An animal must be able to respond to environmental stimuli
Sensory receptors and motor effectors
Sensory receptors can detect the stimulus
Motor effectors can respond to it
Nervous system
Consists of neurons and supporting cells
Types of neurons
Sensory neurons (afferent neurons)
Motor neurons (efferent neurons)
Interneurons (association neurons)
Sensory neurons
Carry impulses from sensory receptors to the central nervous system (CNS)
Motor neurons
Carry impulses from the CNS to effectors—muscles and glands
Interneurons
Located in the brain and spinal cord, help provide more complex reflexes and higher associative functions, including learning and memory
Peripheral nervous system (PNS)
Consists of sensory and motor neurons
Components of the somatic nervous system
Motor neurons that stimulate skeletal muscles to contract
Components of the autonomic nervous system
Motor neurons that regulate the activity of the smooth muscles, cardiac muscle, and glands
Sympathetic and parasympathetic divisions
Counterbalance each other in the regulation of many organ systems
Structure of a typical vertebrate neuron
Extending from the cell body are many dendrites, which receive information and carry it to the cell body
A single axon transmits impulses away from the cell body
Many axons are encased by a myelin sheath, with multiple membrane layers that insulate the axon
Small gaps, called nodes of Ranvier, interrupt the sheath at regular intervals
Schwann cells
Form myelin sheaths in the PNS
Oligodendrocytes
Form myelin sheaths in the CNS
Myelinated axons
Form the white matter in the CNS
Unmyelinated dendrites and cell bodies
Form the gray matter in the CNS
Myelinated axons in the PNS
Bundled together, much like wires in a cable, to form nerves
Nerve impulse transmission
Neurons have a charged cellular membrane, and the charge can change in response to neurotransmitter molecules and environmental stimuli
Ion channels
Allow ions to enter or exit the neuron, have different configurations: open, closed, and inactive
Voltage-gated ion channels
Change their structure in response to voltage changes, regulate the relative concentrations of different ions inside and outside the cell
Membrane potential
The difference in total charge between the inside and outside of the cell
Transmission of a signal between neurons
Carried by a chemical called a neurotransmitter
Transmission of a signal within a neuron
Carried by a brief reversal of the resting membrane potential called an action potential
Neurotransmitter molecules bind to receptors in a neuron's dendrites
A change occurs in the resting membrane potential which in turn initiates action potential
Action potential travels along the neurons up to the axon terminal
Triggers the release of neurotransmitters
Neurotransmitters travel across intercellular junctions known as synapses
To reach another neuron, muscle cells or gland cells
Axons
Transmit neurotransmitters to another axon
Synapses
Intercellular junctions between neurons
Synaptic transmission
Generally affects only the postsynaptic cell that receives the neurotransmitter
Hormone
A regulatory chemical that is secreted into extracellular fluid and carried by the blood and can therefore act at a distance from its source
Endocrine glands
Organs specialized to secrete hormones
Endocrine system
The organs and tissues that produce hormones
The blood carries hormones to every cell in the body
Only target cells with the appropriate receptor for a given hormone can respond to it
Hormone receptor proteins
Function in a similar manner to neurotransmitter receptors, specifically bind the hormone and activate signal transduction pathways that produce a response to the hormone
Paracrine regulators
Molecules released and act within an organ on nearby cells, do not travel through the blood to reach their target
Autocrine signaling
Cells release signaling molecules that affect their own behavior
Senses in humans
Olfaction (smell)
Gustation (taste)
Equilibrium (balance and body position)
Vision
Hearing
General senses (somatosensation)
Temperature
Pain
Pressure
Vibration
Senses related to somatosensation
Vestibular sensation (spatial orientation and balance)
Proprioception (position of bones, joints, and muscles)
Kinesthesia (limb movement)
Sensory transduction
The process of converting a stimulus (such as light, or sound, or the position of the body) into an electrical signal in the nervous system