Every Conscious Action Is Governed by the Nervous System
Chapter 7 Nervous System
Every conscious action is governed by the nervous system
Functions
· Communication between distant parts of the body
· Collection and integration of stimuli (both internal and external
· Formation and initiation of appropriate response
· Responsible for very rapid responses
· Crucial role in maintaining homeostasis
· Works in a concerted effort with the endocrine system (slower, longer-lasting responses)
The nervous divisions
· 2 divisions:
o Central nervous system (CNS): Brain and spinal cord
o Peripheral nervous system (PNS): All of the neurons that extend from the CNS
How the two work together
· Information is passed from sensory receptors to PNS
o Special senses (sight, hearing, smell, taste)
o General sensory receptors (touch, light, external heat, pain)
o Visceral receptors (monitoring internal organ function)
· Information moves from PNS to CNS
· Information is processed by the CNS and the appropriate response is determined
· Motor response is sent from CNS via PNS to appropriate muscles or glands
The peripheral nervous system
· Divided into 2 systems:
o Somatic
o Autonomic
· The PNS: Somatic division
o Serves the skin, skeletal muscles, and tendons
o Mainly involved with conscious actions but also responsible for reflex responses
o Automatic responses are called reflexes
· The PNS: Autonomic division
o Regulates the activity of involuntary muscles (cardiac and smooth) and glands to maintain homeostasis
o Divided into 2 divisions:
§ Sympathetic - coordinates the body for the “fight or flight” response by speeding up metabolism, heart rate, and breathing while down-regulating other functions
§ Parasympathetic - counters the sympathetic system by promoting a relaxed state by slowing metabolism, heart rate, and breathing and returning other functions to normal
The nervous system
· 2 cell types: neurons and neuroglia
· Expanding on neurons
o 3 types of neurons:
o Sensory – take impulses from sensory receptor to CNS
o Interneurons – receive information in the CNS and send it to a motor neuron
§ These essentially connect the sensory and motor neurons
o Motor – take impulses from the CNS to an effector (i.e. gland or muscle fiber)
· Nerve impulses move from the dendrite through the cell body and then down the axon
· From the axon terminus, the signal is transferred to the next neuron
· Nerve impulses
· Neurons function because without any impulse, they already have a resting potential
o This requires creating a charge imbalance across the cell membrane
o The charge imbalance is generated by the Na+/K+ pump (active transport requiring ATP)
o The ions remain in place because of gated channels
o When neurons “fire” in response to signals, there is a rapid reversal of the charge differential across the membrane
o Nerve impulses move from the dendrite through the cell body and then down the axon
o From the axon terminus, the signal is transferred to the next neuron
· Neurotransmitters
o Between adjacent neurons are small spaces called synapses
o Neurotransmitters are released by one neuron and received by the next neuron
· Synaptic integration
o Integration is the summation of the inhibitory and excitatory signals received by a postsynaptic neuron
o This occurs because a neuron receives many signals
The central nervous system
o Consists of the brain and the spinal cord
o Both are protected by:
o Bones – skull and vertebral column
o Meninges – 3 protective membranes that wrap around CNS
o Cerebral spinal fluid (CSF) – space between meninges is filled with this fluid that cushions and protects the CNS
o Both are made up of 2 types of nervous tissue:
o Gray matter – contains cell bodies and nonmyelinated fibers
o White matter – contains myelinated axons
The CNS: Brain
Four major parts:
1. The brain stem
o Midbrain – relay station between the cerebrum and the spinal cord or cerebellum; reflex center for sneezing, hiccuping, swallowing, coughing
o Pons – a bridge between the cerebellum and the CNS; regulates breathing rate; reflex center for head movements
o Medulla oblongata – reflex centers for regulating breathing, heartbeat, and blood pressure
2. Diencephalon
Includes the:
Hypothalamus – helps maintain homeostasis (responds to or initiates hunger, sleep, thirst, body temperature, and water balance) and controls the pituitary gland
Thalamus – 2 masses of gray matter that receive all sensory input except smell; involved in memory and emotions; also visual and auditory startle reflexes
Pineal gland – secretes melatonin that controls our daily rhythms
3. Cerebellum
o Receives and integrates sensory input from the eyes, ears, joints, and muscles about the current position of the body
o Functions
o Maintains posture
o Coordinates voluntary movement
o Allows learning of new motor skills (i.e. playing the piano or hitting a baseball)
4. Cerebrum – the lobes
o Cerebrum – largest portion of the brain
o Divided into 4 lobes/hemispheres:
o Frontal lobe: primary motor area and conscious thought
o Temporal lobe: primary auditory, smell, and speech area
o Parietal lobe: primary somatosensory and taste area
o Occipital lobe – primary visual area
o Cerebrum – the cerebral cortex
o Cerebral cortex – thin, outer layer of gray matter:
o Primary somatosensory area – sensory information from skeletal muscle and skin
o Primary motor area – voluntary skeletal muscle
o Association areas – integration occurs here
o Processing centers – perform higher level analytical functions, includes areas involved in speech
Higher mental functions
o Learning – what happens when we recall and use past memories
o Memory – ability to hold a thought or to recall past events
o Short-term memory – retention of information for only a few minutes
o Long-term memory – retention of information for more than a few minutes and includes the following:
o Episodic memory – persons and events
o Semantic memory – number and words
o Skill memory – performing skilled motor activities (i.e. riding a bike)
o Language – depends on semantic (words and their meanings) memory
Drugs and drug abuse
o Drugs have two general effects on the nervous system - affect the limbic system or promote the action of a certain neurotransmitter
o Most drug abusers take drugs that affect dopamine and thus artificially affect the reward circuit to the point they ignore basic physical needs in favor of the drug
o Nicotine causes the release of dopamine