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