Nervous System (7 & 8)

LO1: Describe the structure and function of neurons and glial cells of the CNS and PNS, and the functional organisation of these cells in the nervous system

LO2: Describe the anatomy of the meninges and their role in protecting the CNS

LO3: Describe the anatomy of the blood-brain barrier and its role in protecting the CNS

LO4: Describe the major anatomical regions and function of the cerebral hemispheres, cerebellum and brainstem

LO5: Describe the major anatomical regions and function of the spinal cord and spinal nerves

LO6: Describe the subdivisions of the peripheral nervous system and their function

LO7: Describe the features of the neuron anatomy that relate to the generation and propagation of an action potential

LO8: Describe how cells communicate and how this relates to the structure and function of a chemical synapses

LO9: Understand the nature and role of neurotransmitters in neural communication and how drugs can be used to mimic or block neurotransmitter function

The Structure of the Nervous System

Cells in the Nervous System

• Neuroglia are non-neuronal cells that support the functions of neuron’s

  • Astrocytes: Transport nutrients such as glucose to neuron’s

  • Oligodendrocytes: Produce myelin in the CNS

  • Schwann cells: Produce myelin in the PNS

  • Microglia: Immune cells of the CNS because they phagocytose debris after injury or pathogens to prevent infection.

  • Ependymal cells: Line ventricles and central canal of spinal cord to produce CSF to cushion and provide the brain with nutrients.

Meninges

• First layer of protection is provided by the skull and vertebral column

• Beneath the bone there are three layers of connective tissue containing CSF

  • Dura matter: Outer, tough, fibrous connective tissue attached to the cranium.

  • Arachnoid matter: Middle layer, attached to dura matter with a space between it and the pia matter called the subarachnoid space for CSF.

  • Pia matter: Inner thin layer of connective tissue attached to brain.

Blood Brain Barrier

• Brain receives 15% of blood supply due to neuron’s high metabolic rate (20% of our oxygen and 50% glucose)

• Brain cannot store glucose like the liver

• Capillaries are made up of a single layer of endothelial cells.

• Movement of molecules usually occurs through small pores in the endothelia cells that made up the capillary walls

• In the CNS they are held tightly together to restrict the movement of molecules and protect the CNS from harmful things in the blood creating the blood brain barrier.

• Also limits the movement of important molecules like glucose so they must be transported across, making it selectively permeable.

Cranial Nerves

Spinal Cord

• Connects CNS to PNS

• 40 - 50 cm long and 2 cm at the widest point (mid thoracic)

• Medulla oblongata to 2/3’s down the vertebral column

• Remain nerves continue down column as cauda equina

• 31 neural segments give rise to 31 pairs of spinal

  nerves.

• Emerge from the vertebral column through an opening

  called neural foramen between adjacent vertebrae.

• CNS: Brain and spinal chord

• PNS: Spinal and peripheral nerves

  • Sensory division (PNS to CNS):

    • Somatic receptors: Position, touch, pressure, pain and temp

    • Specialised receptors: Smell, taste, vision, balance, and hearing

    • Visceral receptors: Internal organs

  • Motor division (CNS to PNS):

    • Somatic nervous sytem: Skeletal muscles (voluntary)

    • Autonomic nervous system: Smooth muscle, cardiac muscle, glands, adipose tissue (involuntary)

      • Sympathetic: Fight or flight

      • Parasympathetic: Rest and digest. Relaxation.

      • Enteric Nervous System: Complex network of neuron’s that governs gastrointestinal system (second brain for peristalsis and enzyme secretion)

        

Myelin Sheath

• Schwann cell or oligodendrocyte wraps around the axon

• Cytoplasm is gradually squeezed out

• 50 - 150 stacked layers of membrane

• Myelin is made of lipids which is a poor conductor of electricity insulating the axon.

• Action potentials regenerate at the nodes of ranvier where the is no axon sheath.

Communication of Cells

• Juxtacrine signalling

  • Membrane bound proteins interact with each other and ligands in the extracellular matrix to communicate with ajacent cells.

  • Signals travel via hydrophobic membrane channels/pores (gap junctions) between cells.

• Paracrine signalling

  • Local communication between cells close together

  • Release of neurotransmitters from one cell which diffuse to the next cell

• Autocrine signalling

  • Chemical signals released by a cell bind to the receptors on the same cell

• Endocrine signalling

  • Chemical signals released by endocrine cells into the circulatory system to communicate with distant target organs.

Chemical Synapses

• Action potential arrives at the terminal

• Triggers calcium (Ca2+) to enter the presynaptic cell

• Ca2+ triggers exocytosis of neurotransmitter

• Neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic cell

• Response triggered in postsynaptic cell

• Response terminated by removing neurotransmitter from synaptic cleft

Neurotransmitters

• To be a neurotransmitter a substance must be

  • Present (and usually synthesised) within the presynaptic neuron

  • Released in a regulated fashion (usually via exocytosis from synaptic vesicles) following stimulation of the pre-synaptic neuron

  • Receptors for the substance must be present on the post-synaptic target cell

  • Mechanisms must be present to remove or inactivate the substance

• Exogenous Neurotransmitters

  • Agonist: Similar to an endogenous (natural produced in body) neurotransmitter that it binds to receptor on the pre/post synaptic neuron's and activates them e.g. morphine & heroin (opiates) mimicking the effect of natural endorphins.

  • Antagonists: Substance that is similar enough that it binds and blocks the receptor but doesn’t activate it e.g. curare binds and blocks muscle receptors causing muscle paralysis