Nervous system, eye and brain

• Organisms increases their chances of survival by responding to changes in their environment
• Changes in environment (stimuli) are detected by receptors
• The initiation of a nerve impulse leads to an associated response by effectors by means of a coordinator (usually the brain)
• The nervous system is composed of a central nervous system and peripheral nervous system
• The central nervous system is composed of the brain and spinal cord
• The peripheral system

  • Voluntary nervous system (somatic): spinal nerves and cranial nerves
  • Automatic nervous system

  1. sympathetic: spinal nerves only
  2. parasympathetic: spinal nerves and cranial nerves
• Spinal nerves are composed of both sensory and motor neurons
• The nervous system involves the transmission of electrical impulses along neurons
• Nervous responses are rapid, very specific and short-lived
• Sensory neurons

  • transmit impulses from receptors that are stimulated by a specific stimulus to the central nervous system
• Motor neurons

  • transmit impulses away from the CNS to the appropriate effector either a muscle or gland
• Relay neurons

  • act as a link between sensory and motor neurons and are situated in the central nervous system
  • also called intermediate or coordinator neurons
• Cell body contains the nucleus and most of the other cell organelles. There may be more than one nerve fibre extending from the cell body.
• Dendrons conduct impulses towards the cell body they are small and break up into small terminal branches at the end
• Axons conduct impulses away from the cell body they are thinner than dendrites and may be several metres long
• Schwann cells surround the axon forming a myelin sheath which insulates the cell
• Node of Ranvier is the region of the axon not insulated by the Schwann cell
• Synaptic knobs are present at the end of the axons and allow communication with other neurons or the effector via a neurotransmitter
• A simple reflex:

  • a reflex is a rapid response to a stimulus
  • reflexes are automatic (involuntary)
• Receptors detect the stimulus and transmit impulses along the sensory neuron to the spinal cord (or brain)

  1. the sensory neuron synapses with a relay neurone which passes impulses to the motor neurone
  2. Impulses are transmitted along a motor neurone to the effector which contracts
• Endocrine system

  • involves the release of hormones from endocrine glands
  • hormones travel in the bloodstream to target cells affecting their physiological activities
• Hormones act on cells by two mechanisms:

  1. peptide hormones are polar molecules that cannot pass through the cell membrane; they interact with specific receptors. this produces a secondary messenger that activates or inhibits specific enzyme activities therefore regulating metabolic pathways
  2. steroid hormones pass through the cell membrane. they interact with receptor sites on transcription factors and increase or decrease the rate of gene expression
• Nervous control

  • electrical and chemical
  • transmission via nerve impulses and chemical at synapse
  • pathway in specific via nerve cells
  • rapid transmission and response
  • often short term changes
  • response often very localised
• Hormonal control

  • chemical only
  • transmission release into the blood transport via circulatory system
  • pathway none specific via blood but target cells specific due to specific receptors
  • slower transmission and relatively slow acting
  • often long term changes
  • response may be widespread e.g. growth
• The automatic nervous system is part of the peripheral nervous system which controls activities inside the body that are normally involuntary e.g. heart rate, peristalsis, sweating
• Autonomic nervous system

  • consists of motor neurons passing impulses to the smooth muscle of internal organs
  • control centers in the medulla and hypothalamus; receive sensory information and coordinates this information from other parts of the nervous system to produce the appropriate response
  • two divisions; sympathetic and parasympathetic
• Sympathetic nervous system

  • Area of influence: effects diffuse
  • Transmitter substance: noradrenaline effector
  • Conditions when active: dominant during danger, stress
  • General effects: increased metabolic rate, increased blood glucose, raised sensory awareness
• Parasympathetic nervous system

  • Area of influence: effects localised
  • Transmitter substance: acetycholine at effector
  • Conditions when active: dominant during rest, controls routine body activities
  • General effects: decrease blood glucose, decrease rhythmic activity, restore awareness to normal levels
• The sympathetic and parasympathetic nervous system generally have antagonistic effects on the organs they supply
• Nerve impulses

  • identical whether it is traveling along the optic, auditory or olfactory nerve
  • the specific region of the brain that receives the impulses and therefore interprets them as a particular sensation
• All or Nothing Law

  • states that a stimulus must be above a certain threshold level or intensity for an impulse to be generated
  • any stimulus below this threshold value will result in a greater frequencies of impulses than a weak stimulus above the threshold
  • the amplitude of the of the impulse always remains the same
• Resting potential:

  • both sodium and potassium ions can diffuse through the neurone membrane
  • maintaining an electrochemical gradient therefore requires active transport
  • sodium-potassium pump actively transports sodium ions out of the neurone and potassium ions in against their concentration gradients which requires ATP
  • the membrane is more permeable to the diffusion of potassium ions than sodium ions so more positively charged ions are outside the cell
  • in a polarised neurone there are more sodium ions outside it than potassium ions inside
• A nerve impulse (steps 1-5)

  1. an action potential involves a change in the potential across the axon membrane from polarised (-70mV) to a positive inside value of +40mV called depolarisation
  2. depolarisation results from an increase in the permeability of the neurone membrane to sodium ions as sodium ion channel proteins in the membrane open
  3. a small concentration of sodium ions diffuse in down a concentration gradient
  4. this causes the membrane potential to be slightly less negative
  5. the threshold potential is reached
• A nerve impulse (steps 6-8)

  6. change in voltage across the membrane; more voltage-sensitive sodium ion channels to open further, increasing the permeability of the membrane to sodium ions so more diffuse in (positive feedback)

  7. when the membrane potential reaches peak voltage, sodium ion channels close

  8. voltage-sensitive potassium ion channels in the membrane open before peak voltage is reached; potassium ions diffuse out the axon
• A nerve impulse (step 9)

  9. sodium-potassium pump restores the resting potential (repolaristation) by actively transporting sodium ions out the neurone and potassium ions back into the neurone
• The refractory period leads to separate discrete impulses and determines the maximum frequency of impulses along a neurone
• Refractory period

  • absolute refractory period is when the neurone cannot normally transmit an impulse
  • due to the fact that the voltage sensitive ions gates have to return back to the resting state after repolarisation which takes time
  • redistribution of the ions that have moved during the action potential in the neurons may also need to take place
  • possible that super-threshold stimuli can cause an action potential and contribute to relative refractory period
• Transmissions speed of impulses

  • two main factors

  1. axon diameter: greater the diameter the greater the speed of impulse
  2. myelination: transmission impulses faster as the action potential jumps from one gap to the next (saltatory conductance)
• The synapse is a functional connection between neurones (or a neurone and a muscle)
• The synaptic knob has:

  • numerous synaptic vesicles containing a neurotransmitter chemical e.g. acetylcholine
  • numerous mitochondria which provide ATP for the synthesis of the neurotransmitter