Nervous system

• CNS

  Central Nervous System
• PNS

  Peripheral Nervous System
• Neurons

  Nerve cells
• Synapses

  Specialised structures that allow information to pass from one cell to another
• Nervous system

  • Only found in multicellular animals
  • Grouping of nerve cells or "neurons"
  • Highly differentiated (cannot divide and reproduce) and specialised cells
  • Electrically excitable cells (change of membrane voltage is necessary for cell response)
  • Communicate with other cells via specialised structures called synapses
  • Coordinate sensory information from body or environment with resulting actions from different parts of the body
• Neurones

  • Cell body: where organelles are located
  • Dendrites: receive signals from other neurones
  • Axon: transmits signals to other neurones
  • Synapse: allow information to pass onto to next cell
• Categories of neurons based on location of nerve cell body

  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)
• Central Nervous System (CNS)

  • Brain
  • Spinal cord
• Peripheral Nervous System (PNS)

  • Cranial nerves
  • Spinal nerves
  • Ganglia
• Central Nervous System (CNS)

  • White matter: bundles of axons
  • Gray matter: cell bodies
  • Cavities filled with cerebrospinal fluid
• Vertebrate brain

  • Forebrain: regulates sleep, olfactory inputs, learning and complex processing
  • Midbrain: coordinates sensory input
  • Hindbrain: coordinates involuntary activities
• Mammalian brain

  • Cerebrum: information processing (learning, emotion, memory, perception, voluntary movement), right & left cerebral hemispheres, corpus callosum connects hemispheres
  • Brainstem: basic, autonomic survival behaviors, medulla oblongata controls breathing, heart & blood vessel activity, digestion, swallowing, vomiting, transfers info between PNS & CNS
  • Cerebellum: coordinates movement & balance, motor skill learning
• Sensory system (PNS)

  • Made of "afferent" neurons
  • Relays information from the body or the environment to the CNS
• Integrative system (PNS)

  • Made of "interneurons"
  • Connect CNS and PNS
  • Most abundant neurons in the body
• Sensory neurones

  • Activated by sensory input from the environment (heat, pain etc)
  • Most are pseudo-unipolar, with one axon split into two branches
• Types of sensory receptors

  • Mechanoreceptors (physical force)
  • Thermoreceptors (temperature)
  • Chemoreceptors (dissolved chemicals)
  • Nociceptors (pain)
  • Proprioceptors (positional information)
• Interneurones

  • Connect motor and sensory neurones, and transfer signals between them
  • Can also communicate with each other to form circuits
  • Localised to the CNS in most animals
• Somatic and Autonomic systems (PNS)

  • Made of "efferent" neurons
  • Relays information from the CNS to the body
  • Controls skeletal muscles and gland secretion
• Autonomic Nervous System

  • Controls involuntary actions
  • Sympathetic division: "fight or flight" involuntary response
  • Parasympathetic division: "rest and digest" involuntary response
• Somatic or Motor System (PNS)

  • Consists of afferent sensory neurons and efferent motor neurons
  • Controls voluntary actions i.e. skeletal muscles
• Motor neurones

  • Part of the central nervous system
  • Connect to muscles, glands and organs throughout the body
  • Transmit impulses from spinal cord to skeletal muscles to control movement
  • Lower motor neurones: travel from spinal cord to muscle
  • Upper motor neurones: travel between the brain and spinal cord
  • Are multipolar (one axon and several dendrites)
• Glial cells or Neuroglia

  • Involved in feeding, insulating and protecting the neurons
  • Smaller than the neurons (10 glial cells / 1 neuron)
  • Can divide and reproduce
  • 5 types: Astrocytes, Microglia, Oligodendrocytes, Schwann cells, Ependymal cells
• Myelin sheath

  • Lipid-rich substance surrounding the axons to insulate them
  • Casing made of oligodendrocytes and Schwann cells extensions
  • Myelin wraps the nerve in segments, with gaps called Nodes of Ranvier
• Myelin sheath

  • Increases the speed of electrical impulses by forcing them to "jump" from one node of Ranvier to another
  • Acts as a "fueling station" for the axon after the generation of electrical impulses
  • Coordinates the transport of cytoskeletal proteins and organelles
  • Demyelination is the hallmark of multiple sclerosis and other neuro-degenerative diseases
  • Similar structure found in some invertebrates (shrimps, annelid worms)
• Cell signalling

  • The generic term for cells communicating with one another
  • A single cell may be sending and receiving multiple signals at once
  • Cells mostly "detect" signals by protein receptors on their surfaces
  • The receptor cell then decides how to interpret the signal
• How a neuron generates a signal

  1. By altering membrane potential

  2. Ions (K+, Na+, Cl-) are unequally distributed between the inside and outside the cell

  3. The inside of the cell is negatively charged with reference to the outside

  4. This voltage is called membrane potential
• Resting potential

  • Potassium concentration is higher in the cell, while sodium is higher outside the cell
  • This gradient is maintained by a sodium-potassium pump (which uses ATP to maintain this gradient)
  • These concentration gradients are a form of chemical energy
• How a neuron generates a signal

  1. A signal is received by a receptor cell

  2. The triggering event leads the receptor cell to convert the energy of the signal into an electrical signal by letting positively charged ions flow into the cell body and depolarising the cell membrane

  3. If the cell body gets positive enough that it can trigger the voltage-gated sodium channels found in the axon, then the action potential will be sent
• Voltage-Gated ion channels

  • Open and close in response to stimuli, allowing movement of ions and therefore changes in membrane potential
  • Ion channels in neurons are voltage-gated ion channels i.e. they open and close when the membrane potential is at a certain level
• Ion channels activity

  1. Resting membrane potential

  2. A nerve impulse causes Na+ to enter the cell, resulting in depolarization

  3. At the peak action potential, K+ channels open and the cell becomes hyperpolarized
• Generation of action potential

  1. If sodium ions influx, more sodium channels open as they are voltage gated

  2. This temporarily changes the membrane potential significantly

  3. Once initiated an AP has a magnitude independent of the strength of the stimulus

  4. AP are an all or none response
• Generation of action potential

  1. If a large enough stimulus is present an action potential (AP) is generated

  2. AP has a constant magnitude and regenerates the same potential in adjacent areas of the membrane

  3. AP can therefore spread along the membrane
• Voltage-Gated ion channels

  These open and close in response to stimuli, this allows movement of ions and therefore changes membrane potential
• Ion channels in neurons

  • They are voltage-gated ion channels i.e. they open and close when the membrane potential is at a certain level
• Generation of action potential

  1. If a large enough stimulus is present an action potential (AP) is generated

  2. AP has a constant magnitude and regenerates the same potential in adjacent areas of the membrane

  3. AP can therefore spread along axons/dendrites for long distance communication
• Refractory period

  • After an AP has been generated there is a lag phase where no more APs can be generated; the refractory period
  • The refractory period is due to inactivation of voltage-gated sodium channels
  • This recovery time ensures all AP are the same and sets the maximum frequency at which impulses can occur
• Transmitting AP – chain reaction

  1. The sodium inflow in the rising AP phase creates a current that depolarises the adjacent region

  2. This process is repeated until the end of the axon/dendrite (synapse)

  3. Magnitude will be the same at every location

  4. The zone immediately behind the AP is in the refractory period so APs can't go backward!
• Frequency of AP

  • AP has a constant magnitude but a neuron can produce hundreds of APs a second
  • The information about the environment is encoded in frequency not amplitude
  • Strong stimulus = more frequent AP
  • Small stimulus = less frequent AP
• Saltatory conduction

  • In myelinated axons, the myelin means voltage-gated sodium channels are restricted to gaps between the myelin called nodes of Ranvier
  • Myelin increases AP conduction speed
  • This process is known as saltatory conduction
• Electrical synapses

  Have gap junctions and electrical current flows directly from pre to post-synaptic neuron