PSYC 101 Notes

Lecture Objectives

• Identify nervous system components
• Understand how NS communication occurs
• Be familiar with function of PNS
  • Somatic nervous system
  • Autonomic nervous system
    • Sympathetic nervous system
    • Parasympathetic nervous system
• Be familiar with structure and function of CNS

Function of the Nervous System

• The nervous system directly controls all of our behavior and thoughts.
• The human brain consumes about 20 W.
• ChatGPT is estimated to run on 29,000 Nvidia A100 GPUs (each about 400 W).

Topics

• Basic Physiology
• Elementary building block: The Neuron
• The Axon
• The Action Potential
• Channels
• The Synapse
• Glia

The Neuron

• Santiago Ramón y Cajal "neuron doctrine" (1882-1934, Nobel Prize 1906)
• A nerve is a bundle of nerve fibers or axons.
• Axons are part of nerve cells or Neurons.
• Components of a Neuron:
  • Dendrites
  • Nucleus
  • Soma
  • Axon hillock
  • Axon
  • Synaptic terminals

Action Potential

• An action potential is a rapid rise from the resting potential $-70 mV$ to peak potential of $+40 mV$, followed by a return to resting potential.
• Takes about $3 - 4 ms$.
• Action potential travel along the axon as a cascade of opening and closing of ion channels.
• Voltage sensitive $Na+$ (Sodium) and $K+$ (Potassium) channels
• Clarification: two types of $K+$ channels: some always open, some (voltage sensitive…) only at the tail end of the action potential to force a quicker return to baseline (also causing the undershoot).

Channels

• Think of the channels like tiny batteries, or better switches that can turn on and off.
• When all channels are closed, the membrane potential is $0 mV$.

Resting Potential

• $-70mV$

Action Potential

• $+40mV$
• The action potential is a combination of $K+$ and $Na+$ potential. They are opposite of each other, but the $NA+$ potential is stronger.
• A purely $Na+$ driven potential would be approx. $+140 mV$.
• At the end of the action potential more $K+$ channels open This lets the membrane potential return faster to the resting potential.

Basic Types of Channels

• Voltage gated channels
  • Sodium
  • Potassium
  • Calcium
• Ligand gated channels
  • Chloride
  • Calcium
  • Sodium
• Sodium Potassium Pump
  • Pumps 2 $Na+$ out of the cell and 3 $K+$ inside, against the gradient.
  • Uses ATP.

The Synapse

• Action potential open voltage gated $Ca+$ channels.
• Increased $Ca+$ lets synaptic vesicles fuse with terminal membrane.
• Neurotransmitter (Glutamate, GABA, Ach, Dopamine) is released into the cleft.
• Neurotransmitter binds on ligand gated ion channels ($Na+$).
• Postsynaptic voltage change (EPSP, IPSP).

Neurotransmitters

• Small molecules released by the presynaptic neuron into the synaptic cleft
• They bind on receptors of the postsynaptic neuron, where they open ion channels resulting in a postsynaptic potential (PSP)
  • EPSP: (excitatory): typically $Ca2+$ or $Na+$ channels
  • IPSP: (inhibitory): typically $Cl-$ or $K+$ channels
• Acetylcholine (ACh): neuromuscular junctions; autonomous nervous system; memory; decision making; nicotinic acetylcholine receptors
• Glutamate (excitatory) & GABA (inhibitory): super common, memory…
• Dopamine: Emotions, movements (Parkinson) …
• Serotonin: Mood, hunger, pain, depression ….
• Adrenalin, Noradrenalin, Norepinephrine, Endorphins, Aspartic Acid, Histamines ….. and many many more ……

Glia Cells

• Glia (glial cells)
  • Surrounds and holds neurons in place
  • Supports neurons
  • Repair
  • Nutrient supply
  • Extracellular voltage control
  • Do not fire action potentials.
  • Probably play some other role in information processing.
  • Don’t outnumber neurons 10:1, but maybe 2:1.
• Astrocytes (astroglia)
  • Star shaped glial cells
  • Brain and spinal cord
  • Form blood brain barrier

Myelin Sheath

• Myelin sheath around axons (Glia).
• Transduction of action potentials can be slow ($1m/s$), speed increases linear with diameter.
• Giant squid axon, $1mm$ diameter $25m/s$.
• Optic nerve with 1 million axons, would be $4 cm$ in diameter.
• Solution: Myelin sheath wraps around the axon.
• It increases nerve pulse transduction.
• Human radial nerve (arm muscles) $0.013 mm$ diameter but $71m/s$.
• Insects nerve cells are not myelinated.
• Multiple Sclerosis: inflammation of myelin sheath in brain and spinal cord.

Neurons

• Neurons come in vastly different shapes and sizes
• Axons in Blue Whales are up to 20m long.

Outline of the Major Structure of the NS

• Peripheral Nervous System (PNS)
  • Somatic NS (Perception, Motor action)
  • Autonomic NS (sympathetic, parasympathetic)
• Spinal cord
  • Ventral and dorsal root
• Hindbrain
  • Medulla oblongata, Cerebellum
• Midbrain
  • Reticular formation, Tectum
• Forebrain
  • Thalamus, RF and sleep

Outline of the Nervous System (NS)

• Central NS
  • Brain
  • Spinal cord
• Peripheral NS
  • Somatic NS
    • Efferent nerves (muscles)
    • Afferent nerves (perception)
  • Autonomic NS
    • Sympathetic
    • Parasympathetic
  • Enteric NS

Somatic NS

• Somatic NS
  • Efferent nerves (muscles)
  • Afferent nerves (perception)
    • Touch
    • Temperature
    • Nociception
    • Proprioception
    • Not: vision, smell or hearing and vestibular (not spinal cord!)
  • Voluntary body movements
  • Skeletal muscles

Spinal Cord

• Mixture of nerves and ganglions
• Connects brain to rest of body (via PNS)
  • Sensory nerves transmit info from body to brain (via somatic N/S) dorsal (back) spinal cord
  • Motor nerves transmit info from brain to body (via somatic N/S) Ventral (front) spinal cord
  • Autonomic Nervous system
• Segmented organisation

Hindbrain and Midbrain

• Phylogenetic older sections of the brain, all our ancestors possessed
• “Involved into everything”
• Control many vital functions of the body. Bridge between Hindbrain and spinal cord and PNS.
  • Cerebellum
  • Pons
  • Medulla oblongata
  • Tectum
  • Cerebral peduncle
  • Midbrain
  • Hindbrain
  • Brainstem

Reticular Formation

• Maintains consciousness and regulate activity states (ascending part)
• Vasomotor control
• Integrating vestibular and motor signals

Hindbrain: Cerebellum

• ‘little cerebrum’
• Finely folded convolute surface
• Cerebrum: ~15-25 billion
• Cerebellum: ~50-100 billion
• motor control
• Memory
• …more
• Computational resource

Cerebellar Cortex

• 3 layers
  • Granular Layer = ~70 billion granule cells
    • Each receives only 4-5 synaptic inputs (pyramidal cells in the cerebrum get about 10 000 on average)
    • > 200 granule cells for every input fiber into the cerebellum!
    • Amongst very smallest of neurons
  • Purkinje Layer = ~15 million Purkinje cells
    • Highly branched and extensive dendritic tree
    • The parallel fibers run through these dendritic trees, with each Purkinje cells receiving up to 200,000 synapses from the granule cells.
    • Axons project to the deep cerebellar nuclei, and from there out of the cerebellum
    • Amongst the biggest of neurons

Cerebellum

• ‘Cytoarchitecture is homogeneous (mostly)
• Connectivity structure is heterogeneous
• => TYPE of info a cerebellar subregion receives is unique => but the COMPUTATION is constant => UNIVERSAL CEREBELLAR TRANSFORM
• Theoretical Role of the Cerebellum = comparing & updating internal expectations/models with external context/behaviour —> reduced behavioural variability
• Cerebellar damage —> dysmetria (‘loss of order’)
  • The site of the damage determines in what domain that ‘dysmetria’ manifests
  • Dysmetria of thought (DoT): “In the same way as the cerebellum regulates the rate, force, rhythm and accuracy of movements, so may it regulate the speed, capacity, consistency and appropriateness of mental or cognitive processes” Schmahmann, 1991

Midbrain

• Reticular formation … Tectum and tegmentum
• Sensory processes and the control of voluntary movements
• Substantia nigra (Parkinsons disease)
• Red nucleus (motor function)
• Pedunculi Important fiber tracts:
  • cortico-spinal,
  • cortico-pontine and cortico-bulbar tracts

Substantia Nigra and Parkinson Disease

• Substantia nigra: high melanin concentrations.
• Dopaminergic Pars compacta vs. pars reticulata
• Parkinson disease degeneration dopaminergic neurons lack of dopamine
• L-DOPA (Levodopa), dopamine precursor
• Dopamine agonists (replacements….)
• Electrical stimulation
• L-DOPA is also used as a recreational drug, bodybuilding “dietary supplement” Increased DOPA levels euphoric