HUMB1001 Lecture Notes - Lecture 1: Reflex Arc, Spinal Nerve, Cranial Nerves
How body functions are controlled
Two main control systems
1. The Nervous System
❖ For rapid response
• Action potentials and neurotransmitters
❖ Controlled consciously (with cerebrum)
❖ Subconsciously (via somatic or autonomic reflexes)
2. The Endocrine System
❖ For slower, more prolonged response
• Uses hormones
❖ Via feedback loops
Nervous System
• 12 pairs → Cranial nerves
• 31 pairs → Spinal nerves
Coronally Cut Cerebrum
❖ Cerebral Cortex
o 2-4mm of grey matter
o Follows the contours of the gyri and sulci
o Increases amount of grey matter for a given area
o Conscious thought and recognition of stimuli
Conscious Control
❖ For sensory information
❖ Sensory/afferent neuron
❖ To CNS for integration
o Spinal cord
▪ Sensory tracts (ascending)
o Brain
▪ Through brainstem
▪ To Cerebral cortex
▪ Conscious awareness of stimuli
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Conscience Control
❖ Requires Central Nervous System
o Nerve impulse travels further
o Many more synapses (connections)
o Decision needs to be made by conscience brain before a response occurs
o More synapses slower the reaction time!
Enter the Reflex!
A reflex is an involuntary response to a stimulus:
❖ Rapid
o Minimal synapses
❖ Predictable
o Same response every time
❖ Unlearnt
o Uses a specific neural pathway (reflex arc)
❖ Cannot be improved by repetition
Reflex Arc Components
1. A sensory receptor detects a stimulus
2. A sensory neuron conducts action potentials through the nerve and dorsal root to
the spinal cord
3. In the spinal cord, the sensory neuron synapses with an interneuron. (An
interneuron is not involved in a monosynaptic reflex arc)
4. The interneuron synapses with a motor neuron
5. A motor neuron axon conducts action potentials through the ventral root and spiral
nerve to an effector organ
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Reflexes
• Bypass the conscious part of the brain
o Can occur without higher brain centre involvement
• Minimal synapses
o Allows for a quick response
Reflexes can be classified functionally as either
a) Somatic
b) Autonomic (visceral)
Somatic Reflexes
Deal with skeletal muscles
❖ Remove the body from painful stimuli
❖ Keeping balance/posture
❖ Involves either the spinal cord or brain as the integration centre
E.g. Patellar Reflex
Autonomic Reflexes
❖ Involve the control of
o Smooth muscle (e.g. wall of blood vessels or bronchioles)
o Cardiac muscle (e.g. the heart)
o Body viscera (organs such as GUT wall, salivary glands)
❖ Used for maintaining bodily functions
o E.g. HR, BP, Blood CO2 levels
o Movement of food through the digestive system
o Defecation and Micturition relflex
❖ Work on same principles as other reflexes
o Can involve spinal cord
▪ E.g. micturition reflex, defecation reflex (sacral)
o Brain stem through Autonomic NS
▪ E.g. HR, BP, etc.
Brain Stem
Composed of the medulla oblongata, pons and midbrain.
❖ Primitive part of the brain
❖ Has many important integration centres
o important for vital functions and reflexes
❖ All information coming from or go to the spinal cord must travel through this region
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Document Summary
Two main control systems: the nervous system. For rapid response: action potentials and neurotransmitters. Subconsciously (via somatic or autonomic reflexes: the endocrine system. For slower, more prolonged response: uses hormones. Nervous system: 12 pairs cranial nerves, 31 pairs spinal nerves. Cerebral cortex: 2-4mm of grey matter, follows the contours of the gyri and sulci. Increases amount of grey matter for a given area: conscious thought and recognition of stimuli. To cns for integration: spinal cord, sensory tracts (ascending, brain, through brainstem, to cerebral cortex, conscious awareness of stimuli. Requires central nervous system: nerve impulse travels further, many more synapses (connections, decision needs to be made by conscience brain before a response occurs, more synapses slower the reaction time! A reflex is an involuntary response to a stimulus: Unlearnt: uses a specific neural pathway (reflex arc) Reflex arc components: a sensory receptor detects a stimulus, a sensory neuron conducts action potentials through the nerve and dorsal root to the spinal cord.
