MBB1 - Structure and Action of Neurons and the Anatomical Divisions of the Brain

features of a typical neuron

soma (cell body), dendrites, myelin sheath, axon (inside myelin sheath), terminal buttons

action potential

rapid change in the membrane potential of the neuron caused by the movement of ions (when a neuron fires)

ions

charged molecules

membrane

edge of cell

membrane potential

difference in total charge between the inside and outside of a cell

what is the resting membrane potential

-70mV

polarise

extreme separation

Step 1 of an action potential

-sodium channels open

-sodium ions flow rapidly into the neuron through sodium channels

-influx of positive ions increases charge within the neuron, increasing the membrane potential in a process known as depolarisation

Step 2 of an action potential

-if there is sufficient sodium influx to depolarise the membrane potential beyond the threshold of excitation of -50mV, this triggers the opening of potassium cahnnels

-potassium flows out of the neuron while sodium is still entering, causing further depolarisation of the membrane potential

Step 3 of an action potential

-after approx. 1 millisecond, the actions potential reaches its peak and the sodium channels become refractory (close)

-the charge inside the neuron is more positive (+40mV) than the outside

-sodium channels will not be able to open again until the membrane potential returns to the resting potential

Step 4 of an action potential

-as the potassium channels are still open, potassium continues to exit the neuron

-this makes the charge within the neuron increasingly negative as the proportion of positive ions decreases

-hyperpolarisation means the charge is more negative than the resting rate during the refractory period

Step 5 of an action potential

-once the membrane potential returns to the resting rate of -70mV, the potassium channels close and the sodium channels reset so that another depolarisation can cause them to open again

-due to hyperpolarisation, a subsequent action will require a greater sodium influx immediately after the neuron has fired

what causes sodium channels to open

-change in the charge around the membrane (depolarisation), eg. when signals such as neurotransmitters are received

what does the sodium-potassium pump cause

a greater concentration of sodium ions outside the neuron, a greater concentration of potassium ions inside the neuron

rate law of action potentials

stronger stimulus = faster threshold for activation = more frequent action potentials

neuron firing as an 'all or none' event

will always go from -70mV to +40mV - can't have a bigger action potential and will always flow down the neuron in the same way - will never be faster or bigger

what can change in terms of action potentials

the gap between individual action potentials (frequency/rate)

what determines the strength of a neural signal

frequency of firing

rostral/anterior

towards the nose - front

caudal/posterior

toward the tail - back

dorsal/superior

toward the back - top

ventral (inferior)

toward the belly - bottom

lateral

away from the midline

medial

toward the midline

ipsilateral

on the same side of midline

contralateral

on the opposite side of midline

corpus callosum

thick section of the brain that consists of large bundles of axons that connect two hemispheres

homotopic connections

connect complimentary region of the other hemisphere

heterotopic

communicate to a different brain region

callosotomy

procedure to cut the corpus collosum to stop severe epileptic seizures - prevents communication between hemispheres

3 major divisions of the brain

forebrain, midbrain, hindbrain

subdivisions of the forebrain

telencephalon and diencephalon

what does the telencephalon consist of

the limbic system, basal ganglia and cerebral cortex

cerebral cortex

-the largest structure of the human brain - divided into two cerebral hemispheres

-inner 'white matter' is pale because of high proportion of axon fibres covered in fatty myelin layer

-outer 'grey matter' is where the neurons synapse and connect together

what is the basal ganglia responsible for

coordinating involuntary movement, particular aspects that are highly automatised or involuntary (eg. walking)

-different to cerebellum because basal ganglia refer to highly automatised movements

what happens in Parkinson's patients

the basal ganglia are dysfunctional, leading to weakness, tremors, limb rigidity, poor balance and difficulty initiating movements

limbic system

-neural system including the hypothalamus, thalamus, hippocampus and amygdala

-thought of as the 'emotion circuit'

-first proposed by James Papez in 1937

what does the diencephalon consist of

thalamus and hypothalamus

thalamus

major relay station for sensory inputs to cerebral cortex - divided into several nuclei

-eg. when playing tennis, info (sight of ball) will be relayed to areas controlling hand movement

hypothalamus

controls autonomic nervous system and endocrine (hormone) system - regulates survival behaviours (fighting, feeding, fleeing, mating)

what does the midbrain consist of

mesencephalon

mesencephalon

-located within the brainstem at the topmost region, directly above the hindbrain

-connects pons and cerebellum with the forbrain

-important role in motor movement, particularly movement of the eye and in auditory/visual processing

-consists of tectum/tegmentum

hindbrain subdivisions

metencephalon and myelencephalon

metencephalon components

cerebellum and pons

cerebellum

-'little brain'

-helps coordination of movement - different to basal ganglia as it uses visual feedback to fine tune movement to carry out a particular task

-damage to it causes problems with walking and leads to jerky, poorly coordinated movements and problems retaining balance

pons

-lies on the ventral surface of the brainstem

-contains several nuclei important in regulating sleep and arousal

-also relays info from cerebral cortex to the cerebellum

myelencephalon

-medulla

-links hindbrain to the spinal cord and contains neurons important for autonomic function like respiration and heart rate

-damage makes it impossible to live

What are the lobes of the cerebral cortex?

frontal, parietal, occipital, temporal

primary visual cortex

the medial and lateral region of the posterior occipital lobe whose primary input is from the retina.

where is peripheral vision processed?

anterior occipital lobe that extends into the calcarine fissure.

What did Hubel and Wiesel find in 1981?

Neurons show orientation selectivity

parietal lobe

attention and spatial awareness on the dorsal surface- the "where" pathway.

temporal lobe

auditory processing and complex visual processing on the ventral surface- "what"

primary auditory cortex

- superior part of the temporal cortex and a section within the Sylvian fissure.

- receives auditory info. from the cochlea (tonotopic map)

primary somatosensory cortex

- area of the parietal lobe where messages from the sense receptors are registered (somatotopic)

- posterior to the central sulcus

primary motor cortex

- the section of the frontal lobe responsible for voluntary movement

- located on the precentral gyrus

frontal lobe

associated with reasoning, planning, parts of speech, movement, emotions, and problem solving

- Railway man Phineas Gage (1848) - damage to frontal lobe, had severe personality changes