Nervous Control in humans
There are two ways through which our body coordinates and responds:
1. Nerve impulses in nervous system
2. Hormones in endocrine system
The nervous system is made up of two major organ systems: The central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises of the brain and spinal cord. The nerves emerge out of the CNS to make the peripheral nervous system. Nerve fibers and receptors make the PNS. The nerve fibers are made up of nervous cells, known as neurons. The receptors are sensory cells. Sensory cells are present in the nose, eyes, tongue, skin, and ears. Nerve impulses are electrical signals that pass along the neurons. Dendrides are nerve fibers that take the message towards the cell body, while the axons are nerve fibers that take the message away from the cell body.
Sense organs are a group of receptor cells that respond to a specific stimuli, like light, sound, touch, etc.
The eye can move in the socket independently.
Cornea, the exposed part, is the bulge of the eye. It helps in the refraction of the light rays entering into the eye, providing an angle. Due to the presence of cornea, the light rays bend when there is a medium present. The sclera is the first layer of the eye which makes the buldge. It is the tough outer coat that protects the eye from danger. The choroid is the second layer of the eye, and it is made up of blood vessels that give the dark color to prevent the inner reflection. The blood vessels are responsible for the transport and exchange of nutrients/gases.
The retina is sensitive and the innermost part of the eye. It is made up of light receptors. It is made up of two types of cells: rods and cone cells. The cone cells, present on the fovea, are sensory to bright light, while the rod cells, present on the edges, are sensory to dim light. There are three types: red, green and blue. All of these blend in to produce an image. They are sensitive to high light intensities. The retina is concentrated with these cone cells. The rod cells are also of three types, but opposite to those of cone cells. The three types in rod cells are gray, black and white. They all blend in to produce an image, and they are sensitive to low light intensity.
The retina has two important spots, known as the blind spot and the yellow spot. Once the image passes through the cornea, the image is upside down, small and mirror image. The optic nerve transmits this message to the brain which corrects it. The yellow spot, also called the fovea, is a spot present on the retina concentrated in the cone cells. It is the part of the retina where light is focused when you look straight at an object. It tends to make the image sharper, and it lacks rod cells. The blind spot is the spot from where the optic nerve emerges out. There are no light-sensitive cells present in it, and it doesn't have the rod and come cells. Due to the lack of sensory cells in it, signals are not sent to the brain from there.
Aqueous humor is the watery fluid present behind the cornea of the eye. It provides a medium for refraction and also provides a supply of nutrients. It supports the cornea and the front chamber of the eye.
The colored part of the eye, iris, is a gate-like structure present being the aqueous humor. The iris controls the quantity of light entering the pupil. It expands and contracts to control how much light enters the eye. Iris is made up of two types of muscle: circular muscle and radial muscle. They are an antagonistic pair of muscles. At high light intensities, the circular muscle contracts and the radial muscles relax, reducing the pupil size. At low light intensities, the circular muscles relax while the radial muscles content. This dilates the pupil, allowing more amount of light to enter the eye. The pupil is a structure made by the iris when light enters the eye. It is a circular opening that lets light in the eye.
Figure (i) Structure of Human Eye, Credit: Brainly.in
A neuron is a nervous cell that helps carry impulses. It is a specialized cell that changes its shape due to its function. The cell body if the neuron contains the nucleus.
There are 3 types of neurons:
1. Sensory/Receptor neuron
2. Intermediate/Relay neuron
3. Motor/Effector neuron
It is an automatic, involuntary reaction our body provides in response to danger. The Reflex action starts from the receptors i.e. taste, sight, pressure, touch, hearing receptors. All the receptors under the skin are closely associated with the sensory neuron. If we feel a touch, receptors send messages to sensory neurons, which transmits the message to the relay neuron to the motor neuron. Lastly, it reaches the effector which respond accordingly.
There are two types of effectors:
1. Muscles: They contract and relax, for e. g. In knee jerk.
2. Glands: They release secretions, for e.g. Pancreas secretes insulin when there is high glucose concentration in the blood.
Hormones are chemical substances produced by the glands. They are always carried in the blood, and they alter the activity of one or more specific target organs.
Glands are body organs that secrete some kind of hormones or chemicals. Ductless glands in the body secrete hormones. These then diffuse into the blood in blood plasma, targeting the specific organ. They are known as endocrine glands.
Adrenaline is a hormone secreted by the adrenal gland in 'fight or flight' situations. It is secreted at times of extreme fear, happiness, anxiety, phobia or tension. It's target organs are heart lungs, eyes and the skin.
- Increased heart rate
- Increased blood glucose concentration
- Increased vasodilation in muscles
- Dilated pupils so more light can enter the eyes
- Paler skin as blood is diverted away from it
- Increased metabolic rate as more glucose reaches the body for respiration
- Increased respiration rate
These effects last for a short time. If it lasts for a long time, the person may experience headaches, stomach aches, slow motion, and high blood pressure.
Insulin and Glucagon
These are the hormones that regulate blood glucose concentration. When there is high glucose concentration in the blood, the pancreas senses the stimulus and secrete insulin which goes to the liver, changing glucose to glycogen. When there is low levels of glucose in the blood, the pancreas secretes glucagon which goes to the liver and changed the glycogen to glucose. The glucose dissolves in blood plasma and maintains sugar levels.
Diabetes Mellitus Type 1
Whenever we take in glucose, the excess is concerted to glycogen by insulin. As soon as the beta cells of the pancreas sense the increase in glucose concentration, they secrete insulin which goes to the liver. Due to certain reasons, some people cannot produce insulin. The excess glucose is excreted out with urine which has a lot of side effects.
- Excessive thirst/hunger
- Excessive urine
- A high flow of excess glucose in urine
- Sweet-smelling breath
- Heart diseases
- Cataract (Eye disease)
- Hardening of arteries
- Premature aging
- Injection of insulin through genetic engineering
- Person should decrease the intake of glucose and fatty acids.