SENSORY RECEPTION

Stimulus - a detectable change in the internal/external environment of an organism that produces some change in that organism
Response - a change in an organism that results from an internal/external stimulus
Selection pressure - an environmental force altering the frequency of alleles in a population

The ability to respond to a stimulus increases the chances of survival for an organism. The organisms that survive have a better chance of reproducing passing on their favourable alleles to their offspring. There is therefore always a selection pressure favouring organisms with more appropriate responses.
Stimuli are detected by cells or organs know as receptors that transfer the energy for the stimulus into one that can be processed by an organism and lead to a response. An effector is an organ that responds to stimulation by a nerve impulse resulting in a response.

Stimulus → Receptor → Coordinator → Effector → Response

A taxis is a simple response whose direction is determined by the direction of the stimulus. If the organism moves towards the stimulus it is known as positive taxis and if it moves away from the stimulus it's known as negative taxis.

A kinesis is a form of response in which the organism doesn't move towards or away from the stimulus. Instead, the more unfavourable the environment, the more rapidly the organism moves and the more rapidly it changes direction. Therefore a kinesis results in an increase in random movements aimed at returning the organism to favourable conditions in order to increase their chances of survival.

A tropism is a growth movement of part of a plant in response to a directional stimulus. If the plants grow towards the stimulus e.g. light it's known as positive phototropism or if it grows away from the stimulus e.g. gravity it's known as negative geotropism.

NERVOUS CONTROL

Neurone - a nerve cell, comprising a cell body, axiom and dendrites which is adapted to conduct action potentials
Central nervous system (CNS) - made up of the brain and the spinal cord
Peripheral nervous system (PNS) - made up of pairs of nerves that originate from either the brain or the spinal cord
Sensory neurones - carry nerve impulses from receptors towards the CNS
Motor neurones - carry nerve impulses away from the CNS to the effectors
Voluntary nervous system - carries nerve impulses to body muscles and is under voluntary (conscious) control
Autonomic nervous system - carries nerve impulses to glands, smooth muscle and cardiac muscle and is involuntary
Reflex - a type of involuntary response to a sensory stimulus
Reflex arc - a pathway of neurones involved in a reflex arc involving just 3 neurones
Synapse - a junction between neurones in which they don't touch but have a narrow gap, the synaptic cleft, across which a neurotransmitter can pass

The nervous system has two major divisions: the CNS and the PNS. The PNS is divided into the sensory nervous system and the motor nervous system. The motor nervous system is further subdivided into the voluntary nervous system and the autonomic nervous system.

The spinal cord is a column of nervous tissue that runs along the back and yes inside the vertebral column for protection. Emerging at intervals along the spinal cord are pairs of nerves.

Main stages of reflex arc such as withdrawing the hand from a hot object:

The stimulus - heat from the hot object
A receptor - temperature receptors in the skin on the hand which create a nerve impulse in a sensory neurone
A sensory neurone - passes the nerve impulse to the spinal cord
An intermediate neurone - links the sensory neurone to the motor neurone in the spinal cord
A motor neurone - carries the nerve impulse from the spinal cord to a muscle in the upper arm
An effector - the muscle in the upper arm, which is stimulated to contract
The response - pulling the hand away from the hot object

Reflexes and important because:

They are involuntary and therefore don't require the decision-making powers of the brain, which leaves it free to carry out more complex responses. The brain is therefore not overloaded with situations in which the response is always the same
They protect the body from harmless stimuli-they're effective from birth and don't have to be learnt
They are fast, because the neurone pathway is short with only 1 or 2 synapses (which are the slowest link in a neurone pathway)

CONTROL OF HEART RATE

The autonomic nervous system controls the involuntary activities of internal muscles and glands. It has two divisions:

The sympathetic nervous system: stimulates effectors and so speeds up any activities. It helps us cope with stressful situations by heightening our awareness and preparing us for activity (fight-or-flight)
The parasympathetic nervous system: inhibits effectors and so slows down any activity, it controls activities under normal resting conditions. It's concerned with conserving energy and replenishing the body's reserves

The actions of the parasympathetic and sympathetic nervous system oppose each other-they are antagonistic.

Changes to the heart rate are controlled by a region of the brain called the medulla oblongata which has two centres:

A centre that increases the heart rate, which is linked to the sinoatrial node by the sympathetic nervous system
A centre that decreases the heart rate, which is linked to the sinoatrial node by the parasympathetic nervous system

Which of these centres is stimulated depends on information they receive from two types of receptor which respond to either chemical or pressure changes in the blood.

Chemoreceptors are found in the walls of the carotid arteries (the arteries that serve the brain) and the aorta. They are sensitive to changes in the pH of the blood that result from changes in the concentration of CO₂ (in solution CO₂ forms an acid and therefore lowers the pH). This works because:

When the blood has a higher than normal concentration of CO₂, its pH is lowered
The chemoreceptors in the wall of the carotid arteries and the aorta detect this and increase the frequency of nerve impulses to the centre of the medulla oblongata that increases the heart rate
This centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node which, in turn, increases the heart rate
The increased blood flow that this causes leads to more CO₂ being removed by the lungs and so the CO₂ level of the blood returns to normal
As a consequence the pH of the blood rises to normal and the chemoreceptors reduce the frequency of nerve impulses to the nebula oblongata
The medulla oblongata reduces the frequency of impulses to the sinoatrial node via the parasympathetic nervous system, which therefore decreases the heart rate to normal

Pressure receptors in the walls of the carotid arteries and the aorta also can affect the heart rate:

When blood pressure is higher than usual: they transmit a nerve impulse to the centre in the medulla oblongata that decreases the heart rate. This centre sends impulses via the parasympathetic nervous system to the sinoatrial node of the heart, which decreases the rate at which the heart beats
When blood pressure is lower than usual: they transmit a nerve impulse to the centre in the medulla oblongata that increases the heart rate. This centre sends impulses via the sympathetic nervous system to the sinoatrial node of the heart, which increases the rate at which the heart beats

ROLE OF RECEPTORS

Generator potential - depolarisation of the membrane of a receptor cell as a result of a stimulus
Ligament - a fought, fibrous connective tissue, rich in collagen, that joins bone to bone
Pacinian corpuscle - sensory receptor that responds to changes in mechanical pressure
Tendon - fought, flexible, but inelastic, connective tissue that joins muscle to bone
Visual acuity - the ability of the eye to detect fine details within small distances of each other

Like all sensory receptors, Pacinian corpuscles:

Are specific to a single type of stimulus: responds only to mechanical pressure
Will produce a generator potential by acting as a transducer: transducers convert the information provided by the stimulus into nerve impulses that can be understood by the body. All receptors convert the energy of the stimulus into a nerve impulse known as a generator potential

Pacinian corpuscles are found deep in the skin in fingers and the soles of feet. They also occur in ligaments, tendons and joints, where they allow the organism to know which joints are changing direction. The single sensory neurone of a Pacinian corpuscle is at the centre of layers of tissue, each separated by a gel.

The sensory neurone that ends at the centre of the Pacinian corpuscle has a special type of sodium channel in its plasma membrane, called a stretch-mediated sodium channel. Their permeability to sodium changes when they change shape, for example by stretching.
The way the Pacinian corpuscle works:

In its normal resting state, the stretch-mediated sodium channels of the membrane around the neurone of a Pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the neurone of the Pacinian corpuscle has a resting potential
When pressure is applied to the Pacinian corpuscle, it changes shape and the membrane around its neurone becomes stretched
This stretching widens the sodium channels in the membrane and sodium ions diffuse into the neurone
The influx of sodium ions changes the potential of the membrane-it becomes depolarised-and produces a generator potential
The generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones, to the central nervous system

The light receptors cells of the mammalian eye are found on its innermost layer: the retina. There are two main types of receptor cells in the eye: rod and cone cells which both act as transducers by converting light energy into the electrical energy of a nerve impulse.

Rod Cells
Rod cells can't distinguish different wavelengths of light and therefore produce images in black and white only. There are many more rod cells than cone cells.

Three rod cells share a single sensory neurone and therefore they can respond to light of very low intensity. This is because a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached.

As a number of rod cells are attached to a single bipolar cell (retinal convergence) there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were attached to each bipolar cell. As a result, rod cells allow us to see in low light intensity (i.e. at night) but only in black and white. As a consequence of retinal convergence, the light received by the rod cells will only generate a single impulse regardless of how many neurones are stimulated. This means that they cannot distinguish between the separate sources of light that stimulated them. Two dots close together will appear as a single dot. Rod cells therefore have low visual acuity.

In order to create a generator potential, the pigment in rod cells (rhodopsin) must be broken down and low-light intensity is sufficient to cause this.

Cone Cells
There are 3 different types of cone cells, each responding to a different wavelength of light. Depending on the proportion of each type that is stimulated we can see images in full colour.

Cone cells are each connected to their own sensory neurone which means that the stimulation of a number of cone cells can't be combined to help exceed the threshold value and so create a generator potential. As there is no retinal convergence in cone cells, if 2 separate cone cells are stimulated, the brain will be able to distinguish the two separate sources of light. Therefore cone cells have good visual acuity.

Light is focused by the lens on the part of the retina opposite the pupil, called the fovea. Therefore the fovea receives the highest intensity of light so cone cells but not rod cells are found at the fovea. The concentration of cone cells diminishes further away from the fovea.

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