BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 1
Terms you should understand: somatosensory pathways, somatosensory cortex, somatotopic organization, cortical receptive field, dorsal columns, anterolateral tracts, thalamus, medial lemniscus, tonic, phasic, basal ganglia, cerebellum, motor cortex, pre-motor cortex, brain stem, bulbar motor nuclei, vestibular nuclei, pyramidal tracts, corticospinal tract, extrapyramidal tracts, dorsolateral tracts, ventromedial tracts.
I. Sensory information from the body (as opposed to the organs of the special senses, such as the eyes, ears, tastebuds, etc.) is processed both within the spinal cord and by the brain. A. Some sensory input produces behavioral output by way of spinal reflexes (e.g., stretch reflex, flexion reflex), which cause behavior even before the information reaches the brain. B. Different modalities of sensory information are handled differently by the nervous system. Information is carried along different pathways, and it is sent to different parts of the brain for processing. (e.g., somatic touch and pain). C. Sensory information is sent to specific regions of the cerebral cortex, where aspects of the signal are analyzed and information from different sensory modalities is integrated. D. Somatosensory pathways (somatosensory = sense of body, mostly skin and muscle) 1. Somatosensory cortex. a. Sensory information is arranged in an orderly, somatotopic manner.
Fig. 6.1
BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 2
b. Like receptor neurons, each cortical cell has a receptive field, but it is quite different– usually much more complex--from the receptive field of a sensory neuron because each cortical neuron receives input from many neurons. 2. Information from a particular sensory neuron may reach the cortex along parallel, but independent, pathways between the periphery and the cortex. a. The pathways usually involve different number of synapses along axons with different conduction velocities, so some pathways are slower than others. b. Example: touch information is carried both by the dorsal columns through the medial lemniscus (Fig. 6.2) and by the anterolateral tracts (Fig. 6.3). Neurons within these tracts synapse in different nuclei within the brain, so although all touch information eventually gets to the cortex, it does so by different routes. c. The final connection to the cortex from both pathways is the thalamus.
Fig. 6.2. Pathway of touch sensation: through the dorsal columns.
BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 3
Fig. 6.3. Pathway of pain and temperature sensation: through the anterolateral system.
BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 4
E. Other cortical sensory areas process other sensory information. 1. Different aspects of a stimulus are recognized by different cortical neurons: a. Light vs. heavy touch b. Moving (phasic) vs. stationary (tonic). c. Large vs. small. 2. Some cells in integrative areas receive input from more than one sensory modality, e.g., touch, temperature, and information on body position all converge on some neurons.
Comparison of how touch and pain information is transmitted between the periphery and the cerebral cortex ______
Touch Pain, temperature
Receptors 1. Tonic mechanoreceptors Free nerve endings 2. Phasic mechanoreceptors
Position of sensory Dorsal Root Ganglion Dorsal Root Ganglion neurons' somata
Major pathways in spinal cord 1. Ipsilateral dorsal columns 1. Synapse onto dorsal horn (with somatotopic map) interneurons, which carry signal across midline 2. (Intersegmental synapses) 2. (Intrasegmental synapses) 3. A little information is 3. Information carried in carried in the contralateral contralateral anterolateral anterolateral tract. tracts, e.g.,
Major pathways in brain 1. Dorsal column nuclei, go 1. Spinothalamic tract ends 2. Across midline in thalamus in medial lemniscus, to 3. Ventral posterior 2. Thalamic pain neurons thalamus (a relay nucleus), to project broadly to 4. Somatosensory cortex cortex and brain stem (map is somatotopic) 5. Cortical feedback regulates information transfer along pathway to cortex.
Cortical processing 1. Primary projection areas Diffuse endings in many send information to regions of the brain, 2. Secondary projection areas including somatosensory and information eventually cortex and association reaches the cortex. 3. Association cortex
F. Neurons in association cortex receive inputs from sensory cortex, from other parts of the cortex, and from deeper structures. BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 5
G. Feedback from the cortex allows the input pathways to be modulated. Cortical efferents from all areas send fibers back to the various levels of sensory processing, thus modulating the information that is fed forward to the cortex.
Please note: the purpose of this figure is not to make you memorize all these connections, but rather to show you the richness and complexity of connections that participate in just the primary processing of somatosensory information.
Fig. 6.4 BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 6
II. Motor pathways (efferent pathways) carry information away from the centers of processing (for example, away from the cerebral cortex or the spinal cord). A. Movement is controlled by activity in one or more of these hierarchically arranged structures (arranged from most peripheral to most central): spinal cord, brain stem, motor cortex, pre-motor cortex. 1. There are connections among all of these regions, as well as a serial pathway from one to the next. 2. Other structures in the brain: basal ganglia and cerebellum, for example. 3. Circuits that produce some very simple behavior are located entirely within the spinal cord; e.g. the stretch reflex, flexion reflex, and the alternating contraction of antagonistic muscles in the limbs during locomotion. 4. Interneurons contained entirely within one or a few spinal segments form the circuits that produce flexion in response to pain and the movements made during locomotion. i. The brain modulates their activity, but input from the brain is not required to produce the patterned behavior. ii. Experiment: a cat that has had the connection between its brain and spinal cord entirely cut will still walk on a treadmill.
B. Nuclei in the brain stem ( e.g., the vestibular nuclei that receive information from the sensory receptors in the inner ears and from other organs that sense body position) are crucial to maintaining posture. 1. Postural adjustments occur continuously throughout our waking hours. 2. Most motor signals in the spinal cord originate from neurons with their somata in brain stem nuclei (also called bulbar motor nuclei). 3. These neurons are part of a group of pathways called the extrapyramidal pathways (more about these pathways and why they have that name soon). BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 7
C. The motor cortex is an important higher center for generating "voluntary" activity. 1. Many fibers from motor cortex travel directly to spinal motor neurons via the corticospinal tract. These fibers generate voluntary somatic movements. 2. Corticospinal axons run in the pyramidal tracts in the brain. Pyramidal axons cross the midline in the decussation of the pyramids, located in the medulla. Hence, the right side of the brain controls the left side of the body.
Fig. 6.5
3. Some fibers from motor cortex synapse in the bulbar motor nuclei. For example, bulbar motor fibers control voluntary facial movements. 4. Like somatosensory cortex, motor cortex is somatotopically arranged. Neurons that affect the same part of the body lie near to one another. BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 8
! ! ! ! neurons) Extrapyramidal tract
Fig. 6.6 corticospinal Pyramidal tract Pyramidal (
5. Once corticospinal fibers enter the spinal cord, they divide into two tracts on each side of the cord: axons in the dorsolateral tracts (aka lateral coticospinal tracts) synapse onto motor neurons that control the movement of limbs; axons in the ventromedial tracts (aka ventral columns, and anterior corticospinal tracts) synapse onto motor neurons that control movement of the trunk muscles. 6. Similarly, some bulbospinal axons run in the ventromedial tract and others in the dorsolateral tracts; these bulbospinal axons are major extrapyramidal tracts. 7. Some—but not all—neurons are named for the location of its soma within the central nervous system. 8. Charles Sherrington, around the turn of the 19th century, demonstrated that - surprisingly - a lot of the signals descending from the cortex are inhibitory, not excitatory. D. Premotor cortex: 1. Receives input from is from association areas of the cortex and from the basal ganglia (see below) 2. Most of the output is to the motor cortex. 3. Its function: such things as programming movements, identifying targets in space, and for choosing a course of action among many possibilities. E. Other major structures in the brain that contribute to shaping motor activity: 1. Cerebellum: smoothes motor output, apparently by comparing descending motor signals with sensory input that results from the consequences of motor activity. 2. Basal ganglia: lesions in the basal ganglia cause involuntary movements and disturbances in posture, e.g., Parkinson's disease. • Some neurons with their somata in the basal ganglia send axons into the spinal cord, as part of the extrapyramidal pathways. BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 9
F. Neurons carrying sensory information synapse on cells at every level of motor processing. 1. There is feedback between the input and the output branches of the nervous system at every level. 2. Interneurons at every level allow these connections to be either excitatory or inhibitory. G. Motor pathways in the CNS are organized anatomically according to function. 1. Motor neurons in the ventral horn of the spinal cord are organized according to the muscles that they innervate. a. The somata of motor neurons that excite proximal (trunk) muscles lie medially; the somata of neurons that excite limb muscles lie laterally. b. The somata of motor neurons that excite extensors lie ventral; the somata of neurons that excite flexors lie more dorsally (but still in the ventral horn). 2. Trunk muscles mediate postural reflexes and activities; the processes of the neurons that excite the motor neurons controlling these muscles run in the ventromedial tracts and mediate postural responses. (Remember, the ventromedial tracts, or ventral columns, contain both pyramidal and extrapyramidal axons.) a. These neurons branch widely; many terminate bilaterally. b. They end in many spinal segments over long sections of the cord. c. They, therefore, exert diffuse control and can coordinate activity in many segments at once. 3. Muscles in the limbs produce more complex, detailed motions. a. Axons of the neurons that excite motor neurons innervating these muscles run in the dorsolateral tracts. (Again, remember that these tracts contain axons from both the pyramidal and extrapyramidal pathways.) b. These descending neurons innervate a smaller number of motor neurons and terminate in a small number of spinal segments, rendering their innervation pattern more precise. c. They, therefore, exert more specific control. BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 10
Sensory paths to somatosensory cortex (red, blue) Outputs from motor cortex and somatosensory output to motor cortex pyramidal tract = green extrapyramidal tracts = blue
Connections among basal ganglia, cerebellum and motor cortex
Fig. 6.7. Summaries of the connections among sensory and motor areas of the brain. BIPN100 F15 Human Physiol I (Kristan) Lecture 6. Sensory and Motor Pathways p. 11
Neuronal pathways - Summary of properties
1. Most kinds of information are carried along more than one pathway through the nervous system, and these pathways carry information independently of one another. This pattern is called parallel processing.
2. All pathways cross the mid-line somewhere. Part of your job is to learn where each one crosses.
3. All pathways between the periphery and higher centers within the central nervous system— on both the sensory and the motor side—consist of several neurons linked by synapses to form a chain with synapses. (The stretch reflex is an exception to this rule.) Part of your job is to learn where the synapses are located along each pathway we talk about.
4. The thalamus is a major center for receiving sensory information and transmitting it to the cortex.
5. All sensory information is sent to more than one part of the brain. Part of the description of how specific sensory information is handled is where the information travels and the location of the synapses in the chain.
6. In many locations in the CNS, neurons are connected to one another reciprocally; that is, each makes synapses onto the neurons that makes synapses onto it.
7. All of the information processing in the cortex is done by interneurons that connect with other interneurons, both within the same cortical region and between cortical regions.
8. Topological relationships are maintained in many pathways as information travels from one neuronal center to another
NOTE: This summary describes some properties that neuronal pathways have in common. In addition to knowing these general properties, you need to know the specific components of real pathways and understand how they fit these generalities.