Nervous System


The nervous system contains a specialised network of cells called neurons and coordinates the actions of complex organisms via the transmission of electrochemical signals

The nervous system can be divided into two main parts:

  • Central Nervous System (CNS):  Made up of the brain and the spinal cord
  • Peripheral Nervous System (PNS):  Made of peripheral nerves which link the CNS with the body's receptors and effectors


Divisions of the Nervous System

The Human Brain


Nerves

There are three main types of neurons in the nervous system:

  • Sensory Neurons:  Conduct nerve impulses from receptors to the CNS (afferent pathway)
  • Relay Neurons:  Conduct nerve impulses within the CNS (also called interneurons or connector neurons)
  • Motor Neurons:  Conduct nerve impulses from the CNS to effectors (efferent pathway)


Structure of a Motor Neuron


The Stimulus-Response Model

The basic pathway for a nerve impulse is described by the stimulus-response model

  • A receptor converts a stimulus into a nerve impulse, which is transmitted by a sensory neuron to the CNS (spinal cord)
  • Relay neurons within the CNS will transmit this signal to a control centre (usually the brain), where the information is processed
  • Motor neurons will transmit a resultant nerve impulse from the CNS to an effector organ (a muscle or gland), eliciting an appropriate response


The Stimulus-Response Pathway


A reflex is a rapid and involuntary response to a stimulus and results from an even simpler pathway called a 'reflex arc'

  • Reflex actions do not involve the brain in the decision making process - instead sensory information is relayed directly to motor pathways within the spine
  • This results in a reaction without conscious thought, which may be important in survival situations when quick reactions are necessary to avoid permanent damage or pain


Transmission of a Nerve Impulse WITHIN a Neuron

  • Transmission of a nerve impulse within a neuron occurs via the movement of an electrical potential along the length of the neuron
  • When a neuron is not firing, the charge difference across the membrane is negative (-70 mV) - this is known as the resting potential
  • When a neuron is firing, the charge difference changes to become slightly positive (~ 30 mV) - this is known as the action potential
  • The change in membrane polarity when the neuron is firing (from resting potential to action potential) is called depolarisation 
  • Restoration of the resting potential is known as repolarisation


Generation of a Resting Potential

  • The sodium-potassium pump (Na+/K+ pump) maintains the electrochemical gradient of the resting potential (-70 mV)
  • It is a transmembrane protein that uses active transport to exchange Na+ and K+ ions across the membrane (antiport mechanism)
  • It expels 3 Na+ ions for every 2 K+ ions admitted (in addition, some of the K+ ions will leak back out of the cell)
  • This makes the inside of the membrane relatively negative when compared to the outside (-70 mV = resting potential)


Transmission of an Action Potential

  • Sodium and potassium channels in nerve cells are voltage-gated, meaning they can open and close depending on the voltage across the membrane
  • In response to a signal at a sensory receptor or dendrite, sodium channels open and sodium enters the neuron passively
  • The influx of sodium (Na+ in) causes the membrane potential to become positive (depolarisation)
  • If a sufficient change in membrane potential is achieved (threshold potential), adjacent voltage-gated sodium channels open, generating a wave of depolarisation (action potential) that spreads down the axon
  • The change in membrane potential also activates voltage-gated potassium channels, causing potassium to exit the neuron passively
  • The efflux of potassium (K+ out) causes the membrane potential to become negative again (repolarisation)
  • Before the neuron can fire again, the original distribution of ions (Na+ out, K+ in) must be re-established by the Na+/K+ pump
  • The inability to propagate another action potential during this time (refractory period) ensures nerve impulses only travel in one direction

Saltatory Conduction


Generation of an Action Potential


Transmission of a Nerve Impulse BETWEEN Neurons

  • The junction between two neurons is called a synapse, it forms a physical gap between the pre-synaptic and post-synaptic neurons
  • An action potential (electrical signal) cannot cross the synaptic gap, so it triggers the release of chemicals (neurotransmitters) to continue the signal


Chemical Transfer Across Synapses

  • When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels
  • Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitters) with the plasma membrane
  • The neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft
  • Neurotransmitters bind to appropriate neuroreceptors on the post-synaptic membrane, opening ligand-gated channels
    • Excitatory neurotransmitters (e.g. noradrenaline) open ligand-gated sodium channels (depolarisation)
    • Inhibitory neurotransmitters (e.g. GABA) open ligand-gated potassium or chlorine channels (hyperpolarisation)
  • The combination of chemical messengers received by dendrites determines whether the threshold is reached for an action potential in the post-synaptic neuron
  • Neurotransmitter molecules released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)


Overview of Synaptic Transfer


Drugs and Toxins

Neurotransmitters can have a variety of responses depending on the cell type activated:


  • Drugs and toxins may affect animal behaviour and physiological responses by effecting the transmission of nerve impulses between neurons
  • Drugs that stimulate the nervous system are called agonists, while drugs that inhibit the nervous system are called antagonists
  • Drugs are commonly used to treat medical conditions or recreationally (to alter mood and perception)
  • Toxins are toxic substances (poisons) produced by organisms which can cause pain, paralysis and even death