Nervous System

• Nervous System

  Its a specialised network of cells in our body and primary communication systems. Its based on chemical and electrical signals whereas the Endocrine system is based on hormones. The NS has two main functions: to collect, process and respond to information in the environment and to coordinate the working of different organs and cells in the body.
• Central Nervous System:Structure and functions

  CNS is made up of the brain & spinal cord. The brain is the centre of conscious awareness. The cerebral cortex is highly developed in humans, this separates our higher mental functions from those of animals. The brain is divided into two hemispheres. The spinal cord is the extension of the brain and is responsible for reflex actions. It passes messages to and from the brain and connects nerves to the PNS.
• Peripheral Nervous System:Structures and Functions

  The PNS transmits messages via millions of neurons to and from the NS

  Its further subdivided into

  • Autonomic Nervous System - governs vital functions in the body eg. breathing, heart rate, digestion, sexual arousal and stress responses.
  • Somatic Nervous System-governs muscle movement and receives information from sensory receptors.
• Neurons 1/2

  There are 100 bilion nerve cells in the human nervous system, 80% are located in the brain. By transmitting signals electrically and chemically, these provide the nervous system with its primary means of communication.
• Neurons 2/2

  • Sensory neurons carry messages from the PNS to the CNS. Long dendrites and short axons. They're located in the PNS in clusters called ganglias.
  • Relay neurons connect sensory neurons to motor neurons or other relay neurons. Short dendrites and short axons. 91% of all neurons are relay neurons, most being in the brain and visual system.
  • Motor neurons connect the CNS to effectors : muscles or glands. Short dendrites and long axons. Cell bodies may be in the CNS but long axons form part of PNS.
• Neuron Structure

  
• Synaptic Transmission

  Each neuron is separated from the next by an extremely tiny gap - a synapse.
• Synaptic Transmission : Chemical transmission

  Signals within neurons are transmitted electricully but signals between them are transmitted chemically across a synapse. When the electrical impulse reaches the end of the neuron (pre synaptic terminal) it triggers the release of neurotransmitters from synaptic vesicles. Once a neurotransmitter crosses the gap its taken up by a post-synaptic receptor site on the next neuron so the impulse only ever travels in one direction. The chemical message is converted back into an electrical impulse and the process of electrical transmission begins.
• Synaptic Transmission : Neurotransmitters

  Are chemicals that diffuse across the synapse to the next neuron in the chain. Many neurotransmitters have been identified and each has its own molecular structure that fits perfectly into a post synaptic receptor site eg serotonin - affects mood and social behaviour.
• Synaptic Transmission : Excitation and Inhibition

  • Adrenaline - usually excitatory, increases the positive charge of the post synaptic neuron, making it more likely to fire.
  • Serotonin - usually inhibitory, increases the negative charge of the post synaptic neuron making it less likely to fire.
  • Dopamine - equally likely to have an excitatory effect or an inhibitory effect.
• Synaptic Transmission : Summation

  Excitatory and Inhibitory influences are summed and must reach a certain threshold in order for the action potential of the post-synaptic neuron to be triggered. If the net effect of the neurotransmitters is inhibitory then the post-synaptic neuron is less likely to fire (no electrical signal is transmitted.) Its more likely to fire if the net effect is excitatory.
• The Human Nervous System is divided into the Peripheral Nervous System (PNS) and the Central Nervous System (CNS).

  • CNS - divided into the brain which is the centre of consciousness and the spinal cord which passes messages to and from the brain
  • PNS - further divided into the Autonomic Nervous System (ANS) which governs vital functions such as breathing, digestion, heart rate and the Somatic Nervous System (SNS) which controls muscle movement and receives information from sensory receptors.
• • Autonomic Nervous System divided into the Sympathetic Nervous System: fight or flight and the Parasympathetic Nervous System: rest and digest.
• Fight or Flight

  -the stressful situation is identified and the amygdala activates

  -amygdala send a a distress signal to the hypothalamus

  -activates the pituitary gland which allows the triggers the sympathetic nervous system

  -SNS stimulates the adrenal medulla and secretes adrenaline into the bloodstream

  -this causes physiological changes:increased heart rate, sweating, dry mouth, shuts down some systems

  -after the perceived stressor is gone, the body goes back to the parasympathetic rest and digest state after about 20 mins
• Structure of neuron

  
• Reflex arc

  The knee-jerk is an example of a reflex arc:

  A stimulus, such as a hammer, hits the knee. This is detected by sense organs in the PNS which convey a message along a sensory neurone. The message reaches the CNS where it connects

  with a relay neurone. This then transfers the message to a motor neurone. This then carries the message to an effecter such as a muscle, which causes the muscle to contract and, hence, causes the knee to move or jerk.