Human Nervous System and Taste

Control and Coordination

• Different parts of the body perform different functions, but the body works as one
• This is due to coordination systems within the body
• Two types of coordination systems exist: the nervous system and the endocrine system
• The nervous system uses electrical impulses (nerve impulses) to coordinate activities rapidly
• The endocrine system uses chemical messengers (hormones), carried in the bloodstream, to control slower activities

Comparing the Nervous and Endocrine systems

• | Feature | Nervous system | Endocrine system |
• |---|---|---|
• | Parts of the system | Brains, spinal cord, nerves/neurones | Glands |
• | Type of message | Electrical impulse | Chemical (hormone) |
• | Method of transmission | Neurones | Bloodstream |
• | Effectors | Muscles, glands | Target cells in specific tissues |
• | Speed of transmission | Very fast | Slower |
• | Duration of effect | Short (until electrical impulses stop) | Longer (until the hormone is broken down) |

The Nervous System

• The nervous system consists of:
  • Central nervous system (CNS):
    • Brain
    • Spinal cord
  • Peripheral nervous system (PNS):
    • Cranial nerves
    • Spinal nerves
    • Peripheral nerves
  • Neurones (nerve cells):
    • Sensory neurones (carry impulses from receptors to CNS)
    • Motor neurones (carry impulses from CNS to effectors)
    • Intermediate neurones (relay neurones, connect sensory and motor neurones)
• A bundle of neurones is known as a nerve
• Neurons coordinate the activities of sensory receptors, decision-making centres in the CNS, and effectors like muscles and glands

Neurones

• There are three main types of neurone: sensory, relay, and motor

• Sensory neurones carry impulses from receptors to the CNS

• Intermediate (relay) neurones are entirely within the CNS and connect sensory and motor neurones

• Motor neurones carry impulses from the CNS to effectors (muscles or glands)

• Each neurone has a different structure

• Motor neurones

  • Have a large cell body at one end within the spinal cord or brain
  • Have a nucleus in the cell body
  • Have many highly-branched dendrites

• Sensory neurones

  • Have a cell body that branches off in the middle
  • Have branches near the stimulus or in a spinal ganglion

• A neurone has a long fibre known as an axon:

  • It is insulated by myelin, a fatty sheath
  • Myelin is made by Schwann cells wrapping around the axon, with small uninsulated sections (nodes of Ranvier)

• Electrical impulses jump from node to node (saltatory conduction) – this speeds up transmission

Sensory Receptor Cells

• Receptor cells are transducers, converting one form of energy (light, heat, or sound) into an electrical impulse within a sensory neurone

• They are often found in sense organs such as the eye, taste buds, etc., and influence the sensory neurone.

• When stimulated, receptor cells become depolarised.

• Only strong stimuli will elicit a sufficient depolarisation to send an impulse.

• The strength of a stimulus affects how quickly impulses are fired, not the size of the impulse.

• Types of sensory receptors include Mechanoreceptors, Chemoreceptors, and Photoreceptors

Chemoreceptors

• Chemoreceptors detect chemicals in the internal and external environments.
• Taste is a form of chemoreception.
• Taste buds contain chemoreceptor cells that detect different tastes (sweet, sour, salty, bitter, umami).
• Dissolved chemicals interact with receptors
• This changes the shape of chemoreceptors causing sodium channels in receptor cells to open.
• This results in an action potential transmitted to the brain.

The Reflex Arc

• The reflex arc coordinates fast responses to stimuli, involving sensory, intermediate, and motor neurons
• This pathway allows quick responses to potentially harmful stimuli

Resting Potential

• The resting potential of a neuron describes the voltage difference across the cell membrane of a neuron that is not transmitting an impulse.
• This potential is usually around -70 mV
• Several factors contribute to maintaining the resting potential including sodium-potassium pumps, ion diffusion, and the permeability of the membrane to different ions, as well as other factors

Action Potential

• Action potentials describe a rapid change in voltage across a neuron membrane, resulting in a propagating wave of depolarization along the axon.
• Key steps include depolarization (increase in sodium), repolarization (decrease in sodium and increase in potassium), and hyperpolarization.

Repolarisation

• Repolarisation involves voltage-gated potassium channels opening, causing potassium ions to leave the axon rapidly, restoring the resting potential
• Repolarization is followed by hyperpolarization, making the membrane briefly more negative than its resting potential.

Refractory Period

• The refractory period is the period after an action potential when a new action potential cannot be easily initiated
• It is essential for ensuring that action potentials travel in only one direction along a neuron
• It limits the frequency at which action potentials can be produced

Synapse

• A synapse is the junction between two neurons
• Synaptic transmission
  • electrical impulses cannot 'jump' across synapses
  • chemical messengers called neurotransmitters are released from vesicles at the presynaptic membrane
  • neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
  • this stimulates the postsynaptic neurone

Cholinergic Synapse

• A specific type of synapse that uses acetylcholine (ACh) as its neurotransmitter
• Steps in cholinergic synapse function:
  1. Action potential arrives at synaptic knob.
  2. Calcium ions enter, triggering vesicle fusion and ACh release.
  3. ACh diffuses across the synaptic cleft and binds to receptors.
  4. Sodium channels open, causing depolarisation of the postsynaptic membrane.
  5. Acetylcholinesterase breaks down ACh, terminating the signal.

Muscle Contraction

• Striated muscle is composed of muscle fibres
• Sarcomeres are the contractile units within myofibrils
• The sliding filament model describes how thin (actin) and thick (myosin) filaments interact to shorten the sarcomere.
• Proteins like troponin, tropomyosin play crucial roles in regulating myosin-actin interactions.
• Calcium ions trigger muscle contraction by causing a shape change in troponin which moves tropomyosin, exposing myosin binding sites on actin
• ATP hydrolysis provides energy for the myosin heads to repeat the cycle.

Control and Coordination in Plants

• Plant hormones, often called plant growth regulators, influence many aspects of plant growth, including elongation growth
• Auxin is a plant hormone essential for growth, often produced at the tips of roots and shoots
• Auxin stimulates proton pumps, lowering the pH and activating expansins to loosen cell walls
• This increases cell expansion by osmosis.

Explore the fascinating functions of chemoreceptors and taste buds in humans. This quiz covers basic tastes, the role of synapses, and the components of the nervous system, providing a comprehensive overview of sensory and neural processes.