L4 Human Physiology

🧠 Concepts Introduced

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Organization of nervous systern

Describe the components of nervous system and direction of information flow. Describe the basic anatomy and functions of cells in the nervous system. Describe how electrical signals are produced and carried in the nervous system Describe cell-to-cell communication across chemical synapses through neurotransmitters

The nervous system

Subdivided into 2 anatomical parts: Central nervous system

• Brain and spinal cord.

Peripheral nervous system

• Afferent (sensory) & efferent (motor) division.

Cells of the nervous systems

Neurons:

• Transmits electrical impulses
• Structure include: Cell body Dendrites Axons

Glial cells:

• Provide structural and metabolic support for neurons.

Classification of neurons

1. bipolar
2. pseudounipolar
3. multipolar

Organisation of the nervous system

functional classification

Glial cells in the nervous systems

oligodendrocyte allow transmission of impulse to be faster

astrocyte support neurons, forms blood brain barrier to prevent toxins or infections from blood to contaminate the brain

ependymal cells produces cerebrospinal fluid

microglial cells are the brain's specialized immune cells. They act as macrophages, moving around to clear out cellular debris, dead neurons, and damaged tissue.

Myelinated neurons

Oligodendrocytes and Schwann cells • provide support, insulation around axons and speed up signal transmission

Transmission of electrical signals

Basis of resting membrane potential. Describe how graded potentials in neurons trigger action potentials. Describe the ionic basis of an action potential. Describe propagation of action potentials in neurons.

Action Potential

cell membrane is highly permeable to K+ (more K+ leakage channel) slightly permeable to Na+ (less Na+ leakage channel) impermeable to large negatively charged proteins

resting membrane potential is -70mV extracellular fluid has higher concentration of Na+ and Cl- intracellular fluid has lower concentration of Na+ and Cl- but higher concentration of K+ and negatively charged proteins

stimulus causes membrane potential to reach threshold of -55mV this causes depolarisation of membrane as voltage gated sodium ion channel opens and more sodium ions diffuse into the cell this causes the intracellular environment to be more positively charged compared to extracellular environments

at +30mV, repolarisation occurs as voltage gated sodium ions channel close and voltage gated potassium ion channel open, potassium ion diffuse out of the cell

voltage gated potassium ion channel closes slowly causing hyperpolarisation. excess potassium ions continue to diffuse out of the cell

to reestablish resting potential back to -70mV sodium-potassium pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell

Refractory Period (time taken for membrane to recover) from depolarisation to reestablishment of resting potential

• Sodium channels quickly become inactivated & unresponsive to stimulus
• voltage - gated sodium channel shut down
• neuron cannot fire again even if there're strong stimulus

Membrane will not undergo another action potential throughout this

• give cells time to reset
• avoid cell immediately firing again + confusion in signal
• this also ensures that nerve impulses travel in one direction only
• next part of axon can be activated to ensure message move forward & not backward

all or none any stimulus that exceeds the threshold, will produce action potential

propagation of action potential conduction along unmyelinated fibres

• depolarisation of one area of membrane causes local current flow between neighbouring areas on the membrane
• this current flow causes depolarisation immediately adjacent to the site of the original stimulus
• shorter current flow

conduction along myelinated fibres

• depolarisation can only occur at the nodes of ranvier
• increase local current flow
• action potential jumps from one node of Ranvier to another node of Rnavier (saltatory conduction)

Synaptic Transmission & Gap Junction

action potential reaches axon terminal
voltage gated calcium ion channels open
calcium ions diffuse into cell
synaptic vesicle containing neurotransmitter fuse with pre-synaptic membrane
neurotransmitter released into synaptic cleft and diffuse across the synaptic cleft
neurotransmitters bind to the receptor on post-synaptic neuron causing depolarisation on post-synaptic neuron

neurotransmitters removed from synapse by:

1. being reabsorbed by presynaptic membrane from axon terminal
2. moving away through diffusion
3. being degraded by certain enzymes

plus Connexon

Excitatory and Inhibitory Neurotransmitters

Excitatory neurotransmitters-Increase the likelihood that a receiving neuron will fire an action potential. Inhibitory neurotransmitters decrease that likelihood. Eg: GABA

Inhibitory postsynaptic potential (IPSP Results in the hyperpolarization of the postsynaptic membrane. May be caused by the opening of postsynaptic K+ channel. May be caused by the opening of postsynaptic CI- channel. Lowers the membrane potential, less likely to fire an action potential. Eg: GABA, Glycine

Graded Potential

Variable strength Amplitude decreases with distance from cell body Multiple graded potential can summate to create an action potential. No threshold

Brain

Midbrain

Brainstem - vital link between the spinal cord and higher brain regions

• Consists of midbrain, pons, and medulla.
• Area where most cranial nerves arise.
• Contains centers that control cardiovascular, respiratory, and digestive function.
• Regulates equilibrium and postural reflexes.
• Controls overall degree of cortical alertness.
• Centers that govern sleep are housed within the brainstem and hypothalamus.

Midbrain

• Control of eye and other body movement
• Processes auditory and visual signals.

Pons

• Connects cerebellum and cerebrum.

Medulla Oblongata

• White matter includes somatosensory (ascending) and corticospinal (descending) tracts
• Gray matter - control involuntary function e.g. blood pressure, breathing, swallowing and vomiting

Cerebellum

uderneath cerebrum

convolution (parallel ridges) grey matter in outer region white mater in inner region

posture & balance & fine coordination connected to inner ear (receive sensory)

stooped posture + cannot balance + no coordination jerky & uncontrolled movements

Action potential not originate from the cerebellum (originates form mortor cortex) so still can move without it but will be no balance, jerky and uncontrolled

gets information from:

1. stretch receptor
2. inner ear
3. cerebrum

Diencephalon

Thalamus

• Receive sensory fibers from optic tract, ears, spinal cord and motor information from cerebellum.
• Projects fibers to cerebrum, where information is processed.
• Serve as relay and integrating station.

Hypothalamus

• Center of homeostatic regulation.
• Influence autonomic nervous system and endocrine functions.

Functional Areas

frontal lobe - thinking, personality, language, control of voluntary movement

• broca's area (speech production)

parietal lobe - process sensory information such as temperature and touch

temperal lobe - receive & process auditory information

• wernicke's area (speech comprehension)

occipital lobe - vision

insula - recognition of different senses & emotion, addiction & psychiatric disorders

functional area: sensory area - receive & interpret impulses from receptors motor area - control voluntary movements association area - concerned with intellectual & emotional processes & interpret information from senses

White matter

Consists mostly of myelinated axons. Appears white due to myelin sheaths. Carries signals between different parts of brain.

3 types:

• connect various areas of the cortex within the same hemisphere
• carry impulses between left & right hemisphere
• connect cortex to other part of the brain or to spinal cord

Grey matter

Consists of nerve cell bodies, dendrites and unmyelinated axons Receive and integrate information

Protective structure of CNS

1. bones cranium > rigid so protects brain

vertebral collumn > rigid, protects spinal cord against physical impact

2. meningeal layers > (covers the surface of the brain & spinal cord) dura matter: protect the brain from moving around brain - stick close to thebrain spinal cord - has padding layer for more flexibility movement to bend

arachnoid matter: space for cerebrospinal fluid to flow loose mesh of fibres

pia mater: to protect brain from infection

3. cerebrospinal fluid from ventricles, circulating in cavity of brain fills up space in arachnoid mater brain immersed in CSF acts shock absorber, lessens impact, cushioning blows transport nutrients to the cells of brain & spinal cord carries away waste support weight of brain that is suspended in cranium

4. Blood-brain barrier: Highly selective permeability of brain capillaries. Prevent harmful substances in blood from entering the interstitial fluid.

Metabolic requirements of neural tissue

Oxygen

• Passes freely across blood-brain barrier.
• Brain receives 15% of blood pumped by the heart.

Glucose

• Brain responsible for about half of body's glucose consumption.
• Membrane transporters move glucose from plasma into the brain interstitial fluid.
• Hypoglycemia leads to confusion, unconsciousness, and death