PSYC1002: Neuroscience

What are the divisions of the nervous system?

How is the central nervous system protected?

• CNS consists of the brain and spinal cord

• the CNS is protected by:

  1. bone: hard casing around the brain

  2. meninges: 3 membranes that wrap around the brain and protect the brain from the inside surface of the skull

  3. blood brain barrier: a selective semi-permeable border of cells that protects the brain from harmful substances in the blood while allowing essential nutrients to pass through

How does the meninges protect the CNS?

It consists of 3 layers which protect the brain:

1. dura mater: thick outer membrane which surrounds the outside of the brain

2. arachnoid mater: a spongy membrane for shock absorption (hence the web-like appearance) which is filled with cerebrospinal fluid. large vessels which supply blood to the brain travel through this

3. pia mater: thinner, fragile, inner membrane which follows the contours of the brain

how does the blood brain barrier protect the CNS?

• the brain uses a lot of blood so if there’s toxins in the blood, the brain is highly susceptible to them

• the blood brain barrier protects the brain from toxins as a border of capillaries

• these capillaries supply blood to the CNS and have special walls which restrict the entry of many chemicals into the CNS

• capillaries in the brain are smaller than in any other party of the body, so less material passes through

what is the spinal cord and its role in the nervous system?

• the spinal cord is a cable of neural fibres with “roots” branching off

• it is the interface between the CNS and PNS

• it produces spinal reflexes- movements triggered entirely by the spinal cord receiving a message (rather than through the brain)

  • e.g. touching a hot surface- you react quickly because the message doesn’t need to be sent to the brain first

how does regeneration differ in the CNS vs PNS?

• PNS can regenerate, CNS cant, so damage to the CNS is permanent

• if the CNS (brain) could regenerate, we wouldn’t have memory

What are the divisions of the PNS?

• PNS is the rest of the nervous system outside of the brain and spinal cord

• it’s divided into:

  • sensory nervous system: nerves which deliver sensory info to the brain (afferent)

  • motor nervous system: nerves which deliver info from CNS to muscles to generate movement (efferent)

what are the divisions of the motor nervous system?

autonomic: non-voluntary bodily functions

somatic: voluntary bodily functions

what are the divisions of the autonomic nervous system?

sympathetic: activated in a stressful situation, heart beats harder and faster

parasympathetic: activated in a relaxed situation, heart beats more gently and slower, blood vessels dilate

enteric: controls gut movement and digestive processes

why does your mouth get dry in a stressful situation?

Your sympathetic nervous system gets activated and shuts down the parasympathetic situation, which makes you salivate

what is the enteric’s systems role?

• located in the walls of the gastrointestinal tract

• controls digestive activity (peristalsis and secretion of enzymes)

• senses physical and chemical conditions of the gut

• interacts with the brain yet can also function independently

• uses neurotransmitters including dopamine and serotonin

what are the ventricles in the brain?

• the human brain is not solid and consists of ventricles

• ventricles are cavities containing cerebrospinal fluid

• ventricles are the ‘sewerage system’ of the CNS by collecting waste for removal

what happens if the ventricular system gets blocked?

• this system can get blocked in a developing foetus or occasionally in adults

• this can be corrected surgically in both foetuses and adults

• for a developing foetus, there’s good recovery due to the high neuroplasticity of the developing brain

• in adults, blockage causes a build up of cerebrospinal fluid called hydrocephalus which inflates the ventricles and squashes the brain

what are the subdivisions of the human brain?

• brain stem

• cerebellum

• thalamus

• hypothalamus

• basal ganglia

• limbic system

• neocortex

• corpus callosum

what is the role of the brain stem?

controls life-supporting functions through controlling the autonomic nervous system (heart rate, blood pressure, breathing)

what happens to the brain stem after a head injury?

• a head injury breaks the blood vessels, causing intracranial bleeding which raises the pressure inside the skull

• there’s only one opening in the head, the foramen magnum, where the spinal cord joins the brain stem

• an injury pushes the brain towards this opening, crushing the brain stem

• thus, a head injury can lead to a coma or death

• treatment involves drilling a hole in the skull to relieve pressure in the brain

what is the role of the cerebellum?

• cerebellum means small brain

• it controls precise movements, including learned ones

• contains a high density of neurons (70% of the brain’s neurons) yet is only responsible for 10% of the brain’s volume

• receives sensory input and knows what signals need to be sent out to make the movement accurate and precise, and does this rapidly

what is the role of the thalamus?

• located at the centre of the brain on top of the brainstem

• is the sensory relay meaning it sends information to the cortex to experience the sensations

• thalamus allows humans to participate in selective attention through controlling what sensory input we pay attention to

what is the role of the hypothalamus?

• hypothalamus is below the thalamus

• it’s involved in hormonal regulation, motivation controls and behaviours such as feeding and sex

• it’s connected to and controls the anterior pituitary which controls glands and hormone production

what is the role of the limbic system?

• regulates emotional experiences and memories

• consists of the hippocampus, amygdala, olfactory bulb and cingulate cortex

• memories and emotions are closely linked- you’re more likely to remember more emotional experiences as your brain acknowledges those moments as important

what is the role of the basal ganglia?

• consist of the caudate nucleus, globus pallidus and putamen

• it’s responsible for action and thought processes, specifically decisions about what action to take

• works in conjunction with other brain areas e.g. cortex

what does damage to the basal ganglia cause?

Leads to neurodegenerative diseases which target the basal ganglia:

• Parkinson’s disease: results in difficulty making movement as the basal ganglia is not assisting in movement

• Huntington’s disease: results in excessive movement as the basal ganglia is not prohibiting certain movements

• damage to certain parts can cause dementia

what is the role of the neocortex?

• neocortex is a 6 layer 1cm thick convoluted sheet on top of the brain

• appears wrinkled due to folds on the cortex which allow the cortex to fit into the head

• having lots of cortex is beneficial for processing information and planning movement and action

• cortex cannot get thicker so instead gets bigger and folds

what lobes make up the neocortex?

1. frontal lobe: planning and executive functions and movement

2. parietal lobe: representing space for action

3. temporal lobe: memory and language

4. occipital lobe: vision

what are the primary motor functions which the cortex performs?

• The cortex performance primary motor functions

  • primary motor cortex: at the back of the frontal lobe, neurons send axons to the spinal cord which send signals to your muscles to move, responsible for fine movement and control (e.g. movement of hands)

  • somatosensory cortex: at the front of the temporal lobe: relating to tactile senses

  • primary auditory cortex: in the temporal lobe, receives auditory information from the thalamus, resulting in auditory experiences

what is the corpus callosum?

connects the left and right hemispheres, allowing one unified experience

what is comparative neuroanatomy and its significance?

• studies brain structure and function across species

• nervous system is almost a defining feature of all animals (except sea sponges)

• jellyfish have 5000-10000 neurons organised into networks that allow it to respond to its environment

• in insects, the nervous system has a more complex organisation

  • specialisation of some neurons

how does neuroanatomy differ in vertebrates?

• vertebrates have a separate PNS and CNS

• nervous system is organised into forebrain, midbrain, hindbrain and spinal cord

• telencephalon is a component of the forebrain and varies largely between different animals

• among vertebrates, there’s a large difference in relative size of different regions in the brain

• only mammals have a neocortex- birds have something similar but it’s not the same 6 layer structure

what does a neuron consists of?

• dendrites: thinner filaments extending from the cell body which receive info from other neurons

• soma: cell body, including the nucleus

• axon: thick filament extending from the cell body

• myelin: insulating layer coating the axon

• axon terminals: end of the axon which forms connections to other neurons

how does an action potential travel?

• neurons are covered in a semipermeable lipid membrane to control ion concentration

• polarization: at rest, the inside is negatively charged compared to the outside and the neuron is polarised

  • this resting potential is maintained by the Na-K pump which deposits 3 Na into extracellular space and 2 K into intracellular space

• depolarization: during an action potential, ion channels open so ions can enter and exit

  • there’s an influx of sodium into the neuron, shifting internal charge to positive

  • depolarization lasts a few milliseconds

• hyperpolarization: potassium channels are opened and membrane potential shifts to more negative

  • the neuron cannot receive another action potential during this

how are action potentials measured?

• neurons contain both digital and analog elements but we process them through digital (discrete) scales meaning an action potential either occurs or doesn’t

• once a neuron’s input reaches a certain threshold, an action potential is generated

• alternatively, analog scales measure continuously

how are action potentials propagated?

• action potentials don’t affect the whole neuron simultaneously

• action potentials are localised to a small segment of the membrane then spread along the membrane

• signal travels from dendrites to cell body to axon to axon terminal

what is the myelin and what is its purpose?

• myelin is an insulating material which coats axons

• myelin prevents depolarization by covering ion channels so when they open, sodium cannot flood in and an action potential does not occur at those parts of the membrane

• the myelin doesn’t coat the entire axon as if it did, an action potential wouldn’t be possible

• there are gaps along the axon, allowing the action potential to jump between these gaps were sodium influx occurs

• this jumping increases the speed of transmission

what are the 2 different pain fibres?

• we have 2 types of pain fibres- one with myelin and one without

• whilst both transmit action potentials, the myelinated a-delta fibres receive the signal faster than the c-fibres

• example: touching a hot surface

  • a-delta fibres receive the signal fast and allow you to react quickly to prevent further harm

  • c-fibres cause the delayed pain sensation

how do neurons interact?

• neurons form small junctions between each other called synapses and communicate across these

• synaptic cleft is 10-20nm wide, making transmission fast

• synapses require an electron microscope to be seen due to their size

what is the process of neurotransmission?

neurons interact through neurotransmission:

1. action potential travels along an axon, reaching the axon terminal of the pre-synaptic neuron

2. arrival of the action potential triggers the opening of calcium channels and the entry of calcium ions

3. this triggers vesicles containing the neurotransmitter to migrate towards the pre-synaptic membrane

4. the vesicle fuses with the pre-synaptic membrane, releasing the neurotransmitter into the synapse

5. the neurotransmitter crosses the synaptic cleft and binds to receptors on the post-synaptic membrane, eliciting a response

   • binding between the neurotransmitter and receptor is highly specific

6. this binding interaction generates an electrochemical force which changes the structure of the protein to open the ion channel in the post-synaptic membrane

what are the types of neurotransmitters?

Neurotransmitters can be excitatory or inhibitory

• excitatory: increases the likelihood of an action potential occurring e.g. glutamate

• inhibitory: decreases the likelihood of an action potential occurring e.g. GABA

  • this is because GABA binds to a chloride ion channel, opening it during binding

  • as chloride ions are negatively charged, its influx into the cell hyperpolarizes the membrane, preventing an action potential

why and how are neurotransmitters removed from the synapse?

• if neurotransmitters linger in the synapse and continues to stimulate receptors, the synapse wont be able to send any new signals

• to stop the neurotransmitter from lingering here:

  • enzymes in the synapse target and destroy neurotransmitters

  • OR re-uptake occurs- the pre-synaptic membrane has pumps which suck up the neurotransmitter from the synapse

how do drugs reduce the time of neurotransmitters in the synapse?

• psychoactive drugs reduce the amount of time neurotransmitters spend in the synapse by interacting with the mechanisms responsible for this

• drugs can target the enzyme which destroys the neurotransmitter, prolonging the effects of the neurotransmitter- e.g. early antidepressant medication

• modern antidepressants stop the re-uptake process, prolonging the neurotransmitter’s effect

• other drugs act on the receptor itself

how do some drugs action on the receptor?

• some drugs mimic the neurotransmitter, as they’re chemically similar to it

• these drugs bind to the receptor directly to open the ion channel that the neurotransmitter otherwise would

• other chemically similar drugs bind to the receptor but don’t open the ion channel but simply stop the neurotransmitter from accessing that receptor, blocking its action

what are the types of drugs that act on neurotransmitters?

drugs which interact on neurotransmitters are either agonists or antagonists:

• agonists: increase the effects of a neurotransmitter

• antagonists: decrease the effects of a neurotransmitter

how do recreational drugs impact the brains neurotransmission?

• Opiates (heroin, morphine, codeine) mimic the brain's opioid neurotransmitters

  • Drugs bind to opioid receptors, stimulating them as endogenous NT would

• Cocaine, amphetamines and ecstasy promote transmission of dopamine, noradrenaline and serotonin

  • Drugs promote transmission of dopamine, noradrenaline and serotonin by increasing their release or preventing their reuptake

• Nicotine

  • Stimulates acetylcholine receptors (nicotinic receptors)

• Caffeine

  • Blocks adenosine receptors, stopping adenosine from stimulating the receptor

how do therapeutic drugs impact the brains neurotransmission?

• Benzodiazepines (e.g. valium)

  • Enhance inhibitory effects of GABA

  • They bind to a different location on the same GABA receptor, amplifying its effect

  • Barbiturates and alcohol do a similar thing

• Anti-schizophrenic drugs

  • Most work by binding to dopamine receptors and blocking them

• Antidepressant drugs

  • Enhance serotonin and noradrenaline transmission

  • Modern ones function by blocking reuptake