The Nerve Cell

Nerve Cell

The Nerve Cell The basic unit of the nervous system is the nerve cell, or neuron. The basic function of the neuron is to transmit information. There are approximately 28 billion neurons in the human body penetrating every tissue in every part. Neurons vary greatly in size and shape, with the longest ones—those that extend down the leg as part of the sciatic nerve—measuring over one meter. All nerve cells have a similar structure.

A basic nerve cell consists of a cell body, an axon, and many dendrites. Dendrites are thread-like branches that increase the surface area of the cell making it possible for it to receive many connections with adjoining nerve cells. Signals picked up by the dendrites travel through the cell and continue along the axon where they are transmitted to the next cell. Synaptic bulbs on the ends of the axons make connections with other nerve cells, with tiny gaps or synapses between the cells.

Muscle cell Fig. Anatomy of striated muscle Muscle cell Fig. Anatomy of striated muscle. The fundamental physiological unit is the fiber.

There are three types of muscles in the body: - 1. smooth muscle, 2. striated muscle (skeletal muscle), 3. cardiac muscle. Smooth muscles are involuntary (i.e., they cannot be controlled voluntarily). Their cells have a variable length but are in the order of 0.1 mm. Smooth muscles exist, for example, in the digestive tract, in the wall of the trachea, uterus, and bladder. The contraction of smooth muscle is controlled from the brain through the autonomic nervous system.

Cardiac muscle is also striated, but differs in other ways from skeletal muscle: Not only is it involuntary, but also when excited, it generates a much longer electric impulse than does skeletal muscle. Correspondingly, the mechanical contraction also lasts longer. Furthermore, cardiac muscle has a special property: The electric activity of one muscle cell spreads to all other surrounding muscle cells, owing to an elaborate system of intercellular junctions.

Red Blood Cells (RBC's or erythrocytes) Biologically, blood is a liquid tissue, transporting materials and protecting us against disease. Suspended in the watery plasma are red and white blood cells, serving different functions, but both of vital importance to our bodies. RBC's are responsible for carrying oxygen (O2) and removing carbon dioxide (CO2) from about 30 trillion cells in the human body. In normal blood RBC's account for about 45% of the total volume. On average, we have about 5 million red cells per cubic millimeter of blood.

Produced in the red marrow of bones, RBC's arise from a single type of cell, called a stem cell. During formation the nucleus is lost and organelles degraded, allowing more internal space to be filled with haemoglobin, the O2 carrying protein abundant in red cells. Without a nucleus RBC's can never divide. Lacking in organelles, they can only survive for about 120 days, before being 'eaten' by white cells. Some 3 million RBC's die and are scavenged by the liver every second.

Structure RBC's are biconcave shaped discs (as seen in this electron micrograph), about 7.5µm diameter. The shape gives an increased surface area, allowing the cell to contain more haemoglobin. The small size and large surface area both increase the rate of diffusion of O2 and CO2. The reddish colour of RBC's is derived from haemoglobin. The protein globin (containing 4 polypeptide chains) combines with haem, which contains an iron atom. One O2 molecule binds to each iron atom. Therefore, each haemoglobin molecule can bind 4 O2 molecules. Cell shape is maintained by a protein cytoskeleton. RBC's are very flexible and can squeeze through minute gaps, such as capillary walls.

Function Haemoglobin in RBC's carries O2 from the lungs and transports it to cells throughout the body. In a reversible process haemoglobin binds O2 in the lungs to form oxyhaemoglobin, which is taken to the cells where it gives up it's O2. Haemoglobin also carries CO2 from the cells, which is returned to the lungs. 95% of CO2 generated is carried by RBC's (containing the enzyme carbonic anhydrase to speed up the process). About 5% of CO2 is dissolved in blood plasma It is essential that blood pH is maintained, and haemoglobin acts as a powerful buffer in maintaining a pH of about 7.4.