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Coordination and Response

A nerve impulse is an electrical signal that passes along nerve cells called neuron

THE HUMAN NERVOUS SYSTEM
  • It is made up of two parts: Central Nervous System (CNS), and Peripheral Nervous System (PNS)
  • CNS: The brain and the spinal chord, which have the role of coordination
  • PNS: The nerves, which connect all parts of the body to the CNS
  • Sense organs are linked to the PNS: They contain groups of receptor cells; when exposed to stimulus they generate an electrical impulse, which passes along peripheral nerves to the CNS, triggering a response
  • Peripheral nerves contain sensory and motor neurons
  • Sensory neurons transmit nerve impulses from sense organs to the CNS
  • Motor neurons transmit nerve impulses from the CNS to effectors (muscles or glands)
  • Neurons are covered with a myelin sheath, which insulates them to make transmission of the impulse more efficient
  • Relay neurons pick up messages from other neurons and pass them on to other neurons
  • The cytoplasm (mainly axon, and dendron) is elongated to transmit the impulse for long distances

A Typical Neuron (Photo Credit: LumenLearning.com)

Reflex Arc

  • A reflex arc describes the pathway of an electrical impulse in response to a stimulus
  • Relay neurons are found in the spinal cord, connecting sensory neurons to motor neurons

    Photo Credit: TeachMePhysiology.com

Synapse

  • Neurons do not connect directly with each other, there is a gap called synapse
  • A synapse is a junction between two neurons

    Photo Credit: LumenLearning.com

How an Impulse Triggers the Release of a Neurotransmitter from Vesicles into the Synaptic Gap

  • When an impulse arrives along the axon of the sensory neuron, it cause theses vesicles to move to the cell membrane and empty their contents into the synaptic cleft
  • The neurotransmitter quickly diffuses across the tiny gap, and attaches to receptor muscles in the cell membrane of the relay neuron
  • This can happen because the shape of the neurotransmitter molecules is complementary to the shape of the receptor molecules
  • The binding of the neurotransmitter with the receptors triggers a nerve impulse in the relay neuron
  • This impulse sweeps along the relay neuron, until it reaches the next synapse
  • Here, a similar process occurs to transmit the impulse to the motor neuron
  • Synapses act like one-way valves
  • There is only a neurotransmitter on one side of the synapse, so the impulse can only go across from that side
  • Synapses ensure that nerve impulse only travel in one direction
  • Many hard drugs, e.g. heroin, act upon synapses
Sense Organs

Sense organs a groups of receptor cells responding to specific stimuli: light, sound, touch, temperature, and chemicals

THE EYE

Photo Credit: PCEyeGlasses.com

Functions of the Parts of the Eye

Pupil Reflex in Response to Light Intensity

Photo Credit: BioNinja.com

  • The reflex action changes the size of the pupil, to control the amount of light entering the eye
  • In bright light:
    • The retina detects the brightness of the light entering the eye
    • An impulse passes to the brain along the sensory neurons and travels back to the muscles of the iris along motor neurons, triggering a response
    • Circular muscles contract; radial muscles relax, so the iris gets bigger
    • The pupil constricts (gets smaller) so less light falls on the retina to prevent damage
  • In dim light:
    • The retina detects the brightness of the light entering the eye
    • An impulse passes to the brain along sensory neurons and travels back to the muscles of the iris along motor neurons, triggering a response
    • Radial muscles contract, circular muscles relax, so the iris gets smaller
    • Pupil size is increased (dilated) to allow as much light as possible to enter the eye

Accommodation of the Eye to View Near and Distant Objects

Photo Credit: Weebly.com

  • To focus on a distant object:
    • Slightly diverging rays of light enter the eye
    • Ciliary muscles relac
    • Suspensory ligaments are pulled tight
    • Lens become thin
    • The thin lens bends the light rays slightly
  • To focus on a nearby object:
    • Greatly diverging rays of light enter the eye
    • Ciliary muscles contract
    • Suspensory ligaments slacken
    • Lens gets fatter
    • The thick lens bends the light rays greatly

Distribution of Rods and Cones in the Retina of a Human

  • Rods are found throughout the retina, but none in the center of the fovea or in the blind spot
  • Cones are concentrated in the fovea

Function of Rods and Cones

  • Rod cells are sensitive to dim light, but they do not respond to color
  • Cone cells are able to distinguish between the different colors of light but they only function when the light is quite bright
  • There are three different types of cones, sensitive to red, green, and blue lights
  • Rods allow us to see in dim light, but only in black and white, while cones gives us color vision
Hormones
  • Hormones are chemical substances, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs

ADRENALINE

  • There are two adrenal glands, one above each kidney
  • They make a hormone called adrenaline
  • When you are frightened or excited, the brain sends impulses long a nerve to the adrenal glands which secretes adrenaline into the blood
  • Adrenaline causes the heart to beat faster, supplying oxygen to the brain and muscles more quickly
  • This provides them more energy for fighting or running away
  • Adrenaline also increases breathing rate, so that more oxygen can enter the blood in the lungs
  • Adrenaline also causes the pupils in the eyes to widen
  • Adrenaline secretion increases when someone is scared

Role of Adrenaline

  • Adrenaline helps us cope with danger by increasing the heart rate thus supplying oxygen to the brain and the muscles more quickly
  • This increases the rate of metabolic activity and gives more energy for fighting or running away
  • The blood vessels in the skin and digestive system contract so that they carry very little blood and more blood goes to the brain and muscles
  • Adrenaline also cause the liver to release glucose into the blood
  • This provides extra glucose to the muscles, thus more respiration and more energy is released for contraction

Function of Insulin, Testosterone and Estrogen

  • Insulin reduces the concentration of glucose in the blood
  • Estrogen causes the development of female secondary sexual characteristics, helps in the control of the menstrual cycle
  • Testosterone cause the development of male secondary characteristics

Comparison of Nervous and Hormonal Control Systems

Homeostasis
  • Homeostasis is the maintenance of a constant internal environment
  • It is important that the internal environment of the body is controlled
  • Maintaining a constant internal environment is called homeostasis
  • The nervous system and hormones are responsible for this
  • These are some of the internal conditions that are controlled:
    • NEGATIVE FEEDBACK
      • A change from normal, for instance, an increase in blood glucose levels triggers a sensor, which stimulates a response in an effector
      • However, the response in this case is the secretion of the insulin hormone, which would eventually result in glucose levels dropping below normal
      • As glucose levels drop, the sensor detects the drop and instructs the effector (pancreas) to stop secreting insulin (negative effect)
      • This is negative feedback – the change is fed back to the effector
    • CONTROL OF GLUCOSE CONCENTRATION IN THE BLOOD
      • The liver is a homeostatic organ; it controls the levels of glucose
      • Two hormones, insulin and glucagon, which are secreted by the pancreas control blood glucose levels

Role of Insulin in Controlling Blood Glucose Levels

  • When blood glucose levels are high, then insulin is secreted by the pancreas; insulin passes the bloodstream to the liver
  • Insulin stimulates the liver to absorb glucose
  • Insulin converts glucose to glycogen
  • Insulin increases the rate of respiration; so more blood glucose is absorbed by cells and used up, to reduce blood glucose levels

Role of Glucagon in Controlling Blood Glucose Levels

  • When blood glucose levels drop below normal, glucagon is secreted by the pancreas
  • Glucagon passes the bloodstream to the liver
  • Glucagon converts glycogen to glucose in the liver; glucose is then released into the bloodstream
The Skin

Photo Credit: LumenLearning.com

Maintenance of a Constant Internal Body Temperature in Humans

  • Humans maintain a body temperature of 37°C
  • A part of the brain called the hypothalamus keeps the internal temperature constant by acting like a thermostat
  • If the temperature is above or below 37°C, the hypothalamus receives information from thermoreceptors in our skin and sends electrical impulses, along nerves, to the parts of the body which have the function of regulating our body temperature
  • When cold, the body produces and saves heat in the following ways:
    • Shivering: Muscles in some parts of the body contract and relax very quickly. This produces heat and is called shivering
    • Metabolism may increase
    • Hair stands up
    • Vasoconstriction: The arterioles, which supply the skin blood to the skin, capillaries become narrower, thus less blood flows in them and thus less heat is lost to the air by radiation
  • When hot, the blood loses more heat in the following ways:
    • Hair lies flat; no insulation
    • Vasodilation: The arterioles capillaries gets dilated, thus more blood flows through them and thus heat is readily lost from the blood into the air by radiation
    • Sweating: Sweat glands secrete sweat on the surface of the skin, which evaporates, taking heat from the skin with it, thus cooling the body
    • Metabolism slows down
Tropic Responses
  • Gravitropism is a response in which parts of a plant grow towards or away from gravity
  • Phototropism is a response in which parts of a plant grow towards or away from the direction from which light is coming
  • Shoots normally grow towards light
  • Roots do not usually respond to light, but a few grow away from it
  • Shoots tend to grow away from the pull of gravity, while roots normally grow towards it

Control of Plant Growth by Auxins, Weedkillers

Auxins in Action (Photo Credit: Weebly.com)

  • Auxins are plant growth substances, produced by the shoot and root tips of growing plants
  • Auxins in the shoot stimulate cell growth by the absorption of water
  • Auxins in the root slow down cell growth

Auxin in Phototropism

  • If a shoot is exposed to light from one side
    • More auxins are moving in the shaded side (from the tip of the shoot)
    • On this side, cells are stimulated to absorb more water, plant grows more
    • Shoot bends toward the light
    • This is called positive phototropism
  • If a root is exposed to light in the absence of gravity
    • More auxins are moving in the shaded side (from the tip of the root)
    • On this side, cells are stimulated to absorb less water, plant grows less
    • Root bends away from the light
    • This is called negative phototropism

Auxin in Gravitropism

  • If a shoot is placed horizontally in the absence of light
    • Auxins accumulate on the lower side of the shoot, due to gravity
    • Cells on the lower side grow more quickly
    • The shoot bends upwards
    • This is called negative gravitropism
  • If a root is placed horizontally in the absence of light
    • Auxins accumulate on the lower side of the shoot, due to gravity
    • Cells on the lower side grow more slowly
    • The shoot bends downwards
    • This is called positive gravitropism

Effects of Weedkillers

  • Weedkillers (herbicides) are synthetic plant hormones, similar to auxins
  • If these chemicals are sprayed on to plants, they can cause rapid, uncontrolled growth and respiration, resulting in the death of the plant
  • Some plant species are more sensitive than others to synthetic plant hormones, so weedkillers can be selective
  • Many weedkillers kill only broad-leaved plants (dicotyledons), leaving grasses (monocotyledons) unharmed
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