Nervous System Function and Coordination in Animals
Role of the Nervous System in Coordination
Relationship with Environmental Stimuli
The nervous system plays a crucial role in coordinating responses to environmental stimuli. It receives information, analyzes it, prepares a response, and executes it. Stimuli are changes in environmental conditions, both internal and external.
Components of Nervous System Function
- Stimulus: A physical or chemical change in the environment.
- Sensory Receptor: Specialized cells that transform stimuli into nerve impulses. Sensory organs receive external information, while sensors receive internal information.
- 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 are involved in this relationship.
Neurons and Glial Cells
The nervous system is composed of neurons and glial cells.
Neurons
Neurons are star-shaped cells with a cell body (soma) containing the nucleus, branched extensions called dendrites, and a long extension called the axon that branches at its end. Nerve impulses leave the neuron through the axon and enter through dendrites. Axons connect with other neurons or effector organs. Axons are often wrapped by Schwann cells, which may contain myelin.
Myelin Sheath
The myelin sheath insulates the axon. Gaps between Schwann cells are called Nodes of Ranvier, which allow nerve impulses to travel faster.
Nerves
Axons bundled together and wrapped in fibrous tissue form nerves. Sensory (afferent) nerves carry impulses from receptors to the central nervous system, while motor (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 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 a synapse, it causes the release of neurotransmitters from vesicles in the synaptic button. These neurotransmitters cross the gap and bind to receptors on the next neuron’s dendrites, triggering a new impulse.
Speed of Nerve Impulse Transmission
Transmission speed depends on axon diameter (larger diameter, faster speed) and the presence of myelin. Myelin allows saltatory conduction, where the impulse jumps between Nodes of Ranvier, increasing speed.
Anatomy of the Nervous System
Simple Animals
Simple animals like polyps and jellyfish have a diffuse network of neurons. Annelids and mollusks have nerve nodes, with larger cerebroides in the head.
Vertebrates
Vertebrates have a central nervous system (CNS) and a peripheral nervous system (PNS) made up of nerves.
Central Nervous System (CNS)
The CNS, protected by the skull and spine, consists of the brain and spinal cord.
Brain
- Brain: Responsible for higher-level functions like instinct, learning, and memory.
- Cerebellum: Controls automatic behaviors and movement precision.
- Medulla Oblongata: Controls internal organ function.
Brain Evolution in Vertebrates
- Increased brain size and decreased cerebellum size.
- Appearance of cerebral hemispheres (absent in fish, distinct 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 and is protected by the spine. It contains gray matter (rich in neurons) and white matter (axons connecting to the brain). The spinal cord coordinates reflexes.
Involuntary Acts (Reflexes)
Components of a Reflex Action
- Sensory Neuron: Receives stimulus and transmits impulse to the spinal cord.
- Association Neuron: Relays impulse from sensory to motor neuron within the spinal cord.
- Motor Neuron: Transmits impulse from spinal cord to effector organ (muscle).
Autonomic Nervous System
The autonomic nervous system controls internal organs. It consists of the sympathetic system (active during alertness) and the parasympathetic system (active during rest).