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Human Biology > Nervous System
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Neurones

  • Soma: nerve cell body / contains nucleus, cell organelles / synthesise of neurotransmitter
  • Dendrites: branching extensions from nerve cell soma
    • Stimulated by other neurones \ transmit impulses towards soma
  • Axon: single extension that extends from the soma to the target cell
    • Myelinated axon is surrounded by Schwann cells (myelin sheath)
    • Have nodes of Ranvier and are rich in myelin (lipid)

Table 16-7-1


NEURONE

STIMULATED BY

TRANSMITS IMPULSES TO

STRUCTURE

Sensory

Receptor

Relay neurone

Cell body in root cell ganglion

Relay

Sensory neurone

Motor neurone

No axon

Motor/effector

Relay neurone

Gland, muscle
(effector organ)

Schwann cells

Change In Membrane Permeability Leading To The Generation Of An Action Potential

  • Stimulus reaches threshold
  • Voltage-regulated sodium channels open / influx of Na+ / down electrochemical gradient / +ve feedback
  • Depolarisation / inside becomes +ve / membrane potential reverses
    • //Depolarisation opens sodium channels in adjacent membrane
  • Potassium channels open (slower than Na+ gates) / diffusion of K+ ions out of neurone
  • Repolarisation
  • Sodium channels close
  • Hyperpolarisation due to overshoot in movement of K+out of the cell
    • //Membrane potential is lower than resting potential
    • //Interior of the cell becomes -ve \ membrane is more permeable to K+ ions than to Na+ ions
  • Sodium-potassium pump restores RESTING POTENTIAL
  • [GRAPH] Highest positive membrane potential is the action potential

The Role Of The Neurone Membrane In The Establishment Of A Resting Potential

  • MEMBRANE IS POLARISED: inside of axon is more -ve than outside
  • A resting potential of -70mV is maintained by [EXAM BYA7 JUN2002]
    • Negatively charged proteins/large anions inside axon
    • Membrane more permeable to K+ ions than to Na+ ions / K+ ions move out faster than Na+ ions diffuse in
    • Sodium/potassium pump / Na+ ions pumped out faster than K+ ions pumped in
  • Electrochemical gradient determines movement of ions
    • K+ cannot move down its conc. gradient
    • Build up of positive Na+ outside membrane repels K+
  • Imbalance of negative ions causes potential difference/voltage
    • Cl- cannot move down its conc gradient
    • Negatively charged proteins in cytoplasm repel Cl-

The All-Or Nothing Nature Of Nerve Impulses

  • Once action potential starts, it travels to a synapse
  • Stimulus must cause sufficient movement of Na+ and K+ to depolarise the membrane and
  • cause an action potential
  • Threshold stimulus → impulse that causes an action potential
    • Stimulus transmits an impulse at a constant and max strength
    • Transmission is independent of any intensity of the stimulus
    • High frequency of impulses / more amount of sodium entry / more ATP
  • Subthreshold stimulus → stimulus weaker than a threshold stimulus
  • Summation → series of subthreshold stimuli cumulate to cause an action potential

Refractory Period

  • Represents a time during which the membrane cannot be depolarised again
    • During repolarisation and hyperpolarisation
    • Membrane is impermeable to Na+ ions / sodium ion channels closed
    • Sodium ions cannot enter axon
    • K+ ions move out as membrane is more permeable to K+ ions
    • Membrane becomes more negative than resting potential
  • Nerve impulses can only travel in one direction
    • Action potential can only depolarise the membrane in front
    • Membrane behind is recovering from refractory period (previous action potential)
  • Limits frequency with which neurones can transmit impulses

Factors Affecting the Speed of Conductance: Myelin, Axon Diameter, Temperature

  • Impulses travel faster in myelinated neurones → SALTATORY CONDUCTION
    • Schwann cells prevent diffusion of ions
    • Flow of current between adjacent nodes of Ranvier
    • Thus, depolarisation only at nodes of Ranvier
    • Action potential jumps from node to node
  • Temp affects speed of conduction of impulses
    • Higher temp increases rate of diffusion of ions
  • Impulses faster in an axon with larger diameter
    • Small cells / large surface area:volume ratio / ion leakage weakens membrane
    • Myelin stops ion leakage \ diameter only important for unmyelinated neurones

Synaptic Transmission

  • Synaptic cleft (gap) of 20μm separates two neurones at a synapse (junction of 2 neurones)
    • Presynaptic membrane is at the end of a neurone
    • Postsynaptic membrane is at the next neurone in the chain
  • Synaptic knob of a presynaptic neurone contains
    • Neurotransmitters in small vesicles
    • Mitochondria to produce ATP needed for neurotransmitter synthesis

Aspects Of Synaptic Transmission

  • Unidirectional
    • Neurotransmitter always travels from pre- to postsynaptic membrane
    • Thus, flow in one direction only, action potential only in postsynaptic neurone
  • Summation
    • Several presynaptic neurones release neurotransmitter
    • Cumulative effect reaches a threshold to depolarise postsynaptic membrane
    • E.g. rod cells when they synapse with relay neurones in the retina
    • Spatial summation
      • Several impulses arrive at one neurone via several synapses
      • Cause sufficient depolarisation / open sufficient sodium ion channels
      • For threshold to be reached
    • Temporal summation
      • Several impulses arrive at same neurone via same synapse
      • Threshold → action potential
  • Inhibition
    • More inhibitory postsynaptic potentials IPSPs than excitatory postsynaptic potentials EPSPs
    • Reduces membrane potential / makes more negative
    • Hyperpolarisation of postsynaptic membrane
    • Cancels effect of action potential when several synapses

The Mechanisms Of Transmission At An Excitatory Synapse

  • Nerve impulse reaches synaptic knob/presynaptic membrane/neurone
  • Depolarisation opens Ca2+ gates / calcium ions enter
  • Ca2+ causes vesicles containing neurotransmitter to fuse with membrane
  • Release of neurotransmitter / into synaptic cleft / by exocytosis
  • Diffuse across synaptic cleft
  • Neurotransmitter binds to specific receptors in postsynaptic membrane
  • Sodium channels open / sodium ions enter
    • Depolarisation of postsynaptic membrane
    • Threshold causes an action potential along postsynaptic neurone
  • Neurotransmitter are quickly removed from the postsynaptic membrane
    • Diffuse out of the synaptic cleft
    • Taken up by presynaptic membrane by endocytosis
    • Enzymes break down neurotransmitters into inactive substances

Knowledge Of Transmitters Limited To Acetylcholine And Noradrenaline

  • Acetylcholine released by
    • motor neurones on to muscle cells
    • neurones in the parasympathetic division of the ANS (autonomic nervous system)
  • Noradrenaline is released in the sympathetic division of the ANS
  • //The international name for noradrenaline is now norepinephrine

The Agonistic And Antagonistic Effects Of Chemicals On Synaptic Transmission

  • Agonists/antagonists are similar in shape to neurotransmitter
    • Fit into specific protein receptors of postsynaptic membrane
  • Agonists → same effect as neurotransmitter
    • Anatoxin produced by some algae and mimics effect of acetylcholine
    • Swallowing H2O contaminated with anatoxin causes continuous salvation in mouth
  • Antagonists block action of neurotransmitter
    • Prevent neurotransmitter from binding with their receptor sites
    • High blood pressure can be treated by drugs called β-blockers
      • Antagonist of adrenaline-receptors on membrane of muscle cells in heart
    • Curare blocks action of acetylcholine at the junction of nerves and muscles
      • Useful as a general muscle relaxant in patients undergoing major surgery

Spinal Reflexes

  • Stimuli: Change in internal or external environment
  • Reflexes → automatic, same, fixed response to a stimulus
    • Dilating of pupils in dim light
    • Salvia production when tasting food
    • Withdrawing part of your body from a painful stimulus
  • Spinal reflex/Reflex arc is the nervous pathway of a reflex
    • Reflex arc involves spinal cord rather than brain (quicker action)
    • May involve brain
      • Control of muscles in iris
      • Tension in suspensory ligaments of eye
    • In-born but can be adapted through learning
      • Relay neurones carry impulses from reflex arc up ascending tracts in white matter to the brain
      • New relay neurones form in brain that stimulate motor neurone
      • They control e.g. muscles that control speech
    • Rapid and autonomic action
    • Large number of receptors, sensory, relay and motor neurones involved
  • Human spinal cord is a hollow tube of nervous tissue
  • 31 pairs of spinal nerves enter and leave spinal cord
    • Grey matter contains unmyelinated neurones (responsible for grey colour)
      • Synapse of relay neurone with sensory and motor neurone
    • White matter contains myelinated neurones (appear white)
      • Motor and sensory neurones
      • Is made up of interneuronal axons (tracts)
    • Sensory neurones enter spinal cord via dorsal root (back)
      • Their cell bodies are in the dorsal root ganglion
    • Motor neurones leave the spinal cord via the ventral root (front)

The Pathway and Adaptive Value of a Simple Spinal Reflex Involving 3 Neurones

  • Stimulus causes receptor to generate nerve impulse along SENSORY NEURONE
    • Moves along dendrite, dorsal root, to the cell body
    • Cell body is in the dorsal root ganglion, outside the cord/CNS
    • From cell body along axon
  • RELAY NEURONE in grey matter synapses with sensory and motor neurone
  • Impulse leaves spinal cord by ventral root of the spinal nerve
  • Moves along axon of MOTOR NEURONE to an effector
    • E.g. release neurotransmitter into muscle cell → contract
  • Brain receives information concerning sensory stimuli by other relay neurones
    • Their long fibres run through ascending (to brain) and descending tracts in the white matter of the spinal cord

Autonomic Nervous System (ANS)

Outline of the Functions of Parasympathetic and Sympathetic Divisions of the ANS

Table 16-10-1: The ANS controls internal glands + muscles beyond conscious control


Target Organ/Tissue
Effect on organ:
Motor neurone releases:

Parasympathetic Stimulation
Inhibitory effect / relaxation
Acetylcholine (ACh)

Sympathetic Stimulation
Excitatory effect / stress
Norepinephrine (noradrenlaine)

Iris of eyes

Constricts pupil

Dilates pupil

Bronchi, bronchioles

Constricts tubes

Dilates tubes

Blood vessels

- Dilates blood vessel
- Lowers blood pressure

- Constricts blood vessels
- Raises blood pressure

Heart

- Lowers heart rate
- Lowers stroke volume

- Raises heart rate
- Raises stroke volume

Intercostal muscles

Lowers breathing rate

Raises breathing rate

Salivary glands

Stimulates secretion of salvia

Inhibits secretion of salvia

Gut

Stimulates peristalsis

Inhibits peristalsis

Sweat glands

No effect

Increases sweat production

 

Specific Physiology in the Context of the Control of Heart Rate

  • Heart rate is controlled by cardiac centre in MEDULLA
  • Cardio accelerator centre CAC RAISES heart rate
    • Sympathetic neurones run from CAC down descending tracts
    • Along spinal nerve to sinoatrial node SAN
    • Release NORADRENALINE onto target cells
  • Cardio inhibitor centre CIC LOWERS heart rate
    • Parasympathetic neurones from CIC leave brain via VAGUS NERVE
    • Releases ACETYLCHOLINE onto target cells
  • CAC and CIC are always active, alter rate of depolarisation of SAN
  • Autonomic nervous system controls rate of heart contraction
    • Contracting muscles pressing on veins force blood towards heart causing greater filling of the ventricles which makes the heart beat faster and stronger
    • If blood pressure rises too far above normal, pressure receptors in the aorta and carotid (artery) sinus send nerve impulses to the CIC
    • CIC send inhibitory nerve impulses to the CAC and the SAN
  • Heart is myogenic, heart beats are initiated by electrical activity in the SAN, followed by the atrioventricular node and atrioventricular bundle
  • BETA BLOCKERS: bind to receptors in myocardium / prevent epinephrine (adrenaline) from binding to the myocardium / reduce rate of contractions