The Role of the Nervous System in Relationship and Coordination
Relationship Function
The nervous system plays a crucial role in relating an organism to its environment. This involves receiving environmental stimuli and developing appropriate responses. Information from the environment is analyzed by the nervous system, triggering the preparation and execution of a response.
Components of the Relationship Function:
- Stimulus: Changes in environmental conditions, both internal and external, that trigger a response.
- Sensory Receptor: Specialized cells that transform stimuli into nerve impulses. Sensory organs receive external stimuli, while sensors receive internal stimuli.
- Coordinating Center: Interprets nerve impulses from sensory receptors and generates a response.
- Effector Organ: Receives nerve impulses from the coordinating center and carries out the response. This can be a muscle (for movement) or a gland (for secretion).
The nervous and endocrine systems work together to facilitate this relationship function.
The Nervous System
The nervous system consists of neurons and glial cells.
Neurons:
Neurons are star-shaped cells with a cell body (soma) containing the nucleus. They have branched extensions called dendrites that receive signals and a long extension called the axon that transmits signals.
Nerve impulses travel from the neuron through the axon and enter other neurons through dendrites. Axons are often wrapped by Schwann cells, which may contain a lipid substance called myelin. Myelin acts as an insulator and speeds up nerve impulse transmission. Gaps between Schwann cells are called Nodes of Ranvier, where the nerve impulse jumps, further increasing speed.
Nerves:
Axons bundled together form nerves, which are wrapped in fibrous tissue. Sensory or afferent nerves carry impulses from sensory receptors to the central nervous system, while motor or efferent nerves carry impulses from the central nervous system to effector organs.
Nature of the Nerve Impulse
The nerve impulse is an electrochemical current that travels along the neuron’s membrane. A resting neuron has a positive electrical potential outside the membrane and a negative potential inside.
Synapses:
Synapses are connections between neurons, separated by a synaptic gap. When a nerve impulse reaches the synapse, it triggers the release of neurotransmitters from the synaptic button (enlarged end of the axon). Neurotransmitters cross the synaptic gap and bind to receptors on the dendrite of the next neuron, initiating a new nerve impulse. This process ensures one-way transmission of the nerve impulse.
Speed of Nerve Impulse Transmission:
The speed of nerve impulse transmission depends on the diameter of the axon (larger diameter means faster transmission) and the presence of the myelin sheath. Myelin allows for saltatory conduction, where the impulse jumps between Nodes of Ranvier, significantly increasing speed.
Anatomy of the Nervous System
Invertebrates:
Simple animals like polyps and jellyfish have a diffuse network of neurons. Annelids and mollusks exhibit some ganglia (clusters of neurons), with the cerebral ganglia in the head being larger.
Vertebrates:
Vertebrates have a central nervous system (CNS) and a peripheral nervous system (PNS) composed of nerves.
Central Nervous System:
The CNS is located dorsally and protected by the skull and spine. It consists of the brain and the spinal cord.
- Brain: Responsible for higher-level functions like instinct, learning, and memory.
- Cerebellum: Controls automatic aspects of behavior and movement precision.
- Medulla Oblongata: Controls the function of internal organs.
Vertebrate brain evolution shows:
- Increased brain size and decreased cerebellum size.
- Appearance of cerebral hemispheres, absent in fish and well-differentiated in reptiles, birds, and mammals.
- Appearance of cerebral convolutions (surface folds that increase surface area) in mammals.
Spinal Cord:
The spinal cord connects to the brain through the medulla oblongata and is protected by the spine. It contains gray matter (rich in neurons) and white matter (composed of axons connecting the spinal cord to the brain). The spinal cord coordinates reflexes, which are rapid, involuntary responses that do not require brain processing.
Involuntary Acts (Reflexes)
A reflex action involves:
- Sensory Neuron: Receives a stimulus and transmits it as a nerve impulse to the spinal cord.
- Association Neuron: Located in the spinal cord, receives the impulse from the sensory neuron and transmits it to a motor neuron.
- Motor Neuron: Receives the impulse from the association neuron and transmits it to the effector organ (muscle).
The muscle contracts in response to the impulse, producing movement.
Autonomic Nervous System
The autonomic nervous system controls the internal organs. It consists of nerves that originate from the medulla oblongata or spinal cord. It has two components:
- Sympathetic System: Prepares the body for “fight or flight” situations (increased heart rate, etc.).
- Parasympathetic System: Promotes “rest and digest” functions (slows heart rate, etc.).
The balance between these two systems ensures proper functioning of the internal organs.