The nervous system forms a communication and coordination system throughout an animal’s body and it is essential to the survival of living things. The nervous system can be broken down into two further sections – the central nervous system and peripheral nervous system. The central nervous system includes the brain and the spinal cord while the peripheral nervous system includes the cranial nerves from the brain, spinal nerves from the spinal cord and the receptors. The peripheral nervous system can be further broken down into different sections- the somatic nervous system and the autonomic nervous system. The autonomic nervous system is further simplified to the sympathetic division and the parasympathetic division.
The method at which the nervous system would work is pretty straight forward.
- First, the body would detect a change – the stimulus
- Following via the nerve endings of the sensory neurones, the message is sent to the central nervous system.
- The CNS would receive and process the information and decide the response
- Response would be sent via motor neurones to the effector where it would be carried out.
In general, there are three different types of neurones – motor neurones, interneurones and sensory neurones. These different neurones each have different abilities and functions.
- The motor neurones have many fine dendrites which carries impulses towards the cell body through a single long axon which carries impulses away from the cell body.
- The interneurones have many short fibres and their main function is to relay messages across a long distance.
- The sensory neurones lastly have a single long Dendron which brings impulses towards the body and a single long axon which carries the impulses away from the CNS.
Neurones all have very special structure or adaptations which would allow them to complete their job better and be more efficient in terms of conveying messages.
- The dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body.
- The axon is a fibre that transmits signals towards other neurons
- The myelin sheath is an insulator that provides resistance to current flow between the axon membrane and the fluid surrounding the axon – these cells are called support cells or neuroglia cells (Schwann cells).
- The nodes of Ranvier are breaks in the myelin sheath which help to pass the action potentials faster along the axon by saltatory conduction
The presence of the myelin sheath affects the speed of transmission of the action potential. The junctions in the sheath are spaced at intervals of 1-2 mm. Only at these nodes is the axon membrane exposed. Elsewhere along the fiber, the electrical resistance of the myelin sheath prevents depolarization so the action potentials are forced to jump from node to node. This is called saltatory conduction and it greatly increases the rate of transmission. However, even with non-myelinated fibres, it is possible to speed up the passage of action potential. Large diameter axons transmit action potentials much more speedily than do the narrow ones.
There is a disease called multiple sclerosis which is the gradual degradation of the myelin sheath that takes place, rendering the axon improperly myelinated or totally bare. These would result in the interference and slowing down of the rate at which impulses are transported towards and away from the CNS.
The neurones within the nervous system transmit impulses along pathways called the reflex arc. A generalized reflex arc is as shown in Fig 2. The reflex arc connects a sense organ with a muscle or gland via the neurones.
- Firstly, the sense organ detects a stimulus which is a form of energy such as sound, light or mechanical pressure which is converted into an impulse in the nerve fiber of a neurone that serves the sense cell.
- Once generated, the impulse travels along the fibers of the sequence of neurones of the reflec arc to an effector organ.
- When it arrives at the effector, the impulse causes a response, for example flexing a muscle.
The simplest form of response in the nervous system is called a reflex action. It is a rapid automatic, but short lived response to a stimulus. It is an involuntary response as it is not generally controlled by the brain’s decision makin centers. In a reflex action, a particular stimulus produces the same automatic response everytime, for example jerking your hand away from scalding water.
In verterbrates, and particularly in mammals, there is a complex nervous system. Within the nervous system there are many reflex arcs and they are all connected to the control center – the brain. The brain is a highly organized mass of interneurones connected to the rest of the nervous system by motor and sensory neurones. With a nervous system of this type, complex patterns of behavior are common, in addition to reflex actions. This is because – impulses that originate in a reflex arc also travel to the brain and the impulses may originate in the brain and be conducted to effector organs. Consequently, much activity is initiated by the brain, rather than merely being a response to an external stimuli. Also, many reflex actions may be overruled by the brain, and the response modified – for example not dropping a hot object because of its value. Therefore we can deduce that the nervous system has two roles.
- Quick and precise communication between the sense organs that detect stimuli and the muscles or glands that respons with changes.
- Coordination and ontrol of the body’s responses by the brain.
Impulses are transmitted along nerve fibers, but it is not an electrical current that flows along the wires of the nerves. Rather, the impulse is a momentary reversal in the electrical potential difference in the membrane of the fibers. That is, it is a change in the position of positive and negative charged ions between the inside and outside of the membrane. This reversal travels from one end of the neurone to the other in a fraction of a second. Between impulses the neurone is said to be ‘resting’. Actually, this is far from case as the ‘resting’ intervals between impulses the membrane of a neurone actively creates and maintains an electrical potential difference between its inside and outside.
Two processes together create the resting potential difference across the neurone membrane.
- There is active transport of potassium ions into the cell and sodium ions out of the cell across the membrane. The ions are transported by potassium and sodium pumps which uses energy from ATP/ So potassium and sodium gradually concentrate on the opposite side of the membrane, however this in itself make no change to the potential difference
- There is also facilitated diffusion of potassium and sodium ions back in. The important point here is that the membrane is far more permeable to potassium flowing out than to sodium ions returning. This causes the tissue fluid outside the neurones to contain many more positive ions than the cytoplasm inside. As a result, the inside becomes more and more negatively charged as compared with the outside. This resting neurone is said to be polarised. The difference in charge or potential difference is about -70mV. This is known as the resting potential.
The action potential, the next event, sooner or later is the passage of an impulse. An impulse or an action potential is triggered by a stimulus arriving at a receptor cell or sensitive nerve ending. The energy of this stimulus causes a temporary and local reversal of the resting potential. The result is that the membrane is briefly depolarized.
The change in potential across the membrane happens through pores in the membrane, these are called ion channels because they allow ions to pass through. One type of channel is permeable to sodium ions and the other is permeable to potassium ions. These channels are globular proteins that span the entire width of the membrane. They have a central pore, with a ‘gate’ that can open and close. During resting potential all these channels are closed.
- The energy of the stimulus opens the gates of the sodium channel in the plasma membrane. This allows the sodium to quickly diffuse in, down their electrochemical gradient. So the cytoplasm inside the neurone fibre quickly becomes progressively more positive and with respect to the outside. This charge reversal continues until the potential difference has altered from the outside. This charge reversal continues until the potential difference has altered from -70 mV to +40 mV. At this point, an action potential then travels along the whole length of the neurone fibre. At any point of the fibre, it exists for only two-thousandth of a second, before the membrane starts to re-establish the resting potential; therefore, action potential transmission is exceedingly brief.
Almost immediately after an action potential has passed, the sodium channels close and the potassium channels open so that potassium ions can exit the cell, again down their electrochemical gradient into the tissue fluid outside. This causes the interior of the neurone fibres to become less positive again. Then the potassium ion channels also closes. Finally the resting potential of -70 mV is re-established by the action of the sodium/potassium pump and the process of facilitated diffusion.
The Refractory period
For a brief period after the passage of an action potential, the neurone fibre is no longer excitable. This is called the refractory period. It lasts only 5-10 milliseconds in total. During this time, firstly, there is a large excess of sodium ions inside the neurone fibre and it is impossible for more to enter. As the resting potential is progressively restored, however, it becomes more possible for an action potential to be generated again. Because of the refractory period, the maximum frequency of impulses is 500 – 1000 per second.
The all or nothing principle
Obviously the energy carried by various stimuli may be widely different – think of the difference between a light touch and the pain of a finger hit by a hammer. A stimulus must be at or above a minimum intensity, known as the threshold of stimulation, in order to initiate an action potential. Either a stimulus depolarizes the membrane sufficiently to reverse the potential difference in the cytoplasm fully – from -70 m to +40 mV – or it does not. If not, no action potential at all arises and no message is sent along the fiber. With all sub-threshold stimuli, the influx of sodium ion is quickly reversed, and the resting potential is re-established. However, as the intensity of the stimulus increases, the frequency at which the action potential passes along the fibre increases. For example, with a very persistent stimulus, action potentials pass along a fibre at an accelerated rate, up to the maximum possible permitted by the refractory period. This means that the effector recognizes the intensity of a stimulus from the frequency of the action potentials.
Junctions between neurones
The synapse is the point where the ends of two neurones meet. It consists of a swollen up synaptic knob of the axon of one neurone – presynaptic neurone and the dendrite or cell body of the next neurone. At the synapse, the neurones are extremely close, but they are not in direct contact. Between them there is a tiny gap called the synaptic cleft, about 20 nm wide. Because there is a gap here the action potential cannot cross it. Here, another form of transmission must carry the impulse. Transmission across the synaptic cleft is not electrical, but chemical. The impulse is carried from one side of the gap to the other side by specific chemicals known as transmitter substances. These substances are all relatively small, diffusible molecules. They are produced in the Golgi apparatus in the synaptic knob and are held in tiny vesicles prior to use.
Acetylcholine is a commonly occurring transmitter substance; the neurones that release acetylcholine are known as cholinergic neurones. Another common transmitter substance is noradrenalin. The brain uses the transmitter glutamic acid and dopamine among others.
- At the arrival of an action potential at the synaptic knob opens calcium ion channels in the presynaptic membrane, and calcium ions enter from the synaptic cleft.
- The calcium ions cause vesicles of transmitter substance to fuse with the presynaptic membrane, releasing the transmitter substance into the synaptic cleft.
- The transmitter substance diffuses across the synaptic cleft. In the postsynaptic membrane thee are specific receptor sites containing a receptor protein for each transmitter substance. Each of these receptors also acts as a channel in the membrane for a specific ion.
- As a transmitter molecule binds to its receptor protein, this instantly opens the ion channel, allowing specific ions to pass through.
For instance when a molecule of acetylcholine attaches to its receptor site, a sodium ion channel opens. As the sodium ions rush into the cytoplasm of the post synaptic neurone, depolarization of the postsynaptic membrane occurs. As more and more molecules bind to their receptors, it becomes increasingly likely that this depolarization will reach the threshold level. When it does, an action potential is generated in the postsynaptic neurone. The process of building up to an action potential in postsynaptic membrane is called facilitation.
Meanwhile, the transmitter substance on the receptors is immediately deactivated by enzyme action, causing the ion channel of the receptor protein to close again. The resting potential in the postsynaptic neurone can then be re-established. Meanwhile, the inactivated form of the transmitter diffuses back across the gap, re-enters the presynaptic knob, is resynthesized into transmitter substance, and packaged for reuse.
The vertebrate brain develops in the embryo from the anterior ends of a simple tube called the neural tube. This tube enlarges to form three primary structures known as the forebrain, midbrain and hindbrain. The various parts of the mature brain develop from the selective thickening and folding of the walls and the roof. These enlargement processes are most pronounced in mammals and a striking feature is the enormous development of the cerebral hemispheres which are an outgrowth of the forebrain. When tissues inside the brain are examined, the parts where cell bodies are grouped together appear grey, so they are known as ‘grey matter’. Areas where myelinated nerve fibers occur together appear whiter, so they are called white matter. White and grey matters are present in both the brain and the spinal cord. Grey matter makes up the interior of the brain and white the exterior. However, in the cerebral hemisphere and cerebellum, there are additional layers of grey matter.
Blood capillaries are also present throughout the nervous tissue. However, in the brain, the capillary walls form a barrier against many of the dissolved substances in the blood. This means that only essential substances such as oxygen and glucose can pass through. This is called the blood/brain barrier. Other substances dissolved in the plasma that might affect the brain’s neurotransmitters in brain synapses. This barrier is therefore important for maintenance of normal brain functions.
The human brain controls all body functions, apart from those under the control of simple spinal reflexes mentioned earlier. According to current understanding of brain functions, it achieves this by
- Receiving impulses from sensory receptors
- Integrating and correlating incoming information in association centres
- Sending impulses to effector organs causing bodily responses
- Storing information and building up an accessible memory bank
- Initiating impulses from its own self-contained activities, ‘personality’ and emotions, and enable us to imagine, create, plan, calculate, predict and abstractly reason.
Within the brain, it is segmented into different parts which would handle different tasks.
- The hypothalamus is a part of the forebrain which is exceptionally well supplied with blood vessels, monitor and controls body temperatures and the levels of sugar, amino acids and ions. Feeding and drinking reflexes and aggressive and reproductive behaviours are also controlled here. It works with the pituitary gland to control the release of hormones.
- The cerebral hemisphere is an extension of the forebrain that forms the bulk of the brain and coordinates most of the body’s voluntary actions, together with many involuntary ones. These hemispheres have a vastly extensive surface which is achieved by extensive folding of the surface such that it forms deep grooves.
- The cerebellum, part of the hind brain has an external surface of grey matter. It is concerned with the control of involuntary muscle movements of posture and balance. Here, precise and voluntary manipulations including hand movements, speech and writing are coordinated.
- The medulla, the base of the hind brain, houses the regulatory centres concerned with maintaining the right heart rate, ventilation of the lungs and temperature. In the medulla, the ascending and descending pathways of nerve fibres connecting the spinal column and brain cross over. As a consequence, the left side of our body is controlled by the right side of the brain vice versa.
In addition to trauma, various neurological disorders such as depression and Alzheimer’s disease can also affect brain function. Nearly 20 million American adults are affected by depression; about two thirds of them are women. Two broad forms of depressive illness have been identified: major depression and bipolar disorder.
- People identified with major depression may experience persistent sadness, loss of interest in pleasurable activities, changes in body weight and sleep patterns, loss of energy and suicidal thoughts. While all of us feel sad from time to time, major depression is extreme and more persistent, leaving the sufferer unable to live a normal life. If left untreated, symptoms may become more frequent and severe over time.
- Bipolar disorder or manic-depressive disorder, involves extreme mood swings. The manic phase is characterized by high self-esteem, increased energy, a flood of thoughts and ideas and extreme talkativeness, as well as behaviours that often court disaster, such as increased risk taking, promiscuity, and reckless spending. In its milder forms, this phase is sometimes associated with great creativity, and some well-known artists, musicians and literary figures with bipolar disorders. The depressive phase is marked by sleep disturbances, feeling of worthlessness, and decreased ability to experience interest and pleasure.
Alzheimer’s disease is a form of mental deterioration or dementia, characterized by confusion, memory loss and a variety of other symptoms. Its incidence is often age related, rising from about 10% at age 65 to about 35% at age 85. This disease is progressive as patients become gradually become less able to function and eventually need to be dressed, bathed and fed by others.
The spinal cord is a cylindrical structure with a tiny central canal. The canal contains cerebrospinal fluid and I continuous with the fluid-filled spaces – ventricles – in the center of the brain. The cerebrospinal fluid brings nutrients to the spinal cord and helps to cushion the CNS. The cord consists of an inner area of grey matter surrounded by white matter. The spinal cord is surrounded and protected by the vertebrae of the backbone, with 31 pairs of spinal nerves that emerge at regular intervals along the length of the spinal cord. The spinal nerves bifurcate into two – the ventral and dorsal roots. The central root contains only motor neurones and the cell bodies of these are found in the grey matter of the spinal cord. The dorsal root only contains sensory neurones. Cell bodies of the sensory neurones in the dorsal root aggregate in a small swelling called the dorsal root ganglion.
In the junction between each pair of vertebrae, two spinal nerves leave the cord, one to each side of the body. The role of the spinal cord is to relay action potential between sensory organs and effector organs of the body, and also between them and the brain, so that the reflex action may be overridden, for example the toleration of a hot object.
Peripheral Nervous System
The vertebrate nervous system is divided into two functional components: the somatic nervous system and the autonomic nervous system. Neurones of the somatic system carry signals to and fro the skeletal muscles, mainly in response to external stimuli. When you walk for instance, these neurones carry commands to make your legs move.
- The somatic nervous system is said to be voluntary because many of its actions are under conscious control. In contrast, the motor neurones of the autonomic nervous system regulates the internal environment by controlling smooth and cardiac muscles and the organs and glands of the digestive, cardiovascular, excretory and endocrine system.
- The autonomic nervous system contains two sets of neurones with opposing effects on most body organs.
- One set, called the parasympathetic division, primes the body for activities that gain and conserve energy for the body. These effects include stimulating the digestive organ, decreasing the heart rate and narrowing the bronchi, which correlates with a decreased breathing rate.
- The other set of neurones called the sympathetic division tends to have the opposite effect, preparing the body for intense energy consuming activities such as fighting, fleeing or competing in a strenuous game. When the division is stimulated, the digestive organs are inhibited, the bronchi dilate so that more air can pass through and the adrenal glands secrete hormones epinephrine and norepinephrine.
Relaxation and the fight-or-flight response are opposite extremes. Your body usually operates somewhere in between, with most of your organs receiving both sympathetic and parasympathetic signals. The opposing signals adjust an organ’s activity to a suitable level. As carriers of command signals, the motor neurones of the parasympathetic and sympathetic systems constitute lower levels of the nervous system’s hierarchy.
The 5 Senses
Sensory input is the process of using receptors to sense the environment and send information about it to the CNS (central nervous system). Sensory input is the process of using receptors to sense the environment and send information about it to the CNS to be integrated and acted upon. Sensory receptors, such as the sensory cells in the eyes and taste buds of your tongue are tuned to the condition of both external world and internal organs. Sensory receptors detect stimuli such as chemicals, light, sound, cold, heat and touch. The sensory receptors in the eyes for instance detect light energy. In the process of sensory transduction, for example in a tongue:
- When sugar molecules first come into contact with the taste-buds, they bind to membrane receptors of the sensory receptor cells
- This would trigger a signal transduction pathway that causes some ion channels in the membrane to close and others to open, changing the flow of ions and alters the membrane potential.
- Each receptor cell forms a synapse with a sensory neuron. When there are enough sugar molecules, a strong receptor potential is triggered, making the cell release enough neurotransmitter to increase the rate of action potential generation in the sensory neurone.
Based on the type of signals to which they respond, we can group sensory receptors into five general categories – Pain receptors, thermo-receptors, mechanoreceptor, chemoreceptor and electromagnetic receptors. These five types of receptors work in various combinations to create the five human senses.
- Pain receptors may respond to excessive heat or pressure or to chemicals released from damaged or inflamed tissues. Prostaglandins are local regulators that increase pain by sensitizing pain receptors. Aspirin and ibuprofen reduce pain by inhibiting prostaglandin synthesis.
- Thermo-receptors in the skin detect either heat or cold. Other temperature sensors located deep in the body monitor the temperature of the blood. The body’s thermostat is the hypothalamus. Receiving action potentials both from surface sensors and from deep sensors, the hypothalamus keeps a mammal’s or bird’s body temperature within a narrow range.
- Mechanoreceptors are highly diverse. Different types are stimulated by different forms of mechanical energy such as touch and pressure. All these forces produce their effects by bending or stretching the plasma membrane. When the membrane changes shape, it becomes more permeable to positive ion and the mechanical energy of the stimulus is transduced into a receptor potential.
- Chemoreceptors include the sensory cells in our nose and taste buds which are attuned to chemicals in the external environment, as well as some internal receptors that detect chemicals in the body’s internal environment. Internal chemoreceptors include sensors in our arteries that monitor our blood, with some sensors detecting changes in pH and other detecting changes in oxygen concentrations.
- Electromagnetic receptors are sensitive to energy of various wavelengths, which takes such forms as magnetism and light. For example, photoreceptors detect the electromagnetic energy of light, and the nonhuman sense of electroreception and magneto-reception rely on other types of electromagnetic receptors.
The structure of the human eyeball is very complex.
- There is first an outer layer of connective tissue called the sclera, at the front of the eye, the sclera becomes the transparent cornea
- The sclera surrounds a pigmented area called the choroid, and at the front of the eye, the choroid forms the iris
- The opening in the center of the iris is called the pupil
- Behind the pupil is the dislike lens which is held in position by ligaments
- At the back of the eyeball is the retina, a layer just inside the choroid that contains the photoreceptor cells
- The optic nerve connects the retina with then brain
- There are two fluid chambers that make up the bulk of the eye, the large chamber behind the lens that is filled with jelly-like vitreous humor and a smaller chamber in front of the lens that contains a thinner liquid called aqueous humor
- A thin mucous membrane called the conjunctiva helps keep the inner surface of the eyelids moist. The conjunctiva lines the inner surface of the eyelids and then folds back over the white of the eye.
- A gland above the eye secretes tear, a dilute salt solution that is spread across the eyeball by blinking and that drains into the ducts leading to the nasal cavities. The eyelid helps to spread the moisture over the eyeball so as to prevent it from drying.
The human eye is a remarkable sense organ that is able to detect a whole multitude of colors.
- The cornea lets light into the eye and also helps focus the light.
- The muscles of the iris regulate the size of the pupil, controlling the amount of light that enters.
- After going through the pupil, light passes through the lens. The lens focuses light onto the retina by bending light rays
- Focussing is done by changing the shape of the lens, the thicker the lens, the more sharply it bends light. The shape of the lens is controlled by muscles attached to the choroid
- When the eye focuses on a nearby object, these muscles contract, pulling the choroid layer of the eye inwards, towards the pupil, making the ligaments that suspend the lens slacken
- A blind spot exists for each eye; however it is not possible for a ray of light to fall on both blind spots simultaneously.
The human retina contains two types of photoreceptors, the rods and the cones. Rods are extremely sensitive to light and enable us to see in dim light, though only shades of grey. Cones are stimulated by bright lights and can distinguish color, but they contribute little to night vision. In humans, rods are found in greatest abundance at the outer edges of the retina and are completely absent from the center. In contrast, the retina’s center of focus contains a high concentration of cones.
- Rods contain a visual pigment called rhodopsin, which is derived from Vitamin A
- Cones contain photopsins which absorb coloured bright light. We can perceive a great number of colours because light from each of the colours trigger a unique pattern of stimulation among the three different types of cones – red, green, blue.
When staring at something near to the eye, the eye would automatically adjust in order to allow you to see things clearly. Firstly, the ciliary muscles would contract and the suspensory ligament would slacken, puller the lens, causing it to become thicker and more convex. In the case of looking at things that are far away on the other hand, the opposite would occur the ciliary muscle would relax. The light would thus be refracted and be focused on the retina. Three of the most common visual problems are near sightedness and astigmatism. All three are focusing problems that are easily corrected with artificial lenses.
- People with near sightedness cannot focus well on distant objects, although they can see well at short distances. A near-sighted eyeball is longer than normal. The lens cannot flatten enough to compensate, it focuses distant objects in front of the retina instead of on it.
- Farsightedness occurs when the eyeball is shorter than normal, causing the lens to focus images behind the retina.
Humans and most predators have two eyes, one located on each side of the face. The image that each will see is slightly different because each eye views an object from a different angle. The slight displacement of the image permits binocular vision, the ability to perceive three-dimensional objects and to sense depth.
The ear is composed of three regions – the outer ear, the middle ear and the inner ear. The outer ear consists of a flap-like pinna – the bendable structure that we commonly refer to as our ear – and the auditory canal. A sheet of tissue called the eardrum separates the outer ear from the middle ear. Both outer ear and middle ear are common sites of childhood infections.
The middle ear contains three small bones: the hammer, the anvil and the stirrup. The stirrup is connected to the inner ear through an opening in the skull bone. The Eustachian tube conducts air between the middle ear and the back of the throat, allowing air pressure to stay equal on either side of the eardrum. This tube is what enables you to move air in and out in order to equalize pressure when changing altitude.
The inner ear consists of fluid-filled channels, the cochlea, and a long coiled tube. Our actual hearing organ, the organ is the Corti which is located within a fluid-filled canal inside cochlea. The organ of Corti consists of an array of hair cells embedded in a structure called the basilar membrane. The hair cells are receptor cells of the ear.
- The ear functions through the process of picking up sound waves in the surrounding air by which would be channelled to the eardrum by the pinna and the auditory canal.
- From the eardrums, the vibrations are concentrated as they pass through the hammer, the anvil and the stirrup.
- The stirrup would transmit the vibrations to the inner ear, producing pressure waves in the fluid within the cochlea.
- As the pressure waves pass through the cochlea, it makes the basilar membrane vibrate.
- Vibration of the basilar membrane makes the hair like projections on the hair cells alternately brush against and draw away from the overlying membrane. When a hair cell’s projection is bent, ion channels on its plasma membrane open and positive ions enter the cell, causing the hair cell to develop a receptor potential and release neurotransmitter molecules at its synapses
- In turn, the sensory neurone sends action potentials through the auditory nerves to the brain
Deafness, the loss of hearing, can be caused by the inability to conduct sounds, resulting from middle ear infections, a ruptured eardrum, or the stiffening of the middle ear bones. Deafness can also result from the damage of the receptor cells or neurones. Few parts of our anatomy are more delicate that the organ of Corti, deafness is often progressive and permanent. Frequent or prolonged exposure to very loud sounds can damage or destroy hair cells of the inner ear.
Muscles are basically made up of a hierarchy of smaller and parallel strands. Each muscle fiber is a single cell with any nuclei. Each muscle fiber itself is a bundle of smaller myofibrils. Skeletal muscles are also called striated (striped) muscles because the myofibrils exhibit alternating light and dark bands when viewed under the microscope. A myofibril is a continuous repeat of units called sarcomeres. Structurally, a sarcomere is a region between two dark, narrow lines in the myofibril. Functionally, the sarcomere is a contractile apparatus in a myofibril – the muscle fiber’s fundamental unit of action. During contraction of the muscle, the filaments overlap in the middle of the thick and thin filaments. A muscle can shorten to about one third of its resting length when all its sarcomeres are contracted. The process that makes the thin filaments slide requires energy that is provided by ATP.
- ATP binds to a myosin head, causing the head to detach from a binding site on actin.
- The myosin head gains energy from the breakdown of ATP and changes shape, into a high energy position
- The energizes myosin head binds to an exposed binding site on actin
- The molecular event that actually causes sliding are the power stroke. The myosin ends back to its low energy position, pulling the thin filament towards the centre of the sarcomere
- After the power stroke, the whole process repeats.
The sarcomeres of a muscle fiber do not contract on their own and they must be stimulated by motor neurones. A typical motor neurone can stimulate more than one muscle fiber because each neurone has many branches.
The function of the skeletal system is to provide a rigid internal framework and maintain the shape of the body. The skeletal structures also help to protect the important organs in a body from injury. The brain for example is protected by the skull. The bones also provide points of attachment of skeletal muscles which would pull the bones when contracting, producing movement. The bones marrow within the bones is also in charge of the production of red and white blood cells. Altogether there are three different types of joints – immovable, partially moveable and free moveable.
- The immovable joints are like those present in the cranial bones as the skull bones are fused together at the sutures. These bones are generally in place to protect fragile and important organs of the body.
- The partially moveable joints are joints which allows for only limited movement. These joints include the gliding joints between bones of the vertebral column.
- Free moving joints finally are joints which has the greatest flexibility. It allows for a 360 degrees movement in all planes. Joints like this include the hip joint and the shoulder joint. These joints are also called the synovial joints. This is due to the synovial fluid which is present in the joints. The fluid helps to lubricate the joints, reducing friction when they move against each other, and provide nutrients for the cartilage of at the ends of the bones.
Skeletal muscles are usually a voluntary muscle which is under conscious control; however, it can be involuntary in reflex actions.
Voluntary and Involuntary
Attached to bone
Forms bulk of heart wall
Walls of internal organs
Number of nuclei
Many per cell
One per cell
One per cell