2J. Human Response and Coordination

2.80 understand how organisms are able to respond to changes in their environment

2.81 understand that homeostasis is the maintenance of a constant internal environment, and that body water content and body temperature are both examples of homeostasis

2.82 understand that a co-ordinated response requires a stimulus, a receptor and an effector

Humans

2.86 describe how nervous and hormonal communication control responses and understand the differences between the two systems

2.87 understand that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

2.88 understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses

2.89 understand the role of neurotransmitters at synapses

2.90 describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

2.91 describe the structure and function of the eye as a receptor

2.92 understand the function of the eye in focusing on near and distant objects, and in responding to changes in light intensity

2.93 describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation

2.94 understand the sources, roles and effects of the following hormones: adrenaline, insulin, testosterone, progesterone and oestrogen

2.95B understand the sources, roles and effects of the following hormones: ADH, FSH and LH

This is a very large unit! I personally split it into:

Reflexes+CNS
Homeostasis
Hormones

to make it less confusing when revising.

It’s important you can label these diagrams without any stimuli. Occasionally there are exam questions where you’re given a diagram from this unit and you have to label certain parts so make sure you revise it with active recall.

Reflexes

Reflex = action preformed without conscious thought as a response to a stimulus

Reflex arc

Stimulus = change in internal or external environment, event that stimulates a functional reaction in an organ, tissue, limb, muscle, gland etc.
Receptor = found in sense organs. When stimulated, the receptor sends electrical impulses along nerves in the central nervous system, leading to rapid responses.

eg. light receptor in the eye (retina)
Effector = the organ, tissue, limb, muscle or gland which acts in response to a stimulus and to return the change in environment back to normal.

eg.

Touch a hot object with finger
Pain receptors in the skin detect the heat. Information is sent through the spinal cord and signaled to the brain.
Muscles in the arm/hand contract to pull the finger away from the hot object.

Central nervous system

When a receptor is stimulated, the sense organ will send electrical impulses through nerves into and out of the central nervous system.

Brain and spinal cord make up the CNS
They allow for rapid responses to changes in the internal/external environment
It is a network of neurons = nerve cells which carry electrical impulses. Impulses are passed along from one neuron to the next.
Nervous impulses are generated in receptor cells in the receptor organ
Receptors send changes in environment as information in electrical impulses
The impulses are carried along 3 types of neurons (sensory, relay and motor), from the receptor to the muscle or gland (effector)

Movement of impulses through the body

Stimulus is detected by a receptor
Receptor generates impulses in the sensory neurons
Impulses enter the CNS through part of the spinal nerve called the dorsal root
Sensory neurons in the spinal cord connect to short relay neurons by synapses
The relay neurons connect with motor neurons
Motor neurons emerge from the spinal cord through the ventral root
Motor neurons transport impulses to the muscle or gland (effector)

Tip to remembering the order of dorsal and ventral root:

Dorsal comes first because ‘d’ is before ‘v’ in the alphabet!

Neurons

you only need to know the structure of the sensory and motor neuron, not relay

Motor neuron

Passes electrical impulses from the CNS to the effector

Sensory neuron

Nerve cells responsible for passing electrical impulses from the receptor to the effector

Dendron

Forms finer extensions called dendrites where there are junctions with other neurons called synapses

Axon

Central part of the neuron
Carries the electrical impulses to the effector organ

Myelin sheath

Insulates the axon
Speeds up the conduction of impulses

Terminal ends

Fine nerve endings of the axon
Connects to muscle at a special synapse called the neuromuscular junction
- Where impulses are carried from the CNS to the muscle

Synapse

= gap between 2 neurons

Nerve impulses cannot cross synapses. Instead, information is passed across the synapse with chemical messengers called neurotransmitters

Process

Impulse arrives at the end of the presynaptic neuron
Neurotransmitters are released into the synapse
The chemical messenger moves across the synapse by diffusion
Synapse is only 20 nanometers wide so it has a very short diffusion distance and time
On the postsynaptic membrane, the neurotransmitter binds to a receptor molecule
When binding, it stimulates a new nerve impulse in the postsynaptic neuron
When levels of the neurotransmitter in the synapse decrease, stimulation of the postsynaptic neuron ends
The chemical messenger is reabsorbed back into the presynaptic neuron to be reused
The neurotransmitter is broken down by an enzyme. The products are used to re-synthesise more neurotransmitters.

Why can the electrical impulse only travel in one direction?

Because the neurotransmitters can only move in one direction:
The presynaptic membrane has no receptors
There are no neurotransmitters in the postsynaptic membrane

Homeostasis

= Maintaining a constant internal environment

All organisms that are alive can respond to changes in their environment (Sensitivity from MRS GRENC)

Homeotherms

Warm blooded organisms
Maintain body temperature despite changes in the surrounding environment
Physiological changes allow the body to generate or lose heat + maintain body temperature
Ensures chemical reactions can continue to take place at steady, predictable rates
Prevents denaturing of enzymes

Skin and temperature control

Epidermis

Dead cells which stop water loss and protect the body against microorganism invasion

Dermis

Contains sensory receptors
Location of sweat glands, small blood vessels and hair follicles which are all used in temperature control

Hypodermis

Fatty tissue, store of energy
Insulates the body against heat loss

Sweat glands

Produces greater amount of sweat when body temperature rises
Secreted sweat is at body temperature. It has to get warmer to evaporate.
- This energy is supplied by the the body’s heat
- When sweat evaporates, it takes some of the body’s heat and provides a cooling action.
Energy causing evaporation is called the latent heat of vaporisation
In humid atmosphere, the sweat will stay on the skin as it cannot evaporate so it has very little cooling effect.

Hair follicles

In cold conditions, the hairs on the skin surface are pulled uptight
- They trap a layer of air next to the skin
- As air is a poor conductor, the layer acts as insulation
Hair erector muscles attached to the base of each hair relax (so the hair lies flat) or contract (so the hair is held uptight)
In hot conditions, the hairs on the surface lie flat to allow more heat loss
This is not an effective temperature control method as the hairs are not very large and not in great enough quantity

Blood vessels

There are tiny blood vessels called capillary loops in the dermis
When blood flows through them, it radiates heat to the outside and cools the body
Vasodilation occurs in hot conditions
- Arterioles leading to the capillary loops widen to increase flow to the surface
- More heat is radiated by the capillary loops
Vasoconstriction occurs in cold conditions
- Arterioles leading to the surface capillaries constrict so blood flow is reduced
- Less heat is lost by radiation

*insert diagram of vasodilation and vasocontriction*

Metabolism

In cold conditions, the body’s metabolism speeds up, stimulated by adrenaline hormone
Generates more heat from large organs like the liver
Shivering also occurs (rapid contraction and relaxation of the muscle) which generates heat energy

The Eye

Sclera

Tough outer coat of the eye
Becomes transparent at the front, called the** cornea **(which lets light into the eye)

Iris

Flat, coloured ring-shaped membrane
Located behind the cornea
Has a hole called the pupil where light passes through
Controls the amount of light that enters by changing the size of the pupil
Has 2 types of muscles which change the pupil size
- Circular muscles
- Radial muscles

Choroid

Contains pigment cells and blood vessels which prevent light being reflected inside the eye
Located underneath the sclera.

Retina

Inner most layer of the eye.
Light-sensitive, is where the light energy is converted into electrical energy of nerve impulses (light receptor)
Contains receptor cells called rods and cones which react to light by producing impulses in sensory neurons.
- Impulses are passed to the brain by the optical nerve
Rods work well in dim light but cannot distinguish colours. Cones can distinguish colors but don’t work well in dim light.
Rods and cones are found throughout the retina but cones are concentrated in the fovea (area at the centre of the retina)

Accomodation

= Changes which take place to allow us to see objects at different distances
Focusing on a distant object:
- Ciliary muscles relax
- Suspensory ligaments are pulled tight
- Lens becomes flat
Opposite for focusing on near object

Changes in response to light intensity

The iris controls the amount of light which enters the pupil by changing its size
It is important the pupil restricts in bright light to let in less light: light that is too bright can damage rods and cones
In bright light:
- Circular muscles contract (CC)
- Radial muscles relax (RR)
- Pupil constricts
Opposite for dim light
This is a reflexive action. The intensity of light hitting the retina is the stimulus, and the retina is the receptor

Hormones

Help regulate metabolic processes throughout the body
Secreted into the blood through endocrine glands
Transported through the blood to organs

Negative feedback loop: increase in hormone decreases the substance being controlled (e.g. Increased insulin decreases blood glucose concentration)

Adrenaline

Released by adrenal glands
Controlled by positive feedback
Release of adrenaline into the bloodstream causes:
- increase in breathing rate and depth of breathing by expanding the passages of the lungs
- leads to increase in heart rate, blood flow
- increased volume of blood pumped due to greater blood pressure
- pupils dilate
- blood vessels supplying muscles dilate
- converts glycogen to glucose in the liver to increase blood glucose (for aerobic respiration)
Results in more glucose and oxygen being transported to the muscles. More aerobic respiration occurs so greater amount of energy is released
Prepares the body for a fight or flight response (allow quick response when in danger)

Insulin

Controls blood glucose concentration
- If blood glucose is too high, cells get damaged because water is lost by osmosis
Negative feedback loop
Process of controlling blood glucose concentration:
- Pancreas continuously monitors blood glucose conc.
- After a meal with high carbohydrate, blood glucose increases (as glucose is absorbed from the ileum)
- Pancreas makes insulin
- Insulin decreases blood glucose concentration
  - increases glucose absorption from the blood by the liver and muscles
  - converts the excess glucose into glycogen which is mostly stored in the liver (but also some muscles)
  - increase respiration in the liver to use more glucose
- Return to normal glucose concentration
If blood glucose concentration is too low, less insulin is produced by the pancreas

ADH

Stands for anti-diuretic hormone
Made by the pituitary gland
Controls blood water content (internal hydration) by a negative feedback loop
Hypothalamus (part of the brain) detects a change in blood water content. If there is too much water, the pituitary gland releases less ADH
- Less ADH means the nephron reabsorbs less water back into the blood.

Testosterone

Produced in testes
Main male reproductive hormone, controls male secondary sexual characteristics
Stimulates sperm production

Progesterone

Important in the menstrual cycle
Produced in ovary
Completes the development of the uterus lining - thickens and maintains the uterus lining to prepare for implantation of embryo
If the egg is not fertilised, the corpus luteum breaks down and stops making progesterone

Oestrogen

Involved in repairing + thickening the uterus lining in the menstrual cycle
Produced in the ovary
Stimulates production of LH and stops production of FSH
Controls female secondary sexual characteristics

FSH

Stands for follicle stimulating hormone
Produced in the pituitary gland
Stimulates maturing of an egg in the ovary

LH

Stands for luteinising hormone
Produced in pituitary gland
Stops the maturing of an egg cell in the ovary and triggers ovulation (releasing of the egg into the Fallopian tubes)

If you’re confused about the female hormones, go check out our article on reproduction where I explain it in further detail :)

Table to summarise hormones

Hormone	Gland	Target organs	Use
Adrenaline	Adrenal	Liver, heart	Prepare the body for ‘fight or flight’ response when in danger
Insulin	Pancreas	Liver	Control blood glucose levels
ADH	Pituitary	Kidney	Control blood water content
Testosterone	Testes	Male reproductive organs	Stimulates sperm production, controls secondary sexual characteristics in males
Progesterone	Ovary	Uterus	Maintains uterus lining during menstrual cycle and pregnancy.
Oestrogen	Ovary	Female reproductive organs	Stops maturing of egg by stopping FSH production. Stimulates pituitary gland to produce LH. Thickens uterus lining. Controls secondary sexual characteristics in females
FSH	Pituitary	Ovaries	Causes maturing of an egg in the ovary + stimulates ovaries to produce oestrogen
LH	Pituitary	Ovaries	Triggers ovulation of matured egg