Nervous control in humans
Human nervous system is composed of the central nervous system (CNS) and the peripheral nervous system.
The CNS is the brain and the spinal cord, whereas the peripheral nervous system is the nerves and neurons which connect the CNS to the rest of the body.
Nervous impulses are electrical signals that pass along neurons to transmit information to and from the brain.
The general nervous pathway
We have various sense organs in our body. Sense organs have the ability to detect certain sensory stimuli.
- Skin = touch i.e. pain, pressure, temperature etc.
- Eyes = light
- Nose = smell
- Ears = noise
- Tongue = taste
When these sense organs detect stimuli, they send raw information to the brain (CNS) via nervous impulses in the sensory neurons.
The brain then interprets the information. If an action is required, then the brain will send impulses to the relevant muscles via motor neurons to carry out this action. These muscles are called effectors.
Sensory receptor -> Sensory neuron -> CNS -> Motor neuron -> Effector
Example 1
Imagine putting a piece of food in your mouth. You find it to be overly disgusting and you therefore decide to spit it out.
- Your tongue is a sensory organ and it detects the taste of the food that you put into the mouth.
- Nerve impulses from the tongue is transmitted to the brain (via sensory neurons)
- The brain then interprets this information. It is at this level that you feel that the food is disgusting.
- The brain transmits nerve impulses (via motor neurons) to the tongue and mouth muscles, instructing it to spit the food out
Tongue -> Sensory neuron -> CNS -> Motor neuron -> Tongue/mouth muscles
This is an example of a voluntary action i.e. actions which are a result of a conscious decision by the brain.
Example 2
“Imagine touching a very hot pan, causing you to IMMEDIATELY take your hands off it.”
- The skin (sense organ) detects high heat
- Instead of nerve impulses being sent to the brain and awaiting it’s response, the information is transmitted straight from the sensory neurons to the motor neurons
- Motor neurons then transmit impulses to the hand/arm muscles
- The muscles immediately take the hand off the pan
This is a prime example of a reflex arc. If we had awaited the brain to tell the muscles to take our hands off the high heat, then it would take too long.
Instead, we are instinctively able to carry out this action WITHOUT the brain telling us to do so because of this reflex arc.
Tongue -> Sensory neuron -> Motor neuron -> Tongue/mouth muscles
This is an example of an involuntary action i.e. an action which is instinctive and not by choice.
Neurons
There are three different types of neurons that you need to be aware of.
Sensory neurons
Sensory neurons carry impulses from sense organ to the brain (CNS).
Motor neurons
Motor neurons carry information from the brain to the effector
Relay neurons
Relay neurons connect a sensory neuron to a motor neuron in a reflex arc
Reflex arc
As we discussed above, a reflex arc allows an immediate response to certain stimuli by bypassing the brain (CNS).
Synapses
A synapse is a junction between two neurons.
The function of a synapse is to ensure a single direction of neuronal transmission.
Consider impulses passing from neuron A to neuron B. The space between the two neurons is the synapse.
- Neuron A = pre-synaptic neuron
- Neuron B = post-synaptic neuron
The presynaptic neuron (neuron A) has vesicles which contain neurotransmitters. When electrical impulses reach the pre-synaptic neuron (neuron B), these vesicles release the neurotransmitters into the synapse.
The neurotransmitters then diffuse across the synapse and bind to specific, corresponding receptors on the post-synaptic neuron.
This allows the electrical impulses to be transmitted to, and carried along neuron B (post-synaptic neuron).
The human eye
Structure and function
It is important to understand how the eyes work.
- Light rays hit the cornea
- The cornea refracts (bends) light
- Light passes through the pupils
- The lens refracts light further to focus it onto the fovea
- The fovea contains light receptors that transmit electrical impulses to the brain
- The brain interprets these impulses and gives us what we call ‘vision’
- Cornea – Refracts light
- Lens – Focuses light onto the retina
- Iris – Controls pupil size
- Pupil – Controls the amount of light entering the eye
- Optic nerves – Carries impulses to the brainn
- Retina – Contains light receptors called rods (responsible for night vision) and cones (responsible for colour vision)
- Fovea – An area of the retina where most of the light is focused on, and has a very high density of cones
Pupil reflex
Light enters the light via the pupils. While too much light can damage the retina, too little light makes it very difficult to see.
The pupil reflex is designed to appropriately control the level of light that entering the eyes. They become smaller in bright conditions and larger in dim conditions.
There are two muscles of the iris that control the diameter of the pupils.
- Circular muscles
- Radial muscles
Accommodation
Accommodation is how the lens changes shape in order to fine focus the light directly onto the fovea.
It does so by bending/refracting incoming light rays so that it converges onto the fovea.
- Light rays from a distant object are parallel so it takes less effort for the lens to focus it
- Light rays from a close object are divergent so it takes more effort for the lens to focus it
For a distant object, ciliary muscles are relaxed. This allows suspensory ligaments to pull tightly on each side of the lens. The lens therefore becomes stretched and thin. A thin lens has less power, and therefore bends light less.
For a near object, ciliary muscles are contracted. This allows the suspensory ligaments to loosen. The lens thus becomes thicker. A a thicker lens has more power, and therefore bends light more.
Hormones in humans
Hormones are defined as a chemical substances produced by an endocrine gland and carried by blood which alters the activity of one or more specific target organs.
Examples
- Adrenal gland secretes adrenalin
- Pancreas secretes insulin & glucagon
- Testes secrete testosterone
- Ovaries secrete oestrogen
Adrenaline
Adrenaline is a hormone secreted in fight or flight situations.
It increases blood supply to the necessary parts of the body to cope with a dangerous situation.
Adrenaline has multiple effects on the body:
- Increased breathing rate
- Increased heart rate
- Secretion of glucose from the liver
- Pupil dilation
Nervous system vs hormonal system
There are some key differences between the nervous system and the hormonal system.
Hormonal control:
- Chemical transmission
- Blood pathway
- Slow speeds of transmission
- Long-term effect
Nervous control
- Electrical transmission
- Nervous pathway
- Fast speeds
- Short-term effect
Homeostasis
The definition of homeostasis is the maintenance of a constant internal environment.
Positive and negative feedback loops control internal conditions within set limits.
- For example, our optimum body temperature is approximately 37°C – This is the SET VALUE/LIMIT
- If temperature goes beyond 37 then negative feedback brings it back down
- If temperature falls below 37 then positive feedback brings it back up
Control of sugar levels
Blood sugar levels are controlled by two hormones that are secreted by the pancreas
- Insulin – Causes liver to use blood glucose and covert it to glycogen for storage inside liver cells
- Glucagon – Causes the liver to convert glycogen to glucose and release it into the blood
Essentially, insulin causes blood sugar levels to fall and glucagon causes blood sugar levels to rise.
When glucose levels are too high, negative feedback will reduce it back down
- Pancreas secretes insulin
- Liver converts blood glucose to glycogen
- Blood sugar levels fall
When glucose levels fall too low, positive feedback will increase it back up
- Pancreas halts insulin secretion
- Pancreas secretes glucagon instead
- Liver converts glycogen to blood glucose
- Blood sugar levels rise
Symptoms & treatment of type 1 diabetes
Type 1 diabetes is caused by insufficient insulin production.
As you may imagine, this leads to a very high blood glucose level.
The symptoms of type 1 diabetes include
- Increased hunger
- Frequent urination
- Blurred vision
- Tiredness & fatigue
- Unexplained weight loss
The management of type 1 diabetes include
- Controlling sugar intakes
- Monitoring sugar levels
- Insulin injections
Maintenance of internal body temperature
Body temperature is maintained at 36 degrees. The body loses heat when the environment is too hot, and retains heat when the environment is too cold.
The brain has blood temperature receptors that can modulate the temperature accordingly via certain mechanisms.
Skin structure
Mechanisms to retain heat
- Basic insulation
- The fat layer in the skin acts as an insulator
- Skin hair follicles stand up to trap a layer of air around the skin which is also an insulator
- Shivering
- Increased metabolism in muscles increase heat circulation
- Vasoconstriction
- Heat is carried in the blood
- If blood goes near the skin surface, then heat radiates out of the body
- Constriction of the skin arterioles reduce the amount of blood flowing near the skin surface to retain heat in the blood
Mechanisms to lose heat
- Sweating
- Sweat is a mixture of water, salt and urea
- Water evaporates from sweat which causes the skin (and body) to cool down
- Vasodilation
- Dilation of skin arterioles increase the amount of blood flowing near the skin surface to allow more heat to radiate out of the body
Tropic responses
Auxins
Auxins are plant growth substances. They are produced by the tips of root shoots and plants.
Auxins will always move away from sunlight and towards gravity.
- In the shoots, auxins promote plant growth
- In the roots, auxins inhibit plant growth
Gravitropism
Gravitropism is the response in which parts of the plant react to gravity.
- Positive geotropism = Growth towards gravity
- Negative geotripism = Growth away from gravity
Positive geotropism
Positive geotropism is when a plant grows towards gravity (i.e. in roots)
In the absence of light, if a root is placed horizontally, then auxins will accumulate on the lower side due to gravity.
Auxins inhibit cell growth in roots, so therefore the lower half of the root will grow slower than the top. This uneven cell growth causes the root to bend towards the ground.
Negative geotropism
Negative geotropism is when a plant grows away from gravity (i.e. in shoots)
In the absence of light, if a shoot is placed horizontally, then auxins will once again accumulate on the lower side due to gravity.
In shoots, the auxins promote cell growth. Therefore the lower side of the shoot will grow quicker the top. This results in the shoot bending away from the ground.
Phototropism
Phototropism is the response in which plants react to light.
Positive phototropism
Positive phototropism is when a plant grows towards sunlight (i.e. in shoots).
When light is exposed to one side of a shoot, auxins move away and accumulate on the shaded side.
Auxins promote cell growth in shoots, so the shaded side grows quicker than the exposed side. This results in the shoot bending towards the sun.
Negative phototropism
Negative phototropism is when a plant grows away sunlight (i.e. in roots).
When light is exposed to one side of a root, again, auxin once again accumulate on the shaded side.
Auxins inhibit cell growth in roots, so therefore the shaded side grows slower than the exposed side. This results in the root bending away from the sun.
Synthetic hormone 2,4-D
This synthetic hormone is similar to auxins. Spraying these one plants causes uncontrolled growth which can kill the plants. They are therefore used as weedkillers, and are also selective. This means that they work better on some plant species than others.