Psych Week 19 (Bio) - Neurons, Synapses and the Brain

What is cephalisation?

• Nervous tissue concentrated at one end of an organism

How have nervous systems evolved?

• Jellyfish have diffuse nerve nets with no CNS

• Bilaterians (e.g. worms) have a nerve cord and a brain

• Humans have CNS and PNS

What is the broad microstructure of the nervous system?

• Nerve cells (neurons) pass information via the sending and receiving of neurotransmitters

• Glia cells help to support and connect the neurons - ‘glue’ of the nervous system

What did Cajal and Golgi discover about the nervous system?

• That neurons are separable (small gap between one and the next)

• That the nervous system therefore consists of individual neurons

What are the functions of glia cells in the nervous system?

• Provide structure and support - e.g. astrocytes hold neurons in place and supply them with nutrients and oxygen

• Insulate nerve cells - e.g. oligodendrocytes in CNS, Schwann cells in PNS

• Remove dead neuronal tissue and immune defence - e.g. microglia

• Modulate neurotransmission to synapses

Why is the soma of a neuron important?

• Contains cell nucleus with genetic info (DNA)

• Contains cell ‘machinery’ - mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus

Why are the dendrites of a neuron important?

• Receive information from other neurons

• Greater surface area means more info can be received

• Synapses located on dendrite surfaces

Why is the axon of a neuron important?

• Sends information to other neurons

• Distal end has branches that swell at the tip to form a presynaptic terminal (bouton)

• Some axons have boutons along their fibres

• Myelin sheath used to speed up electrical transmission

What are the three types of neurons and their functions?

• Afferent - carry info away from receptors and to the brain

• Efferent neurons - carry info from the brain to the PNS to initiate an action

• Interneurons - connect other neurons

What is an excitatory postsynaptic potential (EPSP)?

• Change in polarisation moving along dendrites to the soma

• Provokes depolarisation (decreases polarisation), so neuron is more likely to fire action potential

What is an inhibitory postsynaptic potential (IPSP)?

• Change in polarisation moving along dendrites to the soma

• Provokes hyperpolarisation (increases polarisation), so neuron is less likely to fire action potential

What is resting membrane potential?

• Neuronal membrane maintains an electrical gradient between the inside and outside of the cell when the neuron is not firing

• Gradient is average -70mV, with the inside of the cell being more negative than the outside

What is summation?

• Postsynaptic potentials add up

• Over space - different dendrites (meet other potentials at a point)

• Over time - same dendrites (followed by other potentials)

How do action potentials propagate down an axon?

• Saltatory conduction

• First potential is at axon hillock, then ‘jumps’ between unmyelinated nodes of Ranvier

• Myelin sheath prevents change leakage

What is multiple sclerosis?

• Neurological condition caused by demyelination of axons in the brain and spinal cord

• Causes problems with vision, walking and fatigue

What must happen for an action potential to be produced?

• Neuron only fires if membrane is depolarised enough for threshold potential is reached (~ -55mV)

What is a synapse?

• Gap that connects presynaptic and postsynaptic neurons

• Action potential reaches end of axon and opens calcium ion gates

• Neurotransmitter travels across synapse and binds to receptors

• Movement of ions in and out of postsynaptic neuron causes action potential (or not)

How do inhibitory neurotransmitters work?

• Bind to receptors that open chloride ion gates, causing Cl- to enter the postsynaptic neuron

• Potassium ion gates open and K+ leave the postsynaptic neuron

• Causes IPSP as neuron gets more negative and therefore hyperpolarised

• e.g. GABA, glycine, ACh

How do excitatory neurotransmitters work?

• Bind to receptors that open sodium gates, causing Na+ to enter the postsynaptic neuron

• Causes EPSP as neuron gets more positive and therefore depolarised

• e.g. glutamate, norepinepherine, ACh

What are ionotropic receptors?

• Open ‘fast’ gates

• Used for vision and hearing

What are metabotropic receptors?

• Open ‘slow’ gates

• Used for taste, hunger, fear

What are neuromodulators?

• Chemicals that affect metabotropic receptors

• Are neurotransmitters, but called neuromodulators as they act slower than neurotransmitters at ionotropic receptors

What is synaptic strength?

• Example of neuroplasticity

• Strength of a synapse is defined by the size of postsynaptic potential

• May vary over time - long-term (learning, memory) or short-term (seconds or minutes)

• e.g. long-term potentiation - intensive use of synapse may lead to stronger effects of postsynaptic membrane

What is synaptogenesis?

• Example of neuroplasticity

• Growth/formation of new synapses

• Creates networks

What is synaptic pruning?

• Example of neuroplasticity

• Elimination of existing synapses

• Increases efficiency of transmission - is a process of learning

What is re-uptake of neurotransmitters?

• Used neurotransmitters returned to the presynaptic neuron by chemicals called transporters

• Concerns serotonin and the catecholamines

• Some are broken down (e.g. ACh)

• Big ones diffuse

How can pharmaceuticals affect synapses?

• Can facilitate synaptic transmission (agonists)

• Can inhibit synaptic transmission (antagonists)

• Can impact amount of neurotransmitter in synaptic cleft

What is the peripheral nervous system?

• Nerves outside of the brain and spinal cord

• Somatic nervous system - conveys info from sense organs → CNS → effectors

• Autonomic nervous system - controls internal organs (sympathetic and parasympathetic)

What are the two top-down routes in the somatic nervous system (CNS to muscles)?

• Dorsolateral tract - controls movement in peripheral areas such as fingers and toes

  • Controls contralateral side (one side) of body

• Ventromedial tract - controls more proximal areas such as neck and shoulders

  • Controls both sides of body

What are the two bottom-up routes of the somatic nervous system (receptors to CNS)?

• Dorsal column medial lemniscal pathway - info about touch, vibration, proprioception

• Spinothalamic tract - info about pain and temperature

What are the two mechanisms for homeostatic regulation?

• Neurohormonal

• Behavioural

• Both used to maintain a negative feedback loop

What makes up the empty space in the CNS?

• Central canal in the centre of the spinal cord

• Four ventricles in the brain

• All filled with cerebrospinal fluid

What is white matter?

• Interhemispheric connections (e.g. corpus callosum connects left and right)

• Cortico-subcortical pathways (cortex to subcortical and motor regions in brainstem and spinal cord)

• Subcortical-cortical pathways (brainstem and thalamus to cortex)

What are phylogenesis and ontogenesis?

• Sequence of events involved in evolutionary development of a species (phylogenesis)

• Process of an individual organism’s growth and development (ontogenesis)

What are the functions of the brainstem?

• Transmits info to and from the brain

• Responsible for simple reflexive behaviour

• Responsible for physiological states

What are the components of the hindbrain (rhombencephalon)?

• Medulla oblongata - vital functions controlled via cranial nerves

• Pons - body balance, vision, auditory processing

• Cerebellum - fine motor skills, coordination and balance, cognitive skills like attention and language

What are the components of the midbrain (mesencephalon)?

• Superior colliculus - multisensory

• Inferior colliculus - main auditory centre

• Substantia nigra - nucleus for reward functions and motor movement

What are the components of the forebrain (prosencephalon)?

• Diencephalon - contains thalamus and hypothalamus

  • Thalamus is sensory (not olfaction)

  • Hypothalamus is homeostasis

• Mammillary bodies - relay for impulses coming from the amygdala and hippocampus

What are Brodmann’s areas?

• System to divide cerebral cortex based on cytoarchitecture

• e.g. into primary, motor, somatosensory cortex

What is the function of the frontal lobe of the cerebral cortex?

• Movement and high cognition, including speech (Broca’s area)

• Primary motor cortex

• Decision-making, impulse control, behaviour in response to reward (prefrontal cortex)

  • Damage to prefrontal cortex changes personality and behaviour (e.g. Phineas Gage who survived a rod through the skull)

What is the function of the parietal lobe of the cerebral cortex?

• Body sensations and spatial localisations

• Primary somatosensory cortex (sensations)

• Parietal association areas (combine info from body senses and vision)

What is the function of the temporal lobe of the cerebral cortex?

• Language, audition, visual association

• Auditory cortex (receives info from ears)

• Wernicke’s area (language comprehension and production)

• Inferior temporal cortex (visual identification)

  • Damage causes prosopagnosia (difficulty recognising faces and objects)

What is the function of the occipital lobe of the cerebral cortex?

• V1 (primary visual cortex a.k.a striate cortex) and other visual areas

• Damage to V1 causes cortical blindness

What is the limbic system?

• Forebrain, emotional and behavioural responses

• Hippocampus (creation of memories and integrating them into stable knowledge)

• Amygdala (emotional behaviours and memories)

• Cingulate cortex (behaviour linked to motivation and learning)