Receptors and Reflexes

●     What are Receptors?

○     Receptor: Structure that is able to detect a change in the body’s internal or external environment

○     Sometimes receptor cells of a particular type grouped together in a sense organ

■     Eg. Eye- light receptors

○     Other receptors are simple nerve endings & spread through parts of the body or even whole body

■     Eg. Pain receptors, temperature receptors in skin

○     When a receptor is stimulated, the body is able to respond to the change. In some cases, this is via an automatic reflex; in other cases the response is more complex.

●     Different Types of Receptors

○     Thermoreceptors

■     Are able to respond to heat & cold

○     Skin and some mucous membranes  (Peripheral)

■     Thermoreceptors that inform brain of external temperature change

■     Thermoreceptors are nerve endings **sensitive to EITHER hot or cold, not both

○     Hypothalamus (central)

■     Area where information is received & used to regulate body temperature

■     Contains thermoreceptors monitoring core body temperature (detect temperature of blood flowing through brain)

○     Cerebrum

■     Information received here as well, so we are consciously aware of temperature of surroundings

○     Osmoreceptors

■     Sensitive to osmotic pressure (determined by concentration of substances dissolved in water of blood plasma)

○     Found in the Hypothalamus

■     Respond to very small changes in osmotic pressure

■     Body’s water content maintained within very narrow limits

○     Chemoreceptors

■     Stimulated by particular chemicals

■     Nose: sensitive to odours

■     Mouth: sensitive to tastes

■     Internal: sensitive to composition of body fluids

■     Blood Vessels: sensitive to pH of blood & concentrations of O2 & CO2

■     Involved in regulation of heartbeat & breathing

○     Touch receptors

■     Found mainly in skin

○     Different types:
1) Sensitive to very light touches

■     Greater concentrations in areas of skin that are more sensitive

■     Lips, fingertips, eyelids, external genital organs

○     2) Sensitive to light touch that bends hair

■     Nerve endings at base of each hair follicle

○     3) Sensitive to pressure & vibrations

■     Located deeper in skin

○     NB Touch receptors close to skin surface & those attached to hairs adapt rapidly & after a short time we are no longer aware of the touch

○     Pain receptors (Nocireceptors)

■     Especially concentrated in skin & mucous membranes

■     Occur in most organs, but NOT the brain

■     Stimulated by:

■     Damage to tissues, such as from cut or heavy bump

■     Poor blood flow to tissues

■     Excessive stimulation from stimuli such as heat or chemicals

■     Pain warns us that damage to tissues is occurring & we can take action to minimise damage

■     Pain receptors don’t adapt (or only slightly) unlike other receptors, pain continues as long as stimulus is present

■     Prolonged stimulation of pain receptors makes pain worse

○     Stretch receptors

■     A sensory receptor that responds to the stretching of surrounding muscle tissue and so contributes to the coordination of muscle activity.

■     Stretch receptors are important for regulating breathing (inspiration and expiration).

●     Reflexes
Reflex: a rapid, automatic response to a change in the external or internal environment

○     Properties of ALL reflexes - SIRS

■     Stimulus is required to trigger a reflex- reflex is not spontaneous

■     Reflex is involuntary- occurs without any conscious thought

■     Reflex response is rapid- only a small number of neurons involved

■     Reflex response is stereotyped- it occurs in the same way each time it happens

○     Some reflexes involve the unconscious parts of the brain, but most are coordinated by spinal cord

○     Nerve impulses from receptors are directed into spinal cord, these are not necessarily carried up to the brain (immediatley).

○     Impulse may be passed to motor neurons at same level in cord or travel few segments before travelling out through a motor neuron

○     Reflexes
Spinal Reflex: reflex carried out by spinal cord alone
Reflex Arc: Pathway a nerve impulse follows in travelling from a receptor to an effector
Spinal Reflex Arc: Nerve impulse from receptor to effector via spinal cord

■     Spinal reflex is involuntary as it doesn’t involve brain, even though contraction of skeletal muscle may occur

○     Impulses are sent to brain so we become aware of what is happening, but this doesn’t happen until after response is initiated

○     Components of a Reflex Arc

■     A receptor reacts to a change in the internal or external environment by initiating a nerve impulse in a sensory neuron

■     A sensory neuron carries a nerve impulse towards the spinal cord

■     There is at least one synapse. The nerve impulse may be directly passed to a motor neuron or there may be one or more interneurons directing the message to the correct motor neuron.

■     A motor neuron carries the nerve impulse to an effector

■     An effector receives the nerve impulse and carries out the appropriate response. Effectors are muscles or glands.

○     Reflex Arc

■     Simple reflex arc involving 3 neurons  - See page 64 of text!

■     Response would occur in a fraction of a second & while occurring, impulses would travel up spinal cord to brain.

■     After response made, person consciously aware of situation

■     Function: protect body from injury

■     Eg. Blinking, sneezing, coughing, constriction of pupil

○     Learnt Reflexes
Protective reflexes present from birth

■     Innate reflexes:  an automatic instinctive unlearned reaction to a stimulus. Determined genetically.

■     Eg. Suckling, chewing, following movements with eyes

■     Acquired reflexes: complex motor patterns that are learned. Learned through constant repetition.

■     Eg. Muscular adjustments required to maintain balance while riding a bike, jamming on brakes in a car, catching a ball

●     COMPARISON OF HORMONAL AND NERVOUS COORDINATION

○     Similarities

■     Some substances function as both hormones & neurotransmitters. Eg. Noradrenaline, ADH, dopamine

■     Some hormones (eg. Ocytocin & adrenaline) secreted by neurons into extracellular fluid

■     Some hormones & neurotransmitters have same effect on same target cells (eg. Noradrenaline & glucagon both act on liver cells to glycogen to be broken down to glucose)

Comparison