obviously divided into an outer cortex of grey matter and an inner layer of white matter. The grey matter consists of numerous Purkinje cells (Fig. 5.2), each of which forms thousands of synapses with other neurons.
Protection of the brain
Cranium
This bony structure forms a tough outer shell to protect the soft brain tissue from physical damage.
Ventricular system
The ventricular system (Fig. 5.7) is derived from the hollow neural tube of the embryo. The lumen of this tube develops into a series of interconnecting canals and cavities or ventricles found inside the brain and spinal cord. The ventricles and the central canal are filled with cerebrospinal fluid (CSF). This also surrounds the outside of the brain, lying in the subarachnoid space (Fig. 5.8). CSF is secreted by networks of blood capillaries known as choroid plexuses lying in the roofs of the ventricles. It is a clear fluid, resembling plasma, but has no protein in it – it is an example of a transcellular fluid. The function of CSF is to protect the CNS from damage by sudden movement or knocks and to provide nutrients to the nervous tissue of the brain and spinal cord.
Meninges
Blood–brain barrier
This is a modification of the neuroglial tissue that connects and supports all the neurons within the nervous tissue. Different types of neuroglial cells surround the blood capillaries, creating an almost impermeable layer, to protect the brain from substances that are harmful to or not needed by the brain. These include urea, certain proteins and antibiotics. Other materials such as oxygen, sodium and potassium ions and glucose can pass rapidly through the barrier to be used for brain metabolism.
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