Module 2

• Organisation of the Nervous System:

  • CNS- Brain and Spinal Cord
  • PNS- peripheral nerves
• Neurons help in transmission of information:
• Neuron structure:

  • Dendrites and Cell bodies- Input zone
  • Axon hillock- Summation zone
  • Axon- conduction zone
  • Axon terminals- Output zone
• Morphological types:

  • Multipolar- multiple processes (most commonly used)
  • Bipolar - two processes from cell body
  • Unipolar- one process from cell body
  • Anaxonic- axonless neuron
• Nervous system is composed of two cell; neuron and glia.
• Types of Glia:

  CNS-

  1. Astrocytes-supply nutrients to brain.
  2. Oligodendrocytes- what makes myelin that covers the axon.
  3. Microglia- immune cells (engulf micro-organism)
  4. Ependymal cells- line fluid-filled spaces; have cilia to circulate CSF.
• Glia supports neurons and provides nutrients and oxygen to the neurons.
• PNS

  • Schwann cells cover the axon with myelin.
  • nodes of ranvier (space in between myelin= increases conductivity).
• Structure of a synapse:

  • electrical signals sent along pre-synaptic neuron and at axon terminals, neurotransmitter (chemical) are released to the synaptic cleft.
  • Chemical neurotransmitter bind to receptors on the post-synaptic neuron which causes an AP to propagate as electrical signal along axon to another synapse.
• Information flow:

  • Afferent- towards CNS or towards brain. Sensory
  • Efferent- away from brain. Motor.
• Information transmitted can be either:

  • Somatic ( voluntary control)
  • Autonomic (involuntary/ automatic) eg. heart rate
• Somatic efferent have 2 neurons:

  • upper neuron which has its cell body in the brain and axon inthe spinal cord. (CNS)
  • lower neuron- cell body is in the spinal cord (CNS) and axon in the spinal nerves ( PNS)
• 2 Synapses in somatic efferent system:

  1. Upper neuron to Lower neuron
  2. Lower neuron to effectors
• Somatic efferent:

  • Axons are myelinated
  • Effectors are Skeletal muscles
• Autonomic Efferent (Involuntary) have two divisions:

  1.Sympathetic- fight/flight, nervous or stress responses

  • effects: ^ pupil size, heart rate, sweating

  2.Parasympathetic- "rest and digest", restful situations

  • Effects: decreased heart rate, pupil size and increased salivation.
• Sympathetic involves 3 neurons:

  • N1- cell body in brain and axon on brain or spinal cord (CNS)
  • N2- cell body in brain/ spinal cord (CNS) and axon (PNS)
  • N3- cell body and axon ( PNS)
• Synapse in Sympathetic Nervous system: N2 --> N3

  • Pre-ganglion has its cell body in the thoracolumbar region (T1- L2), axon is SHORT (PNS) and axon terminals synapse in sympathetic ganglion.
  • Post-ganglion has its cell body in the sympathetic ganglion and its axon is LONG (unmyelinated)
  • Neurotransmitter released to the effector muscles is either Acetylcholine (Ach) or Norepenephrine (Ne)
• Neuron number 3 in autonomic division is unmyelinated.
• Synapse in Parasympathetic division:

  • Pre-ganglion has its cell body in cranial or sacral, its axon is LONG and axon terminal synapse in the parasympathetic ganglion ( near effectors, distant from CNS)
  • Post Ganglion has its cell body in the parasympathetic Ganglion, axon is SHORT.
  • Neurotransmitter released (pre/post- ganglion/to effectors) is only Acetylcholine (Ach)
• Sympathetic chain Ganglia

  • located on either side of the vertebral column
  • 21-23pairs
  • it is where neuron 2 synapse to neuron 3
• Typesof ion gated-channels:

  • Chemically-gated channels- stimulus: chemicals or neurotransmitters. Often located within cell bodies and dendrites.
  • Voltage-gated channels- stimulus: change in voltage. eg @- 60 mV= reaches threshold so Na+ VG Channel opens.
  • Mechanically-gated channels- stimulus: deformation/ stretch of skin.
• External anatomy of spinal cord:

  • start at the foramen magnum just inferior to base of skull. " big hole"
  • lies within a sac made of meningis which sits inside the spinal cavity.
  • Spinal cavity extends all thw ays to coccygeal vertebrae.
  • Spinal cord ends in the Lumbar region (inferior border of L1)
• Non-neural structure of the spinal cord:

  • extends within miningeal sac and is filled with cerebrospinal fluid.
  • Conus medularis lies at the end of the spinal cord and is a non-neural tissue.
  • Filum terminal extends from conus medularis to end of spinal cavity (coccygeal vertebrae)- it anchors the spinal cord and is a non-neural fibrous tissue.
• Spinal nerves exit to the appropriate level of origin. eg. Cervical 1 must exit the inferior border of C1.
• Spinal nerves have 31 segment:

  • most have the same number of nerves as the number of vertebrae with the exception of Cervical which has 8 as 8th nerve exits inferior border of C7
  • Cervical (8), Thoracic (12), Lumbar (5), Sacral (5), Coccygeal (1)
• As Spinal cord ends at Lumbar 1, nerves must grow to reach appropriate exit level. This group of spinal nerves is called Cauda Equina.
• Internal Anatomy of Spinal Cord:

  • Dorsal side (posterior) and is responsible for flow of afferent information to the brain.
  • lower neuron (PNS-CNS) has its cell body in the dorsal root ganglion and its axon exits through dorsal roots.
• Ventral Side (anterior) is responsible for flow of efferent infromation from brain to effectors.

  • Cell body is in ventral horn and axon enters spinal cord through the ventral roots to send info to effectors.
• Dorsal Root Damaged:

  • causes loss of sensation as the brain cannot perceive the pain sent from PNS.
• Spinal nerves branch out as they leave the spinal cord:

  • Dorsal Ramus- dorsal side ( efferent /afferent info)
  • Ventral Ramus- ventral side (efferent/ afferent)
  • both then connects to the sympathetic chain ganglia.
• Structure of the Peripheral Nerve:

  • axons are covered by endoneurium
  • bundle of axon form a fascicle which is covered by perineurium
  • Bundle of fascicles is covered by epineurium.
• Somatic Vs Visceral Sensation:

  • Somatic is detected by receptors in the skin, muscle or joint.
  • Visceral is detected by receptors in the internal organs.
• Sensory Transduction:

  • Convert sensory stimulus into an action potential
  • Example: mechanoreceptors detect deformation as a result mechanically-gated channels open letting Na+ to enter- starts depolarisation. When threshold is reached, AP is sent to the brain which will then let you know that something touched you.
• Types of information encoded by neural activity:

  1. Modality- stimulus detected by receptors
  2. Duration- time period by which AP is fired (afferent)
  3. Intensity- rate or frequency of the firing of AP to in Afferent neurons.
  4. Location- location of receptors.
• Thermoreceptors:

  • nerve-ending with temperature-gated channels
  • stimulus: specific temperature
  • Fast Adapting (Phasic)
• Chemoreceptors:

  • specialised receptor cells with chemically-gated ion channels
  • Stimulus: Different chemical concentration.
• Mechanoreceptors:

  • mechanically gated ion channells
  • Examples:

  1. TACTILE RECEPTORS- respond to light,pressure, vibration, stretch

  • May be phasic ( eg Lamellar Corpuscles) or Tonic (Bulboud Corpuscles).

  2. PROPRIOCEPTORS- respond to stretch and tension

  • Awareness of limb position in relation to trunk and torso without visual aid.

  3. BARORECEPTORS- detects stretch of visceral tubes.
• Nociceptors (sensory receptor)

  • nerve-endings and respond to noxious (harmful) stimuli
  • eg. action after hand on fire.
  • can be temp /chem /mech-gated
  • Tonic (slow adapting)
• Duration (TONIC)

  • slow
  • don't adapt
  • continually active therefore constant AP sent to brain causes causes continual pain.
• Duration (PHASIC)

  • Fast-adapting
  • Normally silent
  • After stimulus= AP is sent to brain but stops quickly if stimuli is not harmful.