nervous system

• cells of multicellular organisms communicate with each other to respond to external stimuli.
• central nervous system (cns) vertebrates: brain and spinal cord. in mammals, the cns is connected to the body by sensory and motor neurones
• sensory neurones carry info as electrical impulses from receptors to cns
• motor neurones carry electrical impulses from cns to effectors
• effectors: muscles and glands. respond to nervous impulses
• receptors are cells that detect stimuli
• effectors respond to nervous impulses and bring a change
• in response to nervous impulses, muscles contract and glands secrete hormones
• stimulus > receptor > sensory neurone > cns > motor neurone > effector > response
• synapses connect neurones
• nerve signals are transferred by chemicals which diffuse across the gap between neurones
• reflexes are rapid, automatic responses to stimuli that don’t involve the conscious part of the brain. they reduce chances of injury
• the passage of information in a reflex is called a reflex arc
• • neurones in reflex arc go through the spinal cord or an unconscious part of the brain
  • stimulus is detected by receptors, and impulses are sent along sensory neurones to a relay neurone in the cns
  • chemicals diffuse across synapses to the relay neurone, and again to the motor neurone
  • impulses travel along motor neurone to effector
  • response is made
• reaction time is the time it takes to respond to a stimulus
• reaction time can be affected by age, gender or drugs
• the sclera is the tough, supporting wall of the eye
• the cornea is the transparent outer layer at the front of the eye. it refracts light into the eye
• the iris has muscles to control the diameter of the pupil (hold) and how much light enters eye
• lens focuses light onto retina (receptor cells sensitive to light intensity + colour)
• shape of lens controlled by ciliary muscles and suspensory ligaments
• optic nerve carries impulses from receptors on the retina to brain