nerves

• Stimulus


  • pressure on skin, chemicals in food, light turning on
• The Human Nervous System

  The human nervous system is composed of two parts, the central nervous system (CNS) composed of the brain and spinal cord and the peripheral nervous system (PNS) consisting of the nerves and ganglia on the outside of the CNS
• Receptor

  Detects stimulus (one form of energy) and converts this to electrical energy to send the information as an electrical impulse to the central nervous system via neurones
• Receptor

  • photoreceptor, thermoreceptor, mechanoreceptor, osmoreceptor/chemoreceptor
• Neurones

  Highly specialised cells that generate and transmit nerve impulses
• sensory neurone :

  Carry impulses from the receptor to the CNS
• Functional types of neurones in mammals

  • Sensory
  • Motor
  • Relay (connector or association)
• In mammals, responses to many external and internal stimuli involve the reception of information and its transfer from a receptor to an effector via the nervous system (or as hormones via the blood in the endocrine system)
• Effector


  • muscles / glands
• Effector

  Receives electrical impulses from the CNS and brings about a response
• motor neurone :

  
  

  Carry impulses from the CNS to the effector organs (muscles/glands)
• relay neurone :

  
  

  Found within the spinal cord, receive impulses from sensory neurones or other intermediate neurones and relay them to motor neurones or other intermediate neurones
• Node of Ranvier, Schwann cell nucleus, Myelin sheath, Axon, Dendrites, Cell Body/centron, Synaptic end bulb/Axon endings, Effector organ
• Sensory – cell body in middle of neurone, whereas motor & relay cell body at the start of neurone. Motor and sensory neurones have a longer axon
• The Spinal Cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. It is protected by the spinal column. Most of the peripheral nerves originate from the spinal cord. The function of the spinal cord is the transmission of neural signals between the brain and the rest of the body. It also contains neural circuits that can independently control numerous reflexes. The spinal cord is made up of grey matter containing nerve cell bodies surrounded by white matter consisting of nerve fibres surrounded by myelin sheath (axons). It is surrounded by membranes called meninges
• Simplest type of response is a simple reflex arc, an inborn response to a stimulus that is rapid, automatic, and beneficial
• Spinal cord structure

  Central area of grey matter containing nerve cell bodies, surrounded by white matter consisting of nerve fibres with myelin sheath, surrounded by meninges
• With any reflex action

  Impulses are sent to the brain via ascending nerve fibres that synapse at the grey matter of the spinal cord
• Reflexes

  • Blink reflex, pupil reflex, knee-jerk reflex
• Motor fibres leaving the spinal cord

  Leave via the ventral roots
• Sensory fibres entering the spinal cord

  Enter on the dorsal root with cell bodies found in the dorsal root ganglia
• Spinal cord

  Contains neural circuits that can independently control numerous reflexes
• Neurone

  An excitable cell that can change its resting potential
• Resting potential is the potential difference across the axon membrane of a neurone when no nervous impulse is being conducted
• Resting potential is maintained via the movement of sodium and potassium ions
• Three sodium ions are pumped out of the axon for every two potassium ions pumped in by the sodium-potassium pump
• Resting potential is around -70mV relative to the exterior of the axon
• The brain may store reflex information or relate it to other sense data
• Neurones transmit electrical impulses along the cell membrane by changing the potential difference across the axon membrane
• Neurones have a resting potential of -70mV
• Voltage-gated potassium channels allow potassium to diffuse back out of the axon
• All or Nothing Law
• Repolarisation

  1. Voltage-gated sodium ion channels close, potassium ion channels open

  2. Potassium ions rapidly diffuse out, reducing potential difference across the membrane

  3. Overshoot causes hyperpolarization
• Refractory period

  1. Concentrations of K+ and Na+ are restored

  2. Axon cannot transmit another action potential for about 1ms
• Action potential generation

  1. Stimulus causes voltage-gated channels to open

  2. Threshold voltage reached, permeability to sodium ions increases, leading to depolarization

  3. Negative charge inside the axon becomes positive

  4. Membrane is depolarized
• An action potential is either initiated or not and is always the same size
• Resting potential generation

  1. Voltage-gated channels allow ions to diffuse back out of the axon

  2. Membrane is more permeable to potassium than sodium

  3. Neurone has a resting potential of -70mV

  4. Some K+ channels allow leakage of K+ ions
• The size of the impulse is independent of the size of the stimulus
• The speed of conduction of the stimulus is not altered by the intensity of the stimulus
• Nerve Impulse Transmission in Non-Myelinated Neurones