Lecture 14: Nervous & Sensory Systems
Review of Neuron Transmission
1. What is the difference between an electrical synapse and a chemical
synapse?
2. What mechanism ensures that an electrical impulse in a neuron is
conducted in only one direction down the axon?
3. What are the steps in neuronal transmission (from presynaptic to
postsynaptic)?
Evolution of the Nervous system
What is driving the evolution of the nervous system?
Cephalization – the formation of a distinct head region with a control center (ganglia
or brain) & sensory structures
Development of the spinal cord
The vertebrate nervous system
The Spinal Cord
The spinal cord carries impulses between the brain and PNS, and contains
interneuron circuits that control motor reflexes
has a butterfly-shaped core of gray matter surrounded by white matter
Pairs of spinal nerves connect with the spinal cord at spaces between the vertebrae
Spinal Reflexes
Afferent axons synapse with interneurons in the gray matter that send axons to the
brain
Efferent axons from interneurons of the brain pass the spinal cord and synapse with
motor neurons in gray matter of the cord
Axons of efferent neurons exit the spinal cord through the spinal nerves
Gray matter of the spinal cord contains pathways involved in reflexes
programmed movements that take place without conscious effort, facilitates a
rapid response
Knee-Jerk Reflex and the Spinal Cord
Central Nervous System (CNS)
How does the CNS function?
integrates incoming sensory information from the PNS into compensating responses
The brain and spinal cord are surrounded and protected by three layers of
connective tissue (meninges) and by cerebrospinal fluid
The fluid cushions the brain and spinal cord, nourishes the CNS, and protects it from
toxic substances
Blood-brain barrier
The seal of tight junctions in the epithelial cells of capillary walls in the brain
prevents most substances in blood (eg: toxins, viruses &bacteria) from entering the
cerebrospinal fluid
The Human Brain
Development of the Human Brain
During embryonic development, the anterior part of the neural tube enlarges into
three distinct regions: hindbrain, midbrain, and forebrain
Adult Brain Regions
The hindbrain develops to:
Cerebellum (integrates sensory & motor signals)
Medulla oblongata (controls respiration and blood circulation)
The brain stem is made up of the midbrain, the pons and medulla
The pons - major information traffic center
The midbrain coordinating reflex responses (involuntary reactions)
Development of Vertebrate Brain
The forebrain develops into the cerebrum (telencephalon)
• Largest part of the brain
• Controls higher functions such as thought, memory, language, and
emotions, and voluntary movements
• The forebrain also forms the thalamus and hypothalamus
The thalamus receives sensory input and relays it to the regions of the cerebral
cortex
• The hypothalamus is the primary center for homeostatic control over
the internal environment
Gray Matter Centers
Regions and Association Areas of the Cerebral Cortex
Peripheral nervous system collects information and carries out responses
PNS
Sensory and motor neurons
Somatic NS stimulates skeletal muscles
Autonomic NS stimulates smooth & cardiac muscles and glands
Sympathetic and parasympathetic NS counterbalance each other
The Somatic Nervous System
Controls body movements that are primarily conscious and voluntary
controls reflexes and shivering, and maintains posture and balance
Motor neurons carry efferent signals from the CNS to the skeletal muscles
Dendrites and cell bodies of motor neurons are located in the spinal cord (CNS)
the somatic portions of the cranial and spinal nerves consist only of axons
The Autonomic Nervous System
Controls involuntary processes:
digestion, sweat glands, and circulation, many functions of reproductive and
excretory systems, and contraction of smooth muscles in all parts of the body
Autonomic pathways include two neurons:
Parasympathetic and Sympathetic Divisions
The autonomic system divided into sympathetic and parasympathetic (have
opposing effects)
The sympathetic division involves stress or danger responses (fight or flight) –
increases heart rate and blood pressure, and prepares the body for emergency
responses
The parasympathetic division involves quiet, low-stress situations – promotes
activities such as digestion
Sensory System
Senses provide information about the environment
Sensory receptors provide information from our internal and external
environments that is crucial for survival and success
Specialized sensory receptors for:
1. Vision
2. Hearing
3. Taste
4. Smell
5. Touch
Overview of Sensory Receptors
Sensory information is conveyed to the CNS and perceived in a four-step process
Sensory Transduction
Sensory receptors respond to stimuli by undergoing a change in receptor potential
a graded membrane potential
Change is caused by changes in the rate of conduction of positive ions (Na+, K+, Ca2+)
across the plasma membrane
Gated ion channels open or close in response to stimuli, altering membrane
potential
if this change exceeds a threshold, an action potential is generated
Sensory transduction involves gated ion channels
Sensory cells respond to stimuli via stimulus-gated ion channels in their membranes
Open or close depending on the sensory system involved
In most cases, a depolarization of the receptor cell occurs
Why?
Analogous to the excitatory postsynaptic potential (EPSP)
Referred to as receptor potential
The larger the sensory stimulus, the greater the degree of depolarization of the
receptor potential and the higher the frequency of action potentials
Five Basic Types of Receptors
1. Mechanoreceptors are stimulated by mechanical forces such as pressure
2. Chemoreceptors detect chemicals or chemical changes (ex: taste buds on the
tongue) – WILL NOT COVER
3. Thermoreceptors react to heat and light energy (ex: temperature receptors in
the skin)
4. Photoreceptors detect the energy of light (ex: photoreceptors in the retina of the
eye) – WILL NOT COVER
5. Nociceptors detect tissue damage or noxious chemicals and register their activity
as pain
Mechanoreceptors: Receptors for Touch and Pressure
Cutaneous receptors
Receptors in the skin
Classified as interoreceptors
Respond to stimuli at the border between internal and external environments
Receptors for heat, cold, touch, and pressure
Proprioceptors detect stimuli used in the CNS to maintain body balance and
equilibrium and to monitor the position of the head and limbs
Types of Mechanoreceptors in Human Skin
1. Phasic – intermittently activated
• Hair follicle receptors, Meissner corpuscles, Pacinian corpuscles
2. Tonic – continuously activated
• Ruffini corpuscles, Merkel’s disks
Nociceptors (pain receptors)
Transmit impulses perceived as pain
Sensitive to noxious substances and tissue damage
Most consist of free nerve endings located throughout the body, especially near
surfaces
Transient receptor potential (TRP) ion channel,
capsaicin receptor – sensation of heat and pain
Hearing: Detection of sound waves
Sound is the result of vibration, or waves, traveling through a medium
Detection of sound waves is possible through the action of specialized
mechanoreceptors that first evolved in aquatic organisms
Lateral Line System in Fish
Sense objects that reflect pressure waves and low-frequency vibrations
Supplements hearing
Bending of stereocilia in the direction of the kinocilium has a stimulatory effect
Bending in opposite direction is inhibitory
Inquiry question
How would the lateral line system of a shark detect an injured and thrashing fish (or
human)?
Ear Structure of Land Vertebrates
Air vibrations are channeled through the ear canal of the outer ear
Cause the tympanic membrane (ear drum) to vibrate and movement of three small
bones (ossicles) in the middle ear
Malleus (hammer), incus (anvil), and stapes (stirrup)
The stapes vibrates against the oval window, which leads into the inner ear
The Inner Ear: Cochlea
The cochlea - bony structure containing part of the cochlear duct
All three chambers are filled with fluid
Pressure waves travel down the tympanic canal to the round window, which is
another flexible membrane
Transmits pressure back to middle ear
Ear Structure of Land Vertebrates
As pressure waves are transmitted through the cochlea to the round window, they
cause the cochlear duct to vibrate
Organ of Corti
Basilar membrane contains sensory hair cells
Stereocilia from hair cells project into tectorial membrane
Bending of stereocilia depolarizes hair cells
Hair cells send action potentials to the brain
Navigation by Sound
A few mammals have the ability to perceive presence and distance of objects by
sound
Bats, shrews, whales, dolphins
Emit sounds and then determine the time it takes these sounds to return
Echolocation
The same principle is used for sonar and radar detection
Detection of Body Position
Invertebrates orient themselves with respect to gravity using a statocyst
Consists of ciliated hair cells embedded in a membrane with calcium carbonate
stones (statoliths)
When the animal moves, statoliths lag behind the movement, bending the sensory
hairs and triggering action potentials
A statocyst, an invertebrate organ of equilibrium
The Vestibular Apparatus - Detection of Body Position
In vertebrates, the gravity receptors in the vestibular apparatus consist of two
chambers in the membranous labyrinth and fluid filled canals
semicircular canals
the utricle (horizontal position)
the saccule (vertical position)
all filled with a fluid called endolymph
Detection of Body Position
Utricle and saccule contain hair cells with stereocilia and a kinocilium
Calcium carbonate-rich otolith membrane
Utricle for horizontal acceleration
Saccule for vertical acceleration
Both types of accelerations cause cilia to bend, thus producing an action potential in
an associated sensory neuron
Hearing and Equilibrium
At the ends of semicircular canals are swollen chambers called ampullae
Groups of cilia protrude into them
Tips of cilia are embedded within a gelatinous cupula in contact with endolymph
fluid of each canal
• Different body angles cause different hair cells and their sensory neurons to
be stimulated
• Spinning causes disruption in the equilibrium of the semicircular canals and
you become dizzy
Senses and Balance
When the head rotates, the semicircular canal fluid pushes against the cupula,
causing the cilia to bend
Bending in the direction of the kinocilium causes a receptor potential
Stimulates an action potential in the associated sensory neuron
Saccule, utricle, and semicircular canals are collectively called the vestibular
apparatus
Sensory System Review
I. Kinds of sensory receptors include
1. Mechanoreceptors
2. Chemoreceptors
3. Thermoreceptors
4. Photoreceptors
5. Nociceptors
II. The four steps by which information is conveyed to the CNS:
1. stimulation
2. transduction
3. transmission
4. interpretation
III. Sensory Transduction:
Gated ion channels open or close in response to stimuli, altering membrane
potential; if this change exceeds a threshold, an action potential is generated.
Exercising our senses