5.2 Human nervous system Flashcards
Human nervous system
The nervous system protects organisms from harm by responding to changes in the environment. It does this by coordinating communication between different parts of organisms.
The human nervous system consists of:
Central nervous system (CNS) – the brain and spinal cord
Peripheral nervous system (PNS) – all of the nerves in the body
The nervous system enables humans to react to their surroundings and to coordinate their behaviour
Information is sent through the nervous system as electrical impulses – electrical signals that pass along nerve cells known as neurones
A bundle of neurones is known as a nerve.
Reflex actions
An involuntary (or reflex) response does not involve the conscious part of the brain as the coordinator of the reaction.
Awareness of a response having happened occurs after the response has been carried out.
Responses are therefore automatic and rapid – this helps to minimise damage to the body.
Some examples of reflexes are: Blinking, sneezing.
Reflex arc
A stimulus can be any change in the environment to which the body needs to respond.
The stimulus is detected by a receptor.
Receptors are found all over the body.
They detect the change in the environment and initiate a signalling process within the body.
The signal is picked up by a neurone (nerve cell).
The coordinator formulates a response (the spinal cord or the CNS).
An effector is a muscle or gland that brings about an action in response to the change in the internal or external environment.
The response can be any action that helps the organism to avoid the harmful situation.
Practical 7 (reaction time)
Aim: to plan and carry out an investigation into the effect of a factor on human reaction time.
Test - ruler drop test.
Brain
The brain is a very complex organ that controls all conscious and unconscious thoughts in order to keep an organism alive.
The brain alongside the spinal cord is part of our central nervous system.
The brain is made of billions of interconnected neurones and is responsible for controlling complex behaviours.
Within the brain are different regions that carry out different functions.
Parts of the brain
Cerebellum - This is underneath the cerebral cortex and is responsible for balance, muscle coordination and movement.
Cerebral cortex - This is the outer layer of the brain which is divided into two hemispheres. It’s highly folded and is responsible for higher-order processes such as intelligence, memory, consciousness and personality.
Medulla - This part is responsible for unconscious activities (e.g. breathing and heartbeat).
Investigating the brain
The brain is an incredibly complex and delicate organ – this makes it extremely difficult for neuroscientists to study it to find out how it works.
Our understanding is limited because the brain is so complex and different regions can’t be studied in isolation.
Our limited understanding means that treating brain damage and disease is very difficult; in addition, any potential treatment carries risks of further damage occurring which can lead to increased problems.
Accidental damage could lead to speech or motor issues, or changes to personality which are permanent.
Brain damage
The brain is delicate, complex, and not well understood.
Therefore, the treatment of brain damage and brain disease is difficult.
By studying patients with brain damage, where part of their brain doesn’t function, neuroscientists have been able to link particular regions of the brain to particular functions.
MRI scanners
MRI stands for Magnetic Resonance Imaging.
MRI scanners are very important diagnostic tools used to study the brain and other regions of the body using magnetic fields and the effect these have on protons in the water molecules of the body
Functional MRIs can produce images of different regions of the brain that are active during different activities like listening to music or recalling a memory (the scanners can detect changes in blood flow – more active regions of the brain have increased blood flow)
MRI scanners have allowed us to learn which areas of the brain are active during different activities, such as moving, speaking and listening.
Electrical stimulation
Electrical stimulation has also allowed us to treat certain disorders of the brain.
Tiny electrodes can be pushed into different parts of the brain, tiny jolts of electricity stimulate these regions and the effects can be observed.
Because the nervous system communicates using electrical impulses, electrical stimulation is used to help treat conditions such as Parkinson’s disease (causes tremors).
Spinal cord
The spinal cord is the other component (part) of the CNS. It is also important in coordinating the response of effectors to changes in the environment.
Sensory neurons
The sensory neurone, which carries the signal in the form of an electrical impulse to the central nervous system (CNS).
Sensory neurones are long and have a cell body branching off the middle of the axon.
Relay neuron
Relay neurones are found inside the CNS and connect sensory and motor neurones.
Relay neurones are short and have a small cell body at one end with many dendrites branching off it.
Motor neurons
Motor neurones carry impulses from the CNS to effectors (muscles or glands).
Motor neurones are long and have a large cell body at one end with long dendrites branching off it.
Nerve cells
Neurones (nerve cells) carry electrical impulses (signals) between receptors, the central nervous system (CNS) and effectors.
Reflex to being stabbed with a pin
The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin.
A sensory neurone sends electrical impulses to the spinal cord (the coordinator).
An electrical impulse is passed to a relay neurone in the spinal cord.
A relay neurone synapses with a motor neurone.
A motor neurone carries an impulse to a muscle in the leg (the effector).
The muscle will contract and pull the foot up and away from the sharp object (the response) when stimulated by the motor neurone.
Synapses
Neurones never touch each other, they are separated by junctions (gaps) called synapses.
Synaptic junctions are incredibly small - around 10nm in size - and electrical impulses cannot cross them.
In a reflex arc, there are synapses between the sensory and relay neurones, and the relay and motor neurones
Chemicals called neurotransmitters (such as dopamine and serotonin) are released into the synaptic cleft and diffuse across it (down a concentration gradient).
How does synapse work
The electrical impulse travels along the first axon.
When an electrical impulse arrives at the end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles.
The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane).
This stimulates the second neurone to generate an electrical impulse that travels down the second axon.
The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent.
Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions.
As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - eg this is where heroin works.
Structure of the nervous system
Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS)
The receptors detect stimuli in the environment
The CNS is the brain and spinal cord
The CNS is the coordinator that coordinates the response of effectors which may be muscles contracting or glands secreting hormones
The pathway through the nervous system is:
Stimulus - receptor - coordinator - effector - response
Adaptations of the nervous system
Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites.
Some human neurones have axons over a metre in length (but only 1 - 4 micrometres wide).
This is far more efficient than having multiple neurones to convey information from the CNS to effectors – less time is wasted transferring electrical impulses from one cell to another.
The axon is insulated by a fatty myelin sheath with small uninsulated sections along it (called nodes) which the impulse jumps along.
Retina scanning
Your retina is full of receptor cells, which are sensitive to both the brightness (light intensity) and the colour of light.
Retina scanning looks at the pattern of blood vessels in your retina to identify you.
Function of the eye
The eye is a sense organ containing receptor cells which are sensitive to light intensity and colour.
The purpose of the eye is to receive light and focus it onto the retina at the back of the eye.
There are two main functions of the eye:
Accommodation to focus on near or distant objects.
Adaptation to dim light.
Iris
The iris controls pupil diameter and the quantity of light reaching the retina. If there isn’t much light then the iris will make our pupils dilate (get bigger).
Retina
Controls the light receptor cells that detect light intensity and colour of light.
Optic nerve
Sensory neurone that carries electrical impulses from the eye to the brain.
Sclera
White outer layer which supports the structures inside the eye. It is strong to prevent damage to the eye.
Cornea
Transparent covering of the front of the eye that refracts light.
Ciliary muscles
Ring of muscles around the lens which relaxes and contracts to change the shape of the lens.
Suspensory ligaments
Work with the ciliary muscles to change the shape of the lens.
Lens
Transparent disc that changes shape to focus light onto the retina.
Vitreous humour
Fluid behind the lens that helps maintain the shape of the eye and lens. It also keeps the retina against the wall of the eye.
Aqueous humour
Fluid in front of the lens that helps maintain the shape of the eye and lens.
Fovea
A region of the retina with the highest density of cones (colour detecting cells) where the eye sees particularly good detail.
Adaptations of the eye
The eye can adapt its structures in response to light intensity.
This adaptation is a reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light.
The reflex action is controlled by two groups of muscle:
The radial muscles
The circular muscles
In dim light, the pupil dilates (widens) to allow as much light into the eye as possible
In bright light, the pupil constricts (narrows) to prevent too much light from entering the eye and damaging the retina
Adaptation of the eye
Dim light - Photoreceptors detect change in environment (dark).
Radial muscles contract.
Circular muscles relax.
Pupil dilates to allow more light to enter the eye.
Bright light - Photoreceptors detect change in environment (bright).
Radial muscles relax.
Circular muscles contract.
Pupil dilates to allow more light to enter the eye.
Accommodation of the eye
Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose.
Changing the shape of the lens alters how much light is refracted .
This is important in making sure that light is focused on the retina of the eye rather than in front or behind it.
The contraction or relaxation of the ciliary muscles brings about the changes.
Focusing on objects
Close objects - The ciliary muscles contract.
The suspensory ligaments loosen.
The lens is then thicker and refracts light rays more strongly.
Distant objects - The ciliary muscles relax.
The suspensory ligaments are pulled tight.
The lens is then pulled thin and only slightly refracts light rays.
Defects of the eye
Two common defects of the eyes are
myopia (short-sightedness).
hyperopia (long-sightedness).
In both defects rays of light do not focus on the retina
Generally, these defects are treated with spectacle lenses (glasses). Which refract the light rays so that they focus on the retina.
Myopia
Myopia is short sightedness
The lens is too thick and curved and the eyeball is too elongated so the distance between the retina and lens is too great. This causes the image to be in focus before reaching the retina. A concave lens can be used to correct light rays so they focus on the retina.
Hyperopia
Hyperopia is long sightedness
The eyeball is too short and the distance between the retina and the lens is too short. This causes the image to be in focus behind the retina. A convex lens can be used to refract light rays so they focus on the retina.
Contact lenses
Hard and soft contact lenses:
These sit on the surface of the eye and are almost invisible, making them ideal for activities like sports.
Soft lenses are more comfortable but carry a higher infection risk.
Laser eye surgery
Lasers can be used to change the shape of the cornea (changing how it refracts light onto the retina).
All surgical procedures have a risk of unexpected damage occurring during the procedure which could lead to worse vision or an infection.
For myopia: the cornea is slimmed down, reducing the refractive power.
For hyperopia: the cornea shape is changed so the refractive power is increased.
Lens replacement surgery
This surgery completely replaces the lens of the eye with a plastic artificial lens (rather than changing the shape of the cornea during laser eye surgery).
The procedure is more invasive than laser surgery and carries a risk of damage occurring to the retina leading to complete sight loss.
Monitoring body temperature.
The human body needs to maintain a temperature at which enzymes work best, around 37°C.
Processes such as respiration release energy as heat; and the body loses heat energy to its surroundings – the energy gained and lost must be regulated to maintain a constant core body temperature.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain.
The thermoregulatory centre contains receptors sensitive to the temperature of the blood.
The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre.
Increasing body temperature
If the body temperature is too low, blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver).
These mechanisms reduce heat loss to the surroundings (with skeletal muscle contraction increasing heat released in the body).
Thermoreceptors in the hypothalamus and skin detect change. The body starts to shiver, vasconstriction occurs and skin hairs. are erected which trap air (an excellent insulator) which increases body temperature.
Decreasing body temperature
If the body temperature is too high, blood vessels dilate (vasodilation) and sweat is produced from the sweat glands.
Both these mechanisms cause a transfer of energy from the skin to the environment, cooling the body down.
Thermoreceptors in the hypothalamus and skin detect change. This increases sweating, vasodilation occurs and hairs lie flat against the skin. This decreases the temperature when the sweat evaporates.
