Neuroscience and Clinical Semester 1 Week 1: The anatomy of the nervous system Flashcards
What are the two broad classes of cells integrated to form brain tissue?
Neurones - around 100 billion in human brain
Neuroglia cells - outnumber neurones 10:1
Specialised features of neurones
- Axon
- Myelin sheath
- Dendrite
- Spines
- Synapse
Axon
- output side of the cell
- Arises from a conical shaped thickening on cell body called the ‘axon hillock’, long narrow process that projects from cell body, can travel distances ranging from microns to metres. - Axonal membrane is excitable, specialised for generation and propagation of action potentials.
Dendrite
- input side of the cell
- The short processes emanating from the cell body, which receive most of the synaptic contacts from other neurones.
- Highly branched, giving rise to a dense network of processes, the ‘dendritic arbour’ [spines].
Spines
- Project from the surface of dendrites
- Receive most of the excitatory input
- Have a neck and head
- Average length = 2 μm
- Contain filaments [actin and α- and β-tubulins]
- Dynamic (change in response to activity)
Synapse
Point of specialised contact between two neurones, where chemical signals are transmitted.
What features of a neurone are the same as other cells?
- Nucleus: contains DNA
- Rough endoplasmic reticulum: synthesises proteins
- Smooth endoplasmic reticulum: synthesises fats
- Golgi complex: packages products for transport around the cell
- Mitochondria: powerhouse of the cell
- Lysosomes: waste disposal system
- Cytoskeleton: maintains shape and provides ‘rails’ for transport
Neuronal classification
Neurones can be classified according to:
- number of processes
- length of axon
- shape of cell body
Number of processes
- Bipolar (one dendrite, e.g. purkinje cells)
- Multipolar (more than 1 dendrite, e.g. dopaminergic neurons)
Length of axon
- golgi type I neurone (long axon + large - travels to another brain structure, e.g. pyramidal cells)
- golgi type II neurone (short axon - usually stay in structure where cell is located, e.g. spiny stellate cells)
Shape of cell body
- Ovoid (circular/spherical, e.g. spiny stellate cells)
- Fusiform (fat in middle, thinner edges, e.g. dopaminergic neurons)
- Triangular (e.g. pyramidal cells)
Types of glial cells
CNS:
- Oligodendrocytes
- Astrocytes
- Microglia
PNS:
- Schwann cell
Oligodendrocytes
Have extensions that wrap around axons in the CNS. The extensions are rich in myelin (fatty insulating substance) that forms myelin sheaths which increase the speed and efficiency of axonal conduction. Myelinated segments of axon = internodes; unmyelinated = nodes of Ranvier
Astrocytes
- Star-shaped
- Communicate with each other through gap junctions.
- Ensheath synapses and dendrites
- Project processes to cell somas (neuron cell body)
- Cover blood vessels in the brain.
- Maintain tight junctions in endothelial cells (which form blood brain barrier)
- Uptake of neurotransmitters (gets rid of them when they’ve done their job)
- Neurovascular coupling
Microglia
- Represent the immune system in the brain, as the brain is an immunologically privileged site because of the BBB (immune system )
- Involved in phagocytosis (consuming things that need to be ‘eaten’)
Schwann cells
- located in the peripheral nervous system.
- forms the myelin sheath, an insulating layer around nerve fibers.
Neuroanatomical directions
- Dorsal (towards the back/top of head) + ventral (towards the chest/bottom of head)
- Anterior/rostral (towards front/nose) + Posterior/caudal (towards bottom/tail)
- Medial (towards middle) + Lateral (away from middle)
Planes of section
- Coronal/frontal plane (ear to ear, front and back sections)
- Sagittal plane (vertical plan dividing brain into left and right sections)
- Horizontal plane (upper and lower sections)
5 major subdivisions of the brain
FOREBRAIN
- Telencephalon (undergoes greatest growth during development)
- Diencephalon
MIDBRAIN
- Mesencephalon
HINDBRAIN
- Metencephalon
- Myelencephalon
‘Encephalon’ = within the head
Brain stem = hindbrain + midbrain
Myelencephalon (medulla oblongata)
- Most posterior division
- Largely composed of tracks that carry signals between brain and body
- Reticular formation - network of around 100 tiny nuclei occupying the central core of the brain stem, from the posterior boundary of the myelencephalon to the anterior boundary of the midbrain. These nuclei are involved in various functions e.g. respiration, sleeping etc.
Nuclei in the myelencephalon
Nuclei that control:
- Dorsal (inspiration/breathing in) and Ventral (rhythm) respiratory groups
- Hypoglossal nucleus -> tongue musculature
- Nucleus ambiguus -> blood pressure and heart rate
Part of the autonomic nervous system
Nucleus ambiguus gives rise to part of the vagus nerve.
Vagus nerve is part of the parasympathetic nervous system which controls heart muscle, smooth muscle (e.g. blood vessels) and glands (e.g. adrenal glands)
Metencephalon: Pons
- contains nuclei which are involved in sleep + arousal
- e.g. locus coeruleus - provides noradrenaline (neurotransmitter) to rest of brain, which is involved in level of alertness
Metencephalon: Cerebellum
Cerebellum - ‘little brain’
- integrates sensory information and uses this to modify motor output
- Coordinates and smoothes movements
- Damage = jerky/exaggerated movements, poor coordination (alcohol ‘turns off’ the cerebellum)
Mesencephalon: tectum
Tectum:
- Dorsal surface of midbrain
- Composed of two pairs of bumps: Inferior colliculi (posterior pair) and superior colliculi (anterior pair)
- Inferior - part of auditory system
- Superior - visual-motor function (e.g. rapid eye movements)
